Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session GP1: Poster Session IV |
Hide Abstracts |
Room: Adam's Mark Hotel Grand Ballroom I & II 2:00pm |
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GP1.00001: EDUCATION AND OUTREACH, HIGH SCHOOL RESEARCH, AND UNDERGRADUATE RESEARCH |
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GP1.00002: Teaching plasma physics with computational physics Scott Robertson A senior level course in either plasma physics or in numerical methods may not interest enough students to satisfy The Dean, however, a combined course of one-semester has been found to generate sufficient enrollment. Plasma physics is particularly well-suited for marriage with numerical methods, however, a combined course requires omitting some plasma physics topics. The computational part of the course uses Mathcad, an easy-to-learn spreadsheet. The students download numerical routines (http://debye.colorado.edu/phys4150) that plot potential contours and field lines, follow particle trajectories in magnetic fields, find roots of dispersion relations, evaluate dispersion functions, and the students see more advanced routines for the sheath and presheath, the Fokker-Planck equation, WKB ray tracing, and the Vlasov equation. The course follows F. F. Chen's text and the computational part of the homework often requires applying the written routines in new ways. [Preview Abstract] |
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GP1.00003: Plasma Science and Applications at the Intel International Science and Engineering Fair Lee Berry The Coalition for Plasma Science (CPS) has established a plasma prize at the Intel International Science and Engineering Fair (ISEF). This year's prize was awarded for projects in simulated ball lightning and plasma thrusters. The CPS is a broadly-based group of institutions and individuals whose goal is to increase the understanding of plasmas for non-technical audiences. In addition to the ISEF plasma award, CPS activities include maintaining a website, http://www.plasmacoalition.org; developing educational literature; organizing educational luncheon presentations for Members of Congress and their staffs; and responding to questions about plasmas that are received by the CPS e-mail or toll-free number. The success of these activities depend on the voluntary labor of CPS members and associates. These volunteers include the ISEF judges, whom the APS/DPP and the IEEE/PSAC helped identify. Please send an e-mail to the CPS at CPS@plasmacoalition.org for information if you would like to become involved in spreading the good word about plasmas. [Preview Abstract] |
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GP1.00004: Promoting Plasma Physics as a Career: A Generational Approach James Morgan A paradigm shift is occurring in education physics programs. Educators are shifting from the traditional teaching focus to concentrate on student learning. Students are unaware of physics as a career, plasma physics or the job opportunities afforded to them with a physics degree. The physics profession needs to promote itself to the younger generations, or specifically the millennial generation (Born in the 1980's-2000's). Learning styles preferred by ``Millennials'' include a technological environment that promotes learning through active task performance rather than passive attendance at lectures. Millennials respond well to anything experiential and will be motivated by opportunities for creativity and challenging learning environments. The open-ended access to information, the ability to tailor learning paths, and continuous and instantaneous performance assessment offer flexibility in the design of curricula as well as in the method of delivery. Educators need to understand the millennial generation, appeal to their motivations and offer a learning environment designed for their learning style. This poster suggests promoting a physics career by focusing on generational learning styles and preferences. [Preview Abstract] |
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GP1.00005: An Overview of Science Education and Outreach Activities at the Princeton Plasma Physics Laboratory J. DeLooper, A. DeMeo, A. Post-Zwicker, P. Wieser, J. Morgan, C. Phillips, C. Ritter, G. Czechowicz, E. Starkman As a Department of Energy Laboratory, the Princeton Plasma Physics Laboratory (PPPL) has an energetic science education program and outreach effort. This overview describes the components of the programs and evaluates this effort during the last several years. The primary goal is to inform the public regarding the fusion and plasma research at PPPL and to excite students so that they can appreciate science and technology. The public's interest in science can be raised by news media publicity, tours, summer research experiences, in-classroom presentations, plasma expos, teacher workshops, printed and web-based materials. The ultimate result of this effort is a better-informed public, as well as an increase in the number of women and minorities who choose science as a vocation. Measuring the results is difficult, but current metrics are reviewed. The science education and outreach programs are supported by a dedicated core group of individuals and supplemented by PPPL staff, friends and family members who help with various outreach and educational activities. Supported by U. S. DOE Contract DE-AC02-76CH03073/ab [Preview Abstract] |
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GP1.00006: Recent Initiatives from the Science Education Program at PPPL Andrew Post-Zwicker, John DeLooper, James Morgan, Christine Ritter In the past year, the Science Education Program (SEP) has initiated or enhanced several new collaborations for K-16 students and teachers as well as the general public. Most of these programs are housed in the Plasma Science Education Laboratory, whose upgraded facilities opened in September 2005. The Plasma Camp program was reconfigured for middle school teachers while the ``Energy in the 21st Century'' enrichment program for high school students was expanded. The new middle school Science Bowl was also improved. A collaboration with a ``special needs'' school now includes teacher training and curriculum development centered on energy-related topics. Finally, new programs with local science museums will include remote video conferencing from the NSTX control room for Master Teachers, a tabletop plasma experiment, and new plasma displays for the general public. Along with education programs, student research in the laboratory concentrates on an ECR sputter source and transport measurements in a dusty plasma. [Preview Abstract] |
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GP1.00007: Education Demonstration Equipment A. Nagy, R.L. Lee Several GA Fusion Education Program plasma related demonstration items were developed this year. A 120~V~ac powered electromagnetic coil shows eddy current levitation over an aluminum sheet and continuously changing magnetic force interactions using additional permanent magnets. A 300~V~dc plasma device, with variable current capability and analog data ports, is used to develop plasma I/V plots. An on-demand (via push button) fully enclosed 24~in. Jacob's ladder provides air plasma and buoyancy effects. A low cost Mason jar vacuum chamber filled with inert gas shows pressure and gas species plasma characteristics when excited by a Tesla coil. These demonstration items are used in the Scientist-In-the-Classroom program, GA facility tours, and teacher seminars to present plasma to students and teachers. Three very popular Build-It workshops were held to enable teachers to build these items and take them back to their classroom. [Preview Abstract] |
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GP1.00008: Promoting Pre-College Science Education R.L. Lee, A. Nagy The Fusion Education Program, with support from DOE, has begun its 12th year of interactions among scientists, teachers, and students. Recently, scientist visits to classrooms have become a major part of the program as the extensive DIII-D operating schedule has reduced opportunities for on-site facility tours. Educator workshops have grown to include hands-on construction of magnetic coils and plasma demonstration devices. Topical content in fusion, plasma, radioactivity, and the electromagnetic spectrum continue to provide a strong subject base to the program. During last year, workshops on some of these topics were given to teachers at local meetings, such as the annual SDSEA meeting in San Diego, STEP in Riverside, CA, the annual APS/DPP meeting, and at our summer Build-it Days workshops. Science stage shows for 4-8th grades have expanded to Kauai County, HI, following the Riverside STEP model. Materials distribution requests remain strong. New materials available for teaching and the overall program status will be presented. [Preview Abstract] |
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GP1.00009: Observation of Dust Cloud Phenomena in an Argon D.C. Glow Discharge Emily Margolis, Andrew Post-Zwicker A structural novelty has been observed in dust clouds in a low temperature D.C. glow discharge. A cloud comprised of silica particles 3 to 5 microns in diameter is levitated in an Argon plasma, illuminated with an 8 mW Helium-Neon laser. Striations, or dark bands, as well as various other voids have been previously documented in dusty plasmas. However, in this study, another configuration of empty space has been observed in which a void acts as a partition, dividing the cloud into two sections. Together the separate and distinct halves exhibit the qualities of a single cloud. The lower portion is organized into the typical lattice structure while the particles in the upper section only act as a liquid. An average cloud contains both lattice and liquid like regions. A scan over the cloud with a cylindrical lens expanded laser beam proves the existence of a void between the pieces. A qualitative analysis of the clouds will be presented, including plasma parameters and images. [Preview Abstract] |
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GP1.00010: From Order to Chaos - Chaos in Tokamaks Due to Tearing Modes Kyle Alt, Joshua Moloney, Iris Tavarez, Esther Uduehi, Halima Ali, Alkesh Punjabi In this and the next paper, we show how tearing modes create chaos in the ohmically heated tokamaks with standard q-profile, and how we can build barriers inside the chaos in these tokamaks. We have constructed a new symplectic map to calculate trajectories of magnetic field lines in generic tokamaks. The map is given by \[ \psi _{n+1} =\psi _n -{k\partial \chi (\psi _{n+1} ,\theta _n )} \mathord{\left/ {\vphantom {{k\partial \chi (\psi _{n+1} ,\theta _n )} {\partial \theta _n }}} \right. \kern-\nulldelimiterspace} {\partial \theta _n },\theta _{n+1} =\theta _n +k{\partial \chi (\psi _{n+1} ,\theta _n )} \mathord{\left/ {\vphantom {{\partial \chi (\psi _{n+1} ,\theta _n )} {\partial \psi _{n+1} }}} \right. \kern-\nulldelimiterspace} {\partial \psi _{n+1} }. \] Poloidal flux, $\chi $, is the generating function for the map, the toroidal flux, $\psi $, is the action, and the poloidal angle, $\theta $, is the angle. We use the standard safety factor profile for the tokamaks. We apply the magnetic perturbations (m,n)={\{}(3,2),(2,1){\}}, each with the same amplitude $\delta $. When $\delta $=0, we see invariant tori. For $\delta $ from 1X10$^{-4}$ to 7.5X10$^{-4}$, tori are destroyed and islands are formed. For $\delta \quad >$ 7.5X10$^{-4}$, islands overlap, and finally create full-scale chaos. In the next paper, we show how we can erect a barrier inside this chaos to control transport. This we do by adding a term of order $\delta ^{2}$ to the generating function. This work is supported by the US DOE DE-FG02-02ER54673 and NASA SHARP PLUS. [Preview Abstract] |
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GP1.00011: Order from Chaos: Creating Transport Barriers in Tokamaks Iris Tavarez, Joshua Moloney, Kyle Alt, Esther Uduehi, Halima Ali, Alkesh Punjabi In the previous paper, we showed how tearing modes create chaos in tokamaks. In this paper, we show how erecting barriers can control this chaos. We have constructed a new symplectic map to calculate trajectories of magnetic field lines in tokamaks. The map is given by \[ \psi _{n+1} =\psi _n -{k\partial \chi (\psi _{n+1} ,\theta _n )} \mathord{\left/ {\vphantom {{k\partial \chi (\psi _{n+1} ,\theta _n )} {\partial \theta _n }}} \right. \kern-\nulldelimiterspace} {\partial \theta _n },\theta _{n+1} =\theta _n +k{\partial \chi (\psi _{n+1} ,\theta _n )} \mathord{\left/ {\vphantom {{\partial \chi (\psi _{n+1} ,\theta _n )} {\partial \psi _{n+1} }}} \right. \kern-\nulldelimiterspace} {\partial \psi _{n+1} }. \] Poloidal flux, $\chi $, is the generating function for the map, the toroidal flux,$\psi $, is the action, and the poloidal angle $\theta $ is the angle. We use the standard safety factor profile for the ohmically heated tokamaks. We apply the magnetic perturbations (m,n) = {\{}(3,2),(2,1){\}}, each with the same amplitude $\delta $. As shown in the first paper, these perturbations lead to the creation of chaos within the tokamak for values of $\delta $ above 7.5X10$^{-4}$. Barriers are created through the addition of a term of order $\delta ^{2}$ to the generating function. This term can transform chaos at the barrier location into a good magnetic surface. We show that this newly created barrier surface is impermeable to surrounding field lines and therefore can prevent chaos on one side of the barrier from crossing to the other side. This invariant torus inside the chaos can help reduce transport in tokamaks. This work is supported by the US DOE DE-FG02-02ER54673 and NASA SHARP PLUS. [Preview Abstract] |
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GP1.00012: Creation of Barriers Inside Chaos in Tokamaks with Mixed Symplectic Representation Joshua Moloney, Halima Ali, Alkesh Punjabi The simplest map that represents the topology of one and a half degree of freedom Hamiltonian and near Hamiltonian systems is the Simple Map (SM), given by \[ x_{n+1} =x_n -k{\partial S(x_{n+1} ,y_n )} \mathord{\left/ {\vphantom {{\partial S(x_{n+1} ,y_n )} {\partial y_n ,}}} \right. \kern-\nulldelimiterspace} {\partial y_n ,}\quad y_{n+1} =y_n +k{\partial S(x_{n+1} ,y_n )} \mathord{\left/ {\vphantom {{\partial S(x_{n+1} ,y_n )} {\partial x_{n+1} }}} \right. \kern-\nulldelimiterspace} {\partial x_{n+1} }. \] Behavior of such systems near the hyperbolic fixed point is generic. The unperturbed generating function for the SM is $S_0 =\frac{1}{2}x_{n+1}^2 +\frac{1}{2}y_n^2 -\frac{1}{3}y_n^3 $. Here we derive the perturbed Simple Map that includes the effects of resonant modes (m,n)={\{}(4,3),(4,2){\}} in single-null divertor tokamaks with q$_{edge}$ =3 corresponding to the map parameter k=0.6. We show that the perturbed map is symplectic. We calculate generation of chaos due to the resonant modes. We derive the perturbed map with a barrier inside the chaos by addition of a term of the order $\delta ^{2}$ to create an invariant torus at the location of the barrier. We show that the perturbed map with barrier is also symplectic. It is already shown that an invariant torus can be created inside chaos when the unperturbed generating function S$_{0}$ is pure, i.e., it depends only on momentum (see previous two posters). Here we examine whether it is possible to create an invariant torus inside chaos when the unperturbed geometry is mixed, i.e., when S$_{0}$ depends on both the generalized momentum and position. This work is supported by the US DOE OFES and NASA. [Preview Abstract] |
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GP1.00013: Absorption Spectroscopy of Helium Ions in a Helicon Plasma Source Ryan Murphy, Ioana Biloiu, Mike Spencer, Robert Hardin, Earl Scime The scarcity of strong absorption lines in accessible tuning ranges along with plasma saturation due to low ion population densities makes absorption spectroscopy of helium ions notoriously difficult. Helicon plasmas, with their characteristically high ion densities are a good candidate for initial helium ion spectroscopy experiments. Initial measurements of Doppler broadened ion velocity distribution functions (ivdf) involve injecting a tunable infrared diode laser, tuned to 1012.36 nm and chopped roughly at 1kHz, along the axis of a 1.5m long helicon plasma. Two passes through the plasma were used before the intensity was recorded with a bandpass filtered photodiode. This allowed for sufficient absorption to obtain the ivdf measurement, but eliminated the spatial resolution of the diagnostic. Recent measurements using a single pass through the plasma and additional filters to reduce the laser intensity will be presented in this work. The design of the apparatus and improved measurements of the line integrated ivdf will be presented. [Preview Abstract] |
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GP1.00014: Determining energy coupling parameters for use in NEO interdiction J.C. Sanders, A.R. Miles The large population of 2r$>$100m near earth objects (NEOs) presents a certain hazard to life on the earth. It has been proposed that a nuclear device may be used to alter the course of such objects, thereby averting a catastrophic collision with the earth. The asteroid interdiction problem includes a number of parameters. Many can potentially be determined by observation; these include size, composition, trajectory, and required deflection velocity. However, the yield of the nuclear device necessary to provide sufficient impulse to the NEO to avoid a collision must be calculated. Two coupling parameters are needed for this: $\eta _{Y}$, which gives the explosive yield deposited in the object as a fraction of the intercepted yield, and $\eta _{K}$, which gives a conversion efficiency between the energy deposited in the object and the total change in the object's kinetic energy. The Monte Carlo code TART is used to calculate $\eta _{Y}$, and the hydrodynamics code CALE is used to calculate $\eta _{K}$. This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. UCRL-ABS-213611 [Preview Abstract] |
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GP1.00015: Comparison of Two Microwave and Two Probe Methods for Measuring Plasma Density Scott Knappmiller, Scott Robertson, Zoltan Sternovsky Four types of electron density measurements are compared: cylindrical probe, disk probe, microwave cavity, and microwave hairpin. The measurements are made in hot-filament discharges in soup-pot types of plasma devices with and without multidipolar surface magnetic fields. The hairpin and cylindrical probe gave densities which were in close agreement for all conditions with an average error of 7 percent. The cavity measurement was consistently higher (40 percent on average), perhaps as a result of higher density near the filaments. The disk probe was in good agreement with the cylindrical probe and hairpin for plasmas without multidipolar containment, but with multidipolar containment gave densities 21 percent higher on average, perhaps as a result of the higher density of secondary electrons within the plasma. [Preview Abstract] |
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GP1.00016: Propagating Magnetic Wave Plasmoid Accelerator Jedidiah Smith, Matthew Bartone, John Slough A high velocity plasma accelerator has been designed and constructed that has direct application to space propulsion as well as new innovative high energy density approaches to fusion. The Propagating Magnetic Wave (PMW) plasmoid accelerator could also find application as a fueler for ITER, as well as current tokamaks for adding rotational momentum and velocity shear for stability and transport control. Nevertheless, the PMW has a natural application to high power electric propulsion in space. The PMW operates naturally at both high power and efficiency with no need for electrodes or grids, and can also operate over a wide range in both exhaust velocity and propellant mass. To efficiently accelerate plasmoids to high velocities an acceleration method other than the simple tapered coil must be employed. The rapid acceleration of a compact plasmoid is realized through the application of an externally applied propagating magnetic field. Here, the large axial JxB force is generated from the induced azimuthal current inside the plasmoid and the radial component of the external, axially propagating magnetic field. This accelerating force is sustained as long as the plasmoid remains in phase with the wave field. Exit velocities greater than 200 km/sec for plasmoid masses on the order of 0.1 mg are anticipated from the device that is currently being tested, and the results from the initial experiments will be presented. This research is being supported by the U.S. Air Force Research Laboratory, Edwards AFB. [Preview Abstract] |
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GP1.00017: Assessing confinement limitations and scaling for a toroidal electron plasma using the m=1 diocotron frequency D. P. Ryan, Bao Ha, M. R. Stoneking Confinement issues for toroidal electron plasmas are experimentally addressed using the Lawrence Nonneutral Torus. Electron plasmas confined in a partially toroidal trap ($B=200\mbox{G},$ $R_0=43\mbox{cm}$ $a=5\mbox{cm}$) exhibit an unstable $m=1$ diocotron mode. Suppression of this mode is necessary to prevent plasma loss via scrape off on the wall. By detecting the flow of image charge to and from a wall probe, an active feedback circuit suppresses the growth of the mode, extending the confinement time to the order of 40ms. By modulating the feedback and allowing small amplitude diocotron oscillations to grow, measurement of the evolution of the total charge is made. This technique is non-destructive and implies an initial charge of $12\mbox{nC}$ or a mean density of $5\times10^{6}\mbox{cm}^{-3}$. The effects of the magnetic field on the confinement time and charge evolution were measured by scanning the field strength between 100G and 200G. Stronger magnetic fields yielded longer confinement times. Data was also taken for pressures between $2.0\times10^{-7}\mbox{ Torr}$ and $1.2\times10^{-5}\mbox{ Torr}$. Higher pressures resulted in less trapped charge. This work is supported by NSF and USDOE. [Preview Abstract] |
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GP1.00018: Frequency Swept 40 GHz Microwave Interferometer System J.W. Volock, M. Gilmore, J. Herrera, A. Lynn A swept frequency 40 Gigahertz Microwave Interferometer has been constructed to measure line-averaged electron density in the new HELCAT (HELicon-CAThode) plasma device at UNM. This system utilizes many custom circuits, including a 1MHz sawtooth wave generator with a sweepback time less than 50ns, bandpass filters, and IF amplifiers. The system also utilizes easily adaptable mounts we designed and built that include adjustments for optical alignment. The system design and measurement results will be presented. [Preview Abstract] |
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GP1.00019: Maximizing the Detection of Coincidence Gamma-Rays E.G. Bittle, D.M. Herrick, S.J. Padalino For measurements of high areal density in ICF implosions, a tertiary neutron diagnostic using the activation of 12C is under development. The 12C(n,2n)11C reaction occurs as a result of interactions of 12C with high energy tertiary neutrons. The consequent 11C isotope decays to 11B, emitting a positron which results in the production of two back-to-back 511 keV gamma rays upon annihilation. The coincidence detection of the two gamma rays is achieved by placing the activated 12C between a pair of NaI(Tl) scintillation detectors. The dimensions of the 12C sample influence the amount of neutron activation and the efficiency of gamma-gamma detection. To aid in the selection of appropriate 12C dimensions, the efficiency of a gamma-gamma detection system was empirically determined. A 22Na positron source was placed between two 3in x 3in NaI(Tl) detectors. The gamma-gamma detection data was collected, with and without graphite between the source and the detectors, and compared to Monte Carlo MCNPx calculations. The efficiency is found to be directly related to the intrinsic efficiency of NaI(Tl) for the detection of 511 keV gamma rays, the solid angle coverage of the detectors, the reduction in solid angle due to positron annihilation location, and gamma ray absorption in graphite. Funded in part by the US Department of Energy and the Laboratory for Laser Energetics. [Preview Abstract] |
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GP1.00020: Gamma-Coincidence Modeling with MCNPX Jenna Deaven, Anne Emerson, Jamie Leiter, Sharon Stephenson, Kristen Toskes Tertiary neutrons produced in self-sustaining Inertial Confinement Fusion reactions can activate a carbon target through n + $^{12}$C $\to \quad ^{11}$C + $\beta ^{+}$. The subsequent positron-electron annihilations lead to .511-MeV coincidence gammas, and therefore the tertiary neutron yield can be determined by a gamma-coincidence detection experiment. Monte Carlo N-Particle eXtended transport code (MCNPX) is used to model the $^{12}$C experiment, and through a comparison with real data, the geometry for the detector system can be determined. MCNPX is also used to model the non-uniform neutron activation of the $^{12}$C. Results will be presented. [Preview Abstract] |
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GP1.00021: Target Thickness Optimization of $^{12}$C for Tertiary-Neutron Activation at OMEGA Anne Emerson, Jenna Deaven, Jamie Leiter, Lauren Kate McNamara, Sharon Stephenson, Kristen Toskes One way to determine the $\rho $R (areal density) of IFC capsules is through tertiary-induced neutron activation of elements with appropriately high thresholds such as $^{65}$Cu or $^{12}$C. Since the tertiary neutron yield is many orders of magnitude lower than the primary yield, the experiment demands modeling to determine the optimal target thickness. Codes such as MCNPX seem well suited for such a project; however, activation eventually causes gamma-coincidences, which cannot be ``tallied'' in MCNPX. MCNPX can be used to provide particle track information for a known geometry for \underline {single} gammas. External coding is utilized to fabricate pairs (i.e. coincidences). This allows for the determination of the transmission of both gammas in varied thicknesses of $^{12}$C. The optimal thickness depends on the solid angle, the activation, and the attenuation factor appropriate for each gamma. Results will be presented. [Preview Abstract] |
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GP1.00022: Using a segmented electrode to control a dusty plasma Zachary Aldewereld, Jeremiah Williams, Edward Thomas Dusty plasmas are four component plasmas consisting of ions, electrons, neutrals, and charged microparticles. The charged microparticles (i.e., the dust) are trapped and suspended by potential wells in the plasma. In this experiment, a segmented electrode is being developed to dynamically control the dust particles in the plasma. Circuits are being designed to receive known signals and amplify them. These signals can then be delivered to the electrodes inside a chamber in order to create potential wells to trap the dust. These potentials can be modified in real time to allow controlled interactions among the dust particles. The entire system is controlled using LabVIEW. To make it more versatile, all components necessary to run the system are rack-mounted including a computer, the power supplies, the circuits, and plug panels through which the circuits can be connected to power, inputs, and outputs. Preliminary results will be presented at this conference. [Preview Abstract] |
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GP1.00023: Development of a new gas manifold system for ALEXIS Joseph Senne, Ashley Eadon, Edward Thomas The Auburn Linear Experiment for Instability Studies (ALEXIS) is a device dedicated to the study of low frequency (~ion cyclotron frequency) instabilities in plasmas. Originally, gas flows to the 180 cm long, 10 cm diameter device was done using tanks of helium and argon directly connected to ALEXIS. This configuration achieved its goal for inputting gas, yet required manually adjusting the valves on the input devices to control the flow rate of gas into the chamber. Thus, a more stable and remotely controlled method of delivering the gas is desirable. The new approach makes use of computer-controlled mass flow controllers to remotely supply and control the pressure of the ALEXIS device. This new system provides enhanced flexibility (e.g., allowing controlled mixtures of gases), improved safety (gas cylinders moved to a storage room), and a more rapid startup of plasma operations. This presentation discusses the design, construction, and initial operation of the remote gas system for ALEXIS. [Preview Abstract] |
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GP1.00024: Preliminary Studies of Sheared Flows in the Auburn Linear Experiment for Instability Studies (ALEXIS) using a Mach probe Michael Taylor, Jon Jackson, Ashley Eadon, Edward Thomas Sheared flows are an important mechanism for understanding plasma stability in systems ranging from fusion plasmas to the space plasma environment. In the ALEXIS (Auburn Linear Experiment for Instability Studies) device, a Mach probe was designed and fabricated in order to study these sheared flows. The Mach probe consists of two parallel collection plates electrically separated by a ceramic insulator. The collection plates are negatively biased into ion saturation mode. The ratio of the currents through the probe's plates can be used to determine the velocity of the ions through the plasma. A calibration procedure was developed to ensure the proper operation of the probe at different locations in plasma column. The plasma column was scanned at various radial positions and measurements of spatially varying flows have been made. The initial measurements of the flow velocities in ALEXIS and the design of the Mach probe will be discussed in this presentation. [Preview Abstract] |
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GP1.00025: Controlling Surface Dust in a Tokamak Colin V. Parker, Charles H. Skinner, A.L. Roquemore The potential build-up of dust in next-step fusion devices is a safety concern. In order to maintain the dust inventory below safety limits techniques to assess the dust inventory and methods to remove the dust if it approaches the safety limit are required. A novel electrostatic dust detector to measure the quantity of dust landing on a surface has been demonstrated previously\footnote{A. Bader et al., Rev. Sci. Instrum., 75, (2004) 370.}\footnote{C Voinier et al., J. Nucl. Mater. in press (2005).}. We have applied this device to dust removal from a surface in a vacuum chamber. Measurements of the dust removal efficiency as a function of areal density of incident dust and of the fate of the removed material will be reported. [Preview Abstract] |
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GP1.00026: Spectroscopic Studies of Merging Spheromak Plasmas In the Magnetic Reconnection eXperiment A.J. Carver, M. Yamada, H. Ji, S. Gerhardt, A. Kuritsyn, Y. Ren, M. Inomoto Magnetic reconnection, the topological breaking and reconnection of magnetic field lines, occurs in many magnetized plasmas e.g. in the solar corona, Earth's magnetosphere, and tokamaks. The ubiquity of plasmas in the universe and the potential use of current-carrying plasmas in fusion power plants warrants an improved understanding of magnetic reconnection. The Magnetic Reconnection eXperiment (MRX) is dedicated to improving our understanding of magnetic reconnection. We used nine fiber optic guides leading to a spectrometer and CCD camera to measure spectral line widths and shifts along many lines of sight within MRX, hence mapping the MRX ion temperature and torodial plasma velocity. This diagnostic allows us to study flow patterns and ion heating during the merging of two spheromaks. These measurements are compared to measurements from other MRX diagnostics. A.J. Carver was supported by the Department of Energy's Summer Undergraduate Laboratory Internship program. Contract number DE-AC02-76CH03073 [Preview Abstract] |
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GP1.00027: Development and Benchmark Studies of the Indiana Rf Photocathode Source Simulator Daniel Bolton, Chong Shik Park, Mark Hess The Indiana Rf Photocathode Source Simulator (IRPSS) code is being developed to calculate the electromagnetic fields created by an electron beam in a photoinjector, and to simulate the effects of these fields on the beam using time-dependent Green's function methods. In this poster, we show initial IRPSS simulation results for a simplified photoinjector geometry consisting of a semi-infinite cylindrical pipe and cathode using the experimental parameters for the BNL 1.6 GHz photocathode gun [1]. We also show the excellent agreement within a benchmark study between the IRPSS code operating with the same simplified geometry and the analytical solution for a disk-like bunch and its image bunch propagating with uniform velocities in opposite directions with free space boundary conditions. Our benchmark study has demonstrated that the effects of self-fields reflecting from the pipe, as well as the electromagnetic shock fronts due to causality conditions, may be significant in understanding the physics of photoinjectors. [1] K. Batchelor et al, Development of a High Brightness Electron Gun for the Accelerator Test Facility at Brookhaven National Laboratory, EPAC88, Rome, June 1988. [Preview Abstract] |
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GP1.00028: Fast Resistive Bolometry Jeffrey Graham, Setthivoine You, Paul Bellan A bolometer with microsecond scale response time is under construction for the Caltech spheromak experiment to measure radiation from a $\sim $20 $\mu $s duration plasma discharge emitting $\sim $10$^{2}$---10$^{3}$ kW/m$^{2}$. A gold film several micrometers thick absorbs the radiation, heats up, and the consequent change in resistance can be measured. The film itself is vacuum deposited upon a glass slide. Several geometries for the film are under consideration to optimize the amount of radiation absorbed, the response time and the signal-to-noise ratio. We measure the change in voltage across the film for a known current driven through it; a square pulse (3---30A, $\sim $20 $\mu $s) is used to avoid Joule heating. Results from prototypes tested with a UV flashlamp will be presented. After optimizing the bolometer design, the final vacuum-compatible diagnostic would consist of a plasma-facing bolometer and a reference in a camera obscura. This device could provide a design for fast resistive bolometry. [Preview Abstract] |
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GP1.00029: Building a Real-Time, Multi-Channel Spectrometer Felix H. Yu, Gunsu S. Yun, Paul M. Bellan The design and fabrication of a real-time, multi-channel spectrometer will be presented. This spectrometer will incorporate a 76-pin photodiode array and amplifying circuit detector with a 0.22m monochromator. Initial calculations suggest rise/fall times of $\sim $90 ns (zero to 90{\%}) will be sufficient to observe plasma dynamics with a 1 microsecond timescale; this high-speed response time is addressed with appropriate optimized circuitry. Moreover, characteristic photodiode responsivity and estimated plasma power output indicate peak photodiode signals will be on the order of 2 mA, and hence proper low-noise amplification will allow its application in tracking time-evolution of spectral data over a wide level of signal strength. In addition, this spectrometer will feature a wide-bandwidth ($\sim $40 nm) spectral window. [Preview Abstract] |
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GP1.00030: Simulation of Single-Particle Motion in Spheromak Geometries Parthiban Santhanam, Paul M. Bellan A simulation of single-particle motion in electric and magnetic fields has been developed using a `leapfrog' integration scheme in IDL. Calculations of particle trajectories in the Solov'ev solution to the Grad-Shafranov equation indicate good agreement with theory. Phenomena such as grad-B and curvature drifts, mirroring, and the conservation of energy and canonical angular momentum have been observed. It is planned to use this code to simulate single-particle motion in spheromak geometries, so as to aid in an analysis of particle trajectories in the field configurations of experimental spheromaks. [Preview Abstract] |
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GP1.00031: Conservation of Magnetic Moment for Charged Particle Motion in a Time-Dependent Uniform Magnetic Field S.A. Yi, R.C. Davidson, H. Qin The adiabatic magnetic moment invariant for the motion of a charged particle in a spatially uniform, time-dependent magnetic field B(t) is studied numerically. The robustness of the magnetic moment invariant $\mu $ = mv$_{\bot }^{2}$/2B is explored for slowly varying and rapidly varying magnetic field B(t), for a charged particle moving in a long solenoid with time-varying current. The effects of various functional forms of B(t) on the conservation of the adiabatic magnetic moment invariant are examined numerically. In the case of a slowly varying magnetic field, where the time-scale of the change in the magnetic field is much larger than the particle gyroperiod, it is shown numerically that the adiabatic magnetic moment $\mu $ is asymptotic to a recently discovered exact magnetic moment invariant M, which is conserved even for rapidly varying magnetic fields [1, 2]. [1] R.C. Davidson and H. Qin, \textit{Physics of Intense Charged Particle Beams in High Energy Accelerators} (World Scientific, 2001). [2] H. Qin and R.C. Davidson, An Exact Magnetic Moment Invariant of Charged Gyromotion, submitted for publication (2005). [Preview Abstract] |
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GP1.00032: Numerical modeling of magnetic island structure in high-beta NSTX plasmas J. Stimatze, J. Menard Recent investigation into the dynamics of internal kink modes has yielded significant insight into core magnetic island formation and mode saturation in NSTX high-beta plasmas. Using a simplified island model and ultra-soft x-ray (USXR) data from the National Spherical Torus Experiment (NSTX), it is possible to reconstruct the associated mode configuration by fitting island simulation parameters to the recorded data. Existing code to perform these calculations has been logically restructured and improved with detailed documentation, a graphical user interface, and self-documenting programming conventions. Additionally, the simulation has been extended to improve its accuracy through conservative multi-parameter scanning. This improvement allowed further extensions that provided a significant increase in the time-step resolution of the simulation, providing more detailed information on the time evolution of the island and mode saturation. [Preview Abstract] |
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GP1.00033: Silicon Wafer Transmission Window for Electron-Pumped Laser Systems C.S. McGuffey, C.A. Gentile, C. Priniski An electron beam transmission window has been developed as a component for the Electra Krypton Fluoride laser at NRL. Such KrF lasers will be employed in direct drive IFE. The transmission window is composed of a 1/4$''$ thick aluminum anode frame arrayed with 24 silicon wafer windows, 150$\mu$m thick. The window must endure 2.3atm pressure differential, high temperature (400$^{\circ}$C), $\sim$10$^{8}$ shots and allow electron transmission into the KrF lasing medium $>$80\%. A 1.2$\mu$m diamond passivation layer shields from the corrosive fluorine gas. Wafers are bonded to the aluminum frame using RTV sealant. Wafers withstood pressures up to 2.5atm at 600$^{\circ}$C. A prototype 4x6 wafer array was tested at the laser site at NRL. The wafers shattered after 180 shots. Transient thermal stresses or arcing may have caused the initial failure. Conducting RTV will allow heat and charge to dissipate from the wafers. A pulsed laser will be used to investigate the effect of beam power on the wafers. Further diagnostic testing at NRL is crucial to the next advancements. [Preview Abstract] |
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GP1.00034: Transport and Imaging of Fluorescent Dust in a DC Glow Discharge Plasma Will Gannett, Emily Margolis, Everett Schlawin, Andrew Post-Zwicker A fluorescent dust cloud illuminated by a longwave mercury UV lamp rather than the traditional laser has been produced in a DC glow discharge plasma. The luminescence of the dust particles in the wide UV beam allows imaging anywhere in the chamber, making it possible to observe the initial formation of a cloud as well as dust phenomena not in anticipated locations. The luminescence of the dust particles is sufficiently intense to be recorded by a charge coupled device (CCD) camera at 30 fps, which can be analyzed to obtain a two-dimensional velocity profile for the cloud. This velocimetry is far simpler than contemporary laser methods yet provides temporal and spatial resolution sufficient to analyze a variety of dust phenomena, including dust acoustic waves. A comparison of dust types and illumination sources will be presented, as well as observations of dust cloud formation and transport. [Preview Abstract] |
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GP1.00035: Passive spectrographic measurement of plasma electron temperatures using impurity line emissions D.T.S. Cook, E.P. Garate, W.S. Harris, W.W. Heidbrink, E.H. Trask A photodiode-based passive spectrometer was built in order to estimate electron temperature in the FRC experiment at UC Irvine. The electron temperature is calculated by using a ratio of emitted line intensities from oxygen impurities in the plasma. The use of passive spectroscopy to determine electron temperatures is well documented, and has the important benefit of being noninvasive. The use of line intensities is particularly important for us because other spectrographic techniques, such as those based on Stark broadening or Thompson scattering, are difficult to employ in our experiment. Here we present details of design and construction of the spectrometer, as well as details regarding data acquisition and analysis from the FRC experiment at UC Irvine. [Preview Abstract] |
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GP1.00036: Testing the Paleoclassical Model of Electron Heat Transport T.C. Arlen, C.M. Greenfield, H.E. St. John, J.D. Callen A paleoclassical model for radial electron heat transport in magnetically confined plasmas has been proposed to explain the anomalous behavior that has been experimentally observed in tokamaks [1]. Energy transport in the electron thermal channel is not well understood, as evidenced by the fact that the experimentally inferred electron heat transport often exceeds the theoretical classical and neoclassical values by factors of about 10$^4$ and 10$^2$ respectively. The paleoclassical model has recently been implemented in the ONETWO transport analysis and simulation code, allowing comparison between predictions from this model and experimental data from \hbox{DIII-D}. Results of such comparisons will be shown for several different confinement regimes.\par \vspace{0.5em} \noindent [1]~J.D.\ Callen, Phys.\ Rev.\ Lett.\ ${\bf 94}$, 055002 (2005). [Preview Abstract] |
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GP1.00037: The Distribution of Particulate Sizes Observed in \hbox{DIII-D} During Normal Plasma Operation J. Burkart, B.D. Bray, W.P. West Large dust particulates of characteristic size greater than $\sim$30~nm have been observed in the scrape-off layer and divertor regions of the \hbox{DIII-D} tokamak during normal plasma operations from Rayleigh scattering of 1.06~$\mu$ light from the existing Nd:YAG laser array and Thomson scattering system [1]. Here we discuss techniques to estimate the particulate size distribution from the measured pulse height distribution of scattered laser light. The data suggest a double-peaked distribution of particulate sizes. One group of particulates is small enough ($\leq \lambda$/10) that the Rayleigh scattering approximation holds. Estimates of the size distribution of the group of larger dust particulates require the Mie scattering formulation.\par \vspace{0.5em} \noindent [1]~B.D.\ Bray, W.P.\ West, and J.~Burkart, ``Dust Measurements with the DIII-D Thomson System," this conference. [Preview Abstract] |
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GP1.00038: Analytic Electron Density Barrier Model, Including Edge Transport Barrier N.D. Daniels, R.J. Groebner In H-mode, DIII-D density profiles have steep edge gradients that become flatter in the core. The width of the steep gradient region (pedestal) could be due to either fueling by edge neutrals or the presence of a transport barrier or both. An analytic model of the density profile is being developed to include both effects. The model has two transport regions with particle diffusion coefficients $D_c$ and $D_b$, representing the core and transport barrier regions respectively. The model is derived from coupled continuity equations for the electron and neutral densities which are solved in both regions. The resulting analytic model will allow for a study of the combined effects of the transport barrier and fueling depth, and will provide an expression for the width of the density pedestal as a function of the pedestal height. The predictions of this model will then be compared to observed trends in experimental data. [Preview Abstract] |
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GP1.00039: Phase Contrast Imaging of Inertial Confinement Fusion Capsules Using a Compact X-pinch B. DeBono, Z. Karim, K. Waqschal, F.N. Beg, R.B. Stephens The baseline NIF cryo-ignition fusion target contains a 100~$\mu$m thick layer of DT ice inside a cu-doped Be shell. X-ray phase contrast radiography is currently the only known method for verifying the presence and uniformity of this ice layer, but presently available sources require minutes-long exposure, which results in a blurred image due to shell vibrations. A compact x-pinch generator is an excellent source of bright and energetic x-rays, and has the unique advantage of pulsed exposure ($<$1~ns) while being small enough to fit on a tabletop. Initial results from experiments performed to characterize Be coated, Al coated, and plain CH capsules (various diameters and wall thicknesses) are presented. A compact x-pinch device capable of producing 80~kA of current with a rise time of 40~ns was used. X-pinches of various wire materials including W, Mo, and Al were used. The rough spectrum with Ross filter pairs shows x-rays in 1-10~keV range. The phase contrast images of CH capsules reveal a source size of 2-3~$\mu$m. [Preview Abstract] |
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GP1.00040: Investigation of Magnetic Braking of Plasma Rotation by Applied Magnetic Field Perturbations S.A. Driskill, E.J. Strait, R.J. La Haye, G.L. Jackson, H. Reimerdes Stabilization of resistive wall modes (RWM) has been addressed by two methods: rotational stabilization and active feedback stabilization. It has been predicted that the rotational velocity of the ITER plasma will be insufficient to counteract the RWM through rotational stabilization alone. To study active feedback stabilization in current tokamaks such as \hbox{DIII-D}, the plasma rotation must be slowed to below the critical velocity threshold. In the absence of bi-directional neutral beam injectors, this is done by applying an external torque to the system, known as magnetic braking. The effects of magnetic braking on toroidal rotation are investigated using non-axisymmetric coils, capable of producing toroidal mode numbers n=1, 2, or 3. The relation of the rotation drag to the strength, the poloidal mode spectrum, and the configuration of the applied field are analyzed. Finally, the results are compared to both resonant and non-resonant braking models. [Preview Abstract] |
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GP1.00041: Coupled Electron and Ion Heat Pulse Propagation M. Gildner, C.C. Petty, T.C. Luce, J.C. DeBoo Heat transport in a tokamak plasma can be studied by analyzing the effect of a modulated heat source on the temperature profile. Electron cyclotron heating provides a spatially localized heat source that is rapidly thermalized by the bulk electrons. Using perturbative solutions to the linearized Braginskii energy conservation equation, we can determine the (effective) contributions of diffusion, convection, and damping to the heat transport. Previous research has encompassed only the electron heat pulse propagation. We expand the method to include the effects of electron-ion coupling through both collisional exchange and the transport coefficients, which allows us to analyze simultaneous heat pulse propagation in the electron and ion channels. The solutions are compared with experimental data to determine the relative strength of the effects on the heat pulse propagation of different types of electron-ion coupling. [Preview Abstract] |
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GP1.00042: Evaluation of Ion Cyclotron Harmonic Damping on a Non-Maxwellian Distribution Function D. Schaffner, R.I. Pinsker, M. Choi, M. Porkolab In tokamak plasmas, fast Alfven waves are absorbed on electrons by Landau and TTMP damping and on ions by ion cyclotron harmonic damping. Ion cyclotron harmonic damping of fast Alfven waves in magnetized plasma can be calculated analytically given a Maxwellian ion velocity distribution; however, in plasmas with strong neutral beam heating a non-Maxwellian ion distribution is obtained. The absorption of fast waves on a general non-Maxwellian distribution has been evaluated numerically in recent work [1]. Here, the linear theory is reexamined to find a semi-analytic model for absorption on a slowing-down distribution function. Results are compared to the numerical results from [1], to recent experimental data from DIII-D, and to results from the ORBIT-RF code.\par \vspace{0.5em} \noindent [1]~R.J.\ Dumont, et al., Phys.\ Plasmas ${\bf 12}$, 042508 (2005). [Preview Abstract] |
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GP1.00043: Time-dependent Modeling of Feedback Control of MHD Instabilities in DIII-D A. Van Etten, A.M. Garofalo, H. Reimerdes Feedback control of the long-wavelength resistive wall mode (RWM) using magnetic coils in DIII-D has allowed reliable tokamak operation at normalized pressure exceeding the free-boundary limit by up to a factor of 2. The feedback system senses the resonant response of the stable RWM to intrinsic field asymmetries, yet the effects of the feedback action depend strongly on the algorithm used. A ``smart shell" algorithm can only reduce the plasma response to the error field, while a ``mode control" algorithm can effectively remove the error field itself, thus maintaining the fluid rotation speed necessary to stabilize the RWM. Time-dependent simulations using an ideal MHD model support this interpretation. The time-dependent formulation of the feedback model also allows investigation of nonlinear feedback behaviors, such as the effects of voltage and current limits. In the experiment, these hardware limitations are observed to provide system stability under circumstances that, in linear analysis, are predicted to lead to instability. [Preview Abstract] |
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GP1.00044: Investigation of Millimeter Wave Bursts at ECE Frequencies in DIII-D Plasmas S.M. Wu, M.E. Austin Intense bursts of narrowband millimeter wave radiation have been observed in low density H-mode and QH-mode plasmas in the DIII-D tokamak. These bursts occur in the range of second harmonic electron cyclotron frequencies and have bandwidths of 1 to 4~GHz. The narrow frequency width suggests that the bursting is not classic runaway electron emission which typically has a bandwidth of 10~GHz or more. ECE bursting has been observed in three different conditions. The burst occurs simultaneously with edge-localized mode (ELM) precursors, edge harmonic oscillations, or plasma disruption precursors. In the precursor cases the bursts precede the collapse phase and are coincident with similar types of MHD events. The data will be compared to models of stimulated and scattered emission for electrons in the high energy tail of the distribution. [Preview Abstract] |
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GP1.00045: Development of an auto-convergent free-boundary axisymmetric equilibrium solver J.A. Huang, J. Menard Improvements upon the calculation of the magnetic flux for a given current profile in axisymmetric toroidal plasmas using an iterative, modular algorithm coupled with a fast, direct elliptic solver for the Grad-Shafranov equation to reconstruct a free boundary equilibrium solution are implemented and analyzed. The equilibrium algorithm is modified with the application of the von Hagenow method for determining the flux on the computational boundary, greatly reducing the time cost from $O(N^4)$ to $O(N^2 \ln N)$ machine operations. These improvements allow the grid resolution to be increased efficiently and automatically to reduce the maximum Grad-Shafranov error to values needed for accurate stability calculations on a more effective time scale. [Preview Abstract] |
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GP1.00046: WITHDRAWN--Gains Optimization Algorithm for NSTX Power Supply Control System John Smith, Ronald Hatcher A time-saving procedure for optimizing the gains of a power supply control system is described for the NSTX toroidal field (TF) coil. An algorithm is implemented for determination of the optimal gains for prescribed levels of convergence to the reference current over various intervals. We use Simulink to simulate feedback control behavior and plant response while imposing realistic constraints on the power supply and control system models. A MATLAB routine containing the algorithm rates convergence based on weighted considerations of rise-time, steady-state error and overshoot in user-specified regions, adjusting the proportional and integral gains accordingly. NSTX test shot data files provide reference currents for benchmarking the algorithm. Iterative methods of gains optimization have been explored for best convergence efficiency and have yielded excellent agreement with hand-checked gains for simple reference current behavior. Eventual application may include adaptive gain optimization and setting for the NSTX power supply control system. [Preview Abstract] |
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GP1.00047: Development of a Substrate Handling System for ECR Thin Film Deposition Thomas Wedlick, Andrew Post-Zwicker ECR plasma deposition imparts high particle fluxes, transpires at low pressures reducing contamination, has long mean free paths improving deposition precision, and has faster rates than RF deposition. ECR plasmas are typically both larger and denser than RF plasmas, which allow for greater substrate sizes. The ECR deposition system developed generates a copper-sputtering argon plasma with a 2.45GHz 5kW source. Initial plasma characterization is spectroscopic yielding argon and copper neutral and ion densities. SEM micrographs are used to determine deposition rates, thickness, and uniformity within the 6 in. diameter deposition zone. A precise (within 0.05 in.) substrate handling system, comprised of a Genmark elevator and robotic arm, transports the substrate from the loading chamber to the deposition chamber. Sufficient plasma control necessitates accurate argon flow and pressure control (to within 10$\mu $Torr). Details of the ECR deposition system, the substrate handling system, and the argon flow control will be presented. [Preview Abstract] |
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GP1.00048: Comparison of GS2 Turbulence Simulations with Phase Contrast Imaging in Alcator C-Mod Internal Transport Barriers Andrew Long, Darin Ernst, Liang Lin, Miklos Porkolab, Nils Basse Trapped electron mode (TEM) turbulence arises in gyrokinetic simulations of internal transport barriers in Alcator C-mod experiments [1]. C-mod is equipped with a PCI (phase contrast imaging) diagnostic which measures density fluctuations along 32 vertical chords passing near the magnetic axis. The GS2 density fluctuations are output as an integral along field lines. The GS2 poloidal wavelength spectrum is upshifted relative to the PCI spectrum [1]. To make the comparison more direct, we have modified GS2 [2] to calculate electron density fluctuations at the poloidal angles observed by PCI. The longer wavelength modes are more extended along field lines, so that when viewed off the midplane, they are weighted more strongly. Nonlinear simulations are underway, and results will be presented. \newline [1] D. R. Ernst et al., 20th IAEA Fusion Energy Conf. IAEA-CN-116/TH/4-1. also Phys. Plasmas 11(5) 2637 (2004). http://www-naweb.iaea.org/napc/physics/fec/fec2004/datasets/TH\_4-1.html \newline [2] W. Dorland et al., Phys. Rev. Lett. 85(26) 5579 (2000). funding: Contract number DE-AC02-76CH03073. [Preview Abstract] |
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GP1.00049: Measuring the Magnetic Field of the Princeton Magnetorotational Instability Experiment J. Waksman, H. Ji, M. Burin, E. Schartman The Princeton MRI experiment attempts to clearly demonstrate the Magnetorotational Instability in the lab for the first time. This instability has been theorized to be important in angular momentum transport in accretion discs. The research goal is to design and build an array of detectors in order to measure the external magnetic field of the MRI experiment. The amplitude and spatial distribution of magnetic perturbations are crucial for the identification of MRI and the study of its nonlinear saturation. The design includes two types of diagnostic devices. The first is a Hall Probe, designed to measure large, slowly-changing vertical fields imposed externally by coils surrounding the experiment. The second is a magnetic pickup coil, designed to measure small, but quickly-changing magnetic fields, containing information on the modal structure. The B- fields measured with these diagnostics help us to gain a greater understanding of both liquid metal and accretion disc flows. Detailed design and calibration will be reported. Research supported by the DOE (DE-AC02-76-CH03073) and the NUF Program in Plasma Physics and Fusion Energy Sciences. [Preview Abstract] |
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GP1.00050: Analysis of Momentum Transport in Alcator C-Mod Plasmas with No Momentum Input using a Simplified Diffusion Model Charles Bocchino, Yuri Podpaly, John Rice, Alexander Ince-Cushman High resolution x-ray spectrometers have shown propagation of momentum from the edge to the center of Alcator C-Mod plasmas. This propagation was modeled by a one dimensional, source free momentum transport equation (ignoring convection and treating boundary conditions as a step function). Using this model to fit the central velocities of the EDA H-modes and L-modes yields a diffusivity coefficient, which corresponds to a momentum confinement time, $\tau _{\phi }$. Comparing $\tau _{\phi }$ with plasma parameters provides means to determine which, if any, correlations exist for the momentum confinement time. [Preview Abstract] |
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GP1.00051: Stability studies of the NSTX Microtearing Mode J.A. Baumgaertel, M.H. Redi, R.V. Budny, G. Rewoldt, W. Dorland Insight into plasma microturbulence and transport is being sought using linear simulations of drift waves on the National Spherical Torus Experiment (NSTX). Microtearing may cause high electron thermal conductivity and high heat transport. Marginal stability of the microtearing mode is investigated for conditions at mid-radius in a high density NSTX H-mode plasma. The microtearing mode is driven by the electron temperature gradient, and is known to be mediated by plasma beta, magnetic shear [1], and ion collisions [2]. Based on input files for the parallel code GS2, produced by TRXPL following TRANSP analysis, the variability of mode growth rates is examined as functions of electron density and temperature (thus varying plasma resistivity) and collisionalities. A beta scan of the microtearing instability is planned. [1] M. H. Redi, et al. Proceedings of EPS, Taragona, Spain, 2005 [2] M. H. Redi, et al. Proceedings of EPS, St. Petersburg, RU, 2003 [Preview Abstract] |
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GP1.00052: Study of Free Surface Flow of Water in an Open Channel K. McMurtry, L. Berzak, H. Ji The motivation for this experiment is to study free-surface liquid metal flow in the presence of a magnetic field. In the presence of a free surface, liquid metal flow can be qualitatively different from a closed pipe flow by exciting surface waves. This has direct relevance to the proposed concept of a liquid metal wall as the first layer facing the plasma in a fusion reactor, as well as astrophysical applications such as free-surface MHD physics on the surface of neutron stars. As a first step, a water channel was built that is 90 x 16 x 3 cm in dimension. Water from a 24 L tank flows through a 5 cm ID tube to the channel and then to a collecting basin where it is pumped back to the tank. The height of the tank, as well as the diameter of the tube can be adjusted to achieve desired parameters. In order to study the surface waves, laser light is reflected off the surface of the water onto a translucent sheet and then recorded using a CCD camera. Properties of open channel flow, as well as diagnostic techniques will be discussed in addition to possible mechanisms for the excitation of surface waves. This work is supported through the DOE (DE-AC02-76-CH-03073) and the NUF Research Program. [Preview Abstract] |
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GP1.00053: Injection of a Neutral Hydrogen and Helium Beam through Plasma Teresa Bartal, Elizabeth Foley, Fred Levinton A code is presented for a diagnostic neutral helium beam injected through plasma. Through a set of coupled differential rate equations, the code calculates the population fraction of the atoms in eleven different energy states as the beam propagates through the plasma. The differential equations include the effects of ionization, collisional excitation and de-excitation, charge exchange and spontaneous radiation emission as well as laser-induced excitation for the atoms in the helium beam. Through numerical analysis the effects of beam energy and plasma density and temperature on the population fractions of the helium atoms can be observed. Simulations were performed with uniform plasma parameters and certain profiles. A similar code is being developed for a neutral hydrogen beam propagating through plasma in a magnetic field. The code will calculate the population fractions in the n=1,n=2 and n=3 sublevels as their parameters vary with background magnetic and electric fields. This work is being done in support of motional stark effect diagnostic development. [Preview Abstract] |
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GP1.00054: Setup and Calibration of a Normal Incidence Spectrometer for use on the CTIX Elizabeth Merritt, Elmar Trabert, Hui Chen, David Hwang Spectroscopy is a powerful diagnostic tool for examining plasmas and their behavior. In particular, it can be used to determine plasma composition and velocity. I will present the procedure for preparing a normal incidence spectrometer for use on the Compact Torus Injection Experiment (CTIX). The spectrometer uses a 300 line/mm grating and a liquid nitrogen cooled, thinned, back-illuminated charge coupled device (CCD) detector with 1340x1300 pixels of 20 $\mu $m x 20 $\mu $m area per pixel and has a spectral coverage from extreme ultra-violet to visible. To ensure the highest precision possible for this geometry, I focus the spectrometer, imaging the 100 $\mu $m slit onto the CCD camera. Then, the spectrometer is calibrated for wavelengths between 300 nm and 800 nm using a Hg lamp light source. The background noise from the CCD camera is characterized and accounted for during the Hg spectral analysis. [Preview Abstract] |
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GP1.00055: Pulsed Electrical Discharge in a Gas Bubble in Water Erica Schaefer, Sophia Gershman, Oksana Mozgina, Abe Belkind, Kurt Becker This experiment is an investigation of the electrical and optical characteristics of a pulsed electrical discharge ignited in a gas bubble in water in a needle-to-plane electrode geometry. Argon or oxygen gas is fed through a platinum hypodermic needle that serves as the high voltage electrode. The gas filled bubble forms at the high voltage electrode with the tip of the needle inside the bubble. The discharge in the gas bubble in water is produced by applying 5 -- 15 kV, microsecond long rectangular pulses between the electrodes submerged in water. The voltage across the electrodes and the current are measured as functions of time. Electrical measurements suggest a discharge ignited in the bubble (composed of the bubbled gas and water vapor) without breakdown of the entire water filled electrode gap. Time-resolved optical emission measurements are taken in the areas of the spectrum corresponding to the main reactive species produced in the discharge, e.g. OH 309 nm, Ar 750 nm, and O 777 nm emissions using optical filters. The discharge properties are investigated as a function of the applied voltage, the distance between the electrodes, the gas in the bubble (Ar or O$_{2})$. Work supported by the US Army, Picatinny Arsenal, NJ and the US DOE (Contract number DE-AC02-76CH03073). [Preview Abstract] |
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GP1.00056: Particle Diffusion by Waves in Mirror Geometry Tianhui Li, Andrey Zhmoginov, Nathaniel Fisch By shining a spatially-localized single-frequency wave into a mirror-trapped plasma, a resonance with particles of a certain parallel energy might be created. The resonant particles are selectively accelerated so that their perpendicular energy changes in a chaotic manner, creating a diffusion path along the mid-plane velocity space, and more generally, a diffusion path in a larger phase space that includes radial position. The diffusion equation along the diffusion path is solved, illustrating the importance of boundary conditions. We simulate these wave particle interactions, showing how various spatially varying diffusion coefficients effect this diffusion and present some applications of our analysis such as hot ash removal from fusion devices. The work was done under contract \sc{DE-AC02-76CH03073}. [Preview Abstract] |
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GP1.00057: Web Interface Connecting Gyrokinetic Turbulence Simulations with Tokamak Fusion Data A. Suarez, D.R. Ernst We are developing a comprehensive interface to connect plasma microturbulence simulation codes with experimental data in the U.S. and abroad. This website automates the preparation and launch of gyrokinetic simulations utilizing plasma profile and magnetic equilibrium data. The functionality of existing standalone interfaces, such as GS2{\_}PREP [D. R. Ernst et al., Phys. Plasmas 11(5) 2637 (2004)], in use for several years for the GS2 code [W. Dorland et al., Phys. Rev. Lett. 85(26) 5579 (2000)], will be extended to other codes, including GYRO [J. Candy {\&} R.E. Waltz, J. Comput. Phys.186, (2003) 545]. Data is read from mdsplus and TRANSP [\underline {http://w3.pppl.gov/transp}] and can be viewed using a java plotter, Webgraph, developed for this project by previous students Geoffrey Catto and Bo Feng. User sessions are tracked and saved to allow users to access their previous simulations, which can be used as templates for future work. [Preview Abstract] |
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GP1.00058: Spectroscopic Measurements of Flow and Ion Temperature at SSX J. Fung, S.C. Chang, J. Horwitz, B. Coellner, C.D. Cothran, M.R. Brown, M.J. Schaffer The Swarthmore Spheromak Experiment (SSX) studies magnetic reconnection by merging co- and counter-helicity spheromaks. Typical plasma parameters include electron density $n_e\sim 10^{15}$ cm$^{-3}$, temperature $T_i+T_e\sim 30$ eV, and magnetic fields $|B|\sim 0.1$ T. We present data from a new ion doppler spectroscopy (IDS) diagnostic. Our IDS system features a 1.33 m Czerny-Turner spectrometer with a 316 grooves/mm echelle grating and a 32-channel photomultiplier tube array. On any shot, we can observe any of 10 different chords through the plasma with submicrosecond time resolution and an instrument temperature $\sim 6$ eV. Current studies have focused on the evolution of the CIII 229.7 nm line which we observe at 25$^{\mathrm{th}}$ order, with dispersion 0.008 mm/nm using magnifying exit optics. Analysis to determine radial and toroidal velocities, as well as ion temperatures, via Abel inversion is underway. Preliminary results suggest near-Alfv\`enic bi-directional flows due to reconnection. [Preview Abstract] |
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GP1.00059: Ultrasonic Doppler Velocimetry Measurements on the Madison Dynamo Experiment C.M. Jacobson, C.B. Forest, R.D. Kendrick, M.D. Nornberg, C.A. Parada, E.J. Spence Ultrasonic Doppler Velocimetry (UDV) is used to measure components of the velocity field in the dimensionally-identical water version of the Madison Dynamo Experiment. Both the shape and magnitude of the velocity field must be well understood to predict whether the magnetic field will grow or decay. Probe assemblies with a protective silicon rubber transmission plate have been developed to house high-temperature transducers. The probes have been installed in newly-opened ports on the water experiment. The transducers measure the velocity of neutral- buoyancy seed particles, and are oriented such that ultrasonic reflections from the experiment walls and impellers are minimized. UDV measurements are assessed via comparison with Laser Doppler Velocimetry measurements taken at similar positions. [Preview Abstract] |
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GP1.00060: Faraday-Rotation Measurements of Megagauss Magnetic Fields Produced by the Zebra Z-Pinch Generator Christopher Plechaty, Radu Presura Experiments performed at the Nevada Terawatt Facility have applications related to laboratory astrophysics, radiation sources, and fusion research. Some of these experiments require megagauss magnetic fields in a vacuum environment for hot plasma confinement or stabilization. These required ultrahigh magnetic fields will be produced using Zebra, a fast pulse generator, and will be measured using the Faraday effect. The Faraday effect is the rotation of the polarization plane of a light beam traveling through a Faraday-active material along the magnetic field. The amount of rotation observed is proportional to the magnitude of the magnetic field and the length of the Faraday-active material. The proportionality constant is called the Verdet constant. To make local magnetic field measurements that do not perturb the experiment, the setup for the Zebra experiments uses small disks (3 mm in diameter, 2.0 +/- 0.1 mm thick) of flint glass with a Verdet constant of 15 rad/(Tm) at 532 nm. This Verdet constant allows our setup to be sensitive with a broad range of field strengths, from 10 T to 300 T (0.1-3 MG). Progress made with this diagnostic setup shall be presented. *Work supported by DOE/NNSA under UNR grant DE-FC52-01NV14050. [Preview Abstract] |
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GP1.00061: Finite Element Modeling for Megagauss Magnetic Field Generation David Martinez, Radu Presura Applying external magnetic fields with MegaGauss strength is needed for hot plasma confinement and stabilization. We investigate the possibility of generating ultra-high magnetic fields with the fast z-pinch generator ``Zebra'' for experiments at the NTF. Zebra can produce a load a current of 1 MA in 100 ns. To design appropriate loads we use Femlab\footnote{Femlab 3 -- multi-physics, finite-element modeling program by Comsol AB, 2004} and Screamer\footnote{Screamer -- A Pulsed Power Design Tool developed at SNL by M.~L.~Kiefer, K.~L.~Fugelso, K.~W.~Struve, and M.~M.~Widner.} to simulate the magnetic field. Screamer predicts the load current using a detailed model of Zebra and helps optimize the operation. Using the information from Screamer, Femlab is able to calculate the magnetic field, heating, and stress on the conductor. All these effects must be taken into consideration to determine the integrity of the coil until maximum field is reached. The presentation will include simulation results for single- and multi-turn coils, as well as quasi-force-free inductors. [Preview Abstract] |
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GP1.00062: Use of Lagrangian Radiation-Hydrodynamic Codes to Study Ablative Laser Propulsion M.J. Grosskopf, R.P. Drake, M.R. Taylor Using HYADES computer code developed by Jon Larsen at Cascade Inc., we model different techniques for ablation pressure to provide thrust to a small spacecraft. Large, ground-based lasers would be used to irradiate a chosen material on the craft in order to remove material to propel the craft forward. Both a direct drive and an indirect drive approach to ablating the propellant from the surface are being studied, as well as the effects of laser pulse length and pulse separation on the amount of thrust transferred to the target. This research may have application to the development of a method of launching small satellites more cheaply and efficiently by separating the source of energy from the craft itself. This work was supported by the University of Michigan. [Preview Abstract] |
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GP1.00063: Bias Current Control Via Pulse Width Modulation on the Rotating Wall Machine Sam Stambler, W. Bergerson, C.B. Forest, R. Kendrick, S. Oliva Dynamic current profile control is achieved in the Rotating Wall Machine (RWM) by a recently developed pulse width modulation (PWM) scheme that individually controls the current through as many as seven different plasma guns with up to 1KA each. The system is based around a simple PWM scheme where the current through the plasma is monitored, compared to a reference waveform and the bias voltage is switched on or off to compensate. Inductors provide finite dI/dt allowing the current to oscillate about the desired value in a saw tooth manner. Improved performance is achieved by using a high voltage source to quickly achieve the desired starting current and two lower voltages to then switch between to give smaller dI/dt. This high level of control is required to effectively study the detailed nature of the plasma's instability as well as the effects of complex, time dependent current profiles. The PWM system's high flexibility will allow it to easily adapt to the many changes in plasma characteristics that will occur over the full course of the RWM experiment. Design and experimental data will be discussed. Work supported by the DoE. [Preview Abstract] |
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GP1.00064: Electron Bernstein wave emission measurements using a phased waveguide antenna on the Madison Symmetric Torus S.M. McMahon, J.K. Anderson, C.B. Forest Electromagnetic radiation in the electron cyclotron range of frequencies (ECRF) characteristic of blackbody emission has previously been observed on the Madison Symmetric Torus (MST) reversed field pinch (RFP). As the plasma in the RFP is overdense ($\Omega_{pe} \gg \Omega_{ce}$), the source of the measured radiation is core emitted electron Bernstein waves (EBW) which convert to electromagnetic waves at the plasma boundary. A diagnostic for measuring electron temperature profiles based on the EBW emission is being developed utilizing an antenna with finely adjustable angular resolution. A radiometer receives the emitted waves through a two-waveguide grill antenna designed for launching of power in the 2-4 GHz range. A phase difference imposed between the two arms of the antenna strongly affects the angular sensitivity. Analysis techniques considering frequency dependence and phase shift induced antenna patterns are used to infer temperature using previously developed theory of electromagnetic to EBW coupling efficiency. [Preview Abstract] |
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GP1.00065: Tomographic Reconstruction of Two-Dimensional Plasma Structure in the FRX-L Device Adam Light, Ivo Furno, Glen Wurden, Matthew Leonard, Cameron Bass, Thomas Intrator An optical tomographic system has been developed to analyze plasma structure in the Field Reversed eXperiment -- Liner (FRX-L) device at Los Alamos National Laboratory. FRX-L is designed to produce a high-density field-reversed configuration plasma (FRC) for fusion energy research. Visible light emitted by the plasma provides much information about its internal structure and is measured by several diagnostics, including the tomographic system described in this work. The diagnostic system consists of two optical array holders each equipped with eight 600-$\mu $m optical fibers arranged in a fan-like geometry. Line-integrated optical brightness data from the fan arrays are converted to a two-dimensional emissivity map by tomographic inversion. Calibration techniques for the system are described, together with tomographic principles and inversion methods. Inversion routines and preliminary results are presented. [Preview Abstract] |
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GP1.00066: Testing the Paleoclassical Transport Model As a Component of the H-mode Edge Model K. McFarland, G. Bateman, A.H. Kritz The paleoclassical transport model [1] is tested in ASTRA simulations of pedestal formation and ELM crashes at the edge of H-mode discharges [2]. Simulation results for the shape of the pedestal temperature profiles and the frequency of ELM crashes are compared with detailed experimental data from the DIII-D 98889 discharge provided by T. Osborne. In the ASTRA simulation model, flow shear in the pedestal reduces the transport driven by ion drift modes, resistive ballooning modes, and the electron gradient temperature (ETG) mode, while flow shear does not affect the paleoclassical electron thermal transport or the neoclassical ion thermal transport. A limit on the pressure gradient triggers ELM crashes, which abruptly change the temperature profiles in a region that is wider than the pedestal at the edge of the plasma. The inclusion of the paleoclassical model has the effect of making the electron temperature pedestal wider, which is in better agreement with experimental data compared with previous simulations that used only the ETG mode for electron thermal transport in the pedestal. [1] J.D. Callen, Phys. Rev. Lett. \textbf{94} (2005) 055002. [2] A.Y. Pankin et al., Plasma Phys. Control. Fusion \textbf{47 }(2005) 483. [Preview Abstract] |
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GP1.00067: FIELD REVERSED CONFIGURATION, SPHEROMAKS AND STELLARATORS |
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GP1.00068: Principal Attributes of FRCs Sustained by Rotating Magnetic Field Current Drive A.L. Hoffman, H.Y. Guo, R.D. Milroy Field Reversed Configurations (FRC) sustained by Rotating Magnetic Fields (RMF) are distinctly different from the decaying FRCs formed in theta-pinches. The RMF drive reverses particle diffusion, producing very long particle lifetimes, low separatrix densities, and complete reversal of the external confinement field. The density is set by torque balance between the RMF drive and resistive drag on the electrons. The FRCs will increase in poloidal flux and expand radially inside a flux conserver until the compressed external field pressure balances the product of density times temperature. Higher temperatures, which are determined by a balance between RMF produced heating and various loss mechanisms, will automatically result in higher diamagnetic currents and poloidal magnetic fields, without requiring any increase in RMF parameters, and with very little increase in absorbed RMF power. Current drive performance thus increases dramatically with increasing plasma temperature. Temperatures in present TCS experiments are limited primarily by radiation ($\sim $80{\%}) and conduction/convection ($\sim $20{\%}). Recent experiments show that conduction/convection losses can be greatly reduced using anti-symmetric RMF drive, and extensive modifications are being made to TCS to reduce impurities and radiation losses, so large increases in overall performance can be expected. [Preview Abstract] |
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GP1.00069: Antisymmetric RMF Current Drive in FRCs R.D. Milroy, H.Y. Guo, A.L. Hoffman, L.C. Steinhauer Rotating Magnetic Fields (RMF) can be used to both form and sustain Field Reversed Configurations (FRC). The addition of even a small simple transverse magnetic field to an FRC tends open the field lines, leading to a concern that confinement could also be compromised. Calculations show that both electron and ion orbits are confined due to the cyclic nature of the field line opening, but rapid electron thermal conduction to the outside world remains a concern unless the outside density can be maintained extremely low. It was shown$^{1}$ that if an antisymmetric RMF is applied, the field lines remain closed for small ratios of vacuum RMF $B_{\omega }$ to external axial magnetic field $B_{e}$. Recent experiments show improved confinement when antisymmetric RMF is applied, and an analysis of these results$^{2}$ shows that field lines remain closed for much larger ratios of$ B_{\omega }/B_{e}$ due to fact that the RMF only partially penetrates the FRC. With these encouraging results, antisymmetric RMF will soon be tested further on the new TCS upgrade experiment. We will extend the analysis to include a representation of the antisymmetric RMF that more accurately accounts for the antenna geometry, study the sensitivity of the results to precise symmetry, and determine the threshold for the opening of field lines. $^{1 }$S. A. Cohen and R. D. Milroy, Phys. Plasmas, \textbf{7}, 2539, (2000) $^{2 }$H. Y. Guo, A. L. Hoffman, and L. C. Steinhauer, Phys. Plasmas \textbf{12}, 062507 (2005) [Preview Abstract] |
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GP1.00070: UHV Modifications {\&} Cleaning Techniques to Allow High Temperature Operation in TCS/upgrade K.E. Miller, J.A. Grossnickle, A. Tankut FRCs have been formed and sustained by Rotating Magnetic Fields in the TCS experiment. Steady-state operation has been achieved with many new and exciting features. The plasma density is set by the RMF parameters, but the temperature has been severely limited by high impurity content, with up to 80{\%} of the input power lost to radiation. In order to improve the temperatures and flux levels, TCS is being extensively upgraded. Improvements include UHV qualified bonding of Invar bellows to the quartz tube section necessary to allow RMF application, and the use of wire seals to replace O-rings. The remainder of the system is stainless steel, and heater blankets will allow baking to 200\r{ }C. Provisions are included for Ti/Li gettering and glow discharge cleaning. PVD deposition of Ta and W films is used on surfaces in near proximity to the FRC or its exhaust plasma, and control of recycling will be a priority. Extensive testing using an electron micro-probe and various spectroscopic techniques have been performed to establish appropriate UHV cleaning methods. Detailed data and methodology will be presented. [Preview Abstract] |
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GP1.00071: Impurity Source Studies in TCS J.A. Grossnickle, H.Y. Guo, G.C. Vlases The original TCS experiment has demonstrated the robust ability to form and sustain FRCs in steady-state using Rotating Magnetic Fields (RMF). RMF parameters set the plasma density, but the temperature was severely limited by radiation, which was determined to be the dominant source of power loss for these plasmas. The total radiated power was strongly correlated with the Oxygen line radiation. This suggests Oxygen is the dominant radiating species. Determining the source of the impurities is an important question that must be answered for the TCS upgrade. Indications are that the primary sources of Oxygen are the stainless steel end cones. A Ti gettering system was installed, however, removal of Oxygen was accompanied by an increase in neutral Hydrogen, which also severely limited performance. These findings factored heavily into the design of the vacuum system and cleaning techniques for the TCS upgrade. The DIVIMP impurity code was modified to run on FRC geometries and showed that core impurity contamination is strongly dependent on electron density and radial transport. Thus, impurity source control is more crucial in TCS than high-density theta pinch formed FRCs. [Preview Abstract] |
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GP1.00072: Plans for SSPX Experiments with Auxiliary Heating and Multipulse Helicity Injection D.N. Hill, T.A. Casper, B.I. Cohen, E.B. Hooper, M.M. Marchiano, H.S. McLean, J. Moller, C. Romero-Talamas, R.D. Wood Recent results from the SSPX spheromak, in which peak electron temperatures Te $\sim $350eV were obtained, provide strong motivation for adding auxiliary heating to study energy transport and pressure limits. At 300eV, 1.8MW of neutral beam injection (NBI) heating would match the ohmic heating in the core plasma to provide a known and controlled heat source for the first time in a spheromak. The addition of NBI would follow by about a year the commissioning of a new modular solid-state programmable capacitor bank late in CY2006. The modular bank will allow multi-pulse operation, extend the buildup time for slowly building discharges, and lengthen the high-current formation pulse in SSPX, all of which are aimed at increasing the magnetic field produced by coaxial helicity injection. The schedule for proposed upgrades and plans for experimental campaigns using the modular bank and NBI heating systems will be discussed. [Preview Abstract] |
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GP1.00073: Time-resolved temperature measurements on SSPX with Te$>$350 eV H.S. McLean, D.N. Hill, J.M. Moller, C. Romero-Talamas, R.D. Wood The recent achievement of electron temperature Te $>$ 350 eV is motivating new methods to time-resolve Te in the SSPX spheromak, primarily to diagnose energy transport in the near-term and to study plasma heating dynamics and beta limits in future neutral beam injection experiments. The most direct method is to increase the rep-rate of the Thomson scattering laser system with the required rep-rate closely coupled to the physics being studied. Rapidly evolving MHD instabilities require very fast time response on the order of several microseconds necessitating a closely-spaced burst of laser pulses. Neutral beam heat deposition studies require measurements spaced out over several milliseconds. Several schemes are described including sequential firing of multiple independent lasers, multiple Q-switching of a single laser, and boosting the output of a high-frequency/low energy laser with several amplifier stages. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
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GP1.00074: Review of Bias Field Operation on SSPX R.D. Wood, D.N. Hill, E.B. Hooper, H.S. Mclean, C.A. Romero-Talamas We have carried out experiments to explore how different vacuum magnetic-field configurations affect plasma performance on SSPX. Six independently controllable magnetic field coils and power supplies can be programmed to obtain a wide range of field line configurations: those with vacuum magnetic field lines passing parallel to the side walls of the SSPX chamber and those with diverging fields passing through the side walls of the chamber. The new coils have greatly expanded the density range over which good spheromak plasmas are observed. This results from the ability to form, using the bias coils, a Penning discharge so that spheromak formation can be obtained at lower input gas pressure. In addition, spheromak theory suggests an increase in field amplification could be expected with magnetic field lines parallel to the chamber walls. We observed about a 20{\%} increase in field amplification suggesting that current flow during formation and the resultant MHD stability play an important role in magnetic field build up. Energy confinement and magnetic field generation during operation with the bias coils will be presented. This work performed under the auspices of the USDOE and the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48 [Preview Abstract] |
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GP1.00075: Internal magnetic measurements in SSPX using insertable probes C.A. Romero-Talamas Shortly after plasma breakdown at the Sustained Spheromak Physics Experiment (SSPX) a central column forms (aligned with the chamber axis), but impulsively bends and disappears seemingly merging around the chamber axis. This short lived central column is not well understood and is conjectured to be important in building helicity and forming closed magnetic flux surfaces. High-speed images of the transient central column have helped study its size and behavior in time, but do not provide details of the B field. For this reason, a multielement magnetic probe is being built to investigate the shape and magnitude of the field during spheromak formation and, in general, throughout a plasma shot. The design is based on a probe currently in use at the Caltech Spheromak Experiment [C.A. Romero-Talamas, P.M. Bellan, S.C. Hsu, Rev. Sci. Instrum. 75, 2664 (2004)] and consists of chip inductors arranged in 25 separate clusters to measure the three components of the field along a linear array that spans the entire radius of the SSPX chamber. The probe is designed to be removable between shots without perturbing the chamber's vacuum. The probe design, installation and operation challenges, as well as preliminary findings, are presented. [Preview Abstract] |
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GP1.00076: Report on the installation of Compact Neutral Particle Analyzer on SSPX for Ion Temperature Measurements Ephrem Mezonlin, Joseph Johnson III, David Hill, Bick Hooper, Harry McLean, Reg Wood Using a Compact Neutral Particle Analyzer (CNPA), the relationship between ion temperatures in the spheromak plasma and turbulent signatures in magnetic field fluctuations and species density fluctuations will be investigated in the SSPX at LLNL. Ion temperature measurements will also be useful for a better understanding of energy confinement in spheromak plasmas. The ion temperatures will be inferred from measurements of the energy distribution of charge-exchanged neutrals. To do this requires a careful vacuum analysis, which has now been performed, to be certain that the installation of the CNPA will not have any detrimental effects on the SSPX vessel. This analysis takes into account the main three components of this system: the gas stripping cell; the CNPA; and the SSPX vessel. Once the installation is completed, we will measure the energy indicators with sub-millisecond temporal resolution in order to correlate Ti with changes in magnetic and emission fluctuations related to the formation and sustainment processes in the spheromak discharge. [Preview Abstract] |
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GP1.00077: NIMROD Simulations of Spheromak Formation, Magnetic Reconnection and Energy Confinement in SSPX E.B. Hooper, B.I. Cohen, C.R. Sovinec The SSPX spheromak is formed and driven by a coaxial electrostatic gun that injects current and magnetic flux. Magnetic fluctuations are associated with the conversion of toroidal to poloidal magnetic flux during formation. After formation, fluctuations that break axisymmetry degrade magnetic surfaces, and are anti-correlated with the core temperature and energy confinement time. We report NIMROD simulations extending earlier work$^{1}$ supporting the SSPX experiment through predictions of performance and providing insight. The simulations are in fairly good agreement with features observed in SSPX and underscore the importance of current profile control in mitigating magnetic fluctuation amplitudes and improving confinement. The simulations yield insight into magnetic reconnection and the relationship of fluctuations to field line stochasticity. We have added external circuit equations for the new 32 module capacitor bank in SSPX that will add flexibility in shaping the injector current pulses and substantially increase the injected currents and the magnetic energy. New NIMROD simulations of SSPX lead to higher temperature plasmas than in previous simulations. *Work supported by U.S. DOE, under Contr. No. W-7405-ENG-48 at U. Cal. LLNL and under grant FG02-01ER54661 at U. Wisc Madison. $^{1}$C. R. Sovinec, B. I. Cohen, \textit{et al}., Phys. Rev. Lett. \textbf{94}, 035003 (2005); B. I. Cohen, E. B. Hooper, \textit{et al}., Phys. Plasmas \textbf{12}, 056106 (2005). [Preview Abstract] |
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GP1.00078: Influence of initial magnetic field configuration on spheromak evolution Giovanni Cone, Carl Sovinec The influence of the initial magnetic field distribution on spheromak formation and closed flux generation upon decay is studied using the \texttt{NIMROD} code. Previous spheromak simulations using the \texttt{NIMROD} code have demonstrated the formation of axisymmetric closed flux surfaces with decay of the magnetic field. The $q$ profile within the closed flux region was non-monotonic with values $q_0\sim0.8$ and $q_{\mbox{\tiny{min}}}\sim0.5$. As the configuration evolved, a $m=1$, $n=2$ mode led to localized magnetic field chaos resulting in a degradation of thermal energy confinement. Given the limited ability to control the evolution of the $q$ profile within the closed flux region of a spheromak, we investigate the possibility of forming spheromak plasmas that avoid this deleterious mode by tailoring the initial magnetic field profile appropriately. Poloidal flux amplification during the formation process involves conversion of injected toroidal flux via a line-tied kink mode. By strengthening or weakening the initial magnetic field along the geometric axis of the flux conserver, we attempt to control the amount of flux amplification to produce higher or lower values of $q$ throughout the closed flux surface region. Simulations are performed using a finite element grid that approximates the geometry of the Sustained Spheromak Physics Experiment. In collaboration with Bick Hooper and Bruce Cohen, Lawrence Livermore National Laboratories. [Preview Abstract] |
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GP1.00079: Spheromak plasma flow injection into a torus chamber and the HIST plasmas Akinori Hatuzaki, Ryohei Nishimoto, Yuji Ohnish, Naoyuki Fukumoto, Masayoshi Nagata The importance of plasma flow or two-fluid effect is recognized in understanding the relaxed states of high-beta torus plasmas, start-up and current drive by non-coaxial helicity injection, magnetic reconnection and plasma dynamo in fusion, laboratory and space plasmas. As a new approach to create a flowing two-fluid plasma equilibrium, we have tried to inject tangentially the plasma flow with spheromak-type magnetic configurations into a torus vacuum chamber with an external toroidal magnetic field (TF) coil. In the initial experiments, the RFP-like configuration with helical magnetic structures was realized in the torus vessel. The ion flow measurement with Mach probes showed that the ion flow keeps the same direction despite the reversal of the toroidal current and the axial electric field. The ion fluid comes to flow in the opposite direction to the electron fluid by the reversal of TF. This result suggests that not only electron but also ion flow contributes significantly on the reversed toroidal current. In this case, the ratio of $u_{i}$ to the electron flow velocity $u_{e}$ is estimated as $u_{i}$/$u_{e }\sim $ 1/2. We also will inject the spheromak flow into the HIST spherical torus plasmas to examine the possibilities to embedding the two-fluid effect in the ST plasmas. [Preview Abstract] |
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GP1.00080: Magnetized Target Fusion using a Field Reversed Configuration at Los Alamos Thomas Intrator, Shouyin Zhang, Glen Wurden, William Waganaar, Richard Renneke, Ivo Furno, Mark Kostora, Leonid Dorf, Scott Hsu, Adam Light, Drew Reese, Charles Beer, Alan Lynn, Mark Gilmore, Richard Siemon, James Degnan, Edward Ruden, Theodore Grabowski, Ronald Miller We present an overview of the Magnetized Target Fusion (MTF) project at Los Alamos National Laboratory. MTF could be a low cost path to fusion, in a regime that is intermediate between magnetic and inertial fusion energy. It requires compression of a magnetized target plasma and consequent heating to fusion relevant conditions inside a converging flux conserver. We hope to demonstrate the physics basis for MTF by translating a Field Reversed Configuration (FRC) target plasma into a compression region. FRX-L is a FRC that has shown substantial recent progress towards high pressure and density ($>20-30$ atmospheres, $5 \times 10^{22} m^{-3}$). Part of our FRC physics investigations of the collisional FRX-L include VUV spectroscopy diagnostics to measure plasma flow. Substantial progress towards implementation of the integrated liner compressed plasma experiment at AFRL include test implosions of a deformable liner, and a second generation engineering design. [Preview Abstract] |
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GP1.00081: Reproducible High Density Field-Reversed Configuration Plasma for Magnetized Target Fusion Shouyin Zhang, Thomas Intrator, Glen Wurden, William Waganaar, Richard Renneke, Ivo Furno, Jaeyoung Park, Scott Hsu, Chris Grabowski, Edward Ruden, James Degnan Field-Reversed Configuration (FRC) plasma will be translated into an imploding metal liner in a Magnetized Target Fusion (MTF) scenario. Field-Reversed Theta Pinch technology is employed with programmed cusp fields at the theta coil ends to achieve non-tearing field line reconnection during FRC formation. In the Field Reversed Configuration Experiment with a Liner (FRX-L), an optimum formation procedure is identified. The well-formed FRC plasma has volume-averaged density of 2 - 4$\times $10$^{22}$m$^{-3}$, T$_{e}$+T$_{i}$ of 300-500 eV, and plasma lifetime between 15-20 $\mu $s. These parameters are very close to the desired parameters of a target plasma for MTF, and they can be reproduced with standard deviation of less than 10{\%} about the mean in consecutive discharges. Recently, the redesigned crowbar switches have reduced the external main field modulation from 52{\%} previously to 21{\%} now. Better FRC performance is expected in on-going experimental campaigns. [Preview Abstract] |
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GP1.00082: Zero-Dimensional Energy Balance on a High Density Field-Reversed Configuration R. Renneke, T. Intrator, G.A. Wurden, S. Hsu, Shouyin Zhang FRCs have been created in the Field-Reversed Experiment with Liner (FRX-L) with density 2-4 x 10$^{22}$ m$^{-3}$, total temperature 300-400 eV, and lifetime on the order of 10 $\mu $s. We will present results of an energy balance on this high-density FRC using the method of Rej and Tuszewski (Phys. Fluids 27, p. 1514, 1984). This will include radiated energy measurements from an end-on wide-view bolometer, average pressure balance data from B-dot magnetic pick-up loops and magnetic flux loops, and profile information from a multichord interferometer. We expect conduction and convection losses to change compared to lower density experiments, due to the high-collisionality regime of these FRCs. Supported by DOE OFES contract W-7405-ENG-36. [Preview Abstract] |
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GP1.00083: Deformable contact liner implosion performed using 8 cm diameter electrode apertures with full axial coverage radiography. J.H. Degnan, D. Amdahl, G.F. Kiuttu, F.M. Lehr, J.D. Letterio, N.F. Roderick, E.L. Ruden, W. Tucker, P.J. Turchi, A. Brown, S.K. Coffey, G.G. Craddock, M.H. Frese, S.D. Frese, B. Guffey, T. Cavazos, D. Gale, T.C. Grabowski, R.E. Peterkin, Jr, W. Sommars, R.E. Siemon, Y.F.C. Thio Full axial coverage radiographic data, agreeing with 2D-MHD, indicate the feasibility of using a varying thickness in a long cylindrical solid liner, driven as a 12 megamp Z-pinch, to achieve factor 16 cylindrical convergence, while using 8 cm diameter aperture electrodes. The Al liner was 30 cm long, with 9.78 cm inner diameter for its full length, 10.0 cm outer diameter for the central 18 cm of its length, and outer diameter increased linearly to 10.2 cm at 1 cm from either electrode, and to 11 cm at electrode contacts. The electrode apertures allow injection of Field Reversed Configurations in proposed future experiments on magnetized target fusion. Sponsored by DOE-OFES. [Preview Abstract] |
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GP1.00084: The Effect of Sheared Toroidal Flow on an FRC's n=2 Rotational Instability Edward Ruden A Field Reversed Configuration (FRC) gains angular momentum until $\alpha =\Omega _{R}/\Omega _{Di}$ (rotational frequency over ion diamagnetic drift frequency) reaches a critical value triggering an instability with azimuthal mode number $n=2$. Questions remain as to whether the observed threshold is explained by a rigid rotor profile. Rotation of the bulk via kinematic viscosity and/or convection can entail significant velocity shear. Rotation results in plasma (centripetal) acceleration supported by an external magnetic field, so the instability may be interpreted as a Rayleigh-Taylor (R-T) mode. The instability is investigated here using an analytic planar R-T model of a Finite Larmour Radius (FLR) plasma with a magnetically transverse sheared flow layer accelerated by the magnetic field. Sheared flow and FLR effects are recognized and synergistic mitigating factors for the R-T instability, but if the sheared layer is too thin to reach the magnetic reversal axis, it is unstable and convection to the magnetic axis can be expected to occur quickly. Once this happens, though, the FRC is stable until the shear factor reaches a high value, at which time the $n=2$ mode goes unstable. This model provides insight into what may be an important feature of FRC stability, although less simplified calculations are needed. Nonetheless, it can be used tentatively to predict stability characteristics of an FRC during compression by an electromagnetically imploded metal cylinder for Magnetized Target Fusion. [Preview Abstract] |
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GP1.00085: Integrated Magnetic System Design for Field Reversed Experiment – Liner L. Dorf, T. Intrator, R. Renneke, G. Wurde, V. Semenov Field Reversed Experiment – Liner (FRX-L) is a magnetized target fusion experiment, in which magnetically confined plasma is compressed by an imploding aluminum flux conserver (liner) to achieve fusion-like conditions. The entire FRX-L construction must comprise three stages – formation, translation, and implosion. The magnetic system design for FRX-L and the eventual integrated liner on plasma experiment . involves diffusion of a time-varying external magnetic field into the region surrounded by a conductive cylindrical surface. It is necessary to protect the multi-turn “guide coils” that create an almost uniform magnetic field along the translation stage from a fast-varying magnetic field required in the formation region. This fast field can induce a very large voltage and current across the guide coils, causing irreversible damage. One of the ways to protect the guide coils is to use metal “flux excluder plates”. These plates should be designed such that they would not produce a considerable dip in the resultant magnetic field profile, Bz(z), as the dip in Bz(z) would prevent FRX-L plasma from translating towards the implosion region. In this talk, we propose a design of the magnetic system that fulfills all major requirements for successful formation, translation, and implosion of the FRX-L plasma. [Preview Abstract] |
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GP1.00086: Overview of the Pulsed High Density (PHD) FRC Experiment* George Votroubek, John Slough, Samuel Andreason, Hiroshi Gota, Richard Milroy Experimental studies are under way on the Pulsed High Density experiment (PHD) that will expand the conventional regime of the Field Reversed Configuration (FRC) to the very compact, high energy density regime to approach fusion. Initial studies explore pre-ionization and formation methods used to create high flux FRCs. By utilizing FRCs formed in a smaller, higher density regime, the requirement on the FRC closed poloidal flux is no greater than what has already been achieved; however, higher initial flux will require less final compression (i.e. lower confining field) to achieve fusion conditions. The high flux FRC source is designed to enable the completion of scaling and confinement studies (determined by the ratio of S$_{*}$, separatrix radius/ion skin depth, to E, the elongation) by enabling the ability to program FRC elongation. The ultimate goal of PHD is to form, accelerate and compress an FRC to a density of 1x10$^{22}$ m$^{-3}$ at a temperature greater than 1 keV. Following energy confinement time predicted by previous FRC scaling, the resulting FRC would have an n$\tau _{E}$ product $\sim $ 5x10$^{18}$ m$^{-3}$s. An overview of the experimental plan and basic approach to fusion conditions will be presented. *Research funded by the DOE Office of Fusion Energy Sciences. [Preview Abstract] |
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GP1.00087: Initial Diagnostics and First Experimental Results of the Pulsed High Density (PHD) FRC Experiment* Hiroshi Gota, Samuel Andreason, George Votroubek, John Slough The source region for the Pulsed High Density Experiment (PHDX) has been constructed, and Field-Reversed Configuration (FRC) plasmas are being produced. The several diagnostic systems include and axial array of 20 pairs of magnetic probes and flux loops, and a 64 channel array optical measurement system for visible bremstrahlung tomography. The tomographic system will be capable of reconstructing the plasma shape and mode structure, and will incorporate information from end-on imaging for improved resolution. This array consists of collimator, optical fiber, optical filter ($\lambda $=520 nm, FWHM= 1 to 10 nm), and Photomultiplier Tube (PMT). The tomographic system and magnetic loop array will be used to investigate the equilibrium and tilt stability of FRCs at high $s$/$E$ ($>$3) where $s$ is the ion collisionless skin depth and $E$ is the plasma elongation. The separatrix radius ($r_{s})$ of FRC plasma is determined by the excluded flux measurement, and it is found that $r_{s}$=0.04-0.05 m ($r_{s}$/$r_{w}$=0.16-0.2) just after the RMF current drive start-up. The time sequence of separatrix shape relatively agrees with the result of that estimated from the line-integrated radiation intensity at different axial positions. We will present the result of both simulation and experimental results from measured FRC plasmas as well as future plans. *Research funded by the DOE Office of Fusion Energy Sciences [Preview Abstract] |
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GP1.00088: Initial Pre-ionization and Formation Results from the Pulsed High Density (PHD) Experiment* Samuel Andreason, Hiroshi Gota, George Votroubek, John Slough The Pulsed High Density (PHD) experiment will form Field Reversed Configuration (FRC) plasmas that are accelerated and compressed to near fusion conditions. PHD has begun initial operations concentrated on forming a FRC with the necessary characteristics for acceleration. This source section is required to produce a FRC with an initial trapped flux greater than 10 mWb and initial plasma inventory greater than 5.5 x 10 $^{20}$ ions. Thus far, the experimental emphasis has been on pre-ionization, formation and rotation. Source FRCs have been formed with the initial plasma provided by an axial electric field breakdown. The effects of Rotating Magnetic Field (RMF) on ionization and rotation are being studied. It is hoped that the RMF can spin up the plasma in a direction opposite to that produced by theta pinch FRC formation. A visible light spectrometer with a helium/deuterium gas fill will allow us to measure this rotation. RMFs have been studied on this experiment in the frequency range of 25 kHz to 400 kHz, bracketing the ion gyro-frequency in the RMF field. Additional methods are available for ionization including ringing theta- pinch and a Cascaded Arc Source. The efficacy of ionization techniques will be determined by a HeNe interferometer and neutral pressure measurement. *Research funded by the DOE Office of Fusion Energy Sciences. [Preview Abstract] |
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GP1.00089: Inductive Plasma Accelerator (IPA) Timothy Ziemba, John Slough, Richard Milroy The Inductive Plasma Accelerator (IPA) is a plasma accelerator/interaction experiment currently under constuction at MSNW. The accelerator will be capable of launching a Field Reversed Configuration (FRC) plasmoid having a mass of up to 0.2 mg with a diameter no larger than about 10 cm. In addition, the accelerator will be designed to attain plasma/plasmoid velocities up to~300 km/s while maintaining high uniformity and purity. Two IPAs will be arranged on a test bed to perform FRC merging experiments. In addition, an interaction chamber will be constucted to produce and implode~a plasma liner for enhanced compression experiments on the merged FRCs thus providing the first experimental test of the plasma liner fusion concept. 2D MHD simulations show expected densities of $>$ 10$^{22}$ m$^{-3}$ with ion temperatures in excess of 800 eV for the merging FRCs. The status of design and construction of the experiment and additional simulation results will be presented [Preview Abstract] |
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GP1.00090: Overview of the Helicity Injected Torus Program A.J. Redd, T.R. Jarboe, W.T. Hamp, B.A. Nelson, R.G. O'Neill, R. Raman, P.E. Sieck, R.J. Smith, G.L. Sutphin, J.S. Wrobel The Helicity Injected Torus with Steady Inductive Helicity Injection (SIHI) spheromak experiment (HIT--SI)[Jarboe, Fus.~Tech., v.~36, p.~85 (1999)] addresses critical issues for spheromaks, including current drive, high-beta operation, and confinement quality. HIT--SI features an optimized high-beta plasma shape and current profile, minimal plasma-wall interaction, and the long-term goal of steady-state operation. HIT--SI has a ``bow-tie'' shaped axisymmetric confinement region (major radius R=0.33m, axial extent of 0.57m) and two half-torus helicity injectors, one mounted on each end of the flux conserver. The flux and loop voltage in each helicity injector are varied sinusoidally and in phase, while the two injectors are 90 degrees out of phase with each other, producing a constant rate of helicity injection. The physical principles of SIHI and the HIT--SI device will be presented, along with descriptions of key experimental and computational results. [Preview Abstract] |
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GP1.00091: Modifications to the HIT-SI Flux Conserver P.E. Sieck, W.T. Hamp, T.R. Jarboe, G.J. Marklin, B.A. Nelson, R.G. O'Neill, A.J. Redd, R.J. Smith, G.L. Sutphin, J.S. Wrobel Operation of the HIT-SI helicity injectors leads to a family of field lines that are not driven directly by the injector voltage. These fields are analogous to the closed toroidal fields inside the current sheet of a “bubble-burst” coaxial gun, but the asymmetric injector geometry on HIT-SI allows these field lines to exit the flux conserver through the midplane diagnostic gap. Furthermore, as relaxation activity arises in the vessel, toroidal modes can push field into this gap. Open field lines are helicity-dissipating\footnote{T. R. Jarboe and B. Alper, Phys. Fluids {\bf 30} (4), p. 1177, 1987} and should be avoided. A copper strap has been installed across the diagnostic gap. This modification makes the flux conserver more complete, reducing the quantity of helicity-dissipating flux. The effect of the strap on the magnetic structure of the plasma will be shown. The strap will be replaced with localized current-carrying bridges to restore diagnostic access through the gap. A design for these bridges will be presented. [Preview Abstract] |
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GP1.00092: Internal Magnetic Field Measurements and Langmuir Probe Results for the HIT-SI Experiment R.J. Smith, T.R. Jarboe, B.A. Nelson, A.J. Redd, W.T. Hamp, R.G. O'Neill, P.E. Sieck, G.L. Sutphin, J.S. Wrobel HIT-SI is a spheromak device in which the plasma is generated and sustained by steady inductive helicity injection. Helicity injection is maintained at a constant rate by means of two AC RFP sources phased in quadrature and connected to the main chamber so as to drive a rotating n=1 mode at 5kHz. A magnetic probe consisting of three separated radial arrays of 3d coils has been designed to allow the direct measurement of the plasma current and induced electric fields using finite differences of the magnetic field components. The probe is insertable at the mid-plane to a depth of 15cm. Langmuir probes have been built to study the edge plasma at the mid-plane and the injector openings. Measurements of the internal magnetic field structure, plasma current, poloidal and toroidal flux distributions along with Langmuir probe results are presented for HIT-SI operations over an operation space of varying injector voltage, injector flux and fill density. [Preview Abstract] |
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GP1.00093: Ion Doppler Spectroscopy and Far Infrared Interferometry on the HIT-SI spheromak R.G. O'Neill, R.J. Smith, A.J. Redd, T.R. Jarboe Ion Doppler Spectroscopy (IDS) is used to measure impurity ion velocity and temperature on HIT-SI. The spectrometer uses a 16 channel photo multiplier to track temperature and velocity continuously through the discharge. The spectrometer can view into the HIT-SI injector region as well as into the equilibrium region. Temperature and velocity data will be presented. A tangentially viewing far infrared (FIR) interferometer is being configured to measure electron density on HIT-SI. The system will use a two optically pumped diflouromethane gas lasers to produce a heterodyne signal. It will provide more power, higher heterodyne beat frequency, and improved signal to noise ratio over the older FIR system it replaces. The system can achieve a heterodyne beat of up to 2 MHz compared to the older system frequency 250 kHz. The increased frequency is required to track density fluctuations on HIT-SI. An update on the status of the FIR system will be presented. [Preview Abstract] |
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GP1.00094: Wall Conditioning Enhancements to the HIT-SI Spheromak W.T. Hamp, T.R. Jarboe, B.A. Nelson, R.G. O'Neill, A.J. Redd, P.E. Sieck, R.J. Smith, G.L. Sutphin, J.S. Wrobel HIT-SI was initially operated in a prototype laboratory to calibrate its magnetic probes, and develop its baseline operating parameters. The prototyping laboratory had a minimal vacuum system and no wall conditioning. As the experiment was transitioned to the main laboratory the HIT-SI team developed several wall conditioning techniques to help provide the best possible environment for spheromak formation and sustainment . These systems include pulse discharge cleaning (PDC), glow discharge cleaning (GDC), and a convection baking system to heat the vessel up to 200$^{o}$C. The motivation for these systems and an analysis of their effectiveness are presented. [Preview Abstract] |
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GP1.00095: A Microwave Interferometer and Polarimeter for the HIT-SI Device J.S. Wrobel, R.J. Smith, T.R. Jarboe, R.G. O'Neill Polarimetry has been proposed on HIT-SI as a non-perturbative internal field measurement system. HIT-SI studies the use of inductive helicity injection for sustainment of a toroidal current in a spheromak. The existing 185 $\mu $m FIR interferometer on HIT-SI [1] is capable of multi-chord density measurements but has an unsuitable wavelength for polarimetry. A 2mm heterodyne system based on an IMPATT diode source is being considered. The sources will be stabilized by a phase locked loop and differential quadrature detection is proposed to minimize non-ideal behavior of mixers. The microwave interferometry and polarimetry system will provide chord averaged density and magnetic field measurements to complement the density profile measurements from the FIR interferometer. The design and modeling of the microwave system will be presented for various HIT-SI spheromak equilibria. \newline \newline [1] Jewell, Patrick D. \textit{et al}., Rev. Sci. Instr. \textbf{74,} 80 (2003). [Preview Abstract] |
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GP1.00096: Ongoing MHD Simulations of HIT-SI Using NIMROD G.L. Sutphin, T.R. Jarboe, V.A. Izzo, B.A. Nelson, A.J. Redd, P.E. Sieck, J.A. Rogers The Steady Inductive Helicity Injected Torus (HIT-SI) is a spheromak concept that uses dual injectors to provide constant steady inductive helicity injection (SIHI), which maintains toroidal current by generating poloidal flux with relaxation current drive. Decaying spheromak equilibrium simulations of HIT-SI using Non-Ideal Magnetohydrodynamics (MHD) with Rotation Open Discussion (NIMROD), a finite element, resistive MHD code, demonstrate flux amplification and relaxation current drive. While NIMROD is designed to handle only axisymmetric geometries, time dependent, non-axisymmetric boundary conditions applied to the axisymmetric confinement region of HIT-SI approximate the interaction with the injectors. Previous driven spheromak simulations show poloidal flux amplification at high Lundquist numbers (S = 516, S=897) through the buildup of the magnetic energy in the n = 0 mode relative to the n =1 mode. At low Lundquist numbers (S = 22), where the resistive diffusion time is high relative to the Alfven time, this poloidal flux amplification does not occur. Current research focused on determining the range of S where this build up starts to occur is presented. [Preview Abstract] |
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GP1.00097: Overview of Caltech Spheromak/Astrophysical Jet/Solar Simulation Experiments Paul Bellan, Setthivoine You, Gunsu Yun, Shreekrishna Tripathi, Deepak Kumar Experiments underway at Caltech address several issues common to spheromaks, solar coronal loops, and astrophysical jets. These experiments use magnetized plasma guns and allow the plasma morphology to self-organize rather be imposed by either external coils or the shape of a surrounding vacuum chamber. The evolution of the plasma morphology/topology is tracked using high speed cameras. A good understanding of the evolutionary sequence has been obtained as well as insights into how this sequence depends on the interactions between MHD forces, convection of magnetic flux frozen into flowing plasma, and collimation resulting from the pile-up of flowing plasma and its embedded flux. These interactions are fundamental to the development of equilibria since flow, mass ingestion, and flux build-up are the means by which equilibrium hydrodynamic pressure and magnetic field are established. [Preview Abstract] |
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GP1.00098: Spectroscopic Study on Collimating Plasma Jets: Space- and Time-Resolved Flow Velocity and Electron Density Gunsu S. Yun, Paul M. Bellan In the Caltech spheromak experiment, current-carrying plasma jets are generated to study formation of spheromaks and astrophysical jets. High-speed camera images and spectroscopic measurements [1] reveal that the plasma jets are very fast ($\sim30~km/s$), collimated and dense ($n_{e}$ increases from $\sim10^{20}$ $m^{-3}$ to $\sim10^{22~}m^{-3}$ during the collimation phase). We believe that the collimation is a result of flow stagnation [1]. If flow decelerates along the jet axis, the toroidal magnetic flux carried by the plasma will pile up like fast traffic running into slower traffic, amplifying the pinch force and thus squeezing the plasma. The spectroscopic system now includes a new device designed to measure jet velocities and densities at multiple positions in a single shot. The device consists of a lens, a diffusing screen, and a linear 12 channel fiber array. The fiber array samples several points on the image of plasma jet formed on the screen by the lens. The new diagnostic will permit observation of velocity and density gradients along the jet axis during the collimation process, thus helping to correlate flow stagnation with density gradient and collimation. [1] S. You, G. S. Yun, and P. M. Bellan, \textit{Dynamic and Stagnating Plasma Flow Leading to Magnetic Flux Tube Collimation}, Phys. Rev. Letter (2005) in press. [Preview Abstract] |
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GP1.00099: Sausage, Kink and Spheromak Formation Setthivoine You, Paul M. Bellan Observations from the Caltech spheromak experiment motivate a revisiting of the sausage and kink instabilities. A collimated, current-carrying magnetic flux tube (center column) is observed to ``detach" from the gun at high $\lambda$ (ratio of gun current to imposed magnetic flux). This detachment appears to be initiated by a sausage instability. Previous measurements [1] indicated the kink instability of this center column is a precursor to spheromak formation. A flux-amplifying kink of the column occurs first, quickly followed by a detachment and spheromak formation. Furthermore, recent results [2] have shown that before kinking, dynamic and stagnating plasma flow is responsible for flux tube collimation -as predicted by the gobble/collimation theory [3]. Ideal MHD linear stability analysis of a diffuse current pinch (using the energy principle and including a skin current and magnetic field relationships from [3]) maps out a $k$ vs $\lambda$ operating space, where $k$ is the inverse length. The space includes $m=0$ and $m=1$ instability regions that are modified from classical thresholds. Measurements of $k$ and $\lambda$ throughout the duration of the plasma discharge then trace a path in the operating space. Hence, the observed evolution of the plasma column may suggest an answer to a novel question: is sausage instability another necessary condition for spheromak formation? [1] S.C.Hsu, P.M.Bellan, PRL, 90, 215002 (2003) [2] S.You, G.S.Yun, P.M.Bellan, PRL, (2005) in press [3] P.M.Bellan, Phys.Plasmas, 10, 5 (2003) 1999. [Preview Abstract] |
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GP1.00100: Spheromak Plasma Density Measurement Using a Quadrature Phase Interferometer Deepak Kumar, Paul Bellan A quadrature phase homodyne interferometer has been developed in double-pass Michelson geometry to measure the density of plasma produced in the Caltech spheromak experiment. The design is specially suited for large vacuum chambers ($\sim $1.5m in diameter) and for experiments with time scales ($\sim $10 $\mu $s) much less than the time scale of acoustic vibrations. Quadrature phase information is generated by the interference of a linearly polarized scene beam and a circularly polarized reference beam. Path length difference between the two beams is approximately 3m. To compensate for this large difference, we utilize a non-fundamental longitudinal mode of the HeNe laser. This ensures a greatly simplified optical alignment procedure. The interferometer detected very high densities ($\sim $10$^{22}$/m$^{3})$ of the central plasma jet column. The high densities have been corroborated by Stark broadening measurements.\footnote{ S. You, G. Yun, P. M. Bellan, Phys. Rev. Lett. (In print)} Motivated by the results of the homodyne interferometer, a prototype heterodyne interferometer is also being developed. It is expected to have much better signal to noise ratio. [Preview Abstract] |
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GP1.00101: Onset and saturation of ion heating by odd-parity rotating magnetic fields in the FRC Samuel Cohen, Alexandra S. Landsman Heating of figure-8 ions by odd-parity rotating magnetic fields (RMF) applied to an elongated field-reversed configuration is explored using numerical and analytic techniques. Energy gain is shown to occur at resonances of the RMF frequency with the figure-8 orbital frequency. Cyclotron orbits and high-energy figure-8 orbits tend to interact regularly with the RMF, leading to little or no heating. There is a range of energies for figure-8 orbits, which have a stochastic interaction with the RMF, showing large gains of energy. The lower bound on this type of heating can be derived using exponential separation of trajectories. It is shown that this bound depends on the frequency ratio, s, where s is the ratio of the RMF frequency to the figure-8 orbital frequency. [Preview Abstract] |
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GP1.00102: Wobble Motion on Field-Reversed Configuration Plasmas Yoshinori Hasegawa, Hiroshi Gota, Kayoko Fujimoto, Toshiyuki Fujino, Tomohiko Asai, Tsutomu Takahashi, Yasuyuki Nogi A wobble motion on a field-reversed configuration (FRC) plasma is investigated in detail. It is found from magnetic and optical measurements of the wobble motion that a few magnetic islands appear inside a separatrix at the formation phase and merge with each other at the axial contraction phase. Due to the radial and azimuthal asymmetries of the merging plasmas, the wobble motion is triggered. Sometimes, FRCs that are large amplitude of the wobble motion and very short lifetime of the configuration are observed. When conducting or resistive rings are installed near the ends of the theta-pinch coil, the symmetrical formation of the plasma is assisted and the amplitude of the wobble motion is reduced. Especially, the resistive rings are expected to control effectively the wobble motion by freezing open field lines into them. From these experiments, it is discussed that the wobble motion originates in the formation phase. [Preview Abstract] |
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GP1.00103: Flow measurements in spheromak merging experiments C.D. Cothran, J. Fung, M.R. Brown, M.J. Schaffer, E.V. Belova Results of time resolved multi-chord ion doppler spectroscopy (IDS) studies at the Swarthmore Spheromak Experiment (SSX) are reported. SSX is capable of producing several types of compact toroidal configurations using one or both opposed spheromak sources. A field reversed configuration (FRC) well described by MHD is produced when counter-helicity spheromaks are merged. After a brief dynamic period, co-helicity merging produces a single large fully tilted spheromak or possibly a short Taylor helix. IDS measurements are complete on each of these configurations; Abel inverted midplane flow profiles will be presented. Magnetic studies indicate the FRC should contain a radially sheared toroidal flow at the midplane to support remnant anti-parallel toroidal fields observed at the ends. IDS measurements during the reconnection phase of FRC formation is characterized by double gaussian lineshapes separated by $0.4 v_A$, consistent with bi-directional reconnection outflow. [Preview Abstract] |
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GP1.00104: Stability Studies of Compact Toroid Plasmas in MRX S.P. Gerhardt, M. Inomoto, M. Yamada, H. Ji, A.J. Carver, A. Kuritsyn, B. McGeehan, Y. Ren A series of recent experiments in the MRX device have focused on the stability of compact toroid plasmas formed by spheromak merging. The equilibrium field is produced by three independent coil sets, allowing flexibility in the plasma shape and external field decay index. The stability of the plasma is studied using internal and external arrays of magnetic pick-up coils. The ion temperature and toroidal plasma rotation are monitored via Doppler spectroscopy with either an optical probe (local measurement) or line-integrated measurements (global measurements). Our FRC plasmas tend to develop instabilities, but careful shaping of the equilibrium field enables a longer lived plasma. In particular, an external field decay index much greater than zero is required to form longer-lived plasmas. Detailed results from the magnetic measurements and comparisons with rigid-body stability theory will be shown. The rigid-body analysis of figure-8 coil stabilization has been expanded to include the effects of a distributed current profile, and a set of ``figure-8'' coils has been designed to stabilize global motions. The design of these coils and results (as available) will be shown. This work is funded by the Department of Energy. [Preview Abstract] |
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GP1.00105: Irvine FRC Plasma Characterization and Upgrades E.H. Trask, E.P. Garate, W.S. Harris, W.W. Heidbrink, A. van Drie Reversed fields of $\sim100$ Gauss have been observed. New three dimensional magnetic probe arrays aid us in analyzing the structure of our fields throughout the formation and evolution of our field reversed configuration. Plasma densities of $2\times10^{13}$~cm$^{-3}$ and temperatures of $\sim2$~eV have been observed with a triple probe diagnostic. Confirmation of the plasma density is being tested with a 30 GHz interferometer. Extensive work has been done to increase both the number of channels in our data acquisition system as well as the sensitivity and reliability of our measurements. We will provide an overview of our current acquisition system as well as planned upgrades. [Preview Abstract] |
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GP1.00106: Current Diagnostics in an FRC with a Toroidal Electric Field W.S. Harris, E.P. Garate, W.W. Heidbrink, E.H. Trask, A. van Drie A time-of-flight neutral particle detector\footnote{D. E. Voss and S. A. Cohen, Rev. Sci. Instrum. {\bf53}, 1696 (1982).} will be used to diagnose the ion contribution to the current in the FRC experiment at UCI. The ions are accelerated due to the EMF induced by a flux coil with a peak field of 1 kG. In addition to ion acceleration measurements, the ion distribution function can also be determined. The diagnostic involves a slotted disk that spins at 20,000 rpm, which allows a snapshot of the toroidal velocity distribution to be measured after the neutrals free stream to an electron multiplier. A Mach probe has also been implemented to measure the plasma flow. In conjunction with these, a Rogowski coil is used to measure the total toroidal current. From these measurements, we can deduce the ion contribution to the current. [Preview Abstract] |
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GP1.00107: Mechanisms of current drive improvement in Rotamak plasmas. Yuri Petrov, Tian-Sen Huang The current driven by the rotating magnetic field (RMF) is observed to be larger than expected from the simplified infinite-cylinder model. The mechanism appears to be different for the two regimes of rotamak operation: the Rotamak-ST (with the applied toroidal field), and Rotamak-FRC (no external toroidal field). In the former case, the larger current is associated with an improved penetration of RMF into the plasma. We inspect the experimental data with the model of the whistler wave mode excitation. The comparison of experimental radial profiles of the RMF components with theoretical profiles shows that the first radial mode is present in our plasmas. In the Rotamak-FRC case, we confirm that the poloidally swirling currents associated with the observed self-generated toroidal field in the Rotamak-FRC case can be responsible for driving the toroidal current in the inner areas of plasma where RMF field is nearly zero. The mechanism is due to the coupling of the radial component of the swirling currents with the axial magnetic field. To match the resulting toroidal current with the measured toroidal current density profile, we had to accept a large anomalous resistivity order of 1 mOhm-m. [Preview Abstract] |
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GP1.00108: Accelerator-Region Gas Puffing Experiments on CTIX Robert Horton, Stephen Howard, Sam Brockington, Russell Evans, David Hwang The technique of gas puffing into the acceleration section of the CTIX device has proven to be a simple and reliable method to increase plasma density by a factor of five or more. The resulting plasma maintains the internal poloidal field characteristic of the spheromak-like compact toroid (SCT), although typically at reduced field strength. With proper choice of timing and gas puff intensity, no special measures, such as external magnetic fields, are required to prevent premature breakdown in the acceleration region. The option of puffing a different gas in formation and acceleration regions allows original and added plasma components to be distinguished spectroscopically. A detailed survey of operating conditions will be presented, in which CTIX operating voltages, formation plasma density, accelerator gas valve timing, duration, and operating pressure, are systematically varied. Major diagnostics include upstream and downstream laser interferometers, axial and azimuthal magnetic field probes, and filtered photodiodes; fast cameras in a drift region downstream from the accelerator; and axially- viewing photodiodes and camera. A goal of the experimental series will be to maximize SCT kinetic energy density, the parameter which determines ability of the SCT to penetrate magnetic fields. Supported by U.S. DOE Grant DEFG0203ER54732 [Preview Abstract] |
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GP1.00109: Overview of Recent Results from HSX David Anderson, A. Abdou, A.F. Almagri, F.S.B. Anderson, A.R. Briesemeister, D.L. Brower, J. Canik, C. Deng, W. Guttenfelder, C. Lechte, K.M. Likin, J. Lore, H. Lu, S. Oh, P.H. Probert, J. Radder, V. Sakaguchi, J. Schmitt, J.N. Talmadge, K. Zhai HSX has demonstrated that the quasihelical symmetry (QHS) does indeed improve single-particle confinement over a non-optimized 3-D configuration, as predicted. Some neoclassical differences have been observed under the present operating conditions. We have demonstrated that quasisymmetry leads to reduced parallel viscous damping. This work is being extended to look at flow damping in the presence of islands that locally break the quasihelical symmetry. We have concluded, by making comparisons of on-axis to off-axis heating, that thermodiffusion may account for profile differences, with low thermodiffusion in the QHS case as compared to the mirror case. Our goals are to increase the density, the magnetic field and heating power to accentuate neoclassical transport relative to anomalous. O-mode heating at B=1.0T will allow us to raise the density, decrease anomalous transport, reduce the population of the electron tail and raise the confinement time. A new mode of operation will allow comparisons of confinement over a wide range of effective ripple in a single device. [Preview Abstract] |
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GP1.00110: Particle Transport in HSX John Canik, David Anderson, Simon Anderson, Joseph Talmadge, Kan Zhai The density profile in the Quasi-Helically Symmetric (QHS) configuration is centrally peaked for both on- and off-axis heating. In contrast, for a magnetic configuration with the symmetry broken (Mirror), the density profile is flat or slightly hollow with on-axis heating and a centrally peaked temperature profile. When the ECH resonance is moved off-axis, the temperature profile becomes flat inside the heating radius, and the density profile becomes peaked. To understand particle transport in HSX, experimental data from a set of absolutely calibrated H$_{\alpha }$ detectors has been coupled to simulations using the DEGAS\footnote{Heifetz, D.B. \textit{et al}, J. Comp. Phys. \textbf{46,} (1982) 309} neutral gas code. These calculations yield the particle source rate, which can be integrated to give the steady state radial particle flux. It is found that in QHS plasmas, the experimental particle flux is much larger than the neoclassical flux. In Mirror plasmas, the neoclassical flux is comparable to experiment in the core (r/a $<$ 0.4). In this region, the thermodiffusive flux is the dominant term in the total neoclassical particle flux, suggesting that neoclassical thermodiffusion is the cause of the hollow density profile in the nonsymmetric configuration. This work is supported by DOE Grant DE-FG02-93ER54222. [Preview Abstract] |
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GP1.00111: Increased Parallel Viscous Damping Near Magnetic Islands in HSX John Schmitt, Joseph Talmadge, David Anderson, Stefan Gerhardt The quasisymmetric field in HSX can be broken due to the interaction of the main n=4, m=1 helical magnetic field with the mode structure imposed by naturally forming magnetic islands at rational surfaces. This interaction gives rise to additional components of the magnetic field spectrum that increase the parallel viscous damping in the vicinity of the islands. We calculate the neoclassical increase in the two plasma flow damping rates on a magnetic surface as well as the local increase in the radial conductivity. Using a biased electrode to spin the plasma, a Mach probe to measure the time evolution of the plasma flow and Langmuir probes to measure the electric field, we present initial measurements to demonstrate the effects magnetic islands have on the quasihelically symmetric field in HSX. [Preview Abstract] |
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GP1.00112: Superthermal electron dynamics in HSX stellarator. A.E. Abdou, A. Almagri, D.T. Anderson, K.M. Likin, J.N. Talmadge Superthermal electrons are generated in HSX stellarator during two different processes, (1) magnetic field ramping and (2) 2$^{nd}$ harmonic X-mode ECRH. The normal magnetic configuration is Quasi-Helically Symmetric (QHS). With a set of auxiliary coils, the quasihelical symmetry can be broken (Mirror and AntiMirror configurations). In this work the resolved hard x- ray emission is analyzed using a CdZnTe detector. The hard x-ray spectra were accumulated in a series of similar ECRH discharges. The behavior of the superthermal electrons has been studied for densities in the range of 0.1 to 1.0 x 10$^{12}$ cm$^{-3}$. The magnetic configuration has also been altered in order to determine the effect of magnetic ripples on characteristic energies and densities of superthermal electrons. Pulse height analysis of the hard x-ray emission shows the presence of x-ray photons with energies as high as 1 MeV during the microwave discharge. The Hard x-ray emission shows inverse nonlinear density dependence and higher x-ray intensities and photon energies in QHS than MIRROR and AntiMirror configurations. The time evolution of the hard x-rays shows enhanced superthermal electron confinement in QHS than the Mirror configuration. Calculations of single particle heating and drift indicated that the improved confinement in the quasisymmetric configuration is responsible for the more efficient heating of superthermal electrons. [Preview Abstract] |
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GP1.00113: ECE spectrum of HSX plasma at 0.5 T Konstantin Likin, Hui Juan Lu, David Anderson, Simon Anderson, John Canik, Joseph Talmadge, Kan Zai, Chuanbao Deng, Calvin Domier, Robert Harvey The spectrum of the Electron Cyclotron Emission (ECE) from HSX is measured with an eight channel radiometer at a magnetic field of 0.5 T. The central region of the plasma is not accessible because the plasma is heated by the extraordinary wave at the second harmonic and a narrow stop band filter at 28$\pm $0.3 GHz is used to suppress the non-absorbed microwave power. A comparison between these data and the results of calculations by CQL3D code is reported. Also a comparison between ECE and Thomson scattering measurements is made. Low frequency ($<$ 70 kHz), narrow bandwidth ($<$ 5 kHz), low amplitude (less than 5{\%}) fluctuations have been measured by the ECE radiometer. The phase difference between ECE signals on the either side of the magnetic axis is about 180 degrees. The amplitude of the fluctuations is almost independent of plasma density and has a maximum at about r/a$_{p }$= 0.4 that corresponds to the region of maximum plasma density gradient. [Preview Abstract] |
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GP1.00114: Energetic-Electron-Driven Alfv\'{e}nic Modes in HSX C. Deng, D.L. Brower, D.A. Spong, A. Abdou, A.F. Almalgri, D.T. Anderson, F.S.B. Anderson, K. Likin, S. Oh, V. Sakaguchi, J. Schmitt, J.N. Talmadge, K. Zhai Coherent, global fluctuations in the range of 20-120 kHz are observed for quasi-helically-symmetric ECRH produced plasmas in HSX. Calculations of the Alfvén continua for the n = 1 mode show the measured fluctuations fall in a gap below the m=1 resonance corresponding to a global Alfvén eigenmode (GAE). Fast electrons associated with 2nd harmonic X-mode ECRH are thought to drive the instability. The measured frequency range as well as scaling with ion mass density is consistent with Alfvenic modes. When quasi-helical symmetry is broken, the mode is no longer observed. Flows generated by a biased electrode modify the mode amplitude and frequency. Under these conditions, the fluctuation is even observed in the conventional stellarator configuration. The relation between mode amplitude, flows and growth rates will be explored. [Preview Abstract] |
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GP1.00115: Measurement of magnetic fluctuations in HSX Stellarator S. Oh, A.F. Almagri, D.T. Anderson, J.N. Talmadge, J.C. Schmitt, D.L. Brower, C. Deng Magnetic fluctuations are a strong feature of HSX ECRH plasma at densities up to 2.0x10$^{12}$ cm$^{-3}$. We observe a coherent mode at 40- 50 kHz with a poloidal mode number m=0 in a broad plasma density range. The mode's amplitude and frequency depend on plasma density and the heating location and the mode is well correlated with density fluctuations. Plasma biasing changes the frequency and amplitude of this mode. We also observe a bursty mode at density below 0.5x10$^{12}$ cm$^{-3}$. At this density range we also observe the presence of hard x-rays generated by a fast electron tail. This mode at frequency of about 120 kHz has an m=1 poloidal structure and has a large impact on the plasma confinement. We observe large decreases in the stored energy and ECE signal, and large increases in the soft X-ray signal at the onset of this mode. This mode is observed only with a near- axis heating location. We plan to measure the toroidal n spectrum of these modes and understand what drives these modes unstable. Initial results of these magnetic studies will be presented. [Preview Abstract] |
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GP1.00116: Structure of Edge Turbulence in the HSX Stellarator Carsten Lechte, Walter Guttenfelder, Joe Talmadge, David Anderson The magnetic field of HSX has a unique helical quasisymmetry, which can be broken by a set of auxiliary field coils. Hydrogen plasmas, produced by up to 130\,kW of ECRH power at 5\,kG, exhibit turbulent behavior which is diagnosed by a 16-pin Langmuir probe moved radially in the plasma, while a reference probe stays fixed in space. Measurements of plasma density and potential are processed with correlation analysis and conditional sampling to find the structure of particle transport events (``blob'') at the plasma edge ($r/a > 0.6$). The density-potential crossphases are used to classify the underlying instabilities (i.e. drift wave vs. interchange.) First results from inside the separatrix show blobs with 2--2.5\,cm diameter moving along with the bulk plasma rotation. The crossphase is small, hinting at drift wave dynamics. When a strong radial electric field is imposed on the plasma (via a bias probe), the radial correlation length is halved and the poloidal $E\times B$ rotation is increased. [Preview Abstract] |
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GP1.00117: Similarity of Edge Turbulence in Various Magnetic Configurations of HSX Walter Guttenfelder, David Anderson, Carsten Lechte, Joseph Talmadge Multi-pin Langmuir probes have been used to measure the edge and SOL characteristics of HSX plasmas in multiple locations and under various magnetic configurations. Auxiliary coils provide the flexibility to change the vacuum magnetic spectrum, well depth, rotational transform, and effective minor radius. The ion saturation current and floating potential probes measure broadband, large level fluctuations (10 -- 40{\%}) in the edge and SOL. Poloidal wavenumbers, measured via two displaced probes, are in the range of 0.5 -- 1.5 cm$^{-1}$, with $\rho _{s}$k$_{\theta }$ = 0.1 -- 0.2. From the estimated density gradient scale lengths in the edge (L$_{n}$ = 2 -- 5 cm), these fluctuation levels are consistent with mixing length type arguments (n`/n $\sim $ 1/k$_{\theta }$L$_{n})$. The phase velocities of the fluctuations follow the E$\times $B velocities inferred from floating potential profiles. The measured turbulent features are similar for the quasi-symmetric and non-symmetric configurations. This work is supported by DOE grant number DE-FG02-93ER54222. [Preview Abstract] |
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GP1.00118: B=1.0T Operation of HSX F. Simon B. Anderson, A.F. Almagri, D.T. Anderson, A.R. Briesemeister, J. Canik, C. Deng, W.A. Guttenfelder, C. Lechte, K.M. Likin, J. Lore, H. Lu, S. Oh, J. Radder, J. Schmitt, J.N. Talmadge, K. Zhai Operation of the HSX device at 1 Tesla is planned for late 2005 with fundamental O-mode ECH at 28 GHz. Many of the current diagnostics carry over from the present B=0.5 T operation, but some modifications to plasma diagnostics need to be addressed. More refined models of the device and ANSYS$^{\textregistered}$ analysis of 1 T coil forces and stresses are underway and are compared to coil motions observed with a set of motion sensors. We are investigating a new mode of HSX operation that allows the effective ripple to be modified over a large range without large changes to the macroscopic magnetic parameters. Coil forces and stresses are significantly increased for this mode of operation (which reduces coil currents in one coil per half-field-period), and ANSYS is being used to carefully examine loads and forces to ensure safe machine operation. New or improved plasma diagnostics for 1 T operation, which include CHERS, ECE, microwave reflectometry, soft X-ray tomography, and Z-eff radial profiles, and the planned upgraded ECH heating/waveguide system, will be presented. [Preview Abstract] |
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GP1.00119: Electron Cyclotron Heating System Upgrades on the HSX Stellarator J.W. Radder, D.T. Anderson, F.S.B. Anderson, K.M. Likin, J.N. Talmadge Plasma formation and heating in the HSX stellarator is accomplished via $2^{nd}$-harmonic, X-mode ECRH at 28 GHz for 0.5T operations. The HSX ECRH system is being upgraded to include a new gyrotron and hybrid quasioptical transmission lines to provide a total maximum heating power of 400 kW. The primary goals of the new system are to increase the electron temperature with increased heating power, to vary the heating profile by heating on separate flux surfaces, and to perform ECRH modulation experiments. Vlasov converters transform the $TE_{02}$ gyrotron output mode to Gaussian beams. A quasioptical systems comprised of metallic mirrors focus the Gaussian beams, correct astigmatisms, and rotate beam polarization for X-mode or O-mode heating. Circular cross-section, smooth-walled waveguides are used to traverse distances equal to a multiple of the $TE_{11}$, $TM_{11}$ beat wavelength. Gaussian beams are launched into the HSX plasma with one stationary ellipsoidal mirror and one rotatable ellipsoidal mirror at toroidally separated boxports. Simulation, testing, and the current status of the new ECRH system will be presented. [Preview Abstract] |
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GP1.00120: Charge Exchange Spectrometer for HSX Kan Zhai, Frederic Anderson, Alexis Briesemeister A new charge exchange recombination spectroscopy (CHERS) diagnostic system is now being installed on the Helically Symmetric eXperiment (HSX). This system will provide local profile measurement of the poloidal and toroidal rotation velocity of the ion impurity, and hence the local electric field, at ten radial locations. It consists of a hydrogen neutral beam, two collection optics systems and corresponding spectrometer-CCD detection systems. The beam, on loan from Madison Symmetric Torus (MST), is a nearly monoenergetic (94\%) 30 keV, 4A neutral hydrogen beam, which has low divergence ($<$1$^{\circ}$), focuses to a 3 cm diameter waist in the center of the plasma, and has a duration of 3 ms. The collection optics collect the radiation emitted by impurity ions (oxygen or helium) after they charge exchange with hydrogen beam atoms. Light is coupled to the fiber optics which then transfer the collected photons to the spectrometer, which uses a fast frame CCD of 30ms full-frame rate attached at the image surface. Since the low ion temperature in HSX (20$\sim$50eV) will cause fine structure spreading around the main spectral feature, we will use the Atomic Database and Analysis Structure (ADAS) to model these processes to interpret CHERS data. Details of the system design and data processing will be presented. *Work supported by US DoE under grant DE-FG02-93ER54222 [Preview Abstract] |
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GP1.00121: Initial Operation of the Compact Toroidal Hybrid Experiment Stephen Knowlton, Gregory Hartwell, Ralph Kelly, Christopher Montgomery, Joshua Peterson, Adam Stevenson, Tyler Dart The Compact Toroidal Hybrid (CTH) is a five-field period, low aspect ratio (R/a $\ge $ 3.5) torsatron that uses ohmic current to investigate current-driven ideal and resistive instabilities and disruptions in stellarators, and to test new 3-D equilibrium reconstruction procedures for helical confinement devices. The device parameters are R = 0.75 m, a$_{VESSEL}$ = 0.29 m, B $\le $ 0.6 T. A continuously-wound coil produces the main helical field, and a set of ten toroidal field coils allows the vacuum rotational transform to be varied in the range $\rlap{--} {\iota }_{VAC} \left( a \right)=0.2-0.5$. Equilibrium, shaping, and ohmic current drive are provided by four independent poloidal field coil sets. Initial tests of plasma generation by electron cyclotron heating were performed at 0.1 T. Following vacuum field mapping studies, second harmonic electron cyclotron heating at 18 GHz will be pursued using an initial diagnostic set including probes, H$_{\alpha }$ monitors, and a microwave interferometer. Ohmic operation with equilibrium reconstruction will be implemented subsequent to these studies. [Preview Abstract] |
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GP1.00122: Vacuum Magnetic Field Mapping in the Compact Toroidal Hybrid J.T. Peterson, G.J. Hartwell, S.F. Knowlton, R.F. Kelly, C. Montgomery The Compact Toroidal Hybrid (CTH) is a recently completed, five field-period, low aspect ratio ($R/a_{PLASMA} \quad \ge $ 3.5, $R$ = 0.75 m, $a_{VESSEL}$ = 0.29 m, $B \quad \le $ 0.6 T) torsatron with a highly flexible vacuum magnetic field configuration for stability studies. Electron beam field-mapping evaluation of the vacuum field configuration is now underway with a movable gun and phosphor-coated screen. These experiments compare the actual magnetic configuration with the design field, verify the planned flexibility and the range of accessible magnetic configurations, and identify and correct vacuum field errors. The main helical field is produced by a continuously-wound helical coil, and the vacuum rotational transform is varied with a set of toroidal field coils. Four independent poloidal field coil sets provide equilibrium control and shaping, and are also used for ohmic current drive. A set of 15 error correction coils addresses the issues of static magnetic islands and break-up of the outermost flux surfaces by small field errors. [Preview Abstract] |
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GP1.00123: Microinstability Comparison of Stellarator Magnetic Geometries G. Rewoldt, L.-P. Ku, W.M. Tang The microinstability properties of seven distinct magnetic geometries corresponding to different operating and planned stellarators with differing symmetry properties are compared. Specifically, the kinetic stability properties (linear growth rates and real frequencies) of toroidal microinstabilities (driven by ion temperature gradients and trapped-electron dynamics) are compared, as parameters are varied. The familiar ballooning representation is used to enable efficient treatment of the spatial variations along the equilibrium magnetic field lines. These studies provide useful insights for understanding the differences in the relative strengths of the instabilities caused by the differing localizations of good and bad magnetic curvature and of the presence of trapped particles. The associated differences in growth rates due to magnetic geometry are large for small values of the temperature gradient parameter eta = d ln T / d ln n, whereas for large values of eta, the mode is strongly unstable for all of the different magnetic geometries. [Preview Abstract] |
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GP1.00124: Stellarator Coil Optimization Ronald F. Schmitt, Allen H. Boozer Coil design is critical to the cost and viability of stellarator fusion reactors. Coil design is a complex inverse problem with many subtleties. The complexity and inefficiency of the coils increases exponentially with the number of magnetic features retained. The physics properties of a stellarator plasma are determined by the shape of the outermost surface. Only a limited set of shape parameters are essential to that design. The optimal coil set for a given plasma equilibrium controls only the essential shape parameters. The study of small perturbations about the essential shape parameters yields the magnetic features that the coils must produce. This study allows the design of a coil set that possesses minimal complexity and a minimal ratio of the magnetic field on the coil surface to that on the plasma surface. Coil set designs for several plasmas are presented. [Preview Abstract] |
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GP1.00125: Equilibrium Reconstruction in Stellarators: V3FIT John Shields, James Hanson, Steven Hirshman, Stephen Knowlton, Lang Lao, Edward Lazarus Equilibrium reconstruction is a crucial capability in the interpretation of tokamak experiments. As stellarator plasma beta or bootstrap currents increase, the configuration of flux surfaces deviates further from that of the vacuum. As a result, there has been a growing need in the stellarator community for a fast, flexible, and easy-to-modify reconstruction code that can be used to determine the 3-D equilibrium state. We have been developing the V3FIT equilibrium reconstruction code in response to this need. Recently, a significant milestone was achieved with the implementation of the V3FIT interferometry/polarimetry diagnostic module, which uses line-integrated quantities to reconstruct both plasma density and magnetic field profiles. This module is not machine-specific, it can accomodate any number of diagnostic beams in virtually any configuration, and it provides data that is both independent and complementary to the information provided by the previously existing magnetic diagnostics module. The implementation of these two important modules has allowed us to begin to shift our focus to the designing of the tightly-coupled algorithms that are expected to be the key to V3FIT's speed and efficiency. [Preview Abstract] |
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GP1.00126: Effects of ECRH on neoclassical transport in stellarators JaeChun Seol, C.C. Hegna The effect of ECRH heating on the neoclassical transport of conventional stellarators is addressed. In the absence of symmetry, neoclassical transport of stellarators is not favorable in the low-collisional regime. This transport mechanism is particularly important in ECRH heated plasmas where a large energetic trapped electron population is produced. However, a self-consistently generated EXB poloidal drift reduces the direct loss of trapped particles. A large radial electric field is built up by energetic trapped particles generated by electron cyclotron resonance heating (ECRH) yielding the ``electron root.'' We present a calculation that proceeds by solving for the lowest order electron distribution function using a Fokker-Planck equation for ECRH. Energetic electron generation is modeled using a quasi-linear model ECRH diffusion. The radial fluxes are calculated by solving the 1$^{st}$ order Fokker-Planck equation. The radial electric field is determined by the ambipolarity condition of the particle fluxes. Implications for the achievement of electron root and associated enhanced confinement regimes will be addressed. [Preview Abstract] |
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GP1.00127: Collisionless Damping of Zonal Flows in Helical Systems H. Sugama, T.-H. Watanabe Zonal flows are observed in numerous natural systems such as atmospheric and oceanic currents while in fusion science they are intensively investigated as an attractive mechanism to regulate plasma turbulent transport. In the present work, collisionless time evolutions of zonal flows in helical systems are investigated by the gyrokinetic theory and simulation. The dependence of the frequency and damping rate of the geodesic acoustic mode (GAM) on the helical geometry is elucidated. Also, we analytically describe collisionless long-time behavior of zonal flows in helical systems after the damping of the rapid GAM oscillations [1]. It is shown that, under the influence of particles trapped in helical ripples, the response of zonal flows to a given source becomes weaker for lower radial wave numbers and deeper helical ripples while a high-level zonal-flow response, which is not affected by helical-ripple-trapped particles, can be maintained for a longer time by reducing their bounce-averaged radial drift velocity. This implies a possibility that helical configurations optimized for reducing neoclassical ripple transport can simultaneously enhance zonal flows which lower anomalous transport. A good agreement between the theoretical predictions and results from the gyrokinetic- Vlasov-simulation code [2] is verified. [1] H. Sugama and T.-H. Watanabe, Phys. Rev. Lett. 94, 115001 (2005). [2] T.-H. Watanabe and H. Sugama, in 20th IAEA Fusion Energy Conference, Vilamoura, Portugal, 2004, TH/8-3Rb. [Preview Abstract] |
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GP1.00128: Configurational study on confinement improvement of CHS plasmas Keisuke Matsuoka, Mitsutaka Isobe, Yasuo Yoshimura, Takashi Minami, Kenichi Gagaoka, Shoichi Okamura, Shin Nishimura, Tsuyoshi Akiyama, Chihiro Suzuki, Akihide Fujisawa, Akihiro Shimizu, Katsumi Ida, Chihiro Takahashi Magnetic well, magnetic shear, symmetries, etc. are key parameters that play important roles in the confinement improvement of helical plasmas. The configuration with both of magnetic well and stellarator shear is regarded as a favorable one, because micro-instabilities could be suppressed due to the magnetic well and the drift reversal. The vacuum magnetic configuration of CHS has the stellarator shear, having a magnetic hill for the inward shifted magnetic-axis-position where the orbit confinement is best in CHS. Fortunately, the magnetic hill in the core region can be converted to the well with the finite-beta plasma. NBI heated plasmas have shown the improved confinement of electrons in the core region. The improved confinement will be discussed in terms of the magnetic field configuration with the stellarator shear and the magnetic well. EBW heating is scheduled to investigate the electron heat transport in such configurations. [Preview Abstract] |
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GP1.00129: Lost fast ion diagnostics by a scintillator probe Masaki Nishiura, M. Isobe, T. Mutoh, N. Kubo, M. Sasao, S. Murakami, M. Osakabe, J. Miyazawa, D.S. Darrow For lost fast ion studies in confined plasmas, a scintillator type lost ion probe is installed into the Large Helical Device (LHD). The scintillator probe can measure the pitch angle and energy of fast ions simultaneously. During the neutral beam (NB) injection into the LHD, the fast ion profile at the edge plasma was measured using the scintillator probe. The measured results will be compared with fast ion orbits estimated from the deposition profile of NBs. While the self-sustained detachment plasma has been observed, the measured lost ion signal level became quite lower, compared with that before the self-sustained one occurred. The frequency of lost ion signal with 5 kHz disappeared during the self-sustained detachment phase. Taking into account the changes in the deposition of NBs, lost fast ion behaviors would be discussed. The new scintillator probe has been designed and will be installed into the different location of the LHD. A periscope with eyepiece, relay, and objective lenses is employed to transmit the scintillator light to an image intensified charge coupled device camera and 3x3 photomultiplier arrays. The design and first result using new scintillator probe will be presented. [Preview Abstract] |
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GP1.00130: Three-dimensional observation of the pellet ablation in LHD Ryuichi Sakamoto, Hiroshi Yamada, Mitsuyasu Hoshino Three-dimensional pellet ablation observation, which uses a stereo-scope vision, has been available with fast camera in Large Helical Device (LHD). A pair of the stereo images, which are taken from different location, has been focused onto a single fast camera through a bifurcated imaging fiber to ensure the simultaneity of the both images. It has been confirmed by three-dimensional observations that the pellets penetrate into hot plasmas with maintaining an initial velocity during ablation. At the same time, intermittent breakaway of the pellet ablatant, which surrounds the pellet substance, has been observed. The breakaway plasmoid has the velocity component opposite to the injection direction. It suggests that a part of the pellet mass is lost before it deposits in the target plasma. [Preview Abstract] |
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GP1.00131: Status of the QPS Experiment J.F. Lyon The Quasi-Poloidal Stellarator (QPS) is a very-low-aspect-ratio compact stellarator with $R$/$a\sim $ 2.7, 1/4--1/2 that of existing stellarators. The dominant magnetic field components are poloidally symmetric in flux coordinates, which allows large E x B poloidal flows for direct suppression of anomalous transport. Nine independent coil currents allow varying neoclassical transport by $\sim $25, degree of poloidal symmetry by $\sim $10, and poloidal viscosity by $\sim $20 for study of anomalous and neoclassical transport, stability limits at beta $\sim $5{\%}, and equilibrium robustness. Physics studies focus on evaluations of viscosities and neoclassical transport coefficients, momentum transport and flow damping, impact of bootstrap current on equilibrium and stability, global ballooning stability, minimization of magnetic islands, measures of poloidal symmetry deviation, and control of the plasma and neutral densities. The experiment has $R$ = 0.95 m, $a$ = 0.3--0.4 m, $B$ = 1 T for a 1.5-s pulse, and $P$(heating) = 3--5 MW. An R{\&}D program is underway that includes conductor testing, an improved cooling concept, potting with cyanate ester resin, vacuum canning, and fabrication of a full-scale prototype modular coil. The most complex of the modular coil winding forms has been cast and is undergoing machining prior to coil winding. Recent progress, relationship to other stellarator concepts, project schedule, and proposed experimental program are presented. [Preview Abstract] |
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GP1.00132: Bootstrap current in quasi-symmetric stellarators A.S. Ware, D.A. Spong, L.A. Berry, S.P. Hirshman, J.F. Lyon This work examines bootstrap current in quasi-symmetric stellarators with a focus on the Quasi- Poloidal Stellarator (QPS). In the design of QPS, a code was used to predict the bootstrap current based on a calculation in an asymptotically collisionless limit. This calculation is believed to be a good approximation of the bootstrap current for low density, high electron temperature ($n \sim 3\times 10^{19}{\rm ~m}^{-3}$, $T_{e}\sim 1$ keV, $T_{i} \sim 0.2$ keV), ECH heated plasmas in QPS but is expected to be much higher than the actual bootstrap current for more collisional ($n \sim 8\times 10^{19}{\rm ~m}^{-3}$, $T_{e} \sim 0.4$ keV, $T_{i} \sim 0.4$ keV), ICH heated plasmas in QPS. Recently, a fluid moments approach has been developed to self-consistently calculate viscosities and neoclassical transport coefficients which can be used to calculate the bootstrap current (in addition to neoclassical flows) for arbitrary collisionality and arbitrary magnetic geometry [1]. The predictions from the asymptotic collisionless formula agree qualitatively with the bootstrap current predicted by the fluid moments calculation for the low density, ECH plasmas in QPS. For the high density, ICH heated plasmas, the shape of the predicted profiles are similar but the asymptotic collisionless formula predicts a magnitude of current $4 \sim 5$ times larger than the prediction from the fluid moments code. Bootstrap currents in NCSX and HSX plasmas are also calculated. \\ \noindent {[1] D. A. Spong, Phys. Plasmas 12, 056114 (2005).}\\ [Preview Abstract] |
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GP1.00133: Moments method based analysis of QPS plasma flows and currents Donald Spong, Andrew Ware, Lee Berry, Steve Hirshman, Jim Lyon The Quasi-Poloidal Stellarator (QPS) achieves approximate poloidal symmetry in magnetic coordinates at low aspect ratio through numerically determined three-dimensional shaping. This symmetry results in a number of novel characteristics with respect to neoclassical plasma flows and currents. To analyze these, a moments based analysis has been developed that utilizes the DKES (Drift Kinetic Equation Solver) code to generate a data base of transport coefficients that relate plasma fluxes, parallel flows, Ohmic and bootstrap currents to thermodynamic forces and electric fields. This analysis has indicated that poloidal flows dominate in most regimes, in contrast to the case in toroidally symmetric devices where toroidal flows dominate. Bootstrap currents are suppressed from their axisymmetric levels and have been checked against asymptotic low collisionality predictions. Parameters and profiles are chosen that lead to unique ion/electron root self-consistent ambipolar electric field solutions over the minor radius. Electric field bifurcations are observed at low densities when the electron temperature $>>$ ion temperature. \underline {Acknowledgement} This work was performed at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05-00OR22725. [Preview Abstract] |
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GP1.00134: Ballooning stability in quasi-symmetric stellarators E. Mondloch, A.S. Ware, R. Sanchez, D.A. Spong, D. del-Castillo-Negrete Global ballooning stability is examined for three different quasi-symmetric stellarator equilibria: the quasi-poloidally symmetric QPS, the quasi-helically symmetric HSX, and the quasi-axisymmetric NCSX. Previous work on ideal MHD ballooning stability of the Quasi-Poloidal Stellarator (QPS) focused on local calculations of stability. In this work, theoretical calculations of global ballooning mode stability in QPS are done using the results of infinite-$n$ ballooning theory and the ray tracing techniques introduced by Dewar and Glasser [1]. For comparison, this method is also applied to HSX and NCSX equlibria. Here, the mode structure of cylindrical and spherical ballooning surfaces in the different devices is examined. In both QPS and NCSX all of the unstable structures are of the localized, ballooning type and are limited to narrow bands of field lines. This is true even well above a marginal stable $\beta$. The range of $\alpha$ (where $\alpha$ is the field line label) for which unstable surfaces are found is broader in QPS than in NCSX while the range of $\theta_{k}$ (where $\theta_{k}=k_{q}/k_{\alpha}$ is the ballooning parameter) for which unstable surfaces are found is narrower in QPS than in NCSX. Results for HSX will also be discussed. The implications for second stability as compared to local calculations of second stability in these devices is discussed. \\ \noindent {[1] R. L. Dewar and A. Glasser, Phys. Fluids 26, 3038 (1983).} [Preview Abstract] |
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GP1.00135: Magnetic Field Line Diffusion Footprints on Nonconformal NCSX First Wall Arthur Grossman The magnetic fields of the free boundary equilibria of the NCSX compact stellarator are calculated using VMEC and tabulated both inside and outside the LCMS by MFBE{\_}2001 in a form suitable for field line tracing by both GOURDON and Kisslinger field line diffusion.~ Poincar\'{e} plots of equilibrium configurations from 0 to 4{\%} beta both with and without island healing are made with emphasis placed on the mapping the edge between the LCMS and first wall.~ Footprints for each configuration on the actual non-conformal wall are found with field line diffusion calculations.~ Comparison is made with previous conformal wall calculations.~ It is found that more intersections occur at the close fitting wall at the 60\r{ } cross-section than at the tips of the 0\r{ } cross-section which has the largest flux expansion of the various cross-sections.~ Angles of incidence peak in the range 6-12\r{ }. Footprints at low diffusion are similar to the high diffusion ones where step size of field line starting points are determined by anomalous thermal diffusivity.~ The starting surface is selected to be 1 to 2 cm inside the LCMS, and footprints are found to be independent of starting surface selection.~ An island structure possibly suitable for an island divertor concept is found in the absence of island healing.~ Other configurations lack islands in the SOL and may require an ergodic divertor concept.~ The variation of the footprint with beta and island healing is presented. [Preview Abstract] |
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GP1.00136: Equilibrium Reconstruction From Magnetic Sensors For The NCSX Stellarator E.A. Lazarus, N. Pomphrey In previous work [1] we have demonstrated that NCSX (National Compact Stellaraator Experiment) will require active control of the helical and poloidal field coils in order to remain on a stable trajectory to high beta while retaining quasi-axisymmetry. We require a set of magnetic sensors that will be sensitive to changes in the equilibrium that represent departures from such a trajectory. That is, we will need to control features of the plasma boundary shape to a specification; that specification itself will vary with the current and pressure profiles. We need to determine a satisfactory set of magnetic sensors for this task. We will report on progress in our capability to reconstruct 3D equilibrium based on a proposed set of magnetic sensors for the NCSX stellarator using the STELLOPT code. The accuracy of such reconstructions is used to determine the adequacy of the sensor set for plasma control. [1] E.A. LAZARUS, et al., Fusion Science \& Tech. 46 (2004) 213. [Preview Abstract] |
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GP1.00137: The Equilibrium $\beta $ Limit in the W7-AS Stellarator A. Reiman, M. Zarnstorff, D. Monticello, A. Weller, J. Geiger The PIES 3D MHD equilibrium code has been modified to allow the imposition of an experimentally determined pressure profile. To model the equilibrium in the W7-AS experiment, the pressure profile has been determined by the data from the Thomson scattering system and from the set of magnetic diagnostics. PIES equilibrium calculations with varying $\beta $ indicate that a stochastic region appears at the plasma edge above a threshold value of $\beta $, and that the width of the stochastic region progressively increases as $\beta $ is further increased. The threshold value for the appearance of the stochastic region and the width of the stochastic region depend on the magnitude of the current in the divertor control coils, $I_{cc}$. The maximum achievable $\beta $ in the experiment also depends on the value of $I_{cc}$, and the achievable $\beta $ correlates with the width of the stochastic region calculated by PIES. The value of $I_{cc}$ has little effect on the shift of the magnetic axis, indicating that the equilibrium $\beta $ limit is not adequately characterized by the often invoked rule of thumb that assumes that the equilibrium $\beta $ limit corresponds to a magnetic axis shift of approximately $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ the minor radius. [Preview Abstract] |
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GP1.00138: The Saturation of Beta in W7-AS M.C. Zarnstorff, E. Fredrickson, D. Monticello, A. Reiman, A. Weller, J.P. Knauer Quasi-stationary, MHD-quiescent discharges with $\left\langle \beta \right\rangle $ up to 3.5{\%} were sustained in the W7-AS for more than 100 energy confinement times.. The achieved $\left\langle \beta \right\rangle $ is limited by confinement, not stability, and is above the linear ideal stability threshold. Neutral beam heating power scans are analyzed for vacuum iota values of 0.445 and 0.575. The scans saturate at $\left\langle \beta \right\rangle $ values of 3.1{\%} and 2.1{\%}, respectively. At low power, both scans show confinement incrementally varying as $\tau _{E}\propto P_{NB}^{-0.5} $. At high $P_{NB} $, the confinement incrementally varies as $\tau _{E}\sim P_{NB}^{-0.8} $. In both scans, only the central $T_e $ responds to increasing power. $T_e$ and $\nabla T_e$ do not change appreciably in the outer region of the plasma as $P_{NB} $ increases, indicating an increase in local thermal diffusivity. The edge$\nabla T_e $ is approximately a factor of two larger for lower iota. The role of the magnetic flux topology has been analyzed using the PIES 3D equilibrium code. It calculates that a stochastic field region forms at the edge with increasing $\left\langle \beta \right\rangle $, and that it reduces the minor radius by $\sim $30{\%} at the saturated $\left\langle \beta \right\rangle $ value. In the higher iota scan, this occurs at lower $\left\langle \beta \right\rangle $ and the stochastic region has a shorter connection length, plausibly explaining the reduced $\nabla T_e $, and lower saturated$\left\langle \beta \right\rangle $. [Preview Abstract] |
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GP1.00139: Plasma effects on the location of the outermost magnetic surface Allen Boozer In both tokamaks and stellarators, the plasma boundary is often defined by the separatrix between magnetic field lines that form toroidal surfaces and field lines that strike solid objects such as the chamber walls or divertor plates. Extremely small magnetic perturbations can move this separatrix radially inward. However, in the presence of a plasma this radial movement of the separatrix causes a complicated spatial distribution of plasma pressure, which gives rise to currents that can greatly modify or even shield out the magnetic perturbations. An estimate is given of the minimum magnetic perturbation that is required to break up toroidal magnetic surfaces while avoiding plasma shielding. [Preview Abstract] |
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GP1.00140: BASIC PLASMA: NON-NEUTRAL, SOURCES, AND OTHER |
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GP1.00141: Calculation of the ``Rotating Wall'' Torque Near a Fixed Point Attractor. T.M. O'Neil, M.W. Anderson A rotating field asymmetry (the so-called ``rotating wall'') is often used to exert a torque on a non-neutral plasma in a Penning trap, spinning the plasma up to high rotation frequency (and high density). In recent experiments, the plasma state was observed to converge to an attracting fixed point where the applied torque balanced ambient torques.\footnote{J.R. Danielson and C.M. Surko, Phys. Rev. Lett. {\bf 95}, 035001 (2005); also see invited talk by Danielson at this conference.} At the fixed point, the nearly uniform plasma rotation frequency differs only slightly from the frequency of the rotating field asymmetry. This paper explains the attractor, using simple dynamical equations for the uniform plasma rotation frequency and temperature.\footnote{T.M. O'Neil and D.H.E. Dubin, Phys. Plasmas {\bf 5}, 2163 (1998).} Also, the paper calculates the torque due to the rotating field asymmetry near the attractor, that is, for small frequency difference. The calculated torque is consistent with the measured torque. [Preview Abstract] |
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GP1.00142: Trapped-Particle-Mediated Collisional Damping of Non-Axisymmetric BGK Modes in Electron Plasmas. A.A. Kabantsev, C.F. Driscoll Weak axial variations in magnetic or electric fields in Penning-Malmberg traps cause a small fraction of the electrons to be trapped locally, with a velocity-space separatrix between trapped and passing electrons. Collisional diffusion across this separatrix then causes surprisingly large transport and damping effects, including the damping of $m_\theta \not= 0$, $k_z = 1$ Trivelpiece-Gould (TG) plasma modes discussed here. These modes would exhibit strong $( \omega / \gamma_{\mathrm L} \sim 1)$ Landau damping at low amplitudes; but they appear as long-lasting $( \omega / \gamma_{\mathrm NL} \sim 10^4 )$ BGK states when strongly excited by a downward-chirped frequency drive. We observe that trapped-particle-mediated (TPM) damping (scaling as $[ \nu_{\mathrm ee} / \omega]^{1/2}$) generally dominates over traditional collisional damping (scaling as $\nu_{\mathrm ee} / \omega $) in limiting the lifetime of the BGK states. The TPM damping is readily enhanced by additional trapping barriers or by wiggle-induced resonant scattering across the trapping separatrix. For linear TG modes, this TPM damping would appear as a ``baseline'' for Landau damping. [Preview Abstract] |
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GP1.00143: Those Ubiquitous Trapped-Particle-Mediated Transport and Damping Effects C.F. Driscoll, A.A. Kabantsev Trapped-particle-mediated (TPM) transport and damping effects often dominate in long pure electron plasmas with low collisionality, even though only a few percent of the particles are trapped by weak magnetic ripples ($\delta B/B \sim 10^{-3}$), or by wall voltage variations ($\delta V_w \sim 0.1$ Volt). Plasma rotation or other currents cause strong phase-space discontinuities at the trapping separatrix, so small velocity scatterings cause large changes in particle orbits and energy. Initial theory work shows that TPM effects scale with the square root of the electron-electron collisionality, so they dominate for low collisionality. To date, TPM experiments have characterized damping of electron plasma modes (adjacent abstract), and damping of the novel ``trapped particle diocotron modes.'' When confinement $\theta$-asymmetries are also present, TPM effects produce strong particle transport and strong damping of conventional diocotron modes. The observed rates for all these processes are related by simple scalings for magnetic field, electron density, and asymmetry strength, since they are all caused by the same separatrix crossings. However, these strikingly simple experimental signatures are only partially understood theoretically. [Preview Abstract] |
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GP1.00144: Diocotron Damping in Ion Plasmas in the Presence of a Few Electrons F.A. Anderegg, D.R. Creveling, C.F. Driscoll Here, we present diocotron damping measurements of quiescent Mg$^+$ ion plasma columns including the effects of a small number of electrons. The ion plasma is contained in a 3 Tesla Penning-Malmberg trap with total number of particles $N \sim 10^8 - 10^9$. For the $m_\theta = 1$, $k_z = 0$ diocotron mode, $\gamma / \omega$ vs $T$ was measured as the temperature was varied over the range $0.1 < T < 10$~eV with an ion column length $L_p = 12$cm. We find $4 \times 10^{-6} < \gamma / \omega < 10^{-4}$ decreasing with temperature for $0.1 < T < 1$~eV, increasing to $10^{-2}$ at $T \sim 10$~eV in the range of 1~eV $< T < 10$~eV. The low temperature damping may represent ``rotational pumping,'' or trapped-particle-mediated asymmetry-induced damping; but the high temperature result is not understood. These new measurements of ion diocotron mode damping will be compared to previous work on diocotron mode damping in pure electron plasmas. Rapid loss of Mg$^+$ plasmas has been observed from overlapping nested electrons, but no instability is observed with a weak transiting electron beam, possibly due to concurrent damping. Ion plasma results will be compared to the analogous case of an electron column with transiting ions. [Preview Abstract] |
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GP1.00145: De-Excitation of High-Rydberg Antihydrogen in a Strongly Magnetized Pure Positron Plasma E.M. Bass, D.H.E. Dubin The rate at which highly excited atoms relax to deeper binding is found with classical theories and simulations. This rate relates to antihydrogen formation experiments where such atoms are formed in pure-positron, Penning trap plasmas.\footnote{G.Gabrielse, N.S. Bowden, P. Oxley, {\it et al.}, Phys. Rev. Lett. {\bf 89}, 213401 (2002); M. Amoretti, C. Amsler, G. Bonomi, {\it et al.}, Nature (London) {\bf 419}, 456 (2002).} The analysis concerns atoms that have passed the kinetic bottleneck at binding energy $\varepsilon \approx 4kT$.\footnote{M.E. Glinsky and T.M. O'Neil, Phys. Fluids B {\bf 3}, 1279 (1991).} Energy loss caused by collisions between atoms and plasma positrons is calculated in two ways: For close collisions, a molecular dynamics simulation gives the energy loss; for large-impact parameter collisions, theoretical expressions based on Fokker-Planck theory are employed.\footnote{Eric M. Bass and Daniel H.E. Dubin, Phys. Plasmas {\bf 11}, 1240 (2004).} For a finite magnetic field, the energy loss rate scales as $1/\varepsilon$, just as for infinite field,$^2$ but with a larger coefficient. A statistical description of energy loss by radiation and Stark mixing will also be discussed. [Preview Abstract] |
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GP1.00146: Modeling Nuclear Fusion in High Energy Density Plasmas Using a Strongly Magnetized Non-neutral Plasma D.H.E. Dubin In the hot dense interiors of stars and giant planets, nuclear reactions are predicted to occur at rates that are greatly enhanced compared to those at low densities. The enhancement is caused by plasma screening of the reacting pairs, increasing the probability of close collisions. However, strongly enhanced nuclear reaction rates have never been observed in the laboratory. This poster discusses a method for observing the enhancement using an analogy between nuclear energy and cyclotron energy in a non-neutral plasma in a strong magnetic field. In such a plasma, cyclotron energy is an adiabatic invariant, and is released only through close collisions that break this invariant. It is shown that the rate of release of cyclotron energy is enhanced by precisely the same factor as that for the release of nuclear energy, because both processes rely on close collisions that are enhanced by plasma screening.\footnote{D. Dubin, Phys. Rev. Lett. {\bf 94}, 025002 (2005).} Simulations measuring the screening enhancement will be presented, and the possibility of exciting and studying burn fronts will be discussed.\footnote{See also adjacent poster by J. Bollinger.} [Preview Abstract] |
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GP1.00147: Penning ion trap experiments at NIST J.J. Bollinger, M.J. Jensen, T. Hasegawa, D.H.E. Dubin We summarize recent work and discuss future plans for experiments with laser-cooled, strongly correlated Be$^{+}$ ions stored in a Penning-Malmberg trap. We measured the equilibration rate of ion cyclotron energy with ion energy parallel to the magnetic field and find that it is enhanced by more than $10^{10}$ over that predicted for uncorrelated plasmas\footnote{Jensen et al., PRL {\bf 94}, 025001 (2005); Dubin, PRL {\bf 94}, 025002 (2005)}. The enhancement is due to screening of the Coulomb repulsion between colliding ion pairs by the surrounding (correlated) plasma and is closely related to the enhancement of nuclear reactions in dense stellar interiors\footnote{Salpeter, Australian J. Phys. {\bf 7}, 353 (1954); adjacent poster by Dubin}. This is the first observation of this enhancement in the strongly correlated regime and it can provide, along with future work, a method to advance our understanding of nuclear reactions in high energy density plasmas. We also describe plans for a different type of experiment where we propose to entangle the internal states of ions in small planar plasmas. The proposed method uses quantum gates developed for a few ions in an rf trap on up to $\sim1000 $ ions in a Penning trap. The resulting ``spin-squeezed'' states can be used to improve the precision of a spectroscopic measurement. [Preview Abstract] |
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GP1.00148: Development of a positron multicell trap. J.R. Danielson, E.K. Friis, T.R. Weber, C.M. Surko There are numerous potential applications of high-capacity and/or portable antimatter traps. Previously, we proposed the design for a high-capacity, multicell Penning-Malmberg (PM) trap for positrons \footnote{ C. M. Surko and R. G. Greaves, Rad. Phys. and Chem. {\bf 68}, 419 (2003); and Phys. Plasmas {\bf 11}, 2333 (2004)}. Here, we discuss electron experiments designed to test the limits of confinement in a single PM cell. Specific issues include operation with potentials $\ge$ 1 kV and trapping and storage of $10^{10}$ particles at densities $\ge 10^{10} \ {\rm cm}^{-3}$. We are also exploring methods for the dynamic manipulation and control of trapped plasmas, and methods to access off-magnetic-axis cells. The results of these studies will be used to finalize the design of a 95-cell trap for $N \ge 1 \times 10^{12}$ positrons. Possible extensions of this design, and expected limits on positron accumulation, will also be discussed. [Preview Abstract] |
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GP1.00149: Space charge phenomena in a low-energy electron beam R. Pozzoli, G. Bettega, M. Cavenago, A. Illiberi, M. Rome', Yu. Tsidulko The formation and evolution of 3D coherent structures in a low-energy electron beam, where the space charge effects are dominant, have been studied experimentally in a Malmberg-Penning trap, using CCD diagnostics. The main control parameters are the spatial distributions, at the source, of the electron density, energy and current, and the magnetic field. The reflection process has been investigated by varying the electron energy and emission current. Sharp or gradual transition to the space charge dominated regime have been found. The longitudinal structure of the beam has been studied by varying the magnetic field or/and the emission current. The behavior of the observed structures is in good agreement with the simulations obtained with a PIC code which solves the Vlasov-Poisson system in the zeroth order drift approximation. [Preview Abstract] |
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GP1.00150: Drift Resonance in High Density Nonneutral Plasmas D.J. Kaup Theoretical studies of the operation of crossed-field electron vacuum devices, such as magnetrons and crossed-field amplifiers (CFA), have usually centered on their initial growth, taking this as an indication of their operating modes. In such an analysis, one solves the equations for the density profile and other features of these devices. However what one actually obtains are only the conditions for the {\it initial} operation of the device. Eventually the rf fields will saturate, at which time, an operating device will settle into a stationary operating regime, called the ``saturation stage,'' which is where the device simply delivers rf power. Here there is a different set of physical interactions occuring. The amplitudes have saturated and the ponderomotive forces and nonlinear diffusion of the initiation stage have vanished. In this saturation stage, we now find three new rf modes appearing, in addition to the two modes of the initiation stage. These three new modes have very fast oscillations in the vertical direction: one fast mode corresponds to a plasma drift wave, while the other two fast modes are cyclotron-like modes. In this presentation, we will describe how the fast plasma drift wave interacts with the slow modes at the diocotron resonance. In particular, we will determine the conversion coefficients for the crossing of the drift mode with the slow modes at the diocotron resonance. [Preview Abstract] |
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GP1.00151: Theory of Resonant Downscattering of Diocotron Modes Travis Mitchell, Ben Chang, Nathan Mattor Magnetized electron columns evolve in $(r,\theta)$ as 2D vortices in an incompressible inviscid fluid. Resonant downscattering in which modes downscatter to lower azimuthal mode number has been experimentally observed to be an important damping mechanism.\footnote{T. B. Mitchell and C. F. Driscoll, Phys. Rev. Lett. {\bf 73}, 2196 (1994).} The phenomenon is a fluid analogue to nonlinear Landau damping. Here, we present a quantitative theory of resonant downscattering of two dimensional diocotron modes on an electron column (or Kelvin waves on a fluid vortex). The principal new result is a quantitative prediction of the scattering rate, which we compare with experimental measurements. Theory and experiment agree well, although the experimental uncertainties are somewhat large. Comparisons of the theory with new measurements featuring higher spatial resolution, currently underway, will be presented. [Preview Abstract] |
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GP1.00152: Collisionless energy absorption in inhomogeneous plasmas in spherical geometry$^{1}$ David N. Walker, Richard F. Fernsler, David D. Blackwell, William E. Amatucci, Sarah J. Messer We are continuing an investigation of the RF impedance characteristics of a small spherical probe immersed in a laboratory plasma. The data taken are from network analyzer measurements of the reflection coefficient obtained when applying a low level RF signal to the probe near floating potential or negatively DC-biased in a low pressure plasma. Surprisingly, the plasma impedance in the sheath surrounding the object becomes ``resistive,'' and energy absorption is observed experimentally, even though the plasma is effectively collisionless. This behavior can be realized by solving Maxwell's equations together with cold fluid equations, and the solutions obtained indicate that the plasma resistance is inversely proportional to the plasma density gradient evaluated at the location where the plasma frequency is equal to the applied frequency. This is consistent with a body of earlier work which concentrated mostly on planar probes.$^{2}$ The interpretation of results is simpler for a sphere and the results agree well with theory. Maximum energy absorption is observed at frequencies generally near one-half the plasma frequency. We calculate collisionless resistance for a derived density profile and compare to laboratory data. $^{1}$ Work supported by ONR $^{2}$Crawford, F.W. and K.J. Harker, J. Plasma Phys., \textbf{8}, 261(1972) [Preview Abstract] |
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GP1.00153: Wire array response to current pulse shape variation on COBRA using laser-triggered switching J.B. Greenly, J.D. Douglass, D.A. Chelinski, D.A. Hammer, B.R. Kusse, R.D. McBride, L.M. Maxson The COBRA Z-pinch pulser uses four independent pulseforming lines with a vacuum adder, to allow considerable variation in the driving pulse shape for investigations of wire array physics. Laser-triggered gas switches control the timing of the four lines. Jitter is $<$5 ns when laser-triggered at 70{\%} or more of self-break. With all four lines switched simultaneously, zero to peak risetime is 95 ns, 10-90{\%} time is 55 ns, peak current is 1 MA. With two lines delayed 120 ns, zero to peak is 205 ns, 10-90{\%} is 165 ns, 0.9 MA peak. The response of wire array loads is being observed, using the pulse regimes described above, as well as the case in which one line is switched out early to make a substantial ``prepulse'' or foot on the load current. COBRA employs an inductive load voltage monitor, a $\sim $60 cm long wire connected from the cathode in the convolute region to ground. This monitor is able to see the resistive voltage associated with wire initiation, and allows inference of Ldot voltage due to the implosion, in a manner similar to the work of Cuneo et al, Phys. Plasmas 11, 2009 (2004). [Preview Abstract] |
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GP1.00154: Equilibrium Profile Measurements of a Nineteen Gun Plasma Source David Hannum, Will Bergerson, Gennady Fiksel, Cary Forest, Roch Kendrick, Steve Oliva, John Sarff, Sam Stambler The rotating wall machine is a linear screw-pinch built to study the role of different wall boundary conditions on the resistive wall mode (RWM). Its plasma is created by a hexagonal array of nineteen guns. The central seven guns can be biased to discharge up to 1 kA of current.\footnote{See the undergraduate poster on pulse width modulation by Stambler {\em et al}} Different MHD instabilities are studied by changing the current and density profiles. At the other end of the 1.2-meter plasma column, a segmented anode yields a rough current profile measurement. Internal magnetic and Langmuir probes have measured radial profiles of $q, T_e, n_e$ and $\Phi_p$ at the top and bottom of the plasma column. The profiles are seen to change from end to end. Additional probes are under construction to create a 3-D map of these parameters along the entire length of the column. This poster presents the radial profiles of $q, T_e, n_e$ and $\Phi_p$ measured in different plasma configurations, and considers their implications on MHD stability and transport. [Preview Abstract] |
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GP1.00155: Onset and saturation of the kink instability in a current carrying, line-tied plasma William Bergerson, Cary Forest, Gennady Fiksel, Roch Kendrick, Dave Hannum, John Sarff, Sam Stambler The MHD stability properties of a line-tied plasma in the Rotating Wall Machine. An internal kink instability is observed to grow when the safety factor $q = \frac{4\pi^2 r^2 B_z}{\mu_0 I_p(r) L}$ drops below 1 inside the plasma. After a brief growth phase, the mode then saturates as a rotating helical equilibrium. The main diagnostics for measuring the MHD stability is a 2D array of 80 radial magnetic field sensors surrounding the plasma column and a segmented anode, which serves to measure current distribution inside the plasma. The 2D array indicates a plasma dominated by n=1, m=1 modes. In addition to the ideal mode, reconnection events are observed to periodically flatten the current profile and alter the magnetic topology. Resistive MHD or reconnection events redistribute the current in the plasma, which itself is observed with the segmented anode. Finally, initial results of a resistive wall surrounding the plasma will be presented. This work was supported by the DoE. [Preview Abstract] |
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GP1.00156: Temperature equilibration in disparate temperature plasmas Michael Barnes, W. Dorland The temperature equilibration time for disparate temperature laboratory and astrophysical plasmas is frequently determined by well-understood collisional processes. In turbulent, weakly collisional plasmas, however, there can be significant anomalous energy exchange among species. Such processes might be important, for example, in two-temperature accretion flows conjectured to exist around some supermassive black holes and also in magnetic confinement fusion devices. We explore the possibility of the existence of a turbulent mechanism for temperature equilibration by considering a disparate temperature pair plasma in slab geometry with spatial inhomogeneity. We employ the linear gyrokinetic equation to derive an instability allowed by the ion-electron temperature difference and carry out full nonlinear simulations with the gyrokinetic code GS2 to determine the effect of this instability on the ion-electron temperature ratio. Extensions to conventional (disparate mass) plasmas will be discussed. [Preview Abstract] |
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GP1.00157: Variational Principles for Nonstationary States of the Vlasov-Poisson System P.J. Morrison, B. Afeyan, V. Savchenko Variational principles are used for studying nonstationary steady states of the Vlasov-Poisson system [1]. Our motivation is to understand the creation and long time stability properties of nonstationary steady states, motivated by recently discovered Kinetic Electrostatic Electron Nonlinear (KEEN) waves [2]. Unlike BGK modes, which are stationary, we seek steady states where the particles interact with a multiple-harmonic self-consistent nonstationary field via a sequence of trapping, detrapping, and retrapping transitions. While KEEN waves were discovered [2] by driving a Maxwellian plasma with a ponderomotive force caused by the beating of two laser beams [2,3], the variational formulations allow the investigation of other mechanisms that might lead to such states, besides a single mode drive of finite temporal duration or a Maxwellian initial state. Criteria for initial states and external fields that lead to nonstationary states can be obtained with variational procedures and reduced descriptions derived from it. [1] P.J.\ Morrison, Phys.\ Plasmas 12, 058102 (2005); H.\ Ye and P.J.\ Morrison, Phys.\ Fluids 4B, 771 (1992). [2] B.\ Afeyan et al., Proc. IFSA (Inertial Fusion Sciences and Applications 2003, Monterey, CA), 213, B.\ Hammel, et al., eds., Am.\ Nuc.\ Soc.\ (2004). [3] M.\ Mardirian et al., Savchenko et al., and Kline et al., this conference. Work Supported by the DOE SSAA Grant DE-FG03-03NA00059. [Preview Abstract] |
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GP1.00158: Calculation of the vacuum Green's function valid for high toroidal mode number in tokamaks. Morrell Chance, Alan Turnbull, Philip Snyder The present evaluation of the Green's function used for the magmetic scalar potential in vacuum calculations for axisymmetric geometry in the vacuum segments of \textsc{gato}, \textsc{pest} and other \textsc{mhd} stability codes has been found to be deficient for moderately high toroidal mode numbers. This was due to the loss of numerical precision arising from the upward recursion relation used for generating the functions to high mode numbers. The recursion is initiated from the complete elliptic integrals of the first and second kinds. To ameliorate this, a direct integration of the integral representation of the function was crafted to achieve the necessary high accuracy for moderately high mode numbers. At very high mode numbers the loss of numerical precision due to the oscillatory behavior of the integrand is further avoided by judiciously deforming the integration contour in the complex plane. Machine precision, roughly 14 -- 16 digits, accuracy can be achieved by using a combination of both these techniques. [Preview Abstract] |
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GP1.00159: Separation of the Magnetic Field into Parts Produced by Internal and External Sources David Lazanja, Allen Boozer Given the total magnetic field on a toroidal plasma surface, a method for decomposing the field into a part due to internal currents (often the plasma) and a part due to external currents is presented. The decomposition exploits Laplace theory which is valid in the vacuum region between the plasma surface and the chamber walls. The method does not assume toroidal symmetry, and it is partly based on Merkel's 1986 work on vacuum field computations. A change in the plasma shape is produced by the total normal field perturbation on the plasma surface. This method allows a separation of the total normal field perturbation into a part produced by external currents and a part produced by the plasma response. [Preview Abstract] |
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GP1.00160: Bremsstrahlung and Radiation Transport in Weakly Ionized Plasma C.N. Nguyen, H.L. Rappaport Numerical simulations of Bremsstrahlung from inhomogeneous plasma have been performed using the Generalized Kirchoff law.$^ 1$ RF absorption is found in the low frequency regime by a finite element code and in the high frequency regime by a ray optics code. A method for separating incident rays into wave packet congruences is described. The particular physical system under study has conducting boundaries embedded within the plasma. The geometrical theory of diffraction is applied to evaluate wave energy in shadow regions. Computations in the penumbra regions where the separation of the electric field into plane and cylindrical waves fails is also discussed.\\ \noindent $^1$See poster of H.L. Rappaport at this meeting. [Preview Abstract] |
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GP1.00161: Generalizations to the Generalized Kirchoff Law H.L. Rappaport The generalized Kirchoff law$^1$ which facilitates the computation of thermal radiation from inhomogeneous plasmas is discussed in some detail. In this poster, the generalized Kirchoff law is derived from a test-particle formalism and the Lorentz reciprocity theorem. The symmetry of the equations describing wave propagation, be they fluid or kinetic, needed to find radiation from Kirchoff's law are shown explicitly. The roles played by plasma waves and equilibrium electric fields in this problem are examined. A method for overcoming the limitations of Kirchoff's law that arise from diffraction is given. A derivation of the test particle source function for electron-neutral collision processes from the master equation of statistical mechanics is also given.\\ \noindent $^1$Usenko, A.S., Phys. Rev. E, V. 58, No. 5. Nov. 1998, p. 6465. [Preview Abstract] |
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GP1.00162: Effect of nonlinear circularly polarized waves on linear instabilities Luis Gomberoff It is shown that nonlinear left-hand polarized waves can either stabilize or destabilize linear right-hand polarized instabilities triggered by an alpha-particle beam in a magnetized electron proton plasma. The stabilization or destabilization depends upon the plasma $\beta_i=v_{th.i}/v_A$, where $v_{th.i}$ is the temperature of the 'i' plasma component, and $v_A$ is the Alfv\'en velocity. It is also shown that the presence of the large amplitude wave can trigger purely electrostatic ion-acoustic instabilities. The unstable waves are supported either by the proton core or by the alpha-particle beam. [Preview Abstract] |
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GP1.00163: Laser induced fluorescence observation of ion velocity distribution functions in a plasma sheath Nicolas Claire, Gerard Bachet, Ulrich Stroth New experimental results obtained by laser induced fluorescence on metastable ion velocity distribution functions (MIVDFs) in electrostatic presheaths and sheaths in multipolar Argon plasmas are presented. The laser power broadening of the MIVDFs has been observed when there is no ion drift. The MIVDFs parallel to the plate in front of which the sheath is formed are Maxwellian with the ions at exactly the ambient temperature. The MIVDFs perpendicular to the plate (PMIVDFs) are in qualitative agreement, for the presheath, with Emmert's predictions: a Maxwellian profile at the center of the device where the potential is zero, with an ion temperature equal to the ambient temperature ($0.027\ {\mathrm eV}$), and a profile made of three pieces at the beginning of the pre-sheath. The PMIVDFs point out that their width is related to the neutral pressure. In the sheath, where the theory is no more valid, the PMIVDFs recover a Maxwellian profile. This ``ion thermalization'' has never been observed in published simulations. The velocity and potential profiles for different plasma conditions have also been measured. [Preview Abstract] |
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GP1.00164: Evolution of the Parallel and Perpendicular Ion Velocity Distribution Function in Pulsed Helicon Plasma Sources Obtained by Time Resolved Laser Induced Fluorescence Earl Scime, Costel Biloiu, Xuan Sun, Forest Doss, Edgar Choueri, Rotislav Spektor, John Heard, Daniel Ventura The temporal evolution of parallel and perpendicular ion velocity distribution functions (ivdf) in a pulsed, helicon- generated, expanding, argon plasma is presented. The ivdf's temporal evolution during the pulse was determined with time resolved (1 ms resolution), laser induced fluorescence (LIF). The parallel ivdf measurements indicate that, in the expansion region of the plasma and for certain operational parameters, two ion populations exist: a fast moving population moving at supersonic speeds (1.1 Mach) resulting from acceleration in an electric double layer (EDL) and a slow moving population (0.7 Mach) generated by local ionization. After 100 ms, although present, the EDL is not fully developed and has not reached steady state. Measurements of the perpendicular ivdf indicate constant radial expansion, with ion speeds of $\sim$ 400 m/s, in the expansion region. [Preview Abstract] |
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GP1.00165: Revisiting the anomalous rf field penetration into a warm plasma Igor Kaganovich Radio frequency waves do not penetrate into a plasma and are damped within a skin layer. However, electrons can transport the plasma current away from the skin layer due to their thermal motion. As a result, the width of the skin layer increases when electron temperature effects are taken into account. It is shown that separating the electric field profile into exponential and non-exponential parts --as it was originally done by Landau - yields an efficient qualitative and quantitative description of the anomalous skin effect [1]. The anomalous penetration of the rf electric field occurs not only for transversely propagating to the plasma boundary wave (inductively coupled plasmas) but also for the wave propagating along the plasma boundary (capacitively coupled plasmas). Such anomalous penetration of the rf field modifies the structure of the capacitive sheath [2]. Recent advances in the nonlinear, nonlocal theory of the capacitive sheath are reported. [1] Igor D. Kaganovich, et al., submitted to IEEE Trans. on Plasma Sci. (2005); http://arxiv.org/abs/physics/0506135. [2] Igor D. Kaganovich, Phys. Rev. Lett., \textbf{89}, 265006 (2002). [Preview Abstract] |
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GP1.00166: Atomic Physics Effects on IEC Ion Radial Flow G.A. Emmert, J.F. Santarius A simple model for the effect of charge exchange and ion impact ionization of background gas on the performance of spherical, gridded IEC devices has been developed. Ions entering the intergrid region are not only accelerated by the falling electrostatic potential, but also produce a source of cold ions through charge exchange and ion impact ionization of the background gas. Charge exchange is treated as a loss of ions with finite energy and a corresponding source of cold ions. The cold ions are also accelerated by the potential and, in turn, produce additional cold ions. A formalism has been developed which includes the bouncing motion of ions in the electrostatic potential well and sums over all generations of cold ions. This leads to a Volterra integral equation for the resulting total cold ion source function. The integral equation is solved numerically, and the energy spectrum of the ion and fast neutral flux is calculated from the cold ion source function. Macroscopic quantities, such as the current collected by the cathode, and the fusion rate between ions and fast neutrals with the background gas, are calculated and compared with representative experimental values for the Wisconsin IEC device. The agreement is generally good. Extensions to the model, such as multiple ion species, are being developed. Research supported by the US Dept. of Energy under grant DE-FG02-04ER54745. [Preview Abstract] |
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GP1.00167: Particle-in-cell simulation of plasma-wall interaction in presence of a strong secondary electron emission Dmytro Sydorenko, Andrei Smolyakov, Igor Kaganovich, Yevgeny Raitses In presence of a strong secondary electron emission (SEE), the electron flux to the wall may greatly increase compared with the case without SEE. This leads to much greater heat loss from the plasma. To study this effect, a 1d3v particle-in-cell code has been developed. The plasma is bounded by two dielectric walls with SEE, the stationary electric field parallel to the walls and magnetic field normal to the walls are applied. Similar configuration can be found, e.g., in the channel of a Hall thruster. The results of simulations with typical thruster plasma parameters show that many conventional assumptions of fluid theories fail: 1) the electron velocity distribution is strongly anisotropic, instead of generally assumed isotropic; 2) secondary electrons from opposite walls form beams well separated from the main part of distribution function in phase space; 3) these beams produce a two-stream instability, which affects their penetration through the plasma; 4) the SEE coefficient due to the plasma bulk electrons can exceed unity without formation of the double layer, because averaged over bulk and secondary electrons SEE coefficient is less than unity; 5) the SEE-induced near wall conductivity across the magnetic field dominates over the collisional conductivity if the external electric field is strong enough. [Preview Abstract] |
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GP1.00168: Kinetic Particle-In-Cell Simulations Of The Presheath With A Magnetic Field And Collisions Olwen S. Cetiner, Malcolm G. Haines A kinetic particle-in-cell code is developed to study the presheath and sheath accounting for velocity dependent collisions of the electrons and ions with neutral Hydrogen and an oblique magnetic field. By comparing the results with simulations of a collisionless plasma with and without an oblique magnetic field, the trends associated with collisions are identified. A particle source, due to ionization, is located in a single cell to differentiate its role. It is demonstrated that the magnitude of the wall normal flow velocity through the system displays a dependence on the ability of the source to accelerate the ions. The variation in the electrostatic potential drops of the different models illustrates a dependence of the magnitude on the difference between the electron and ion particle velocities as they traverse to the surface. These properties result in relatively low values of the electrostatic potential across the presheath and Debye sheath and wall normal flow velocity when a magnetic field at grazing incidence to the surface and collisions are accounted for. [Preview Abstract] |
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GP1.00169: Kinetic analysis of collisionless plasma-wall transition layer Gakushi Kawamura, Atsushi Fukuyama In the transport analysis of the fusion plasmas, the physics of the scrape-off layer is thought to be essential as a boundary condition. The modeling of PWTL (plasma-wall transition layer) is necessary to obtain the condition for the equilibrium state to exist for a given boundary condition. The PWTL requires the kinetic treatment because of the nonlinear behavior of plasma and the strong electric field. Therefore, we describe the densities of electron and ion as functions of the electrostatic potential using the Liouville's theorem and the energy conservation law. We assume collisionless plasma and particles are injected with the shifted Maxwellian. Combined with the Poisson equation, full-kinetic model equations which decide the potential profile are obtained and solved numerically. The solutions are classified as attached and detached states according to the conditions at the injection boundary. In the detached case, the length of the system can be arbitrary chosen and a long plateau is formed in the pre-sheath region, otherwise the maximum length is limited and only the Debye sheath is formed. The solutions are in good agreement with the results of particle simulations. We also investigate the magnetic pre-sheath by using guiding center descriptions. [Preview Abstract] |
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GP1.00170: Calculation of the Ion Extraction Boundary of a Plasma Ion Source Scott Kovaleski The hypothesis that an unmagnetized plasma will form a meniscus sheath above an ion extraction aperture on the electric field magnitude contour, $|E|=E_p=T_e/\lambda_D$, is tested. The hypothesis is assumed to be true, that the ion extraction surface of a single aperture ion extraction optic is given by the vacuum Laplace electric field magnitude contour equal to $E_p$. The ion current extracted from this surface for a single aperture is calculated analytically. The analytically calculated current is compared to a particle-in-cell simulation of ion extraction from a single aperture, for densities ranging from $1.49\cdot10^{14}$ to $1.49\cdot10^{20} m^{-3}$ and electron temperatures of 1.35 to 2.7 eV. The simulated extraction current and extraction area agrees with the calculated current and area very well, with deviations of only 0.1 to 8 \% relative to the simulated value. [Preview Abstract] |
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GP1.00171: Sliding friction experiments at Alise S\'ebastien Hulin, Claude Fourment, Alexis Casner, Tony Caillaud, Fabrice Nicolon, Philippe Nicola\"{i}, Guy Schurtz, Jean-Luc Feugeas, Vladimir Tikonchuk, Xavier Ribeyre, Alain Boscheron, Philippe Canal, Christian Lepage Following recent developments of the fast ignitor concept to achieve Inertial Confinement Fusion, a cone made of a heavy material such as gold is used to guide the match laser into the compressed fuel. Before reaching this point, the DT sphere will be compressed and subsequently its plastic shell will slide along the gold cone. We realized preliminary experiments on the ALISE laser facility to understand the behavior of this shell sliding along a gold wall. We describe in this paper the experimental setup used to accelerate a 10 $\mu $m thick CH foil up to 100 km/s between two gold walls and measure the evolution of the foil's rear side speed. First results will be presented as well as preliminary interpretations. [Preview Abstract] |
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