Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Plasma Physics
Volume 55, Number 15
Monday–Friday, November 8–12, 2010; Chicago, Illinois
Session UP9: Poster Session VIII: Plasma Technology and Other Fusion; DIII-D Tokamak II; Stellarators; Magneto-Inertial Fusion; Beams and Coherent Radiation |
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Room: Riverside West |
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UP9.00001: PLASMA TECHNOLOGY AND OTHER FUSION |
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UP9.00002: Radiation Damage from Atomic to Meso-Scales in Extreme Environments Cris W. Barnes, M.A. Bourke, S.A. Malloy, F.G. Mariam, F.E. Merrill, Michael Nastasi, E.J. Pitcher, D.J. Rej, J.L. Sarrao, J.S. Shlachter A foreboding materials challenge is to be able to withstand the 10--15 MW-year/m${^2}$ neutron and heat fluence expected in the first wall and blanket structural materials of a fusion reactor. Overcoming radiation damage degradation is a key rate-controlling step in fusion materials development. New science, approaches, and facilities are needed at multiple scales. The objective of the new Center for Materials at Irradiation and Mechanical Extremes is to understand, at the atomic scale, the behavior of materials subject to extreme radiation doses and mechanical stress in order to synthesize new materials that can tolerate such conditions. The Matter Radiation Interactions in Extremes (MaRIE) concept is a National User Facility to realize the vision of 21$^{\rm st}$ century materials research and development. The Fission and Fusion Materials Facility (F$^3$) segment of MaRIE proposes to use the present proton linac at Los Alamos with a power upgrade to drive a spallation neutron source that can provide the required radiation environment. Coupled with integrated synthesis and characterization capability, F$^3$ would also provide the capability for in-situ measurements of transient radiation damage, using unique x-ray and charged particle radiography diagnostics. [Preview Abstract] |
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UP9.00003: PIC Simulation for ICF Plasma Sputter Coater W. Wu, H. Huang, P.B. Parks, V.S. Chan, C.C. Walton, S.C. Wilks To satisfy mesh spacing constraint $\Delta/\lambda_{Debye}\leq 1$ particle In Cell (PIC) simulations at 25$\times$ reduced cathode currents levels are used to numerically model the distribution of currents, electrostatic potentials and particle kinetics in a Type II ``unbalanced" cylindrically symmetric magnetron discharge used for Be sputter coating of ICF capsules. Simulation indicates a strong magnetic field confinement of the plasma in the closed field lines region adjacent to cathode, and accompanying cross-field line plasma diffusion into the open-field line region connected to wall/anode. A narrow Charles-Langmuir sheath and a pre-sheath that is $\sim$10$\times$ wider due to the existence of the B-field are observed. The effects of varying boundary conditions, e.g., the separation between the anode/cathode, the anode bias voltage, etc., are studied, which is expected to aid experimentalists in turning these ``knobs" for better coating qualities. We also show that the etch rate due to sputtering of Be targets predicted by the results of our PIC simulations, after rescaling to experimental conditions, agrees with experiments. [Preview Abstract] |
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UP9.00004: Performance Assessment of a Fusion Hybrid Reactor Based on the Gasdynamic Mirror Terry Kammash A preliminary evaluation of the performance of a fusion hybrid reactor whose fusion component is the gasdynamic mirror (GDM) is presented. Since the primary role of the fusion component is to supply high energy neutrons that will induce fission reactions in a blanket surrounding the plasma, it can operate at or below ``break even'' condition. Unlike other fusion devices proposed for this component, the GDM is a linear, cylindrically symmetric device that can operate in steady state making it uniquely suited for this application. We investigate the power producing capability of such a hybrid reactor using the thorium fuel cycle whereby neutrons generated by DT reactions in the GDM impinge on a thorium-232 blanket. These 14.1 MeV neutrons will breed uranium-233 in the blanket and simultaneously burn it to generate power. We treat both the fusion component and the blanket as semi-infinite cylinders so that one-dimensional analysis can be applied. The two relevant equations in this regard are the time evolution of the U-233 density in the blanket, and the neutron one dimensional diffusion equation. We address the steady state operation of this reactor and find that it can produce gigawatts of power per cm, safely since it is ``subcritical,'' and securely since the fuel cycle is proliferation resistant. The approach to steady state will also be presented and analyzed. [Preview Abstract] |
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UP9.00005: Design Criteria and Machine Integration of the Ignitor Experiment A. Bianchi, B. Coppi High field, high density compact experiments are the only ones capable of producing, on the basis of available technology and knowledge of plasma physics, plasmas that can reach ignition conditions. The Ignitor machine ($R_0\cong 1.32 \textnormal{ m}, a \times b\cong 0.47\times 0.83 \textnormal{ m}^2, B_T \leq 13 \textnormal{ T}, I_p\leq 11 \textnormal{ MA}$) is characterized by a complete structural integration of its major components. A sophisticated Poloidal Field system provides the flexibility to produce the expected sequence of plasma equilibrium configurations during the plasma current and pressure rise. The structural concept of the machine is based on an optimized combination of ``bucking'' and ``wedging''. All components, with the exception of the vacuum vessel, are cooled before each plasma pulse by means of He gas, to an optimal temperature of 30 K, at which the ratio of the electrical resistivity to the specific heat of copper is minimum. The 3D and 2D design and integration of all the core machine components, including electro-fluidic and fluidic lines, has been produced using the Dassault CATIA-V software. A complete structural analysis has verified that the machine can withstand the forces produced for all the main operational scenarios. [Preview Abstract] |
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UP9.00006: Prospects for fusion nuclear science facility producing net electricity based on the tokamak, ST, and stellarator R.J. Hawryluk, T. Brown, R.J. Goldston, R. Kastner, C. Kessel, S. Malang, J. Menard, G.H. Neilson, S. Prager, L. Waganer, M.C. Zarnstorff, L. El-Guebaly, M. Sawan, L. Bromberg, T. Gerrity, D. Whyte, T. Burgess, R. Kurtz A potentially attractive next major DT step in fusion development is a device that produces net electricity as quickly as possible in a configuration directly scalable to a power plant. Such a device would accelerate the commercialization of magnetic fusion by both demonstrating net electricity production and also carrying forward a high neutron fluence component testing mission needed to ultimately achieve high availability in fusion systems. This paper will explore three configurations for a pilot plant: the advanced tokamak (AT), spherical tokamak (ST), and compact stellarator (CS). Overall, initial analysis indicates that the CS and AT are the most energy efficient electrically, while the ST is the most compact radially and provides the highest neutron wall loading. This work is supported in part by U.S. DOE Contract {\#}DE-AC02-09CH11466. [Preview Abstract] |
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UP9.00007: Near Term Fission-Fusion Hybrids- Advantages for Fission and Fusion M. Kotschenreuther, S. Mahajan, P. Valanju Fission-fusion hybrids are described with unique advantages relative to fission only systems, for different missions. Innovative designs allow hybrids to incinerate waste, or produce fuel, with far fewer hybrids than would be possible with fission-only fast reactors. In addition, hybrids can perform these missions while addressing concerns regarding proliferation, by using unique fuel cycles, not accessible to fission alone, that use no reprocessing, or greatly reduced reprocessing. These goals can be achieved using fusion devices in the range considered for fusion Component Test Facilities (CTF). The fusion physics and fusion engineering performance necessary for these missions is far less stringent than for a pure fusion power reactor. Unique designs aspects substantially separate the fission and fusion aspects, allowing much more independent development of each. Hence, a suitable hybrid can provide a nearer term, lower risk, application for fusion, and be an additional incentive to implement a fusion CTF device. The fuel cycle possibilities above are verified using Monte-Carlo neutron calculations. Fusion neutron sources with high power densities and modest power levels are sufficient and necessary. [Preview Abstract] |
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UP9.00008: Modeling Electrode Plasma Effects in Particle-in-Cell Simulation of High Power Devices Timothy Pointon A new method for including electrode plasma effects in particle-in-cell simulation of high power devices is presented. It is not possible to resolve the plasma Debye length, $\lambda _{D} \quad \sim $ 1 $\mu $m, but using an explicit, second-order, energy-conserving particle pusher avoids numerical heating at large $\Delta $x/$\lambda _{D} \quad >>$ 1. Non-physical plasma oscillations are mitigated with Coulomb collisions and a damped particle pusher. A series of 1-D simulations show how plasma expansion varies with cell size. This reveals another important scale length,$\lambda _E =T/(eE)$, where E is the normal electric field in the first vacuum cell in front of the plasma, and T is the plasma temperature. For $\Delta $x/$\lambda _{E}$ $< \quad \sim $1, smooth, physical plasma expansion is observed. However, if $\Delta $x/$\lambda _{E} \quad >>$ 1, the plasma ``expands'' in abrupt steps, driven by a numerical instability. For parameters of interest, $\lambda _{E} \quad <<$ 100 $\mu $m. It is not feasible to use cell sizes small enough to avoid this instability in large 3-D simulations. [Preview Abstract] |
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UP9.00009: Electrostatically Balanced Threesomes William Webb Two mathematical series of electrically charged threesomes are developed. All members of both series are electrostatically balanced threesomes. There is a match between the 38 electrostatically balanced threesomes and 38 naturally occurring nuclei. The match is for the electrical charge and electron emission of the 19 least massive naturally occurring beta decaying nuclei and the electrical charge of their resulting 19 least massive stable nuclei. The 38 for 38 perfect match provides insight to the charge arrangement in nuclei. [Preview Abstract] |
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UP9.00010: Relativistic correction of $(v/c)^{2}$ to the collective Thomson scattering Jian Zheng, Chenfanfu Jiang, Bin Zhao Collective Thomson scattering with inclusion of relativistic correction of $(v/c)^{2}$ is theoretically investigated. The correction is rather small when inferring plasma parameters from the spectra off thermal electron plasma waves in the plasmas. Since the full formula of the corrected result is rather complicated, a simplified one is derived for the sake of practical use, which agrees well with the un-simplified one. [Preview Abstract] |
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UP9.00011: Progress on US ITER Diagnostics David Johnson, Russ Feder There have been significant advances in the design concepts for the 8 ITER diagnostic systems being provided by the US. Concepts for integration of the diagnostics into the port plugs have also evolved. A prerequisite for the signoff of the procurement arrangements for these each diagnostic is a Conceptual Design Review organized by the ITER Organization. US experts under contract with the USIPO have been assisting the IO to prepare for these Reviews. In addition, a design team at PPPL has been working with these experts and designers from other ITER parties to package diagnostic front-ends into the 5 US plugs. Modular diagnostic shield modules are now being considered in order to simplify the interfaces between the diagnostics within each plug. Diagnostic first wall elements are envisioned to be integral with these shield modules. This simplifies the remote handling of the diagnostics and provides flexibility for future removal of one diagnostic minimally affecting others. Front-end configurations will be presented, along with lists of issues needing resolution prior to the start of preliminary design. [Preview Abstract] |
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UP9.00012: High resolution measurement of magnetic field by using Lamb-dip LIF spectroscopy Kohei Ogiwara, Yuta Itoh, Yoshiyuki Kato, Masayoshi Tanaka, Mitsutoshi Aramaki, Shinji Yoshimura Since magnetic field is one of important physical quantities in plasma, several kinds of diagnostics for magnetic field have been developed. The measurements utilizing the Zeeman effect have an advantage of measuring the magnetic field directly from the Zeeman shifted frequency. In addition, the disturbance to the plasma can be minimized by using LIF technique. We have developed a measurement system utilizing Lamb-dip LIF spectroscopy, a kind of saturation spectroscopy, for the precise measurement of the magnetic field. A laser beam of a diode laser, is introduced to a plasma and excites metastable argon atoms in the 4s[3/2]$^{o}_{2}$ state. The transmitted laser beam is reflected by a mirror and counter-propagates the plasma exactly along the same path. According to the hole burning effect, Lamb dips are observed on the LIF spectrum. The Lamb dip is Doppler-free and has a narrow spectral width ($\sim $50 MHz). By measuring the Zeeman shift of the LIF spectrum with the Lamb dips, we measured the magnetic field of 10$^{-1}$ T with an accuracy of 10$^{-4}$ T. It is found that the Lamb-dip LIF system is capable of determining the magnetic field more precisely than the conventional procedure, in which the LIF spectrum is decomposed into the sub-level distributions using a fitting tool. [Preview Abstract] |
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UP9.00013: High Field Neutron Source Facility* M. Zucchetti, F. Bombarda, G. Ramogida, B. Coppi, Z. Hartwig Fusion creates more neutrons per energy released than fission or spallation, therefore DT fusion facilities have the potential to become the most intense sources of neutrons for material testing. An Ignitor-like device, that is a compact, high field, high density machine could be envisaged for this purpose making full use of the intense neutron flux that it can generate, without reaching ignition. The main features of this High Field Neutron Source Facility, which would have about 50$\%$ more volume than Ignitor, are illustrated and the R\&D required in order to achieve relevant dpa quantities in the tested materials are discussed, in particular the adoption of superconducting magnet coils. Radiation damage evaluations have been performed by means of the ACAB code for some fusion-relevant materials, like pure iron, ASI316L, EUROFER, SiC/SiC, Mo, Graphite, V-15Cr-5Ti. Values ranging from $1.6 \times 10^{-26}$ to $2.4 \times 10^{-25}$ dpa per source neutron have been obtained. Some full-power months of operation are sufficient to obtain relevant radiation damage values in terms of dpa: the setup of a duty cycle for the device in order to obtain such operation times is the next required step to proceed with the evaluation. *Sponsored in part by ENEA of Italy and by the U.S. D.O.E. [Preview Abstract] |
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UP9.00014: Neutronic Model of a Mirror Based Fusion-Fission Hybrid for the Incineration of Spent Nuclear Fuel and with Potential for Energy Amplification Klaus Noack, V.E. Moiseenko, O. Agren, A. Hagnestall In the last decade the Georgia Institute of Technology (Georgia Tech) published several design concepts of tokamak based fusion-fission hybrids which use solid fuels consisting of transuranic elements of the spent nuclear fuel from Light-Water-Reactors. The objectives of the hybrids are the incineration of the transuranic elements and an additional net energy production under the condition of tritium self-sufficiency. The present paper presents a preliminary scientific design of the blanket of a mirror based hybrid which was derived from the results of Monte Carlo neutron transport calculations. The main operation parameters of two hybrid options were specified. One is the analog to Georgia Techs first version of a ``fusion transmutation of waste reactor'' (FTWR) and the other is a possible near-term option which requires minimal fusion power. The latter version shows considerably better performance parameters. [Preview Abstract] |
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UP9.00015: New Superconductors for Near Term Fusion Devices* S. Berta, B. Coppi, G. Grasso, R. Penco The high density plasma regimes that have been observed and investigated by the Alcator and Frascati Torus programs, and more recently by the LHD helical device, lend themselves to be exploited in the near term for demonstrating D-T ignition using compact, high field experiments like Ignitor. The same plasma regimes can be relied upon in the attempt to devise useful neutron sources for a variety of applications. The relevant machine does not need to be optimized for ignition but to produce the highest usable neutron fluence. Thus, in order to improve the machine duty cycle the adoption of the recently discovered superconducting material MgB$_2$ to supplement the copper material in the high field magnet systems (hybrid solution) has been considered. The MgB$_2$ solution has the advantage of employing gas-He as coolant, like the copper material needed for the highest field regions, although at lower temperature ($\stackrel{<}{\sim} 10$ K), and it is not subject to large temperature excursions during the current pulse. A significant fraction of the volume taken up by copper is replaced by structural material (e.g. steel) that can carry the relevant stresses to less critical regions of the magnets. The largest poloidal (vertical) field coils of Ignitor (5 m diameter) are being re-designed to be made exclusively of MgB$_2$ superconducting cables, a first for a fusion device. *Sponsored in part by ENEA of Italy and by the D.O.E. [Preview Abstract] |
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UP9.00016: The Internal Optimization of Axisymmetric Mirror M.T. Missanelli, Z.H. Guo, X.Z. Tang There has been considerable recent interest in the concept of an economical volumetric neutron source based on the axisymmetric magnetic mirror. The stabilization against interchange modes due to the ``bad'' magnetic curvature is crucial to achieve a MHD stable equilibrium. The axisymmetric mirror is robustly unstable to flute-type modes unless the beta averaged magnetic curvature is positive. The finite Larmor radius effect is later found to be able to prevent the development of small-scale interchange perturbations. Therefore, the magnetic divertor may be introduced into the central cell. It helps to both localize the ``bad'' curvature and increase the ``good'' curvature region. In this work, we present a high-beta mirror MHD equilibrium solver. Given various vacuum fields, with or without divertor, and parallel pressure profiles as functions of the magnetic flux and strength, different 2-D MHD equilibriums are numerically calculated and compared. Then, the stabilization effect of magnetic divertors is investigated by evaluating the Rosenbluth-Longmire integral. A scan of control parameters is performed for the internal optimization of the mirror-confined plasma. [Preview Abstract] |
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UP9.00017: One-Dimensional Burn Control in Fusion Reactors Mark D. Boyer, Eugenio Schuster Control of plasma density and temperature magnitudes, as well as their profiles, are among the most fundamental problems in fusion reactors. Economic and technological constraints may require fusion reactors to operate at operating points for which an active control system may be necessary to stabilize the thermonuclear reaction. In [1], a zero-dimensional (0-D) nonlinear model involving approximate conservation equations for the energy and the density of ion species was used to synthesize a nonlinear feedback controller for stabilizing the burn condition of a fusion reactor. This result is exploited in this work to propose a controller that is able to stabilize the one-dimensional (1-D) burn dynamics. A simulation study is carried out to assess the performance of the controller and its effect on the plasma density and temperature profiles. The long-term goal is to develop model-based controllers for simultaneous kinetic profile regulation and burn condition control. \\[4pt] [1] E. Schuster, M. Krstic and G. Tynan, ``Burn control in fusion reactors via nonlinear stabilization techniques,'' Fusion Science and Technology, vol. 43, no. 1, pp. 18-37, January 2003. [Preview Abstract] |
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UP9.00018: Depth profiles of Helium and Deuterium in tungsten ``fuzz'' using Elastic Recoil Detection Kevin Woller, Dennis Whyte The structure of the surface region of tungsten plasma facing components changes when subjected to helium (He) plasmas and material temperature $>$1100 K. The micron-sized highly porous nano-tendril structure is termed tungsten ``fuzz.'' These changes are potentially detrimental to the performance of tungsten in D-T fusion devices. The morphological evolution of the fuzz is not understood but experimental results indicate that the fuzz grows in depth with He fluence. Elastic Recoil Detection (ERD) has been used for the first time to measure the concentration of the plasma species (He and hydrogen/deuterium) with respect to the W density as a function of depth perpendicular to the surface. ERD was performed on tungsten samples that have been exposed to helium plasma with varying material temperatures and exposure times. The results suggest that the percent atomic concentration of helium is saturated at a very high level, $\sim $0.5{\%} He/(He+W), and is constant throughout the measureable ``fuzz'' depth, even with exposure temperatures $>$1800 K. The implications of these measurements are discussed. Supported by US DoE award DE-SC00-02060. [Preview Abstract] |
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UP9.00019: Overview of progress on the Liquid Metal Experiment J. Rhoads, A. Arthurs, E. Edlund, P. Sloboda, E. Spence, H. Ji A flowing liquid wall is an attractive plasma facing component in fusion devices due to the ability to withstand high heat and neutron fluxes. The Liquid Metal Experiment (LMX) consists of externally driven, free-surface flow through a wide-aspect ratio channel subjected to a strong magnetic field orthogonal to the surface of the flow; similar to the scenario of a toroidally flowing divertor. LMX has been modified to study heat transfer in addition to measuring fluctuations of the surface and mapping the velocity profile in open channel flow. A high-wattage resistive heater and an infrared camera have been installed to observe the effect of a magnetic field on heat transfer. Two position-sensitive diodes are in place to make measurements of the fluctuations of the surface, which can be correlated to underlying turbulent structures and track changes in the k-spectra. Also, an array of potential probes has been implemented in order to map the flow profile as the magnetic field is increased. All of these phenomena must be studied in order to determine how a flowing liquid divertor would respond in a reactor setting. An overview of the modifications and preliminary results will be presented. [Preview Abstract] |
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UP9.00020: Apparatus for Seebeck Coefficient Measurements of Lithium-7 and Lithium-6 Maryann Tung, Vijay Surla, Wenyu Xu, David Ruzic, Dennis Mansfield Lithium, owing to its many advantages, is of immense interest to the fusion community for its use as plasma facing component (PFC) material. Recently, in the center for plasma material interactions, it was shown that the flow of liquid lithium in the presence of magnetic fields is dominated by thermoelectric Magnetohydrodynamic (TEMHD) flow. To accurately describe these observed results, the knowledge of the thermoelectric properties of lithium is essential. To this end, an apparatus for determining the Seebeck Coefficient of lithium was developed. Using this apparatus, the Seebeck Coefficient of lithium as a function of temperature is obtained. The Seebeck Coefficient of Lithium-7, with respect to stainless steel, is found to gradually increase from 16 $\mu $V/K to 35 $\mu $V/K, as the temperature is raised from 300 K to 500 K, which is in good agreement with published values [1]. Furthermore, the Seebeck coefficient of Li-6, for the first time, was measured and the results are presented. \\[4pt] [1] Bidwell, C.C, ``Electrical Resistance and Thermoelectric power of the Alkali Metals,'' Phys. Rev. 28,357 (1924). [Preview Abstract] |
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UP9.00021: Chemical Erosion Studies of Lithiated Graphite Priya Raman, Vijay Surla, David Burns, David Ruzic Lithium evaporation treatments in the National Spherical Torus Experiment (NSTX) have shown dramatic improvements in plasma performance increasing the viability of lithium as Plasma facing Component (PFC) material. In order to understand the complex system of lithiated ATJ graphite, chemical erosion measurements of plain and lithiated ATJ graphite are conducted in the newly built RF plasma facility. A differential pumping scheme is employed and a Residual Gas Analyzer is used for chemical erosion measurements. Target is mounted on a substrate heater (0-500C) and it is connected to a biasing circuitry to allow for temperature dependent studies and energy dependent measurements. To study the effect of lithium on chemical erosion, lithium is evaporated in-situ onto ATJ graphite. The dominant chemical erosion products are known to be CD$_{4}$ and C$_{2}$D$_{2}$. The challenges in measuring C$_{2}$D$_{2}$, as it interferes with N$_{2}$ and CO peaks, are presented. It was found that lithium treatments have suppressed the CD$_{4}$ signal, and the effect of lithium on other peaks is presented. The effect of temperature on chemical erosion is also investigated and it was found that temperature increases the erosion of graphite. [Preview Abstract] |
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UP9.00022: Exposure of lithium-coated molybdenum samples to intense pulsed plasmas H. Ortiz-Uribe, S. Jung, M. Nieto-Perez, D. Ruzic Plasma facing components in fusion reactors are exposed to high flux/high energy plasmas during both normal and abnormal operation of the device. With lithium-coated plasma facing components showing beneficial effects on reactor performance and molybdenum showing good chemical compatibility with it, the study of the Mo/Li system subject to intense plasmas is relevant to establish operational limitations on these surfaces. In this study, 304 stainless steel samples with a molybdenum coating deposited using air plasma spray (APS) were exposed to the intense plasma generated in the DevEX facility at the University of Illinois, with a typical density of 10$^{15}$ cm$^{-3}$ and an electron temperature of tens of eV. The effect of plasma exposure on these surfaces was studied by means of a triple Langmuir probe located near the exposed surface and optical emission spectroscopy techniques to determine plasma parameters such as density, temperature and impurity content on the plasma formed near the target. [Preview Abstract] |
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UP9.00023: Chemical Sputtering of Carbon Revisited: Temperature and Isotope Effects Jonny Dadras, Carlos Reinhold, Predrag Krstic We study chemical sputtering of carbon irradiated by 1-30 eV/D atoms, in a range of temperatures, 300 -- 1200 K by molecular dynamics, using modified potentials of Brenner-Terzoff type. At each temperature and impact energy a quasi-stationary state of the carbon erosion is reached by cumulative bombardment. Dependence of the mass and energy spectra of hydrocarbons, as well as the moiety density (sp, sp$^{2}$, sp$^{3})$, on surface temperature are studied. We also study the chemical erosion of hydrogen-supersaturated carbon kept at 300K due to bombardment by hydrogen isotopes H, D, and T at energies of 1--30 eV. Chemical sputtering yields are found to increase with projectile mass but not as dramatically as that predicted by simpler models based on sequential binary collisions. Only a weak dependence on the mass was found in the number of hydrocarbon moieties of supersaturated surfaces created by cumulative bombardment and, thus, the root of the mass dependence was found to be directly related to the probability for breaking the C--C bonds that attach such moieties. Our results are compared with available experimental data on acetylene, methane, and total carbon sputtering yields. [Preview Abstract] |
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UP9.00024: Molecular Dynamics of Lithium-Carbon Surfaces P.S. Krstic, E. Yang, J. Dadras, P. Kent, A. Allouche, J.P. Allain We study chemistry induced by low-energy deuterium impact of lithiated carbon surface. The processes include evolution of the lithium-carbon surfaces as well as chemical sputtering. New interatomic potentials are developed for the molecular dynamics simulations of the mixed Li-C-H material, based on the Brenner-Tersoff form, with Lenard-Jones corrections. Particular attention is paid to the Coulomb interactions of the charged atoms in this mixed material induced by large difference in the electronegativity of lithium and carbon-hydrogen. We compare our results with available experimental data. [Preview Abstract] |
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UP9.00025: ITER ICH Transmission Line and Matching System Progress* D. Swain, R. Goulding, D. Rasmussen The ITER Ion Cyclotron Heating (ICH) system is required to deliver 20 MW to the ITER plasma for pulse lengths over 3000 s. The US is responsible for the design and fabrication of the transmission lines and matching system. Significant progress has been made in the design and layout of the system in the past year. The conceptual design has been done by the US and approved by the ITER International Organization (IO), and preliminary design has started. More detailed layouts and analysis of the matching system have been done. A new, more detailed matching system design that uses a 3-dB hybrid coupler and two shorting stubs in a phase-shifter mode is being analyzed. In collaboration with the IO, the layout of the transmission lines and matching system has been detailed. In addition, work has started on control algorithms to do real-time matching and control of the system in response to changing plasma conditions. An R{\&}D program to test prototype concepts and components has been started; recent results and R{\&}D plans will be presented. *The submitted manuscript has been authored by a contractor of the U.S. Government under contract DE-AC05-00OR22725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. [Preview Abstract] |
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UP9.00026: ABSTRACT WITHDRAWN |
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UP9.00027: Magnetized plasma flow injection into tokamak and high-beta compact torus plasmas Hiroyuki Matsunaga, Yuuki Komoriya, Hiroyasu Tazawa, Tomohiko Asai, Tsutomu Takahashi, Loren Steinhauer, Hirotomo Itagaki, Takumi Onchi, Akira Hirose As an application of a magnetized coaxial plasma gun (MCPG), magnetic helicity injection via injection of a highly elongated compact torus (magnetized plasma flow: MPF) has been conducted on both tokamak and field-reversed configuration (FRC) plasmas. The injected plasmoid has significant amounts of helicity and particle contents and has been proposed as a fueling and a current drive method for various torus systems. In the FRC, MPF is expected to generate partially spherical tokamak like FRC equilibrium by injecting a significant amount of magnetic helicity. As a circumstantial evidence of the modified equilibrium, suppressed rotational instability with toroidal mode number n = 2. MPF injection experiments have also been applied to the STOR-M tokamak as a start-up and current drive method. Differences in the responses of targets especially relation with beta value and the self-organization feature will be studied. [Preview Abstract] |
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UP9.00028: Modeling of EBW Propagation and Damping in PEGASUS and MST S.J. Diem, J.K. Anderson, M.W. Bongard, R.J. Fonck, C. Forest, A. Redd, A. Seltzman, R.W. Harvey, Y. Petrov Electron Bernstein waves (EBW) can be used for localized heating and current drive (CD) in overdense devices, such as the spherical torus, Pegasus, and the reversed field pinch, Madison Symmetric Torus (MST), located at UW-Madison. Numerical modeling of EBW propagation and damping has been explored using the GENRAY ray-tracing code and the CQL3D Fokker-Planck code in support of current and proposed heating and CD experiments on both devices. In Pegasus, calculations were performed investigating a proposed EBW system for available sources at 2.45, 3.6 and 5.55 GHz frequencies for waves launched 25$^{\circ}$ above the midplane. Preliminary results show between -35 kA/MW to 65 kA/MW can be driven at r/a $>$ 0.5 with the available sources. Edge current profile modification is being explored to improve particle and energy transport in MST. Calculations of 5.5 GHz injection estimate $>$ 10 kA/MW can be driven off axis at r/a $>$ 0.65 via the Ohkawa CD method. The effect of large stochastic particle transport on CD efficiency was investigated by varying the radial transport model included in CQL3D. [Preview Abstract] |
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UP9.00029: Low-Impurity, Electrode-less Pre-ionizer Plasma Gun for Innovative Confinement Concepts James Prager, Timothy Ziemba, Robert Winglee, Jonathan Wrobel There is a need within the fusion community for a pre-ionizer plasma source that does not produce impurities; act as a plasma limiter, either through direct contact with the plasma or open magnetic field lines; or produce significant electromagnetic interference on control and diagnostic systems. The University of Washington, in collaboration with Eagle Harbor Technologies, Inc., has developed a Low-Impurity, Pre-Ionizer Plasma Gun for Innovative Confinement Concepts. The plasma gun has been constructed and integrated with the vacuum chamber. The experimental setup includes a flux conserver so that plasma injection through a flux conserver can be demonstrated, which is important for many applications. Here we present preliminary density, temperature, and velocity data to characterize the plasma gun. [Preview Abstract] |
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UP9.00030: A Robust Modular IGBT Power Supply for Fusion Science Applications Timothy Ziemba, Ken Miller, James Prager A present challenge facing the fusion energy community and particularly the ICC community in its support of the main line tokamak program is the ability to generate increased power levels for pulsed magnets, arc plasma sources, radio frequency heating, and current drive schemes, at reasonable cost. Continuous wave (CW) tube based power supplies are typically large and expensive, making them prohibitive for smaller experimental facilities or not cost effective when only pulsed input power is required. Continued research and development of next generation solid state power supplies could allow for multiple applications with a single well developed, low cost module that could be configured in many ways. Eagle Harbor Technologies has developed, built, and tested a modular solid state power supply based on Insulate Gate Bi-polar Transistor (IGBT) technologies. The power supply utilized a modular, low cost, high power IGBT based system that can be assembled in multiple ways to address a wide range of applications. Testing results demonstrating the prototype abilities in both parallel and series configurations and for both high current and/or high voltage operation will be presented. The prototype cost was shown to be significantly lower than older generation power supplies for similar applications. Future work includes upgrading the prototype for increased power levels ($>$ 10 MW). [Preview Abstract] |
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UP9.00031: DIII-D TOKAMAK II |
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UP9.00032: Addressing the New Challenges of Burning Plasma Physics With Dominant Electron Heating on DIII-D T.S. Taylor, R.J. Buttery Future burning plasma devices will be heated predominantly through the electrons, with low torque injection, core fuelling and collisionality, while steady state operation will demand highly off-axis currents to ensure stability and good transport. In contrast, most present devices heat through the ions, with high torque, core fuelling, and relatively peaked current profiles. These differences have dramatic impacts on the performance, stability and transport in fusion plasmas. Thus an upgrade is proposed to DIII-D to provide dominant electron heating and steady state current profiles via an upgrade to 15~MW electron cyclotron heating (ECH), enabling access to burning plasma relevant parameters in regimes ranging from the ITER baseline to power plant-like $q_{min}>2$ steady state scenarios, with independent control of $T_e/T_i$, rotation, collisionality, current profile and $\beta$. This will provide vital and unique capability to resolve the physics of burning plasmas and device designs, as well as critical tests of fusion simulation in the new regimes for which they must be developed. [Preview Abstract] |
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UP9.00033: Physics Issues for Extending the Pulse Length of High $f_{NI}$ DIII-D Discharges J.R. Ferron, T.C. Luce, P.A. Politzer, J.C. DeBoo, T.W. Petrie, C.C. Petty, R.J. La Haye, S.P. Smith, C.T. Holcomb, F. Turco, M. Murakami, J.-M. Park, Y. In, M. Okabayashi, E.J. Doyle, H. Reimerdes The increase of the total available NB and gyrotron injected energies has enabled study of high $f_{NI}$ discharges with the high $\beta_N$ phase extended to $>$3~s. To minimize $n_e$ for maximum EC and NB CD, discharges were produced after a boronization. Typically $H_{98}\approx 1.5$, but the common decrease in $n_e$ in the later portion of the high $\beta_N$ phase correlates with reduced $\tau_E$. The broadly deposited ECCD appears to improve $n=1$ tearing mode stability, but a 2/1 or 3/1 mode is more likely with longer pulses as profiles are not stationary. Scaling of $J_{NI}$ with $B_T$ was studied to best match $J_{NBCD}$, $J$, and the $P_{beam}$ required for a given $\beta_N$. Short ELM-free phases, perhaps from improved H-mode pedestal stability at high $\beta_p$, and rapid fishbone-like bursts were found to affect discharge stability and capability for $\beta_N$ control. [Preview Abstract] |
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UP9.00034: Progress in Demonstration of the ITER Baseline Scenario on DIII-D E.J. Doyle, J.C. DeBoo, P. Gohil Previous experiments to simulate ITER operating scenarios on DIII-D matched the anticipated ITER design for plasma shape, aspect ratio and value of I/aB, with size reduced by a factor of 3.7. Operating with deuterium plasmas, $H_{98y2}\geq 1$ was obtained while operating at the ITER target $\beta_N$ of $\sim$1.8, i.e.\ key ITER performance targets were met. In 2010, attention changed to focus on investigating the performance of baseline scenario plasmas using H neutral beam injection into He plasmas, as proposed for use in ITER's initial non-nuclear operating phase. With H NBI, baseline scenario plasmas with an I/aB close to that for 15~MA operation on ITER, with $q_{95}\sim 3.2$ and $\beta_N\sim 1.8$ were operated. The confinement in these plasmas was substantially reduced as compared to equivalent D plasmas, requiring $\sim$8.6~MW versus 2.8~MW of NBI, respectively, to maintain the target $\beta_N$. Baseline scenario discharges were also successfully operated with varied levels of net torque input, using a mixture of co- and counter-NB, so as to investigate the effect of plasma rotation on confinement. [Preview Abstract] |
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UP9.00035: Dependence of DIII-D Hybrid Scenario on Dimensionless Parameter C.C. Petty, J.C. DeBoo, J.R. Ferron, R.J. La Haye, T.C. Luce, P.A. Politzer, R. Prater, M.R. Wade, G.R. McKee, L. Vermare, M. Murakami, E.J. Doyle Experiments on DIII-D have studied the dependence of the hybrid scenario on Mach \#, beta, and relative gyroradius. An MHD study varied the central Mach \# between 0.75 and 0.13 using co/ctr NBI; below this lower limit the 3/2 tearing mode slowed down and locked. Interestingly, suppression of the 3/2 mode using ECCD did not allow smaller Mach \# to be accessed because of the onset of a 2/1 mode. A moderate beta degradation of confinement was observed for normalized beta between 1.5 and 2.7. The unfavorable beta dependence existed mainly in the electron channel, perhaps indicating magnetic flutter transport. The relative gyroradius scaling of local transport was close to gyroBohm-like in the core; however, a less favorable edge dependence and broadening of the NBI profile resulted in a global scaling that was closer to Bohm-like. This relative gyroradius dependence is similar to H-mode plasmas at high $q_{95}$. [Preview Abstract] |
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UP9.00036: Progress Report on the DIII-D ECH System J. Lohr, M. Cengher, J.L. Doane, Y.A. Gorelov, H.J. Grunloh, C.P. Moeller, D. Ponce The DIII-D six gyrotron system is a primary experimental tool fulfilling a variety of unique requirements. The system operates at 110~GHz, injecting up to 3.5~MW for pulses up to 5~s in length. The peak injected energy has been 16.6~MJ for a single tokamak pulse. As the capabilities were steadily being upgraded, the system has performed with good reliability. Recently improved procedures for alignment of the rf beams at the waveguide inputs have reduced mode conversion, giving -1~dB transmission losses, the expected value for 90~m waveguide length with 12~miter bends per line. Real time steering of the launcher antennas has demonstrated NTM suppression under control of the DIII-D operating system with current drive swept across the resonant flux surface for the $m/n=3/2$ island. Injected power up to 5~MW will be realized by addition of a 7th gyrotron, by reduction in the number of miters, and by installation of new lower loss miters in the system. Planning has begun for a further significant upgrade to generated power up to 15~MW with a 5\% increase in rf frequency. [Preview Abstract] |
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UP9.00037: Some New Results From Electron Cyclotron Heating Experiments on DIII-D R. Prater, R.J. Groebner, J. Lohr, P.A. Politzer, T.H. Osborne, M.E. Austin, E.J. Doyle, L. Zeng, R.W. Harvey, A.E. White Second harmonic ECH experiments on DIII-D using up to 3.2~MW of power at 110~GHz have shown that very high central electron temperatures, to 15~keV, can be obtained without large deviations from a Maxwellian, according to calculations with the CQL3D Fokker-Planck code, and with good agreement between the Thomson scattering and electron cyclotron emission measurements of electron temperature. In some other discharges the details of the density pumpout when the ECH electron heating is applied are being studied using reflectometry for excellent spatial and temporal resolution. And experiments with ECH applied at the inboard edge of ELMing \hbox{H-mode} discharges show a strong increase of the ELM frequency when the heating location is just inside the last closed flux surface, possibly placing a limitation on applicable models for ELMs. [Preview Abstract] |
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UP9.00038: Improved Efficiency of the ECH Transmission Lines on DIII-D M. Cengher, J. Lohr, I.A. Gorelov, D. Ponce, C.P. Moeller, M. Shapiro The transmission in the waveguide lines of the electron cyclotron heating (ECH) system on DIII-D was measured for the six 110~GHz, 1~MW class gyrotrons. After the angular alignment of the rf beam was improved, the measured transmission loss for 4 of the lines was less than -1.1~dB, close to the -1~dB theoretical value. Measurements showed HE$_{11}$ mode content over 85\% for all the lines, indicating reduced losses from mode conversion. The higher transmission resulted in an 87~kW increase in the power measured at the DIII-D end of the line for the waveguide system with the best improvement. A newly developed 4-port rf monitor was used together with a directional coupler to measure the HE$_{11}$, HE$_{21}$, and TE$_{01}$ mode amplitudes for diagnosis of waveguide performance. The HE$_{11}$ signal was proportional to the transmitted power. The results show that a minimum in the undesired HE$_{21}$ and TE$_{01}$ mode amplitudes corresponds to the best angular alignment. Future plans include reducing the number of miter bends in each line by rerouting the lines and using lower loss miter bends. [Preview Abstract] |
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UP9.00039: Alignment of RF Beams to the Waveguide Transmission Lines at DIII-D Y.A. Gorelov, J. Lohr, M. Cengher, D. Ponce The DIII-D gyrotron system comprises six 110~GHz gyrotrons and generates $\sim$4.5~MW for plasma experiments. Waveguides are circular, corrugated, evacuated and about 90~m in length. Power injected into the tokamak is reduced to $\sim$3.5~MW by transmission line losses partly owing to misalignments. The Gaussian rf beam of a gyrotron couples with 98\% efficiency to the HE$_{11}$ waveguide mode using a single focusing mirror in the matching optics unit (MOU), however the single mirror limits the flexibility of the alignment procedure and mode conversion is quite sensitive to misalignments at the waveguide input, increasing as $\Theta^2$ and $\Delta^2$ for tilt and offset errors, respectively. To improve the alignment, the rf beam was propagated in free space, far from conducting surfaces, after reflection from the MOU mirror. The rf beam position was measured at least at 3 different distances from the mirror for each case. Based on these results, the tilt angle and the offset of the rf beam were calculated and the mirror was readjusted. The process was then repeated until the tilt angle and the offset were minimized. [Preview Abstract] |
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UP9.00040: Transport Analysis of Bat-eared $T_e$ Profile Discharges in DIII-D M.E. Austin, K.W. Gentle, C.C. Petty, T.L. Rhodes, L. Schmitz In DIII-D H-mode discharges created solely with off-axis ECH, the $T_e$ profiles often exhibit a pronounced hollow or ``bat-eared" shape for a long period just after the onset of the heating. The location just inside of the off-axis $T_e$ peaks is seen to be a region of low transport, both from a calculation of diffusivity from transport codes and from an observation of phase jumps in heat pulse propagation. This transport barrier location is also near the $q=1$ surface, as determined by the sawtooth inversion radius. The depth and width of the barrier consistent with the observed $T_e$ measurements is investigated with a simple transport code model. Also, the differences between the bat-eared discharges and the more common discharges with heat-pinch behavior are examined. [Preview Abstract] |
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UP9.00041: Transport Dependence on Safety Factor Profile in DIII-D Steady-state Discharges C.T. Holcomb, J.R. Ferron, T.C. Luce, J.C. DeBoo, A.E. White, T.L. Rhodes, L. Schmitz, F. Turco An analysis of the transport dependence on the safety factor in steady-state scenario discharges is presented based on experimental scans of $q_{95}$ and $q_{min}$ at fixed $\beta_N$ and $B_T$. Electron and ion density and temperature decrease with $q_{95}$. $T_e$ and $T_i$ increase and broaden with $q_{min}$. Power balance calculations show ion thermal diffusivity $\chi_i$ increases with $q_{95}$ and somewhat with $q_{min}$, but $\chi_e$ decreases with $q_{min}$. Measured low-k density turbulence increases strongly with $q_{min}$ and weakly with $q_{95}$ in rough agreement with the $q$-dependence of $\chi_i$ but not $\chi_e$. TGLF drift wave linear stability analysis predicts mid-radius growth rates at all k decrease with increasing $q_{95}$ and increase with increasing $q_{min}$. This disagrees with the observed $\chi_i$ increase with $q_{95}$, is consistent with the increase in $\chi_i$ with $q_{min}$, and is at odds with the observed decrease in $\chi_e$ with $q_{min}$. Calculations of the critical gradient for low-k modes and nonlinear stability analysis with mode coupling will be presented. [Preview Abstract] |
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UP9.00042: Numerical Investigation of the Generalized Plasma Density Diffusion Equation in the Edge Pedestal J.-P. Floyd, W.M. Stacey Momentum balance constraints require that the radial particle flux in the plasma edge satisfies a pinch-diffusion relation, which leads upon substitution into the continuity equation to generalized diffusion equations which also contain temperature gradients, a pinch velocity determined primarily by the electromagnetic forces, and a generalization of the diffusion coefficient. Since both the temperature gradients and the pinch velocities vary rapidly with radius in H-mode tokamak edge plasmas, it is of interest to investigate the numerical solution of such equations in order to gain insight as to whether such generalization of the diffusion equation formalism can be incorporated into large plasma edge codes such as UEDGE and SOLPS. Numerical studies indicate that direct solution of the coupled continuity and pinch-diffusion equations is more accurate than solution of the generalized diffusion equation that results from combining the two. [Preview Abstract] |
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UP9.00043: Particle Transport Analysis Using Modulated Gas Puff Technique in DIII-D L. Zeng, E.J. Doyle, T.L. Rhodes, L. Schmitz, W.A. Peebles, S. Mordijck, J.S. deGrassie By using gas puff modulation techniques, the electron particle diffusion coefficient D, and pinch velocity V, have been experimentally measured as a function of radius in a wide range of plasma operating regimes, including Ohmic, L-mode, H-mode, QH-mode, and RMP ELM-suppressed plasma regimes. The time and spatially resolved measurements from a profile reflectometer system show clear density profile perturbations associated with the gas puff modulations deep into the plasma core in L-mode and Ohmic plasmas. Along with the particle transport data, simultaneous detailed local turbulence measurements have been obtained for the first time. These fluctuation data clearly indicate that core plasma turbulence is significantly modified by external gas puffs, providing a new level of detail for which the plasma response can be modeled. The data also suggest a link to momentum transport. The extensive new set of transport and turbulence data will be compared to the theory-based models (TGLF and GYRO). [Preview Abstract] |
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UP9.00044: Gyrokinetic Eigenmode Analysis of High-Beta Shaped Plasmas J. Candy, E.A. Belli The effect of compressional magnetic perturbations on gyrokinetic stability and transport in moderate and high-beta shaped plasmas is studied using GYRO. In high-beta plasmas, closely-spaced branches of unstable eigenmodes exist, and are difficult and time-consuming to resolve with existing linear initial-value solvers. For this reason, a fast Maxwell-dispersion-matrix eigenvalue solver has been developed and applied to systematic studies of the linear eigenmode spectrum in representative DIII-D and NSTX discharges. As expected, compressional perturbations are mostly negligible in DIII-D but significant in NSTX for which both low-k, hybrid ITG-like/KBM-like modes as well as high-k mode cascades are observed. Finally, we present a transport analysis for both machines in terms of the full 4-3 transport matrix; that is, four transport coefficients (particle flux, momentum flux, energy flux, anomalous exchange) decomposed into three transport channels (electrostatic, transverse electromagnetic, compressional electromagnetic). [Preview Abstract] |
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UP9.00045: Survey of Driftwave Linear Growth Rate Sensitivity to Gradient Scale Lengths J.C. DeBoo, G.M. Staebler Previous experiments in DIII-D were performed to locally vary $a/L_{Te}$ using ECH in order to vary the turbulence associated with the electron temperature gradient, particularly in the intermediate wavenumber regime where trapped electron modes (TEMs) dominated the turbulence. For these low density L-mode discharges, calculations with TGLF of normalized driftwave linear growth rates $\gamma$ indicate that they are largest near $k_y = k_\theta\rho_s = 0.8$ and are most sensitive to $a/L_{Te}$ well above the marginal condition for TEM activity. The growth rates do not always vary smoothly with gradient scale lengths. Systematic variation of $a/L_{Te}$ by a factor of 2, the range in experiment, shows that $\gamma$ smoothly decreases with decreasing $a/L_{Te}$ at $k_y = 0.8$ until nearing the marginal value. In this regime $\gamma$ is sensitive to $a/L_{Te}$, $a/L_{Ti}$ and $a/L_{ne}$ such that $\gamma$ can vary up to a factor of 2 within typical experimental uncertainties of the gradient scale lengths. Strong variation of growth rate with small changes in gradient scale length can also be found at lower $k_y$ where $\gamma$ is most sensitive to $a/L_{Ti}$. [Preview Abstract] |
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UP9.00046: Multi-scale/Multi-field Turbulence Measurements to Rigorously Test Gyrokinetic Simulation Predictions on \hbox{DIII-D} T.L. Rhodes, W.A. Peebles, L. Schmitz, E.J. Doyle, J.C. Hillesheim, L. Zeng, G. Wang, C.H. Holland, G.R. Tynan, A.E. White, G.R. McKee, Z. Yan, J.C. DeBoo, K.H. Burrell, C.C. Petty, D. Mikkelsen The progress in rigorously testing gyrokinetic turbulence simulations through a series of carefully designed experiments is described. A unique array of multi-field, multi-scale turbulence diagnostics is utilized, including new measurements of TEM-scale $\tilde{n}$, turbulence flows, $\tilde{n}_e - \tilde{T}_e$ crossphase, as well as previously available ITG and ETG scale $\tilde{n}$ and low-k $\tilde{T}_e$. Turbulence and transport response to $T_e/T_i$ was quantified for QH-mode, low-rotation Hybrid H-mode, and L-mode cases. Little variation with $T_e/T_i$ of low-k through high-k $\tilde{n}$ was found in L-mode; however, $\tilde{T}_e$ varied strongly. In contrast, low-k $\tilde{n}$ increased substantially with $T_e/T_i$ in the Hybrid H-mode. These and other measurements, including particle transport via gas puff modulation, will be compared to linear and nonlinear gyrokinetic simulations. [Preview Abstract] |
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UP9.00047: Electron Thermal Transport and Multi-scale Turbulence in Low Collisionality H-mode Plasmas L. Schmitz, C. Holland, T.L. Rhodes, G. Wang, J.C. Hillesheim, L. Zeng, W.A. Peebles, E.J. Doyle, G.R. McKee, A.E. White, K.H. Burrell, J.C. DeBoo, J.S. deGrassie, C.C. Petty Electron thermal transport and the role of local ITG/TEM/ETG-scale core turbulence are investigated in high temperature DIII-D H-mode/QH-mode plasmas at ITER-relevant electron to ion temperature ratio ($0.5\leq T_e/T_i \leq 1.2$) and collisionality ($\nu_e^*\sim 0.05$). The $T_e/T_i$ ratio is varied using central ECH ($P_{ECH}\leq 2.7\,$MW). Experimentally determined H-mode electron transport fluxes and turbulence wavenumber spectra are directly contrasted with nonlinear gyrokinetic (GYRO) simulations results. The effects of $E\times B$ shear on core ITG/TEM-scale turbulence are studied at low and high rotation, with the latter leading to reduced electron thermal transport across the entire minor radius. GYRO simulations indicate that a significant portion of the remaining H-mode electron heat flux results directly from short-scale TEM/ETG turbulence. [Preview Abstract] |
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UP9.00048: Multi-field/Multi-scale Turbulence Response to EC Heating of DIII-D Ohmic Plasmas G. Wang, W.A. Peebles, T.L. Rhodes, J.C. Hillesheim, E.J. Doyle, L. Schmitz, L. Zeng, A.E. White, Z. Yan, G.R. McKee, J.C. DeBoo, C.C. Petty, K.H. Burrell, A.W. Leonard, G.M. Staebler, W.M. Solomon Understanding plasma turbulence in existing fusion devices is essential to establishing a predictive capability of turbulence and transport model for future devices like ITER. Historically, such effort has tended to focus on long wavelength density turbulence with little attention given to electron temperature fluctuations and/or short-scale density turbulence. In this work, we report the response of low-k electron temperature turbulence and low-, intermediate-, and high-k density turbulence ($\tilde{n}/n$) to EC heating of DIII-D Ohmic plasmas in the core region. It is observed that $\tilde{n}/n$ of low- and intermediate-k shows little change with ECH, while high-k density and electron temperature turbulence amplitude increased strongly. Results of transport analysis and linear gyro-kinetic stability simulations will also be reported. [Preview Abstract] |
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UP9.00049: Study of Poloidal Variation and Radial Wavenumber of Edge Turbulence in L- and H-mode with PCI on DIII-D J.C. Rost, M. Porkolab, J.R. Dorris, K.H. Burrell The Phase Contrast Imaging (PCI) diagnostic has been used on DIII-D to measure plasma turbulence from 2 to 30 cm$^{-1}$ using three roughly vertical beam paths: tangent at $r/a=1$; $r/a = 0.75$ (Phase~I); and now $r/a = 0.4$ (Phase~II). An optical filter provides localization. Comparison of measurements made in different beam paths shows poloidal variation and spectral structure that one measurement location does not provide. The observed group velocity of the turbulence spectrum decreases away from the midplane at high input power, but is constant at low input power, with no up-down asymmetry in $v_g$ in either case. Measurements in L-mode are consistent with edge turbulence having a finite $k_\theta$ and a range in $k_r$ centered at $k_r=0$. Measurements in H-mode are strongly asymmetric in wavenumber as observed by the PCI, indicating that edge turbulence is large at two radii with different flow velocities, one with a strong positive $k_r$ and one a strong negative $k_r$. [Preview Abstract] |
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UP9.00050: Measurements of Short Wavelength Plasma Fluctuations Using the DIII-D Phase Contrast Imaging Diagnostic J.R. Dorris, J.C. Rost, M. Porkolab, K.H. Burrell The DIII-D Phase Contrast Imaging (PCI) diagnostic has been upgraded and used to measure turbulence in the outer plasma region ($0.7 < r/a < 1$) covering an operational range of 10~kHz through 10~MHz and 2-30~cm$^{-1}$. A novel rotating mask has been used to measure turbulence as a function of propagation angle about the PCI chord. This technique provides localized measurements along the PCI chord for turbulence with $k_\parallel \sim 0$, and an estimate of the turbulence $k_\parallel$ value otherwise. Long wavelength ($\vert k\vert \buildrel<\over\sim 12~$cm$^{-1}$) turbulence is localized to within the instrumental width of the last closed flux surface (LCFS) ($r/a\buildrel>\over\sim 0.9$). Modes with finite (and theoretically unexpected) {\it parallel~wavenumber} have been seen to propagate at angles as large as $k_\parallel/k \sim 0.1-0.4$. Due to the finite $k_\parallel$, these modes cannot be localized with the present techniques. A theoretical explanation for these modes is lacking at the present time. [Preview Abstract] |
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UP9.00051: Geodesic Acoustic Mode Measurements in DIII-D J.C. Hillesheim, W.A. Peebles, T.A. Carter, T.L. Rhodes, L. Schmitz Geodesic acoustic modes (GAMs) are nonlinearly driven, axisymmetric ($m=0$, $n=0$) $E\times B$ flows, which may play an important role in establishing the saturated level of turbulence in tokamaks. Doppler backscattering (DBS) measures the flow of turbulent structures and the level of intermediate-k ($k_\perp\rho_s\sim 1-4$) density fluctuations. Measurements have been made with multichannel DBS systems at toroidal locations separated by 180$^\circ$. Both linear characteristics of the mode and its nonlinear interactions have been studied. Observations include cases where the GAM exists as a persistent mesoscale structure, coherent over $\sim$1/3 of the minor radius; measurements in repeat shots indicate a poloidal dependence of the GAM's radial wavenumber; and bicoherence analysis between the toroidally separated DBS systems has revealed a relationship between the GAM and low frequency zonal flows. [Preview Abstract] |
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UP9.00052: Gyrokinetic Simulation of Residual Stress from Diamagnetic Velocity Shears R.E. Waltz, G.M. Staebler, W.M. Solomon Residual stress refers to the remaining toroidal angular momentum (TAM) flux (divided by major radius) when the shear in the parallel velocity (and parallel velocity itself) vanishes. Previously [1] we demonstrated with gyrokinetic (GYRO) simulations that TAM pinching from the diamagnetic level shear in the $E\times B$ velocity could provide the residual stress needed for spontaneous toroidal rotation. Here we show that the shear in the diamagnetic velocities themselves provide comparable residual stress (and level of stabilization). The sign of the residual stress, quantified by the ratio of TAM flow to ion power flow (M/P), depends on the signs of the various velocity shears as well as ion (ITG) versus electron (TEM) mode directed turbulence. The residual stress from these temperature and density gradient diamagnetic velocity shears is demonstrated in global gyrokinetic simulation of ``null" rotation DIIID discharges by matching M/P profiles within experimental error. \vskip8pt \noindent [1] R.E.\ Waltz, G.M.\ Staebler, J.~Candy, and F.L.\ Hinton, Phys.\ Plasmas {\bf 14}, 122507 (2007); errata {\bf 16}, 079902 (2009). [Preview Abstract] |
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UP9.00053: On the Identification of Interferometer Signals in High-k Scattering Data R. Nazikian High-k scattering is a technique used on multiple devices to diagnose short scale (order of the ion gyroradius) density fluctuations with good spatial resolution. In the last decade the method has been used to try to identify short radial scale Kinetic Alfv\'en Waves (KAWs) that are theoretically prediceted to couple to large scale Alfv\'en eigenmodes such as TAEs and RSAEs. The importance of these KAWs is that they contribute an important dissipation mechanism for Alfv\'enic instabilities. However, the use of high-k scattering is problematic for the interpretation of the KAW because even a small level of stray radiation from the beam can produce an interferometric signal that can be mistaken for high-k scattering. This talk discusses several methods that can be used to identify the interferometer effect based on properties of the complex amplitude of the received signal. Data will be used from DIII-D and NSTX to illustrate the method. [Preview Abstract] |
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UP9.00054: Gyrokinetic Particle Simulations of Energetic Particle Transport by Drift Waves and of Toroidal Alfv\'en Eigenmodes W. Zhang, W.W. Heidbrink, Z. Lin, D.C. Pace, D. McCune Simulations with the gyrokinetic toroidal code (GTC) of fast-ion transport by ion-temperature gradient (ITG) turbulence are compared with experimental measurements for a dedicated discharge with excellent fluctuation and fast-ion data. To facilitate the comparison, TRANSP can now utilize fast-ion diffusion coefficients that are arbitrary functions of energy, pitch, and flux coordinate. The strong energy dependence of transport predicted by GTC is in better agreement with the data than a weaker energy dependence. The excitation of shear Alfv\'en eigenmodes in toroidal systems, such as toroidal Alfv\'en eigenmode (TAE) and energetic particle mode (EPM), and nonlinear transport by these toroidal shear Alfv\'en eigenmodes have also been explored through large-scale gyrokinetic simulations using GTC. [Preview Abstract] |
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UP9.00055: Pitch and Energy Resolved Fast Ion Losses in the \hbox{DIII-D} Tokamak D.C. Pace, W.W. Heidbrink, C.M. Muscatello, Y.B. Zhu, R.K. Fisher, M.A. Van Zeeland, M. Garcia-Munoz A scintillator-based fast ion loss detector (FILD) measures the pitch and energy of energetic ions reaching the wall at approximately 45-degrees below the outer midplane. Losses are observed in connection with various instabilities, including Alfv\'en eigenmodes, energetic particle driven geodesic acoustic modes, and tearing modes. Orbit trajectory calculations based on FILD measurements allow for the identification of ion/mode interactions in phase space. These orbit calculations are part of the development of a synthetic FILD diagnostic that is validated against the well-understood case of neutral beam prompt losses in a specially designed DIII-D discharge. Results from the FILD across an array of plasma parameters will be presented, along with the initial design calculations and physics goals pertaining to a second FILD that will be installed near the outer midplane before the next experimental campaign. [Preview Abstract] |
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UP9.00056: Sawtooth-induced Fast-ion Transport in the DIII-D Tokamak: Observations and Comparison to Theory C.M. Muscatello, W.W. Heidbrink, D.C. Pace, Y.B. Zhu, Ya.I. Kolesnichenko, V.V. Lutsenko, Yu.V. Yakovenko, M.A. Van Zeeland, R.K. Fisher, B.J. Tobias Tokamak sawteeth consist of a reorganization of the plasma magnetic field and various plasma parameters. The extent to which the fast-ion distribution function $F(x,v)$ is influenced can depend on the ions' distribution of pitch and energy as well as the nature of the crash. Recent sawtooth experiments at DIII-D employed the newly extended fast-ion deuterium-alpha (FIDA) diagnostic, 2D FIDA imaging, and the newly commissioned fast-ion loss detector. Consistent with theoretical predictions, the FIDA diagnostic indicates that passing particles are more strongly affected by a sawtooth crash than the trapped population. Furthermore, FIDA imaging reports a depletion of up to 50\% of the central fast-ion density. Extensive experimental data provide a rigorous test bed of theoretical models. [Preview Abstract] |
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UP9.00057: Effect of Test Blanket Module on Triton Burn-up in DIII-D Tokamak Y.B. Zhu, W.W. Heidbrink, M.J. Schaffer Time resolved measurements of triton burnup on DIII-D tokamak have been performed using a newly restored and upgraded 14~MeV neutron emission monitor based on silicon surface barrier diode. Neutron and energetic ion relevant data have been analyzed for the dedicated ITER Test Blanket Module (TBM) mockup plasma experiments. During the TBM on period, no observable change was recorded by a Faraday type fast ion loss collector, but a decrease in D-D neutron yield was usually encountered with a drop in plasma density. With full current applied on TBM coils, coincident reduction in 14 MeV neutron counts was prominent, as well as the ratio of 14~MeV D-T neutrons to 2.45~MeV D-D neutrons. In some cases, these behaviors were accompanied by a slight increase in signal of ion cyclotron emission loops attached behind the plasma-facing graphite protection module. Deterioration in triton confinement was observed to be weaker with partial current applied to the TBM and did not occur at all for no TBM current. [Preview Abstract] |
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UP9.00058: Rapid Shutdown Methods for Runaway Electron Suppression by Large Shattered Pellets and Massive Gas Injection in DIII-D T.C. Jernigan, N. Commaux, L.R. Baylor, N.W. Eidietis, T.E. Evans, D.A. Humphreys, P.B. Parks, J.C. Wesley, E.M. Hollmann, V.A. Izzo, A.N. James, J.H. Yu Massive gas injection (MGI) has been shown to significantly reduce both the heat loads and forces transmitted to the tokamak first wall. However, MGI has been unable to approach the ne required to prevent runaway electron (RE) generation by the avalanche process. Large shattered pellet injection (SPI) has been developed to overcome this limitation in MGI. Application of SPI and MGI techniques using D$_2$ in DIII-D will be compared. Extensions to neon will be discussed. Other techniques for RE suppression explored in DIII-D, such as large shell pellet injection, very high intensity gas injection, and external magnetic perturbations, will be discussed along with key RE diagnostics used. [Preview Abstract] |
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UP9.00059: Runaway Electron Confinement Studies with NIMROD V.A. Izzo, A.N. James, J.H. Yu, E.M. Hollmann, P.B. Parks, L.L. Lao, J.C. Wesley, D.G. Whyte, G. Olynyk, R.S. Granetz Simulations of disruptions in DIII-D, Alcator C-Mod and ITER are performed with NIMROD, using a test-particle orbit-integration module that calculates transport of runaway electrons on the NIMROD 3D fields as the current quench progresses. In the simulation, variation of plasma shape is studied to explore the experimental observation that limited, low-elongation plasmas confine runaways better than diverted plasmas. Cross-comparison of similar rapid shutdown scenarios in the three devices is performed to examine the effects of machine size and total plasma current and stored energy on confinement results. We also present comparisons of pellet-like and massive gas injection (MGI)-like rapid shutdown scenarios in C-Mod, and simulations with applied magnetic perturbations in DIII-D. Measurements of synchrotron emission and x-rays associated with runaway electrons are used to make direct connection between the simulation results and experiments in DIII-D and C-Mod. [Preview Abstract] |
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UP9.00060: A Database Investigation of Advanced Tokamak Mode Disruptivity in DIII-D A.W. Hyatt, D.A. Humphreys, T.C. Luce, P.L. Taylor A fully realized Advanced Tokamak (AT) mode plasma runs steady-state. Therefore the disruptivity of an AT plasma is best described by a probability of disruption per second, rather than per shot. Previous analyses of DIII-D overall disruptivity have estimated $\sim$13\% per-shot-disruptivity for all operating regimes [1], and AT regime per-shot disruptivity of $\sim$5\% [2]. We expand a 3-year DIII-D experimental operation database to add all identified AT discharges for comparison. Each shot in this database that does not reach the end of programmed flattop is analyzed to identify the cause of premature termination. Rampdown is not analyzed. A complete time history of selected data is associated with each shot, so a disruptivity frequency can be calculated for each phase of a shot as a function of the time spent in that phase. We report on the results of this disruptivity analysis for all AT shots, and compare them with similar analysis for the 3 continuous years' shots.\par \vskip6pt \noindent [1] A.W.\ Hyatt, et al., Bull.\ Am.\ Phys.\ Soc.\ {\bf 45}, 279 (2000).\par \noindent [2] T.C.\ Luce, review talk, submitted to Phys.\ Plasmas (2010). [Preview Abstract] |
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UP9.00061: On the Relation Between 3-D Equilibrium and Stability A.D. Turnbull 3-D perturbations can arise from error fields, imposed non-axisymmetric coils, or saturated instabilities. In each case, the plasma response is a key to determining the consequences. The problem can be treated as a dynamic or stability problem or from a nearby perturbed equilibrium approach and the relation between these is considered. The nearby equilibrium approach aims to bypass the detailed evolution and search for the appropriate final state. Since multiple nearby equilibria exist, the key is to assure accessibility of the final state. The simplest approach is to add a perturbation from a stability code or external field to the equilibrium and solve for 3-D force balance. This is an example of the ``Almost Ideal MHD" idea, where one looks for invariants relating the 2-D and nearby 3-D system; the invariants are buried in the numerical details of the equilibrium code, for example imposition of nested surfaces. There is no guarantee the new state is physically accessible. The dynamic approach can guarantee accessibility but is numerically time consuming. However, the constraints can be informed from dynamic simulations. [Preview Abstract] |
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UP9.00062: Rotation-compatible Resistive Wall Mode Modeling in Tokamaks Using Self-consistent Stability Code MARS V.A. Svidzinski, Y. In, J.S. Kim, M.S. Chu, Y.Q. Liu Rotational stabilization of resistive wall modes (RWM) has been observed in experiments. To study a rotation profile influence on RWM stabilization, a self-consistent ideal MHD calculation, including plasma rotation, is in progress using code MARS-F. Specifically, the sensitivity of stability (s) and torque (alpha) parameters defined by Boozer in [1] is studied near the RWM onsets. In a single mode approximation, the plasma response on a perturbed RWM magnetic field can be represented as a simple function of these parameters such that the dependence of the growth rate of the mode on s and alpha can be found analytically. The calculated RWM growth rate dependence on s and alpha will be compared with the results of this simple model. This modeling is expected to help us diagnose the proximity to RWM stability boundary based on a rotation profile data.\par \vskip8pt \noindent [1] A.H.\ Boozer, Phys.\ Rev.\ Lett.\ {\bf 86}, 5059 (2001). [Preview Abstract] |
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UP9.00063: Magnetic Feedback-controlled Error-field Correction and RWM Stabilization Y. In, V. Svidzinski, J.S. Kim, M. Okabayashi, G.L. Jackson, R.J. La Haye, E.J. Strait, J.M. Hanson, M.J. Lanctot, H. Reimerdes, Y.Q. Liu The sensitivity of resonant field amplification to non-axisymmetric error field varies significantly, subject to the proximity to the resistive wall mode (RWM) stability conditions. Recent DIII-D experiments show that simultaneous operation of magnetic feedback-controlled error-field-correction (EFC) and direct feedback on RWM would be a practical solution, not only achieving RWM stabilization, but also performing the necessary EFC in unstable RWM regime. Also, when the bandwidth of the magnetic feedback is broadened beyond wall characteristic frequency, high beta plasmas were observed to have been sustained longer than with a narrow bandwidth feedback. It is conjectured that resonant magnetic perturbations driven by various MHD bursts (e.g.\ off-axis fishbone), as well as uncorrected error field, can be removed quickly by high frequency magnetic feedback control, before magnetic islands are formed. [Preview Abstract] |
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UP9.00064: Improved Dynamic Response of Magnetic Feedback in DIII-D with AC Compensation Lidia Piron, Lionello Marrelli, Piero Martin, Paolo Piovesan, Anton Soppelsa, Jeremy Hanson, Holger Reimerdes, Yongkyoon In, Michio Okabayashi High-$\beta $ tokamaks need robust magnetic feedback to cope with various MHD modes. A new algorithm was tested to improve the DIII-D feedback dynamic response. Magnetic sensor signals include contributions from vacuum sources, such as active coils. In the present algorithm, the plasma response to an applied field is computed by subtracting from the sensor signals the dc component of couplings to the coils. But, when the coil currents vary on fast enough time scales, wall eddy currents modify the contributions to the sensor field with respect to its dc value [1]. Such ac effects can be non-negligible. Transfer functions between coils and sensors were measured and an ac compensation scheme accounting for them was developed. Significant coil current was saved likely due to a better estimate of the plasma response. Ac effects may be more important at high-$\beta $, where uncorrected error fields are strongly amplified. [1] E.J. Strait \textit{et al.} 2003 Nucl. Fusion \textbf{43} 430. *Work supported in part by US DOE under by DE-FG02-04ER54761, DE-FG02-06ER84442, {\&} DE-AC02-09CH11466. [Preview Abstract] |
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UP9.00065: Feedback Control of Resistive Wall Mode Stability Using Neutral Beam Injection Heating J.M. Hanson, H. Reimerdes, M.J. Lanctot, G.A. Navratil, G.L. Jackson, R.J. La Haye, P.E. Sieck, E.J. Strait, M. Okabayashi, Y. In Measurements of the plasma response to externally applied, low-n magnetic fields can be used to determine the resistive wall mode (RWM) stability of the plasma equilibrium. A real-time plasma response measurement has been used as an input to a neutral beam injection (NBI) feedback algorithm to directly control the plasma response amplitude and hence the RWM stability for the first time. A rotating $n=1$ perturbation is applied at a fixed frequency, and the plasma response is calculated by Fourier-analyzing magnetic measurements at the applied frequency and subtracting the known vacuum pickup from the control coils. In contrast to stability analysis methods that rely on calculated equilibria, the plasma response measurement naturally includes both ideal MHD and non-ideal contributions to RWM stability. A zero-dimensional model for NBI control of the plasma response has been developed to aid in optimizing feedback settings. [Preview Abstract] |
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UP9.00066: Stabilization of Disruptive Locked Modes at DIII-D by Means of ECCD and Magnetic Perturbations F.A. Volpe, R.J. La Haye, J. Lohr, R. Prater, E.J. Strait, A.S. Welander, M.J. Lanctot A new technique for disruption avoidance has been demonstrated at DIII-D. Locked tearing modes that would otherwise cause disruptions were stabilized by applying magnetic perturbations to control the toroidal phase and injecting electron cyclotron current drive (ECCD) in the island O-point. The magnetic perturbation was applied before complete locking and used to force the rotating precursor to lock in the optimum location for stabilizing ECCD. In this way the mode was rapidly stabilized. As expected, ECCD was the most stabilizing at the O-point and destabilizing at the X-point. Further, ECCD was more stabilizing than heating alone. In some cases with high NBI torque the mode unlocked when the size was reduced by the ECCD and began to rotate before eventually being stabilized. The technique made it possible to increase normalized beta to values as high as 2.5 and still suppress the locked mode under conditions where disruptive locked modes were otherwise observed at values around 1.7. [Preview Abstract] |
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UP9.00067: Measurement of Tearing Mode Structure and Evolution Through Fluctuation Analysis of the Motional Stark Effect Diagnostic J.D. King, M.A. Makowski, C.T. Holcomb, S.L. Allen, D.N. Hill, W.H. Meyer, R. Geer, M.A. Van Zeeland, T.L. Rhodes, E.C. Morse Fluctuation analysis of the motional Stark effect (MSE) diagnostic allows for the direct internal measurement of $\tilde{B}_z$. Coherent fluctuations from internal tearing modes are easily observed on several MSE channels. At the heart of this measurement is the accurate recovery of MHD sidebands at 2nd harmonic photoelastic modulator (PEM) frequencies plus/minus the MHD toroidal mode frequency of rotation. Single value decomposition (SVD) filtering and analysis has been applied for adjacent MSE channels improving signal-to-noise and enabling successful recovery of sidebands. A sample temporal and spatially resolved $\tilde{B}_z$ measurement of a 2/1 tearing mode is presented. [Preview Abstract] |
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UP9.00068: Linear Plasma Response Model Based on the Solution to a Perturbed Grad-Shafranov Equation A.S. Welander, D.A. Humphreys, M.L. Walker, D.A. Gates, E. Kolemen, B. Xiao Linearized models of the plasma response to variations in conductor currents are important for design of equilibrium shape, position, and stability control. We describe a new plasma response model based on the linearly perturbed Grad-Shafranov equation. Inputs to the model are changes in applied flux from conductor currents, as well as pressure and current profiles. The change of pressure and current profiles can be derived from changes in the total thermal energy content, $W_{th}$ or $\beta_p$, total plasma current, $I_p$ and normalized inductance, $\ell_i$, together with assumptions about the details of the profiles. Alternatively, some or all of these can be supplied as exogenous variables. The flexibility in constraints that can be applied depending on the relevant plasma operating regime or scenario is a key feature of the new model. The model predicts changes in the plasma boundary, as well as changes in the $q$-profile and displacement of $q$-contours. Validation results will be presented based on data from DIII-D, NSTX, and EAST. [Preview Abstract] |
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UP9.00069: STELLARATORS |
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UP9.00070: Density dependence of the alpha particle confinement in the LHD type helical reactor Yoshitada Masaoka, Sadayoshi Murakami In helical systems, high-energy particle trajectory is complicated in a three dimensional magnetic configuration and, thus the confinement of alpha particles is one of the critical issues in designing helical reactor. Based on the recent experimental results of LHD, the high-density plasma scenarios of the LHD type helical reactor are suggested. In this paper, we study the confinement of alpha particles in a heliotron reactor based on the three typical LHD configurations. The GNET code is applied to study the alpha-particle confinement with the energy and pitch angle scattering during the energy slow down. We study the real and velocity space distributions, and the energy and particle loss rates changing the plasma density keeping the total fusion power. It is found that the energy loss rate is strongly reduced, E$_{loss} \quad \sim $ n$^{-2.9}$, in the optimized configuration, base on the R$_{ax}$=3.53m, as the plasma density increases. While the reduction is rather small in the non-optimized configuration, based on the R$_{ax}$=3.75m. [Preview Abstract] |
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UP9.00071: Characterization of magnetic configurations of planar-axis stellarators LHD and CHS in terms of the boundary shape Shoichi Okamura A series of plasma confinement experiments with two stellarator/heliotron devices CHS and LHD in NIFS, Japan has been demonstrating experimental results largely contributing to the magnetic fusion research. The most important result in the physics study was the experimental demonstration of the zonal flow measurements conducted in CHS device. High performance results in LHD device are the stable plasma operation with 5{\%} beta and the achievement of very high-density plasmas well above the equivalent Greenwald limit. CHS and LHD are classified in the planar-axis stellarators, which were designed by tuning parameters of helical coils and a small number of poloidal coils. Because a new trend of the advanced stellarator design is the tuning of configurations based on the boundary shape, the same analyses of magnetic configurations used in CHS and LHD experiments are necessary for the comprehensive understandings of stellarator experiments including non-planar-axis ones. This paper presents comparisons of various configurations of CHS and LHD from the common point of view of the boundary shape analysis. [Preview Abstract] |
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UP9.00072: PIES LHD Magnetic Field Reconstruction Samuel Lazerson, David Gates, Allan Reiman, Don Monticello Reconstructions of the magnetic fields in the Large Helical Device (LHD) by the Princeton Iterative Equilibrium Solver (PIES) code are presented. The pressure profile for the device is experimentally determined from Thomson scattering. Magnetic diagnostics provide magnetic field constraints. Initial conditions are provided by the Variational Moments Equilibrium Code (VMEC). The pies code is then utilized to study magnetic island formation and stochastic regions in the stellarator. Mechanisms for flux surface breakup in stellarators are investigated. These results provide additional benchmarks for comparisons to other equilibrium codes (such as HINT2). [Preview Abstract] |
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UP9.00073: Objectives and Design Parameters of a High-Resolution X-ray Imaging Crystal Spectrometer for the Large Helical Device (LHD) Manfred Bitter, L. Delgado-Aparicio, D. Gates, K.W. Hill, D. Monticello, H. Neilson, N. Pablant, A. Reiman, A.L. Roquemore, S. Morita, M. Goto, H. Yamada, J.E. Rice A high-resolution X-ray imaging crystal spectrometer, whose concept was tested on NSTX and Alcator C-Mod, is being designed for LHD. The instrument will record spatially resolved spectra of helium-like Ar$^{16+}$ and provide ion temperature profiles with spatial and temporal resolutions of $<$ 2 cm and $>$ 10 ms. The stellarator equilibrium reconstruction codes, STELLOPT and PIES, will be used for the tomographic inversion of the spectral data. The layout of this spectrometer and instrumental features, which are largely determined by the complicated magnetic field structure of LHD, will be described. [Preview Abstract] |
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UP9.00074: Heating scheme dependence of electron density profile measured with AM reflectometer in Heliotron J plasmas Kiyofumi Mukai, K. Nagasaki, V. Zhuravlev, T. Fukuda, T. Mizuuchi, T. Minami, H. Okada, S. Kobayashi, S. Yamamoto, Y. Nakamura, K. Hanatani, S. Konoshima, S. Ohshima, M. Takeuchi, K. Mizuno, H.Y. Lee, H. Yashiro, F. Sano Measurement of electron density profile is one of important issues for understanding plasma confinement and transport. A X-mode microwave amplitude modulation (AM) reflectometer for electron density profile measurement is applied to Heliotron J in the 2009 experimental campaign. The carrier frequency of the reflectometer ranges from 33 GHz to 56 GHz, and the modulation frequency is 200 MHz. In this study, time evolution of density profile was measured in 70 GHz ECH or balance-injected NBI plasma by the AM reflectometer. The injection powers are 270 kW (ECH) and 770 kW (NBI, total), respectively. As the results of measurements, in ECH plasma, the reconstructed profile has flat shape at n$_{e}\sim $0.8$\times $10$^{19}$ m$^{-3}$ and has steep gradient at edge region in all measured density region (n$_{e}<$ 3.0$\times $10$^{19}$ m$^{-3})$. The profile of NBI plasma is peaked at any line averaged density under 2.5$\times $10$^{19}$ m$^{-3}$. In presentation, the profile of SMBI plasma will be discussed. In forthcoming experiment, another reflectometer by using O-mode for density fluctuation measurement is installed and the relation between density gradient length and fluctuation will be studied. [Preview Abstract] |
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UP9.00075: Linear gyrokinetic studies in W7-AS and W7-X stellarators with the GS2 code J.A. Baumgaertel, W. Guttenfelder, G.W. Hammett, D.R. Mikkelsen, P. Xanthopoulos, H. Maassberg, J. Geiger, Y. Turkin, W. Dorland, E. Belli The GS2 gyrokinetic code is being used to study microinstabilities and turbulence in non-axisymmetric flux-tube geometries. Non-axisymmetric systems, such as stellarators, have a number of interesting features, like natural reversed magnetic shear and a large number of shaping parameters. These offer possibilities for reducing microturbulence and improving performance. The W7-AS and W7-X designs were partially optimized for neoclassical transport, and significant comparisons have been made between experimental data and neoclassical expectations. However, the turbulent transport has not been studied in detail. We will present initial studies of gyrokinetic instabilities in W7-AS and W7-X equilibria. This work was supported by the SciDAC Center for the Study of Plasma Microturbulence, the DOE Fusion Energy Sciences Fellowship, and Department of Energy Contract DE-AC02-09CH11466. [Preview Abstract] |
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UP9.00076: Current-driven discharges in the Compact Toroidal Hybrid Stephen Knowlton, Gregory Hartwell, James Hanson, Xinxing Ma, Adam Stevenson Control of disruptions remains a critical issue in toroidal confinement, particularly at the scale of ITER. Along with the other benefits of 3D shaping fields, the addition of moderate stellarator (vacuum) transform to the tokamak configuration may allow disruptions to be passively avoided$^{1}$. The effects of the quench may also be reduced by the maintenance of a vacuum equilibrium throughout the disruption. Disruptions of current-driven stellarator plasmas are studied in the Compact Toridal Hybrid (CTH) device (R$_{0}$ = 0.75 m, a $\sim $ 0.2 m, B$_{0} \quad \le $ 0.7 T, $\bar {n}_e $ = 0.2 -- 1.5 x 10$^{19}$ m$^{-3})$, an ECR-heated heliotron with plasma currents I$_{p}\le $ 45 kA. In the range of vacuum transform $\iota _{VAC}$(a) = 0.05 -- 0.1, disruptions leading to a complete loss of plasma can be induced by raising the density above $\bar {n}_e \quad \ge $ 0.9 x 10$^{19}$ m$^{-3}$, comparable to the Greenwald limit for CTH. At higher vacuum transforms, disruptive signatures are not observed despite the radiative decline of the plasma as the density is increased. \\[4pt] [1]. A.H. Boozer, Phys. Plasmas, \textbf{16} (2009) 0058102 [Preview Abstract] |
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UP9.00077: 3D Equilibrium Reconstruction of Current-Driven Discharges in the Compact Toroidal Hybrid with Magnetic Diagnostics A. Stevenson, J. Hanson, G. Hartwell, J. Hebert, S. Knowlton The capability of rapidly reconstructing 3D equilibria in toroidal confinement experiments is important to understand the stability and confinement of fusion plasmas. Plasma reconstructions using the V3FIT 3D magnetic equilibrium reconstruction code [1] are performed using magnetic diagnostics in the Compact Toroidal Hybrid (CTH). CTH is a heliotron device in which the magnetic configuration can be strongly modified by an ohmic plasma current. Currently, signals from several sets of segmented and full Rogowski coils measuring the poloidal field provide the experimental input to V3FIT which utilizes the VMEC equilibrium code [2] to reconstruct 3D plasma equilibria. A movable array of Hall probes is being built to measure the interior poloidal field and will be included in plasma reconstruction to provide better resolution of the current profile. Supported by US DOE Grant DE-FG02-00ER54610. \\[4pt] [1] J. D. Hanson et al, Nucl. Fusion 49, 075031 (2009) \\[0pt] [2] S. P. Hirshman and D. K. Lee, Comput. Phys. Commun. 39, 161(1986) [Preview Abstract] |
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UP9.00078: Characterization of edge biasing in the Compact Toroidal Hybrid stellarator M. Cianciosa, E. Thomas, B.A. Stevenson, G. Hartwell, S. Knowlton Sheared flows arising from spatially inhomogeneous, transverse electric fields are of interest in space, laboratory and fusion plasmas. These flows are source of free energy that can drive or suppress instabilities. In fusion plasmas the presence of edge localized sheared flows are associated with enhanced confinement regimes (H-mode). Using a biasing probe, the radial electric field of the Compact Torodial Hybrid (CTH) stellarator (R$_{0}$ = 0.75 m, a $\sim $ 0.2 m, B$_{0} \quad \le $ 0.7 T, $\bar {n}_e $ = 0.2 -- 1.5 x 10$^{19}$ m$^{-3})$ is modified. Simultaneous measurements of plasma parameters and potential fluctuations are presented for various biasing probe configurations. Changes to the plasma response in the presence of magnetic islands may be presented. [Preview Abstract] |
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UP9.00079: Incorporation of Soft X-Ray Chordal Diagnostics into Equilibrium Reconstruction on the Compact Toroidal Hybrid Experiment G.J. Hartwell, J.D. Hanson, S.F. Knowlton, B.A. Stevenson Signals from Soft X-Ray (SXR) chords normally used for tomographic reconstruction on the Compact Toroidal Hybrid (CTH) torsatron experiment (R = 0.75 m, a $\sim $ 0.2 m, B $\le $ 0.7 T, n$_{e} \quad \le $ 10$^{19}$ m$^{-3}$, T$_{e} \quad \le $ 250 eV) have been incorporated into the V3FIT[1] equilibrium reconstruction code. Four cameras, each consisting of a 20-channel AXUV-20EL photo-diode array view the plasma through 2$\mu $m Be foil. Three cameras view the plasma in one symmetry plane ($\varphi $ =36$^{o})$ while the fourth views the plasma at another symmetry plane ($\varphi $ =0$^{o})$, one-half field period away. Under the assumption of uniform SXR emissivity on a flux surface, the signals provide additional constraints to V3FIT, which primarily uses magnetic data to fit the equilibrium. A description of the V3FIT code additions and capabilities will be given. Use of the signal effectiveness to optimize the placement SXR cameras will be discussed. Equilibrium reconstruction results will be presented. \\[4pt] [1] J. Hanson, S. Hirshman, S. Knowlton, L. Lao, E. Lazarus, J. Shields, Nucl. Fusion, \textbf{49} (2009) 075031 [Preview Abstract] |
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UP9.00080: Overview of HSX Stellarator Experiments K.M. Likin, D.T. Anderson, F.S.B. Anderson, A.R. Briesemeister, D.L. Brower, C.A. Clark, C.R. Cook, C. Deng, J. Lore, J.W. Radder, J.C. Schmitt, J.N. Talmadge, G.M. Weir, R.S. Wilcox, K. Zhai HSX is operating at 1 T with ECRH injected power up to 100 kW. An internal transport barrier (ITB) with a very peaked T$_{e}$ profile has been observed. The working hypothesis on ITB is a suppression of the turbulent transport in the region of strong shear of the radial electric field as predicted by neoclassical theory. A second ECRH system with a steerable launcher has been installed. The RF power modulation from the second source will allows us to measure the local thermal diffusivities. The first measurements of large flows in the direction of symmetry have been made with a CXRS diagnostic. Measurements of plasma currents show their reduced value by the high effective transform. Work is now proceeding on 3D equilibrium reconstruction of the current profile. Long-range correlations are measured during electrode biasing. A laser blow-off technique is under development to inject impurities into the plasma, and the transport characteristics will be determined from spectroscopic measurements and modeling. Ion cyclotron resonance heating is being examined as a route to increased density and ion temperatures. [Preview Abstract] |
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UP9.00081: Measurement, Modeling and Reconstruction of Parallel Currents in the HSX Stellarator J.C. Schmitt, J.N. Talmadge, J. Lore Parallel currents are measured with a set of magnetic diagnostics on the HSX. Measurements show that the Pfirsch-Schl\"{u}ter current is helical due to the lack of toroidal curvature and is reduced in magnitude compared to an equivalent tokamak because of the high effective transform ($\sim $3) in a quasihelically symmetric stellarator. The bootstrap current density is calculated using the PENTA code,$^{1}$ which includes momentum conservation between plasma species. The data shows better agreement with a model that includes momentum conservation. HSX plasmas are heated by a 28 GHz gyrotron which allows the electrons to access the low collisionality regime, while the cold ions are generally in the plateau. In HSX, a 3-D plasma with small symmetry-breaking, the calculations show that for two species in different collisionality regimes, the bootstrap current can be strong function of the radial electric field. In the plasma core, multiple stable electric field solutions to the ambipolarity constraint exist. The large positive electric field, the ``electron-root'' solution, can result in a reduction and even a reversal of the bootstrap current. The measured fields and fluxes are used in the V3FIT$^{2}$ code to reconstruct the current profile. Supported by DOE grant DE-FG02-93ER54222. $^{1}$D.A. Spong, Phys. Plasmas 12 (2005) 056114. $^{2}$J.D. Hanson, et al, Nucl. Fusion 49 (2009) 075031. [Preview Abstract] |
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UP9.00082: Measurement of Flows in the HSX Stellarator Demonstrating the Importance of Momentum-Conservation in Neoclassical Flow Modeling A. Briesemeister, J. Lore, K. Zhai, D.T. Anderson, F.S.B. Anderson, J.N. Talmadge The flow velocity of carbon ions is measured using a Charge Exchange Recombination Spectroscopy (CHERS) system on the Helically Symmetric Experiment (HSX), a quasi-helically symmetric stellarator. Intrinsic parallel flow speeds of up to 20km/s have been measured. The parallel velocity is compared to the predictions of the PENTA code [1-2]. Multiple ion species, including the species used for the CHERS measurements, are included in the calculations. PENTA is a neoclassical code that includes the effects of momentum-conservation, which are often neglected for nonsymmetric stellarators. Without momentum conservation the parallel flow velocity in HSX is under-predicted by approximately an order of magnitude. Agreement is seen between the measured and calculated parallel flows when momentum conservation is included.\\[4pt] [1] D.A. Spong, Phys. Plasmas 12 (2005) 056114.\\[0pt] [2] J. Lore et al, Phys. Plasmas 17 (2010) 056101. [Preview Abstract] |
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UP9.00083: Second ECRH system and upgraded ECE on HSX G.M. Weir, K.M. Likin, F.S.B. Anderson, D.T. Anderson, J.W. Radder A second 200 kW/ 28 GHz ECRH system has been installed on the HSX stellarator. The Varian gyrotron VGA-8050M has a multimode output. The TE$_{02}$ waveguide mode is dominant (about 90{\%} of total power) in the power spectrum. To maximize the RF power that can be delivered to the torus over a pure wave guide transmission line, a hybrid transmission line has been implemented to deliver power to the plasma with correct polarization (O1/X2) and as a well-focused beam. The line consists of a Vlasov mode converter with a set of focusing mirrors, a polarizer, 4'' and 2.5'' dual-mode waveguides, and a launcher. The antenna is a steerable focusing mirror within the HSX vessel that allows power deposition studies, specifically the effect of power deposition on ITB formation in the stellarator. The power from the second gyrotron can be modulated to facilitate local thermal diffusivity measurements using the ECE diagnostic. Upgrades to the 16-channel ECE radiometer will also be discussed. [Preview Abstract] |
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UP9.00084: Density Fluctuation Measurements by Far-Forward Collective Scattering in the HSX Stellarator C. Deng, D.L. Brower, D.T. Anderson, F.S.B. Anderson, K. Likin, J. Lore, J.C. Schmitt, J.N. Talmadge, R. Wilcox, K. Zhai The multichannel interferometer system on the HSX stellarator is reconfigured to perform far-forward collective scattering measurements of electron density fluctuations. The collective scattering system has 9 viewing chords with 1.5 cm spacing. The source is a bias-tuned Gunn diode at 96 GHz with passive solid-state tripler providing output at 288 GHz ($\sim $5 mW). The scattered power is measured using a homodyne detection scheme. Far-forward collective scattering provides a line-integrated measurement of fluctuations within the divergence of the probe beam covering wave number range: $k_\bot <$ 2 cm$^{-1}$. The perpendicular wave number consists of poloidal and radial contributions which vary with chord position. Comparison of density fluctuations measured using scattering and interferometry techniques will be made. Both coherent modes and broadband fluctuations are measured. Comparison of fluctuation amplitude and frequency spectra for quasi-helically symmetric and non-axisymmetric magnetic configuration will be presented. [Preview Abstract] |
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UP9.00085: Measurements of long-range correlations and bicoherence during biasing in HSX Robert Wilcox, Boudewijn van Milligen, Maria Angeles Pedrosa, Mirko Ramisch, David Anderson Using toroidally-spaced Langmuir probes, long-range fluctuation correlations have been measured in floating potential signals during biased discharges in the HSX stellarator. The increase in long-range correlations during biasing occurs in the floating potential signals, but not in the ion saturation current signals. This has been linked to zonal flow formation in the TJ-II stellarator, both in biased discharges and in naturally occurring improved-confinement discharges [1]. Measurements of the auto-bicoherence of the poloidal electric field signals show an increase in broadband 3-wave coupling during biasing, which is analyzed and compared to both biased and naturally occurring enhanced-confinement discharges in TJ-II [2]. Additional measurements of fluctuation moments in HSX are also presented.\\[4pt] [1] M.A. Pedrosa, et al, Phys. Rev. Lett. 100 (2008) 215003.\\[0pt] [2] B.Ph. van Milligen, et al, Nucl. Fusion 48 (2008) 115003 [Preview Abstract] |
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UP9.00086: Upgrade of the HSX Thomson Scattering System K. Zhai, F.S.B. Anderson, D. Anderson At the HSX plasma laboratory, a single-shot 10 channel Thomson scattering system has been implemented and is now operational. A second ECH system permits variation of heating power and location during a plasma discharge. This permits expanded study of internal transport barrier formation and transient thermal conductivity measurements. Efforts to upgrade the current system with multi-shot adjustable time resolution are in progress. The new system will utilize three Nd:YAG lasers, which are 10m along the beam path from the plasma region of measurement. These three laser outputs will be combined to share the same beam path. They will also all use the collection optics and the polychromators of the present system. The integrator digitizers have been upgraded (on loan from the Madison Symmetric Torus group) to accommodate 85 in-memory measurements at intervals down to 65~$\mu $s. Detailed results and progress will be presented at the conference. [Preview Abstract] |
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UP9.00087: Development of Laser Blow-Off Impurity Injection Experiments for the HSX Stellarator C. Clark, D.T. Anderson, F.S.B. Anderson, J.N. Talmadge At HSX, we are assembling an experimental program designed to investigate impurity transport in a quasisymmetric stellarator. ~The laser blow-off impurity injection technique that will be used in this system rapidly deposits a small, controlled quantity of aluminum, iron, boron or silicon into the confinement volume. A mix of bolometers, soft x-ray detectors, and a VUV spectrometer will then be used to track the radial transport of the chosen impurity. ~The data from these measurements will be processed into transport coefficients with the transport code, STRAHL. Simulations of the light emitted by trace levels of injected aluminum using STRAHL and atomic data from ADAS will be shown. [Preview Abstract] |
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UP9.00088: Application of SIESTA to Well and Hill Equilibria in HSX C.R. Cook, S.P. Hirshman, R. Sanchez HSX (the Helically Symmetric eXperiment) is a quasi-helical 4 field-period stellarator at the University of Wisconsin-Madison. Previous Biot-Savart field-line following code work done on HSX shows a large 4/4 island chain in the 6.2{\%} Hill configuration and an 8/7 island chain in the 6.2{\%} Well configuration. However, these simulations were all done in vacuum conditions. SIESTA (Scalable Island Equilibrium Solver for Toroidal Applications) is a three-dimensional magnetohydrodynamic code used to compute equilibria with islands and stochastic regions in finite-pressure, toroidally-confined plasmas. The very strong helical shaping of the HSX magnetic field structure requires a large number of toroidal integration points and toroidal modes. The requirements to obtain good convergence in these simulations are described. The equilibria found running SIESTA on the Well and Hill configurations are discussed and compared to the vacuum field-line results. [Preview Abstract] |
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UP9.00089: Impact of quasi-symmetry on stellarator equilibrium, stability and transport A.S. Ware, K. McGarvey-Lechable Development of three-dimensional toroidal confinement devices with quasi-symmetry of the magnetic field strength in Boozer coordinates is a key component of the U.S. stellarator program. Previous work has led to the design of quasi-helically symmetric (QH), quasi-axisymmetric (QA) and quasi-poloidally symmetric (QP) stellarators but at very distinct aspect ratios, major radii, and plasma $\beta$. In this work, a computational analysis is undertaken to optimize stellarator configurations for a variety of quasi-symmetries, all at the same target aspect ratio, major radius and plasma $\beta$. We compare initial cases of QA, QP, and two QH configurations. The initial optimizations focused solely on enhancing quasi-symmetry. The QP configuration had high magnetic ripple which resulted in a relatively high ballooning stability $\beta$-limits and also relatively high estimates of ripple-induced neoclassical transport. The other three configurations had lower magnetic ripple and consequently lower ballooning stability $\beta$-limits and much lower estimates of ripple-induced neoclassical transport. Subsequent optimizations have focused on simultaneous optimization of degree of quasi-symmetry, higher $\beta$ stability, and lowering ripple transport. [Preview Abstract] |
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UP9.00090: Plasma rotation in a quasisymmetric stellarator Andrei Simakov, Per Helander The equilibrium plasma rotation in a general toroidal magnetic field is nearly always subsonic and is determined by the requirement that the collisional particle transport should be ambipolar in lowest order in the small-ion-gyroradius expansion [1]. Gyrokinetic turbulence can only appreciably modify the rotation on radial length scales of order the ion gyroradius [1,2]. Only in quasisymmetric fields, where collisional particle transport is intrinsically ambipolar, can the plasma rotate freely and then only in the quasisymmetry direction [1]. In particular, sonic rotation velocities are allowed in this case [3]. However, the quasisymmetry is broken when the rotation speed exceeds the diamagnetic speed appreciably, leading to reappearance of the non-intrinsically-ambipolar $1/\nu$-transport regime. Herein, we explicitly evaluate the electron radial particle flux caused by such a quasisymmetry breaking. We find that this flux scales with the fourth power of the rotation Mach number and is therefore expected to be modest in most plasmas of interest.\\[4pt] [1] P. Helander and A. N. Simakov, Phys. Rev. Lett. {\bf 101} 145003 (2008).\\[0pt] [2] P. Helander and A. N. Simakov, Contrib. Plasma Phys. {\bf 50}, 1 (2010).\\[0pt] [3] P. Helander, Phys. Plasmas {\bf 14} 104501 (2007). [Preview Abstract] |
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UP9.00091: Electric field effects in a quasisymmetric stellarator Matt Landreman, Peter J. Catto It was recently shown [1] that the radial electric field $E_r$ can alter the neoclassical ion flow, heat flux, bootstrap current, and residual zonal flow in a tokamak, even if the ExB speed is small compared to the ion thermal speed. Here we show these calculations can be adapted to a quasisymmetric stellarator. The finite-$E_r$ effects can be understood as finite-orbit-width effects associated with variation in the electrostatic potential over an orbit. The tokamak calculation uses $B_{poloidal}/B$ as a small parameter, and in the stellarator case, we use the fact that an analogous ratio of field components can be small. The quasisymmetry calculation also exploits conservation of the ``helical momentum'' $\psi_*$, which is used in place of the toroidal or poloidal flux as the radial variable. The calculations generalize the model collision operator in [1], which keeps only velocity-space derivatives normal to the trapped-passing boundary, even as this boundary is significantly shifted by a large ExB drift. We show $E_r$ in HSX may be sufficient for these effects to be significant.\\[4pt] [1] G Kagan and P J Catto, Plasma Phys. Control. Fusion, 52, 055004 (2010). [Preview Abstract] |
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UP9.00092: Characterization of a Class of Stellarator Steady States Harold Weitzner For a specific class of stellarator appropriate magnetic fields the coupled ion-electron Fokker-Planck equations are solved by a formal expansion in the small Larmor radius parameters. A system of relatively simple ordinary differential equations is given to determine the plasma profile functions, number density, temperature, and electrostatic potential. A low collisionality ordering is used. The magnetic field has stellarator symmetry of $N$ periods in the toroidal direction and is approximated by a closed magnetic line configuration with rotational transform $N/R.$ The magnetic field is approximately quasisymmetric. Finally a small additional field is included which leads to a field without flux surfaces. [Preview Abstract] |
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UP9.00093: Progress Toward Improved Compact Stellarator Designs G.H. Neilson, T. Brown, D. Gates, L.P. Ku, S. Lazerson, N. Pomphrey, A. Reiman, M. Zarnstorff, L. Bromberg, A. Boozer, J. Harris Stellarators offer robust physics solutions for MFE challenges-- steady-state operation, disruption elimination, and high-density operation-- but require design improvements to overcome technical risks in the construction and maintenance of future large-scale stellarators. Using the ARIES-CS design (aspect ratio 4.56) as a starting point, compact stellarator designs with improved maintenance characteristics have been developed. By making the outboard legs of the main magnetic field coils nearly straight and parallel, a sector maintenance scheme compatible with high availability becomes possible. Approaches that can allow the main coil requirements to be relaxed in this way are: 1) increase aspect ratio at the expense of compactness, 2) add local removable coils in the maintenance ports for plasma shaping, and 3) use passive conducting tiles made of bulk high-temperature superconducting material to help shape the magnetic field. Such tiles would be arranged on a shaped, segmented internal support structure behind the shield. [Preview Abstract] |
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UP9.00094: Kinetic shielding of magnetic islands in 3-D equilibria C.C. Hegna Kinetic theory is employed to calculate corrections to analytic predictions of saturated magnetic islands due to pressure gradients in 3-D magnetic configurations. The theory calculates the dominant trapped particle response to 3-D field induced net bounce averaged radial drifts. The associated kinetic response describes plasma currents that flow within magnetic surfaces. In general, these currents have a component that resonates with the helical angle of the magnetic island and affect saturated island sizes through the parallel currents generated to satisfy quasineutrality. The resulting kinetic response generally opposes the effects of singular Pfirsch-Schl\"uter currents that arise at the rational surfaces of general 3-D MHD equilibria. Accounting for both the MHD and kinetic responses, self-consistent magnetic island widths are calculated using Ampere's law. The kinetic effect is largest at lowest collisionality suggesting high-$\beta$ stellarators are more resilient to retaining flux surface integrity at high-temperatures than predictions from conventional MHD based theory would imply. [Preview Abstract] |
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UP9.00095: Investigation of equilibrium plasma beta limits in 3-D magnetic topologies M.G. Schlutt, C.C. Hegna, C.R. Sovinec, E. Held, M. Schlutt 3D MHD equilibria are modeled using NIMROD. A vacuum equilibrium helical magnetic field is loaded into the geometry of a straight stellarator. The symmetry of the vacuum field with a dominant magnetic harmonic can be spoiled by adding small perturbations. These perturbations alter the magnetic spectrum, and produce magnetic islands and regions of stochasticity. Finite-$\beta$ equilibria are generated via numerical simulations that include the effect of a heating source and self-consistent anisotropic pressure transport. A variety of magnetic configurations, including helically symmetric and spoiled symmetry cases, are investigated. To study the stability properties of finite-$\beta$ 3D equilibria, nearly helically symmetric configurations are subjected to a symmetry-spoiling perturbation. If the equilibrium is linearly unstable, MHD modes are triggered. The nonlinear consequences of violating MHD stability is subsequently simulated. These cases are compared to simulations of heated configurations that have no intrinsic symmetry - the equilibrium fields are fully 3D. The connection between high beta properties of systems with saturated instabilities and equilibrium beta limits is discussed. [Preview Abstract] |
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UP9.00096: Grad-Shafranov-like equation for stellarators with a non-planar magnetic axis Antoine J. Cerfon, Felix I. Parra, Jeffrey P. Freidberg, Harold Weitzner We derive an equation for the poloidal flux function in a large aspect ratio stellarator, similar to the Grad-Shafranov equation for toroidally axisymmetric plasma configurations. There are three key elements in our new formulation. (1) We choose an ordering in helical field amplitude and inverse aspect ratio that generalizes the original work of Greene and Johnson [1] to allow for a non-planar magnetic axis. Toroidal effects thus enter the analysis in the same order as helical effects, and more complex 3-D geometries can be treated than in [1]. (2) We use an asymptotic expansion in the small parameter $\delta\equiv|\mathbf{B_{p}}|/B_{\phi}$ , the ratio of the poloidal field to the toroidal field. (3) By exploiting the smallness of $\delta$, we define a new set of poloidal coordinates which can be constructed order by order, effectively transforming the problem into a 2-D Grad-Shafranov-like equation containing coefficients averaged over toroidal angle. \\[4pt] [1] J.M. Greene and J.L. Johnson, Phys. Fluids $\textbf{4}$, 875 (1961) [Preview Abstract] |
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UP9.00097: MAGNETO-INERTIAL FUSION |
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UP9.00098: Magnetized Target Fusion Integrated Engineering Test Shot T. Intrator, J. Sears, P.J. Turchi, W.J. Waganaar, T. Weber, G.A. Wurden, J.H. Degnan, M. Domonkos, C. Grabowski, E.L. Ruden, W. White, D. Gale, M. Kostora, J. Parker, M.H. Frese, S.D. Frese, J.F. Camacho, S.K. Coffey, V. Makhin, R.E. Siemon, S. Fuelling, B.S. Bauer, A.G. Lynn, N.F. Roderick The LANL {\&} AFRL collaboration has carried out the first engineering shakedown demonstration of a Magnetized Target Fusion (MTF) shot. We used a solid, cylinder aluminum flux compressor. The target plasma was created as a high density Field Reversed Configuration (FRC) with closed flux surfaces. After formation, the FRC was expelled to a compression region at 15km/sec. We show some initial data that characterize the target FRC, including some translation data from the Los Alamos FRC experiment FRXL and the FRCHX experiment at AFRL. Data from the implosion shot show that we achieved all our initial objectives. The solid liner realization of Magneto Inertial Fuson is only one of several magnetized, pulsed, fusion schemes that are being pursued. [Preview Abstract] |
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UP9.00099: Field Reversed Configuration (FRC) injection and compression experiments J.H. Degnan, R. Delaney, M. Domonkos, C. Grabowski, F.M. Lehr, P. Robinson, E.L. Ruden, W. White, H. Wood, D. Gale, M. Kostora, W. Sommars, M.H. Frese, S.D. Frese, J.F. Camacho, S.K. Coffey, V. Makhin, T.P. Intrator, G.A. Wurden, J. Sears, P.J. Turchi, R.E. Siemon, S. Fuelling, B.S. Bauer, A.G. Lynn Experiments on FRC injection into an imploding liner are described, both with and without the operation of the imploding liner. The use of an actual liner limits the FRC diagnostics to observing it entering the liner. For proper choice of operating parameters, guided by 2D-MHD simulations, interferometry, magnetic probe, and collimated light probe data indicate that the FRC enters the liner without leaving it. Other operating parameters result in the FRC entering the liner and bouncing out. Field exclusion lifetime, sufficient for injection and implosion-compression system testing, may need to be increased for better quality implosion-compression. Supported by DOE-OFES. [Preview Abstract] |
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UP9.00100: FRC Formation, Translation and Capture Studies for the Field Reversed Configuration Heating Experiment (FRCHX) C. Grabowski, J.H. Degnan, R. Delaney, M. Domonkos, F.M. Lehr, P.R. Robinson, E.L. Ruden, W. White, H. Wood, D. Brown, D. Gale, M. Kostora, J. McCullough, W. Sommars, M.H. Frese, S.D. Frese, J.F. Camacho, S.K. Coffey, V. Makhin, T.P. Intrator, G.A. Wurden, P.J. Turchi, R.E. Siemon, S. Fuelling, A.G. Lynn During FRC compression-heating experiments using imploding solid liners it is difficult to diagnose the FRC once it enters the liner due to limited diagnostic access. To confirm capture of the FRC within the magnetic mirrors inside the liner and obtain measurements of its lifetime, density, temperature and excluded flux radius, numerous non-implosion tests are performed using a surrogate liner with greater diagnostic access prior to the compression heating tests. Such experiments are described along with the experimental setup, and data from interferometry, magnetic probes, and collimated light probes are presented. 2D-MHD simulations have been used to guide the parameter selection for these experiments. This work is supported by DOE-OFES. [Preview Abstract] |
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UP9.00101: Implementation of a RF pre-preionizer for FRCHX Mark Kostora, Edward Ruden An RF pre-preionizer (applied prior to the main preionization discharge) has been implemented for the FRCHX experiment to increase plasma lifetime. The present 46 MHz system is based on one used on previous experiments such as LSX and TRAP at the University of Washington. RF from a 2.5 KW power supply charges conductors on either side of the quartz vacuum chamber, and capacitatively couple to the gas prefill to produce a weakly ionized plasma discharge. A new system is planned using an optimal frequency of 146 MHz to enhance the level of ionization, based on theoretical considerations presented. An important feature is the load match into a composite 50 ohm terminator and the plasma itself during and after gas breakdown. The previous configurations of the RF pre-preionizer resulted in a considerable improvement the lifetime of translated FRC. Changes in the lifetime of the FRC will be correlated with the application of the pre-preionizer circuit. [Preview Abstract] |
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UP9.00102: Toroidal Field Effects in a Magnetized Target Fusion (MTF) Plasmoid Thomas Weber, Jason Sears, Thomas Intrator The FRXL and FRCHX experiments at Los Alamos National Laboratory and the Air Force Research Laboratory seek to understand the physics of a Field Reversed Configuration (FRC) target plasma undergoing compression within a contracting, flux conserving liner. In each experiment, the target FRC is formed using the field-reversed theta pinch method within a conical formation region. This simultaneously forms and accelerates the FRC out of the formation region and into the liner for compression. Large cone angles produce large axial forces, leading to shorter translation timescales, which can ease FRC lifetime requirements, but also result in a plasmoid with a significant degree of toroidal magnetic field. The presence of this field may lead to additional stability and longer lifetime, however, it may also be detrimental to the compression process. The geometry of these experiments allows the cone angle to be altered with relative ease, presenting a unique opportunity for the study of the effects of varying amounts of toroidal field within an MTF plasmoid. [Preview Abstract] |
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UP9.00103: Internal measurements on the Magnetized Target Fusion plasmas and overview of Pulsed Polarimetry techniques R.J. Smith, T. Intrator, G.A. Wurden Progress toward producing internal measurements of n$_{e}$, B and T$_{e}$ using Pulsed Polarimetry on the FRX-L device at LANL is detailed. The present laser, detection scheme and optical layout are described. The FRX-L field reversed configuration (FRC) plasma serves to develop the formation and translation hardware and scenario for Magnetized Target Fusion (MTF) program with n$_{e}$ and B approaching 10$^{23}$ m$^{-3}$ and 5 T and T$_{e}\sim $100 eV. An overview of Pulsed Polarimetry measurement scenarios across the High Energy Density (HED) regimes represented by the MTF program and other HED plasmas is presented. [Preview Abstract] |
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UP9.00104: Interference visibility oscillation in a multimode laser interferometer, and its use in optimizing path lengths Edward L. Ruden, J. Frank Camacho, Alan G. Lynn A 632.8~nm HeNe laser interferometer with multiple single-mode optical fiber-fed probe beams probes a field-reversed configuration (FRC) for AFRL/LANL's magnetized target fusion (MTF) experiment. The long coherence length of the individual laser modes permits the probe optical path length to excede that of the reference beam by $\sim $100~m. This permits the main optical table to be located far from the (highly destructive) experiment with only a short reference path on the table. The multiple modes, however, require that the optical path length difference be an integer multiple of twice the laser resonator length to within a few cm. Fiber nonuniformities make simply measuring the length of the fiber insufficiently accurate, so path length error must be inferred from the interference signal. An oscillation in the interference signal visibility due to mode sweeping on an acoustic time scale provides this information. A model of this phenomenon is presented and corroborated by measurements of the visibility oscillation versus path length error. [Preview Abstract] |
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UP9.00105: Electron Density Measurements of a Field-Reversed Configuration Using Fiber Probe Interferometry J.F. Camacho, A.G. Lynn, E.L. Ruden A HeNe laser interferometer operating at 632.8 nm with two single-mode optical fiber probe beams has been assembled to measure time history of the line-integrated electron density of a field-reversed configuration (FRC) for a magnetized target fusion (MTF) experiment. Our system features probe path lengths many times longer than the reference paths. We have performed simultaneous measurements along two diameters at different axial locations. During plasma formation, translation, and capture tests, the lower probe monitored the formation region, while the upper probe monitored the capture region corresponding to the location of an imploding cylindrical aluminum liner driven by the Shiva Star capacitor bank to compress and heat the FRC plasma. For the actual imploding liner experiment, the upper chord was moved to monitor the translating FRC at the entrance to the liner region. Results from the formation, translation, and capture tests as well as an actual imploding liner experiment will be presented. In addition, interferometer visibility measurements and other factors establishing the viability of our design will be discussed. [Preview Abstract] |
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UP9.00106: Measurements and Computations of the Visible Light Spectrum Emitted from the Ionized Surface of Ohmically Heated Aluminum Rods R.E. Siemon, T.S. Goodrich, B.S. Bauer, S. Fuelling, I.R. Lindemuth, T.J. Awe Measurements have been reported of surface brightness temperature for green light in the UNR rod heating experiments (other papers these proceedings and Invited Presentation by T. J. Awe.). We also measured the entire visible spectrum to compare with a blackbody (T. S. Goodrich, M.S. Thesis, UNR, 2010). Results show a spectrum measurably different than a blackbody. Numerical modeling suggests the following interpretation. Bremsstrahlung is the primary emission and absorption process from aluminum with Z$\sim $3. Deviation from a blackbody occurs under some conditions (lower temperatures early in time) because the optical thickness is less than unity, and the spectrum tends towards that of bremsstrahlung. Under other conditions (higher temperatures later in time) the optical depth can be above unity, but a temperature gradient length comparable to the absorption length alters the emission spectrum. The comparison of experiment and modeling allows an estimate of plasma density, which is difficult to measure directly, and in general adds credence to numerical modeling. [Preview Abstract] |
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UP9.00107: Numerical Modeling of Plasma Formation and Evolution from Thick Aluminum Pulsed with Multi-Megagauss Magnetic Field B.S. Bauer, M.A. Angelova, S. Fuelling, I.R. Lindemuth, R.E. Siemon, W.L. Atchison, T.J. Awe, S.F. Garanin, S.D. Kuznetsov Understanding and predicting the formation and evolution of plasma from metal surfaces driven by intense current is important for both basic science and applications. Radiation-MHD simulations with UP, MHRDR, and Raven are explaining the experimentally observed evolution of thick aluminum pulsed with multi-megagauss field (see Invited Presentation by T.J. Awe). The simulated aluminum expansion agrees with laser-absorption and visible-light images. Surface plasma forms in low density material resistive enough to expand across the magnetic field, yet conductive enough for ohmic heating to exceed expansion cooling. The appropriateness of plasma formation has been checked via analytic calculation. The experimentally observed magnetic field threshold for plasma formation can be computationally reproduced using certain choices of EOS and conductivity. Radiation calculations then reproduce the signals measured by visible and EUV photodiodes. The radiation-MHD simulations have thus achieved results that agree well with observations. [Preview Abstract] |
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UP9.00108: Overview, Status, and Plans of the Plasma Liner Experiment (PLX) S.C. Hsu, T.J. Awe, D.S. Hanna, J.S. Davis, F.D. Witherspoon, J.T. Cassibry, M.A. Gilmore, D.Q. Hwang The Plasma Liner Experiment (PLX) is a multi-institutional collaboration that is exploring and demonstrating the feasibility of forming imploding spherical plasma liners to reach peak pressures $\sim 0.1$~Mbar upon stagnation. The liners will be formed via merging of 30--60 dense high Mach number plasma jets ($n\sim 10^{17}$~cm$^{-3}$, $M\sim 10$--35, $v\sim 50$--70~km/s, $r_{jet}\sim 5$~cm) in spherically convergent geometry. We are aiming for two potential follow-on applications if this work is successful: (1)~assembling repetitive, macroscopic (cm and $\mu$s scale) plasmas suitable for fundamental HEDLP scientific studies and (2)~a standoff driver solution for magneto-inertial fusion. This is a staged project where scientific issues will be studied first at modest stored energies ($\sim 300$~kJ) before attempting to reach HED-relevant pressures (requiring $\sim 1.5$~MJ)\@. This poster provides an overview/status of the project and the research plan, which includes numerical/theoretical and experimental studies of plasma jet formation/acceleration, propagation/merging, liner convergence/stagnation, and laser driven beat waves for magnetizing the imploding liner. [Preview Abstract] |
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UP9.00109: Diagnostics for the Plasma Liner Experiment (PLX) Elizabeth Merritt, Mark Gilmore, Alan Lynn, Bruno Bauer, F. Douglas Witherspoon, Jason Cassibry, Scott Hsu The Plasma Liner Experiment (PLX) is exploring and demonstrating the feasibility of forming HED and MIF relevant imploding spherical ``plasma liners'' that can reach peak pressures $\sim $ 0.1 Mbar at stagnation. Liners will be formed via merging of 30 - 60 dense, high Mach number plasma jets (M $\sim $ 10-35, v $\sim $ 50-70 km/s, jet radius $\sim $ 5 cm) in spherically convergent geometry. Issues include determining parameters (n, T, radius) at stagnation, dynamics of liner formation and convergence, and liner symmetry. Simulations predict wide parameter ranges over the liner evolution, from densities of 10$^{22 }$- 10$^{26}$ m$^{-3}$ and T$_{e}$ $\sim $ T$_{i}$ from 1-500 eV, which necessitate a variety of diagnostics. Diagnostics include multichord visible interferometry and polarimetry, Schlieren imaging, visible and VUV spectroscopy, fast 1D imaging diode arrays, fast cameras, bolometry, and magnetic, electrostatic and pressure probes. This poster overviews and discusses the current status of diagnostic design and implementation. [Preview Abstract] |
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UP9.00110: Pressure and Magnetics Measurements of Single and Merged Jets S. Messer, A. Case, S. Brockington, R. Bomgardner, F.D. Witherspoon We present pressure and magnetic data from both a single full scale coaxial gun and from the merging of jets from several minirailguns. The magnetic probes measure all three components of field, and include an array of probes inside the coaxial gun. Magnetic measurements beyond the muzzle of the gun show the scale of currents trapped in the plasma plume. The pressure probe measures adiabatic stagnation pressure and shows how this quantity decreases with distance from the gun as well as the changes in stagnation pressure through the merge process. Stagnation pressure is influenced by density, temperature, and velocity, and serves as a check on spectroscopic and interferometer measurements. Unlike optical measurements, stagnation pressure is taken at a definite location. These guns are early prototypes of guns to be installed on the Plasma Liner eXperiment at LANL. The jet-merging results are reviewed in the context of what is expected for PLX. [Preview Abstract] |
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UP9.00111: Overview of Plasma Guns for PLX F. Douglas Witherspoon, Richard Bomgardner, Andrew Case, Sarah Messer, Samuel Brockington, Linchun Wu, Raymond Elton, Scott Hsu, Jason Cassibry, Mark Gilmore Plasma guns are being developed for use on the Plasma Liner Experiment (PLX) located at LANL. The collapsing plasma liner will be formed via merging of 30-60 dense, high Mach number plasma jets ($n\sim 10^{16-17}$~cm$^{-3}$, $M\sim$10--35, $v\sim$50--70~km/s, $r_{\rm jet}\sim $5~cm) in a spherically convergent geometry. Small parallel-plate railguns are being developed for this purpose. Each gun will operate at $\sim$300-600~kA peak current, and launch up to $\sim$8000 $\mu$g of high-Z plasma (Ar, Xe) using a $\sim$50~kJ pfn. We are now successfully operating with very fast gas valve injection of Ar, and have already achieved good performance of 1200 $\mu g$ at 42 km/s, and 4000 $\mu g$ at 20-25 km/s at low current. Work is underway to increase both the mass and velocity using higher current. We describe experimental development of the minirailguns and their present and projected performance. We also discuss options for modest size coaxial guns that might achieve the same performance and provide additional control of the plasmoid structure. [Preview Abstract] |
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UP9.00112: High Current Systems for HyperV and PLX Plasma Railguns Samuel Brockington, Andrew Case, Sarah Messer, Richard Bomgardner, Linchun Wu, Raymond Elton, F. Douglas Witherspoon HyperV has been developing coaxial pulsed, plasma railgun accelerators for PLX and other high momentum plasma experiments. The full scale HyperV coaxial gun accelerates plasma armatures using a contoured electrode gap designed to mitigate the blow-by instability. Previous experiments with the full scale gun successfully formed and accelerated annular plasma armatures, but were limited to currents of up to only $\sim$400 kA. In order to increase full scale gun performance to the design goal of 200 $\mu$g at 200 km/s, the pulse forming networks required upgrading to support currents up to $\sim$1 MA. A high voltage, high current field-effect sparkgap switch and low inductance transmission line were designed and constructed to handle the increased current pulse. We will describe these systems and present initial test data from high current operation of the full-scale coax gun along with plans for future testing. Similar high current accelerator banks, switches, and TM lines will also be required to power PLX railguns which are planned to operate at 8000 $\mu g$ at 50 km/s. The design of that experiment may require the capacitor banks to be located as much as 10 feet from the gun. We discuss the available options for low inductance connections for these systems. [Preview Abstract] |
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UP9.00113: High Speed Jet Merging Studies In Support of PLX Andrew Case, Richard Bomgardner, Sarah Messer, Sam Brockington, Lin-Chun Wu, Ray Elton, F. Douglas Witherspoon Formation of an imploding plasma liner for the Plasma Liner Experiment (PLX) requires individual plasma jets to merge into a uniform shell of plasma converging on the target region. In order to understand the dynamics of the merging process, knowledge of the plasma phenomena involved is required. We present here results from a study of the merging of two and three plasma jets in two dimensional (coplanar) and three dimensional geometry. The experiments were performed using HyperV Technologies Corp. one centimeter bore MiniRailguns on a vacuum chamber designed to partially reproduce the port geometry of the PLX vacuum chamber. Diagnostics include fast imaging, spectroscopy, interferometry, deflectometry, fast pressure probes, B-dot probes, and high speed spatially resolved photodiodes, permitting measurements of plasma density, temperature, velocity, stagnation pressure, trapped magnetic field, and density gradients. These experimental results are compared with simulation results from the LSP 3D hybrid PIC code. [Preview Abstract] |
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UP9.00114: Modeling of plasma jet production from rail and coaxial guns for imploding plasma liner formation* R.J. Mason, R.J. Faehl, R.C. Kirikpatrick, D. Witherspoon, J. Cassibry We study the generation of plasma jets for forming imploding plasma liners using an enhanced version of the ePLAS implicit/hybrid model.$^1$ Typically, the jets are partially ionized D or Ar gases, in initial 3-10 cm long slugs at 10$^{16}$-10$^{18}$ electron/cm$^{3}$, accelerated for microseconds along 15-30 cm rail or coaxial guns with a 1 cm inter-electrode gap and driven by magnetic fields of a few Tesla. We re-examine the $B-$field penetration mechanisms that can be active in such wall-connected plasmas,$^2$ including erosion and EMHD influences, which can subsequently impact plasma liner formation and implosion. For the background and emitted plasma components we discuss optimized PIC and fluid modeling techniques, and the use of implicit fields and hybridized electrons to speed simulation. The plasmas are relatively cold ($\sim $3 eV), so results with fixed atomic Z are compared to those from a simple analytic EOS, and allowing radiative heat loss from the plasma. The use of PIC ions is explored to extract large mean-free-path kinetic effects. 1. R. J. Mason and C. Cranfill, IEEE Trans. Plasma Sci. \textbf{PS-14}, 45 (1986) 2. R. Mason, et al., Phys. Fluids \textbf{B, 5}, 1115 (1993). \\[4pt] *Research supported in part by USDOE Grant DE-SC0004207. [Preview Abstract] |
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UP9.00115: Numerical Studies of High-Z Plasma in the HyperV Plasma Guns Linchun Wu, Sarah Messer, F. Douglas Witherspoon, Dale Welch, Carsten Thoma, Mike Phillips, I. Nick Bogatu, Sergei Galkin, Joe MacFarlane, Igor Golovkin Numerical studies of railguns and coaxial guns at HyperV Technologies Corp. include simulations of hypervelocity plasma transport in the gun, plasma expansion out of the nozzle, and two or more jets merging in vacuum. Plasma detachment, merging jets temperature and charge state evolution are examined in these processes. High-Z materials, such as argon and xenon, are used throughout these simulations. The plasma moves with an initial velocity of 0-10 km/s (80-100 km/s for jet merging), the initial number density ranges from 10$^{15}$cm$^{-3}$ to 10$^{18}$cm$^{-3}$, and the merging jets are several centimeters in radius. The LSP code is used to perform the simulations using improved fluid algorithms and equation-of-state models from Voss and atomic data from Prism. [Preview Abstract] |
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UP9.00116: One-dimensional numerical modeling of imploding plasma liners Thomas Awe, David Hanna, Joshua Davis, Scott Hsu, Milos Stanic, Jason Cassibry The Plasma Liner Experiment (PLX) will generate high energy density laboratory plasmas (HEDLP) of unmatched size and lifetime (cm \& $\mu$s), with peak pressures near 1~Mbar, by merging 30--60 spherically convergent hypervelocity plasma jets. PLX is motivated by the interest in fundamental HEDLP science and the need for a standoff driver for magneto-inertial fusion (MIF). Plasma-liner-driven MIF may simultaneously create the magnetized D-T target and the high-Z liner using composite jets. 1D physics issues of PLX liner implosions are discussed, with focus on the scaling of stagnation pressure on liner initial conditions, and atomic physics effects. A goal is to develop scaling laws applicable to more energetic liners and higher peak pressures. 1D results from RAVEN (rad-MHD), HELIOS (rad-hydro with DCA), and SPHC (smoothed particle hydro) will address the above physics issues. Simulations on multiple platforms allow direct comparison between numerical algorithms and EOS and transport models. The pertinence of results to PLX experimental design and application to HEDLP and MIF is emphasized. [Preview Abstract] |
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UP9.00117: Idealized modeling of merging plasma jets in two dimensions using Nautilus John Loverich, Ammar Hakim PLX is a new experiment at LANL investigating imploding plasma liners formed via merging plasma jets. The imploding plasma liner is envisioned as a standoff driver for MIF. In this paper idealized merging argon plasma jets are simulated in 2D using both gas dynamic and MHD models using the Tech-X fluid plasmas code Nautilus. Results indicate that peak pressures of several hundred kilobar can be achieved for PLX-relevant jet parameters. Including a simple optically thin Bremsstrahlung radiation model and plasma targets shows that extremely high densities and magnetic fields can be achieved during jet merging on the order of 1000 times the initial density/field. This result is highly dependent on plasma radiation and therefore further investigations should include detailed ionization processes and multiple species. In addition, more accurate radiation modeling is also necessary to properly describe the radiation transport. At PLX-relevant Mach numbers and densities, atomic physics and radiation are also expected to be important during the jet merging process. [Preview Abstract] |
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UP9.00118: Theory and Modeling of the Plasma Liner Experiment (PLX) J.T. Cassibry, M.D. Stanic, T.J. Awe, D.S. Hanna, J.S. Davis, S.C. Hsu, F.D. Witherspoon High pressures and temperatures may be generated at the center an imploding plasma liner. These phenomena are being studied on the Plasma Liner Experiment (PLX) in which a spherical liner is formed via the merging of plasma jets. The basic physical processes include pulsed plasma acceleration, plasma jet propagation in a vacuum, plasma jet merging, liner formation, liner implosion, stagnation, and rarefaction. Each of these processes is dominated by different physics, requiring different models. For example, $\lambda _{ei}$ at the jet merging radius may be $\sim $1~cm, so that liner formation is partially collisionless, while liner implosion is collision dominated. Further, the liner transitions from optically thin to gray during the implosion. An overview of the theory and modeling plan in support of PLX will be given, which includes 1D rad-hydro, 3D hydro, 3D MHD, 2D PIC, and 2D hybrid codes. We will emphasize our recent 3D hydro modeling, which provides insights into liner formation, implosion, and effects of initial jet parameters on scaling of peak pressure. [Preview Abstract] |
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UP9.00119: Simulation of the Radiative Properties of Merging Plasma Jets in the Plasma Liner Experiment Joseph MacFarlane, I. Golovkin, P. Woodruff, C. Thoma, D. Welch, S.C. Hsu, F.D. Witherspoon In Plasma Liner Experiments (PLX) to be performed at Los Alamos National Laboratory, a set of merging plasma jets will be used as the source of an imploding plasma liner for magneto-inertial fusion studies. To develop a good understanding of the compression and heating of the merging jets, Thoma et al. (this meeting) are performing hybrid particle-in-cell (PIC)-fluid simulations using the LSP code, complementing other efforts using hydrodynamics and two-fluid codes. Prism has been collaborating with Voss Scientific in adding equation-of-state (EOS) and radiation physics algorithms into LSP. In addition, we have updated our SPECT3D Imaging and Spectral Analysis package to post-process LSP output to generate high-resolution spectra and images that can be compared with experimental measurements. Here, we discuss radiation physics simulations relevant to merging plasma jets, including the role of radiation losses for candidate gases. For the plasma conditions expected, we will explore potential spectral and imaging measurements suitable for diagnosing key physical parameters in the experiments. [Preview Abstract] |
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UP9.00120: Simulation of Formation and Implosion of Plasma Liners for Magneto-Inertial Fusion Roman Samulyak, Lingling Wu, Paul Parks Spherically symmetric simulations of the implosion of plasma liners and compression of plasma targets in the concept of plasma liner driven magneto-inertail fusion have been performed using the method of front tracking. Single and double-layer deuterium and xenon liners have been investigated as well as liners to be used in the PLX experiment. By varying target and liner parameters, the implosion process was optimized for maximum fusion energy gain and compared with theoretical predictions and scaling laws. In the most optimal setup, fusion ignition and energy gain of 10 was achieved with energy release of 10 GJ. 3D simulations of the propagation and merger of high Mach number deuterium jets and the formation of liners have also been performed using the FronTier code. The merger of 125, 144 and 625 jets have been simulated and the uniformity and Mach number reduction of the corresponding liners have been investigated. During late stages of the implosion, the Mach number of 3D liners was about half of that of spherically symmetric liners. [Preview Abstract] |
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UP9.00121: Hybrid-PIC Algorithms for Simulation of Merging Plasma Jets in the Plasma Liner Experiment Carsten Thoma, Dale Welch, Robert Clark, Joseph MacFarlane, Igor Golovkin, F. Douglas Witherspoon In the upcoming Plasma Liner Experiment (PLX) at Los Alamos National Laboratory a spherical array of 30-60 jets generated by plasma guns will be merged to form imploding plasma liners. We describe the Hybrid particle-in-cell (PIC) methods implemented in the code LSP for plasma jet simulation and present results of simulations of merging Ar jets. Electron macroparticles are treated as fluid elements which carry an intrinsic temperature while ion macroparticles are treated kinetically. The effective charge state is obtained from EOS tables as a function of the local plasma parameters under the assumption of local thermodynamic equilibrium (LTE). The effect of radiation cooling on the electrons is also included self-consistently into the Hybrid PIC formalism. The LSP results of jet merging simulations will be post-processed using the SPECT3D code to generate simulated radiation flux levels, spectra and images (MacFarlane et al., this meeting). [Preview Abstract] |
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UP9.00122: Plasma Jet Propagation and Stability Modeling for the Plasma Liner Experiment (PLX) J.R. Thompson, N.I. Bogatu, S.A. Galkin, J.S. Kim, D.R. Welch, C. Thoma, J.J. MacFarlane, F.D. Witherspoon, J.T. Cassibry, T.J. Awe, S.C. Hsu The Plasma Liner Experiment will explore the formation of imploding spherical ``plasma liners" that reach peak pressures of $\sim $0$.$1 Mbar upon stagnation. The liners will be formed through the merging of dense, high velocity plasma jets ($n\sim $10$^{17}$ cm$^{-3}$, $v\sim $50 km/s) in a spherically convergent geometry. The focus of this study is jet propagation and stability from the wall to the merging radius using analytic models and the Large Scale Plasma (LSP) code with atomic physics. We will discuss the dominant physics, including cooling due to adiabatic expansion, and charge neutralization involving the plasma equation of state (EOS), whose character transitions between local thermal equilibrium and time-dependent collisional-radiative dominance during the propagation. 1D, 2D and 3D jet propagation and stability modeling will be presented, identifying experimental parameters impacting jet expansion and stability. [Preview Abstract] |
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UP9.00123: Plasma-Jet Magneto-Inertial Fusion Burn Calculations John Santarius Several issues exist related to using plasma jets to implode a Magneto-Inertial Fusion (MIF) liner onto a magnetized plasmoid and compress it to fusion-relevant temperatures [1]. The poster will explore how well the liner's inertia provides transient plasma confinement and affects the burn dynamics. The investigation uses the University of Wisconsin's 1-D Lagrangian radiation-hydrodynamics code, BUCKY, which solves single-fluid equations of motion with ion-electron interactions, PdV work, table-lookup equations of state, fast-ion energy deposition, pressure contributions from all species, and one or two temperatures. Extensions to the code include magnetic field evolution as the plasmoid compresses plus dependence of the thermal conductivity on the magnetic field. \\[4pt] [1] Y.C. F. Thio, et al.,``Magnetized Target Fusion in a Spheroidal Geometry with Standoff Drivers,'' in Current Trends in International Fusion Research, E. Panarella, ed. (National Research Council of Canada, Ottawa, Canada, 1999), p. 113. [Preview Abstract] |
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UP9.00124: Laser-driven Beat-Wave Current Drive in Dense Plasmas with Demo on CTIX Fei Liu, Robert Horton, David Hwang, Ben Zhu, Russell Evans, Sean Hong, Scott Hsu The ability to remotely generate plasma current in dense plasmas hanging freely in vacuum in voluminous amount without obstruction to diagnostics will greatly enhance our ability to study the physics of high energy density plasmas in strong magnetic fields. Plasma current can be generated through nonlinear beat-wave process by launching two intense electromagnetic waves into unmagnetized plasma. Beat-wave acceleration of electrons has been demonstrated in a low-density plasma using microwaves [1]. The proposed PLX experimental facility presently under construction at Los Alamos offers the opportunity to test the method at a density level scalable to the study of HED plasmas. For PLX beat-wave experiments, $CO_2$ lasers will be used as pump waves due to their high power and tunability. For a typical PLX density $n_e=10^{17}cm^{-3}$, two $CO_2$ lasers can be separately tuned to 9P(28) and 10P(20) to match the 2.84THz plasma frequency. The beat-wave demo experiment will be conducted on CTIX. The laser arrangement is being converted to two independent single lasers. Frequency-tuning methods, optics focusing system and diagnostics system will be discussed. The laser measurements and results of synchronization of two lasers will be presented, and scaling to PLX experiments will be given. [1] Rogers, J. H. and Hwang, D. Q., PRL. v68 p3877 (1992). [Preview Abstract] |
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UP9.00125: Laser-driven magnetic flux compression simulation with Nautilus C.D. Zhou, J. Loverich, A. Hakim The Tech-X fluid and MHD framework, Nautilus, is a finite-volume and finite-element shock-capturing code supporting both structured and unstructured meshes. Nautilus is an easy-to-access, parallel, 3D code available to the ICF community. It is incorporated with features relevant to magneto-inertial confinement fusion (MIF) simulations. Laser-driven magnetic-flux compression is an innovative approach to achieve MIF. A cylindrical target with initial seed magnetic field is compressed by energetic laser beams. The magnetic field that is ``frozen-in'' plasma gets compressed with the target. The resulting high magnetic field reduces electron thermal conductivity and improves alpha particle confinement, thus providing an additional thermal insulation of the fuel forming the hot spot. It reduces the energy and driver symmetry requirements for ignition. Features for MIF simulations, such as anisotropic heat conduction , laser ray tracing and energy deposition, have been demonstrated with Nautilus. We discuss our approach to determine the best algorithms for properly modeling laser-driven shock implosions with magnetic fields in conditions relevant to cylindrical MIF. [Preview Abstract] |
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UP9.00126: Plasma Liner Compression of Merged FRCs George Votroubek, Chris Pihl, John Slough Creation of high energy density (HED) plasmas in the laboratory is a topic of ongoing interest in the area of magneto-inertial fusion. One innovative approach to HED plasma creation, and the one being explored in the Plasma Liner Compression (PLC) experiment, is through the compression of a field reversed configuration (FRC) plasma by an imploding plasma liner. Unlike other forms of liner compression, the plasma liner is driven inductively by theta-pinch solenoid coils rather than by an axial z-pinch current. This removes many of the design limitations that a direct electrical connection imposes. Liner masses of 3 mg have been formed in Xenon and have been accelerated to a velocity of 50 km/s. The experiment has been modified to include two theta-pinch FRC formation chambers at opposite ends of the plasma liner chamber. The two remotely formed FRCs are injected into the plasma liner chamber where they merge. Past experiments have shown this to be a reliable method of forming a hot, stable, and stationary FRC target suitable for plasma liner compression. Details of the liner dynamics and initial results of FRC merging in the plasma liner chamber will be discussed. [Preview Abstract] |
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UP9.00127: BEAMS AND COHERENT RADIATION |
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UP9.00128: Frequency upshift of terahertz radiation via flash ionization scheme Noboru Yugami, Takeshi Higashiguchi, Fuminori Suzuki, Masahiro Nakata, Takamitsu Otsuka When plasmas are instantaneously created around an electromagnetic wave, frequency of the wave up-converted to the frequency, which depends on the plasma frequency. This phenomenon is called as the flash-ionization. The theory requires the plasma creation in time much shorter than an oscillation period of the electromagnetic wave. We have demonstrated the proof of principle experiment using the interaction between a terahertz wave and plasmas created by an ultrashort laser pulse, which ensures the plasma creation time-scale much shorter than a period of electromagnetic source wave. We observed frequency up-conversion from 0.2 THz to 0.7--1.5 THz by the irradiance of the laser in ZnSe crystal. The up-conversion to the broad frequencies suggests that the flash ionization occurs in non-uniform plasmas produced in the deeper region of the crystal. The results support the potentiality of developing tunable and short pulse coherent radiation sources with a frequency THz spectral region. [Preview Abstract] |
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UP9.00129: Tunnel Ionization by Two Linearly Polarized Lasers: Generation of THz Radiation Anil Malik, Hitendra Malik, Ulrich Stroth The THz radiation generation has received great deal of attention due to its diverse applications in the field of material characterization, topography and remote sensing, chemical and security identification$^{ }$etc. In order to develop high-power and efficient THz sources, several schemes have been proposed. For example, THz radiation can be generated by superluminous laser pulse interaction with large band gap semiconductors and electro-optic crystals, on synchrotron radiation from bunched electron beams etc. In the present investigation, an analytical study is made for the THz radiation generation based on tunnel ionization achieved by superposition of two linearly polarized femtosecond laser pulses focused on the gas after passing through an axicon. These lasers are considered to have the same frequencies but different field amplitudes and phases. In this mechanism, the initial phase of the envelope plays an important role in optimizing the rate of ionization and subsequently the residual current due to dipole oscillations. Since a nonuniform plasma is produced during the tunnel ionization, the effect of radial variation in the electron density in the plasma channel is studied on the frequency of the THz radiation and on its power. [Preview Abstract] |
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UP9.00130: Analytical and Simulation Study of Laser-Ionized Plasma George Rodriguez, Natalia Krasheninnikova, Mark Schmitt Terahertz generation inside ionization filaments produced by self-focusing of high-intensity short laser pulses in the atmosphere is a complex multi-dimensional nonlinear phenomena [1]. We are modeling these effects in 3-D using the PULSE [2] code that includes both ionization and nonlinear optics physics models. The relative ionization effects from multi-photon, tunneling and collision-driven avalanche ionization have been investigated. Nonlinear optics effects include Kerr self-focusing and self-phase modulation, group velocity dispersion, plasma defocusing and inverse Bremsstrahlung. To validate our models and facilitate a more complete understanding of the physical mechanisms at play, we have studied the effects of experimental parameters including gas species, gas pressure, and laser pulse energy on filament generation. A detailed description of the model and a comparison of our simulation results with experimental results are presented. \\[4pt] [1] M. J. Schmitt, J. Opt. Soc. Am. B 20 719 (2003). \\[0pt] [2] K. Y. Kim et al., Nat. Photonics 2 605-609 (2008). [Preview Abstract] |
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UP9.00131: Efficiency Study of a Terahertz-Range Gyrotron Ruifeng Pu, Oleksandr Sinitsyn, Gregory Nusinovich This study is part of an effort toward designing a 300 kW gyrotron oscillator operating at 670 GHz. Three configurations of cavities designed for operation at the TE31,8 -- mode were considered. Cavity design studies have been performed by using the self-consistent code MAGY, with the account for ohmic losses and velocity spread. It was found that the maximum efficiencies in these three cavities reached 23{\%}, 34{\%} and 35{\%}, respectively. Details of calculation will be presented at the meeting. [Preview Abstract] |
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UP9.00132: Investigation of a 94 GHz Pseudospark-Sourced Klystron David Bowes, Huabi Yin, Adrian Cross, Wenlong He, Alan Phelps, Kevin Ronald, Xiaodong Chen, Daohui Li Work is ongoing at the University of Strathclyde to design, construct and test a klystron amplifier with an operating frequency of 94 GHz using a pseudospark discharge beam source. A pseudospark is a form of low-pressure, high-current discharge which produces an electron beam possessing high brightness and self-focusing properties during the final stages of the discharge. Single-gap pseudospark experiments were performed in order to assess the feasibility of scaling down in size previous work carried out at the University of Strathclyde. Using 1mm cathode and anode apertures with 3mm separation, a 10kV voltage applied at a pressure of 100mTorr resulted in a 4 A beam current, which compares favourably with previous discharges. A 3-cavity 94 GHz klystron has been simulated with the particle-in-cell code MAGIC-2D with pulsed voltage and current inputs in keeping with possible pseudospark discharge traces. Efficiency and gain were simulated at 7.38{\%} and 16.5 dB respectively. [Preview Abstract] |
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UP9.00133: Multi-Megawatt Operation of a 95 GHz Gyrotron Kevin Felch, Monica Blank, Philipp Borchard, Pat Cahalan, Steve Cauffman A 95 GHz gyrotron capable of generating CW power levels in excess of 2 MW is under development at CPI. The gyrotron employs a single-anode electron gun that produces a 90-kV, 75-A electron beam for interaction with the TE$_{22,6,1}$ cavity mode. Output power from the cavity is transformed into a Gaussian beam using an internal converter that consists of a waveguide launcher and three focusing and phase-correcting mirrors. The beam exits the tube through a chemical-vapor-deposition diamond window. The spent electron beam is dissipated in a 40.6-cm diameter collector fabricated from a strengthened copper alloy. The collector nominally operates at a voltage of 61 kV relative to the cathode potential to minimize the amount of power absorbed in the collector and to improve efficiency. In tests on the gyrotron the maximum power achieved thus far is 1.72 MW with 33{\%} efficiency at the nominal beam current of 75 A. At a reduced beam current of 45 A, an output power of 1.4 MW is obtained with over 50{\%} efficiency. We are planning tube modifications to increase the output power level where efficiencies of greater than 50{\%} can still be obtained. [Preview Abstract] |
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UP9.00134: Experimental Research on a 1.5 MW, 110 GHz Gyrotron with a Smooth Mirror Internal Mode Converter D.S. Tax, I. Mastovsky, M.A. Shapiro, R.J. Temkin, A.C. Torrezan Megawatt gyrotrons are important for electron cyclotron heating (ECH) of fusion plasmas, including ITER. These gyrotrons should operate with high efficiency to reduce the prime power required and to ensure good reliability. The gyrotron efficiency is affected both by the physical principles that govern the device and the performance of components like the internal mode converter (IMC), which must convert the electromagnetic cavity mode into a Gaussian beam. An IMC consisting of a helically-cut launcher and three smooth curved mirrors, which is less susceptible to alignment errors than an IMC using mirrors with phase correcting surfaces, was recently tested on a 1.5 MW, 110 GHz, 3$\mu $s pulsed gyrotron operating in the TE$_{22,6}$ mode, and an output beam with 95.8 $\pm $ 0.5 {\%} Gaussian beam content was measured in both hot and cold tests. We are also examining the issue of mode competition in the gyrotron, which can limit the achievable output power and efficiency. The sequence of competing modes excited during the rise time of the voltage pulse has been measured and results are compared with the numerical simulation code MAGY. These results should provide a good test of the accuracy of the code. [Preview Abstract] |
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UP9.00135: High Power Microwave Gas Breakdown Experiments at 110 GHz Alan Cook, Martin Goycoolea, Michael Shapiro, Richard Temkin We report the results of measurements of breakdown in gases induced by megawatt-level W-band radiation, covering the pressure range 5 - 760 Torr. A microwave beam, generated by a 1.5 MW, 110 GHz gyrotron with a 3-microsecond pulse duration, is focused by a lens into a pressurized chamber at a peak intensity of 5 MW/cm$^2$. Breakdown takes place in the volume of gas, without any field-enhancing objects to initiate the discharge. Data is taken in air and argon. Breakdown threshold data is seen to follow a Paschen-type curve of E vs. p, similar to DC breakdown, having a minimum threshold level at a pressure of about 40 Torr in air and 80 Torr in argon. The spatial structure of the breakdown plasma changes from a diffuse, uniform discharge at low pressure to a periodic array of quarter-wavelength-spaced filaments at high pressure. The transition in observed structure coincides with the measured transition between the low-pressure, diffusion-controlled and the high-pressure, collisional breakdown regimes. [Preview Abstract] |
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UP9.00136: Distributed Discharge Limiter Studies for X-band High Power Microwaves David Holmquist, John Scharer, Matt Kirley, B. Kupczyk, John Booske We have constructed and carried out measurements to study rapid gas breakdown with high power X-band microwave radiation. The microwave source is a 25 kW, 9.38 GHz magnetron with a 0.8 microsec pulse width. The discharge chamber is a section of WR-650 (L-band) waveguide with Lexan vacuum windows. Pumping, flanging, tubulation and gas flow control allow pressure variation from below 0.1 to 760 torr with a variety of gas species. Microwaves are transmitted into the test chamber from the output of a WR-90 (X-band) waveguide. Using HFSS simulations, the maximum electric field intensity on the chamber's interior side of the vacuum window is indicated to be 4.1 kV/cm, well above the breakdown for many gases. Experiments using 22 kW pulses have demonstrated that rapid plasma discharges can be obtained between 220-250 torr in Ar and Ne. Discharges form with an 18 ns turn-on delay with 7-14 ns rise times dependent on pressure and have sufficient density and extent to attenuate the transmitted power by 28 dB in neon at 240 torr. The high collisionality $\nu \sim 10^{11} $ s$^{-1}$ in these gases at high pressures allows substantial attenuation of the signal at substantial plasma densities $n_e \sim 10^{12}$ cm$^{-3}$. [Preview Abstract] |
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UP9.00137: Microwave-Plasma Window Experiments and Theory Matthew Franzi, Peng Zhang, Ronald Gilgenbach, Y.Y. Lau, Andrew McKelvey The development of modern High Power Microwave (HPM) sources necessitates protections against damage to sensitive electronic equipment. The primary goal of this research is an electrically transparent window that optimizes transmission of desired low power signals and generates plasma to absorb or reflect potentially damaging HPM pulses. Current experiments at UM attempt to exploit multipactor, inducing surface plasma flashover when exposed to the RF electric fields of HPM. Theory and experiments will characterize conditions necessary to initiate multipactor as well as competing mechanisms such as collisional ionization. Two experimental setups are being investigated: 1) relativistic magnetron at 100 MW and 1.04 GHz, and 2) CW, 1-KW, source at 2.45 GHz. Each experimental system utilizes cross-polarized, interdigitated-copper fingers on the window's surface, capable of a DC bias (0-750V), to seed breakdown and enhance ambient fields. [Preview Abstract] |
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UP9.00138: Negative, Positive, and Infinite Mass Properties in a Rotating Electron Beam David French, Y.Y. Lau, R.M. Gilgenbach, Brad Hoff Electrons rotating under a general combination of axial magnetic field B and radial electric field E have an effective mass in the azimuthal direction which can be positive, negative, or infinite depending upon the magnitude and sign of E [1]. No prior simulations have systematically studied such properties. This work was also motivated by our recent invention of the recirculating planar magnetron [2], where rapid start up utilizes the negative mass instability in the inverted magnetron configuration (that has a positive E). The transition between positive and negative mass is the infinite mass case where the electrons hardly respond to an azimuthal electric field. We present the results of particle-in-cell simulations using MAGIC for general E and B showing positive, negative, and infinite mass behavior of the electron beam. \\[4pt] [1] D. Chernin and Y. Y. Lau, Phys. Fluids 27, 2319 (1984). \\[0pt] [2] R. M. Gilgenbach et al., in this conference; also, Proc. IVEC, p. 507 (2010). [Preview Abstract] |
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UP9.00139: Recirculating Planar Magnetron Modeling and Experiments Ronald Gilgenbach, Matthew Franzi, David French, Brad Hoff, Y.Y. Lau We present simulations and initial experimental plans of a new class of crossed field device: Recirculating Planar Magnetrons (RPM).\footnote{Patent pending} Two geometries of RPM are being explored: 1) Two planar-magnetrons connected by a recirculating section with axial magnetic field and transverse electric field, and 2) Planar cathode and anode-cavity rings with radial magnetic field and axial electric field. These RPMs can be configured in either the conventional or inverted (faster startup) configurations and have numerous advantages for high power microwave generation by virtue of larger area cathodes and anodes. Two and three-dimensional EM PIC simulations show rapid electron spoke formation and microwave oscillation in pi-mode. RPM experiments are planned for the MELBA accelerator at parameters of -300 kV, 1-10 kA and pulselengths of 0.5-1 microsecond. [Preview Abstract] |
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UP9.00140: Buneman-Hartree Condition Revisted D.H. Simon, Y.Y. Lau, R.M. Gilgenbach, W. Tang, B. Hoff, K.L. Cartwright, J.W. Luginsland The Buneman-Hartree (B-H) condition is re-examined in a cylindrical relativistic magnetron using both the conventional, single particle model, and the Brillouin flow model. These two models yield the same result for the B-H condition only in the limit of a planar magnetron. When $b/a$ = 1.3, where $a$ is the cathode radius and $b$ ($>a)$ is the anode radius, the difference in the two models becomes significant. When $b/a$ = 4, the difference is acute, the B-H magnetic field, at a given voltage, in the Brillouin flow model exceeds four times that in the single particle model. Such a difference is always present, whether the voltage is relativistic or not. These results are quantified for $b/a>>$ 1 using Davidson's model [1], conveniently cast in terms of the normalized gap voltage and normalized magnetic flux imposed on the cylindrical magnetron [2]. Comparison with experiments will be reported.\\[4pt] [1] R. C. Davidson \textit{et al}., Proc. SPIE \textbf{1061}, 186 (1989). \\[0pt] [2] Y. Y. Lau \textit{et al}., Phys. Plasmas \textbf{17}, 033102 (2010). [Preview Abstract] |
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UP9.00141: Peer-to-Peer Locking of Magnetrons Edward Cruz, Ian Rittersdorf, Y.Y. Lau, Ron Gilgenbach Coherent power combining of multiple high-efficiency, moderate power magnetrons might be achieved via peer-to-peer locking, in which each magnetron acts as a master of and a slave to all others [1]. In peer-to-peer locking of two magnetrons, dependent on the degree of coupling, the frequency of oscillation when locking occurs does not necessarily lie between the two magnetrons' free running frequencies. Likewise, when the locking condition was violated, the beat frequency is not necessarily the difference between the free running frequencies. These features were observed in our experiments on the peer-to-peer locking of two 1-kW magnetrons [2]. The stability (accessibility) and temporal evolution of the two possible states, together with phase measurements, will be reported. Our theory shows that peer-to-peer locking in the presence of a frequency chirp may be approximately achieved, if the locking condition [1] is satisfied locally. \\[4pt] [1] P. Pengvanich et al., Phys. Plasmas 15, 103104 (2008). \\[0pt] [2] E. Cruz et al., Appl. Phys. Lett. 95, 191503 (2009). [Preview Abstract] |
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UP9.00142: Input Pulse Response in Nonlinear Transmission Line Ian M. Rittersdorf, David M. French, Michael Johnson, Y.Y. Lau, Ronald M. Gilgenbach, Donald Shiffler, Brad Hoff, John Luginsland Nonlinear transmission lines have been demonstrated to be an effective technique for generating high power ultrawideband or mesoband radiation without the need for a vacuum system, electron beam, or magnet. Experiments have been performed at UM on a discrete element nonlinear transmission line that uses ferrite inductors as the nonlinear element. Pulse sharpening of multi-kA input pulses has been observed in addition to the generation of oscillations at the characteristic LC frequency. The response to various input pulses to excite a single inductor on a single stage of the nonlinear transmission line, which may consist of a linear capacitor and a nonlinear inductor, will be modeled. Also studied is the pulse response to a nonlinear telegraphic equation. Results of these modeling efforts as well as experimental results performed at UM will be presented. [Preview Abstract] |
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UP9.00143: Two-dimensional particle-in-cell simulations of a magnetron with strapping and extraction modeled via circuit elements Keith Cartwright, Wilkin Tang, Tim Fleming, Mike Lambrecht A two-dimensional (2-D) magnetron model has been developed to investigate the effects of strapping and extraction on operational parameters of the device, including oscillating frequency and mode separation. The effort began with a study of three-dimensional (3-D) electromagnetic (EM) particle-in-cell (PIC) simulations of magnetrons that indicated a uniform electric field distribution along the axial extent of the slow wave structure (SWS). This uniformity led to the creation of a representative 2-D model consisting of a SWS cross-section that approximately represents the entire SWS cavity, and required significantly less computation time. The model was then augmented with circuit elements to represent both strapping and extraction. The phase velocity and characteristic impedance for both 2-D and 3-D models compared favorably, validating the 2-D approximation. The validated 2-D model is now being used to study the influence of strapping and extraction on magnetron behavior through variation of circuit element values, and will provide insight and guidance for future magnetron design. [Preview Abstract] |
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UP9.00144: Electric Field Screening of Dual Tungsten and Dual Carbon Fiber Cathodes Wilkin Tang, D. Shiffler, K. Cartwright, K. Golby, M. LaCour, Y.Y. Lau Field emitter arrays have the potential to provide high current density, low voltage operation, and high pulse repetition for radar and communication. The emitted current from an individual emitter could be affected by the field screening effect caused by the close proximity of its neighbors. Previous experiments have shown a degradation of current density when the packing density of the field emitter arrays is too high or too low. These experiments were conducted with 1000s of thin film field emitters. Here we describe experiments in a dual-cathode configuration. The experiments used only two field emitters with variable spacings so as to scrutinize the effects on field emission from an immediate neighbor. Analytic model and Particle-in-cell simulations are performed to compare with the experiments that used tungsten and carbon fiber cathodes. [Preview Abstract] |
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UP9.00145: An Improved Self-consistent Fitting Model for Characterizing Field Emitters M.C. Lin, J.P. Verboncoeur Field emission of electrons has attracted a lot of attention due to its wide applications in vacuum nano-electronics as well as understanding the fundamental surface physics. In recent experiments of field emission arrays (FEAs), the Fowler-Nordheim (FN) plots were found to become nonlinear and apart from the prediction of the conventional FN theory. A self-consistent model including the space charge and within the framework of an effective work function approximation demonstrated a good fit to the I-V characteristics of FEAs. Although the theory showed good agreement with the experiments and indicated that space charge effects cause the saturation, a constant effective work function is not enough to characterize all the emission surface properties as one can see a discrepancy in the transition region. A later study employing the effective enhancement factor model showed that geometric enhancement effects are strongly dependent on the applied fields. In this work, we employ both effective work function and effective enhancement factor approximations to improve the fitting model for better characterizing field emitters. [Preview Abstract] |
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UP9.00146: Controlling electron-beam emittance partitioning for future X-Ray light sources Nikolai Yampolsky, Bruce Carlsten, Kip Bishofberger, Leanne Duffy, Steven Russell, Robert Ryne Future hard X-ray free electron lasers (FEL) producing photons with energies significantly higher than 10 keV require electron beams having high transverse brightness. Typically, the constraints on the transverse beam emittance are much more stringent than limitations on the longitudinal beam emittance. Currently used photoinjectors produce electron beams having the same order of magnitude longitudinal and transverse emittances. Even though, the 6-D phase volume of such a beam can be smaller than what is required for FEL lasing, the partitioning between different emittances is not optimal. Poor overlap between radiation and electron phase spaces reduces FEL efficiency and even suppresses lasing. The quality of the electron beam can be described by three eigen-emittances which remain constant under linear beam optics transforms. Manipulating the beam cross-correlations at the photocathode surface allows one to affect partitioning between eigen-emittances. Once the cross-correlations are removed downstream, the eigen-emittances become the actual beam emittances. Therefore, the eigen-emittance partitioning can be used to generate extraordinarily transversely bright electron beams. We demonstrate that transverse-axial emittance partitioning can be performed using the generalized concept of the flat-beam transform. [Preview Abstract] |
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UP9.00147: Towards Recombination Pumped H-Like N X-Ray Laser I. Gissis, A. Lifshitz, A. Rikanati, I. Be'ery, U. Avni, A. Fisher, E. Behar The recombination pumping scheme for soft X-Ray lasers has better energy scaling, than the collisional-excitation pumping scheme. Implementation of an H-like $3\to 2$ Nitrogen recombination laser, at $\lambda \sim $13.4 nm requires initial conditions of 50{\%} fully stripped Nitrogen, kTe$\sim $140eV and n$_{e}\sim $10$^{20}$cm$^{-3}$. The cooling period to below 60eV should be faster than the typical three-body recombination time in order to reach population inversion. Our study aims at achieving the required plasma conditions using a capillary discharge Z-Pinch apparatus. The experimental setup includes capillaries in different radii, coupled to a pulsed power generator of $\sim $60 kA peak current, with a quarter-period of 60 ns. Various diagnostic techniques were applied to measure the plasma conditions, including X-Ray diodes and time-resolved pinhole imaging. Here we introduce time resolved spectroscopic measurements analysed with a multi-ion collisional-radiative model, from which we derive the plasma temperature temporal evolution. The analysis shows a rapid cooling period to below 60eV, demonstrating the feasibility of recombination-pumped capillary discharge lasers. [Preview Abstract] |
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UP9.00148: Revisiting the ion-channel laser for the production of coherent betatron radiation Xavier Davoine, Joana Martins, Frederico F\'Iuza, Ricardo Fonseca, Warren Mori, Luis Silva The Ion-Channel Laser (ICL) [1] is a possible alternative approach to Free Electron Lasers to produce coherent and high intensity light at different wavelengths taking advantage of an ion channel, which can be created in a laser or plasma wakefield accelerator. After comparing the ICL and conventional FEL, we show with PIC simulations that, under certain conditions, the betatron radiation emitted by the electrons oscillating in the ion channel can bunch the electron beam at the radiation wavelength, and coherent radiation is generated and amplified. The power of the coherent radiation increases exponentially until a saturation level. This is possible due to boosted frame simulations in the electron bunch frame, which are three orders of magnitude faster than simulations in the lab frame, and can avoid numerical instabilities associated with numerical Cerenkov radiation. [1] D. H. Whittum et al., Ion-Channel Laser, Phys. Rev. Lett. 64, 2511 (1990) [Preview Abstract] |
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UP9.00149: Short-pulse laser amplification and saturation using stimulated Raman scattering E.S. Dodd, J. Ren, T.J.T. Kwan, M.J. Schmitt, P.B. Lundquist, S. Sarkisyan, E. Nelson-Melby Recent theoretical and experimental work has focused on using backward-stimulated Raman scattering (BSRS) in plasmas as a means of laser pulse amplification and compression [1,2,3]. We present initial computational and experimental work on SRS amplification in a capillary-discharge generated Xe plasma. The experimental set-up uses a 200 ps pump pulse with an 800 nm wavelength seeded by a 100 fs pulse from a broadband source and counter-propagates the pulses through a plasma of length 1 cm and diameter 0.1 cm. Results from initial experiments characterizing the plasma and on short-pulse amplification will be presented. Additionally, we present results from calculations using pF3d [4], and discuss the role of SRS saturation and determine the possible significance of electron trapping with a model implemented in pF3d [5]. [1] G. Shvets, N. J. Fisch, A. Pukhov, and J. Meyer-ter-Vehn, \textit{Phys. Rev. Lett.} \textbf{81} 4879 (1998). [2] V. M. Malkin, G. Shvets, and N. J. Fisch, \textit{Phys. Rev. Lett.} \textbf{82} 4448 (1999). [3] R. K. Kirkwood, E. Dewald, and C. Niemann, \textit{et al.}, \textit{Phys. Plasmas} \textbf{14} 113109 (2007). [4] R. L. Berger, B. F. Lasinski, T. B. Kaiser, \textit{et al.}, \textit{Phys. Fluids B} \textbf{5 }2243 (1993). [5] H. X. Vu, D. F. DuBois, and B. Bezzerides, \textit{Phys. Plasmas} \textbf{14} 012702 (2007). Supported by US DOE and LANS, LLC under contract DE-AC52-06NA25396. LA-UR-10-04787 [Preview Abstract] |
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UP9.00150: High Harmonic Generation from Solid Targets at High Repetition Rate James Easter, Aghapi MOrdovanakis, Bixue Hou, Alec Thomas, John Nees, Karl Krushelnick Harmonics of 800 nm light up to the 18th order are generated from solid targets by focusing 2 mJ, 50 fs pulses to a spot size of 1.7 $\mu $m (FWHM). This is the first demonstration of high-harmonic generation with a very short focal length paraboloid (f /1.4). The harmonics have a low divergence ($<$ 4{\_}) compared to the driving beam and conversion efficiencies ($>$ 10-7 per harmonic) comparable to gas harmonics. No contrast enhancement techniques are employed and the system is capable of operating at 500 Hz. [Preview Abstract] |
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UP9.00151: Injection of super-Alfv\'{e}nic ion beams in a magnetized plasma of the Large Plasma Device Shreekrishna Tripathi, Walter Gekelman, Patrick Pribyl An ion beam source (25 kV, 1.0 A) has been built for performing fusion-relevant studies on fast-ion interaction with a magnetized plasma in the Large Plasma Device (LAPD). The ion beam source comprises a plasma source, accel/deaccel grids, magnetic focusing coils, and neutral pumping system. The plasma is produced by a 0.4--0.6 MHz, 25 kW inductive RF source. A multi-aperture grid system extracts the ion beam (rep rate: 0.3-1.0 Hz, pulse-width: 0.5-1.5 ms). The beam profiles were measured at several axial locations (up to 12 m from the deaccel grid) using a fast-ion collector. These measurements confirm the production of a low-divergence ion beam that forms helical orbits during oblique injection ($\theta < 6^0$). The source fulfills several requirements (e.g., minimal charge-exchange loss of fast-ions) that are specific to its operation on the LAPD. Apart from discussing the development of the fast ion-source, we present initial results on the wave-field spectra excited by hydrogen and helium ion beams in sub-Alfv\'{e}nic and super-Alfv\'{e}nic propagation regimes. [Preview Abstract] |
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UP9.00152: Very Low velocity Ion Slowing Down in Binary Ionic Mixtures Patrice Fromy, Bekbolat Tashev, Claude Deutsch Binary ionic mixtures (BIM) in dense and hot plasmas of specific concern for inertial confinement fusion (ICF) and white dwarf crust are considered as targets for incoming light ions with a velocity smaller than thermal electron one in target.The given BIM formalism worked out within a dielectric approach [1] is specifically investigated in terms of charge-and mass-asymmetry in the target BIM components.Results are scanned w.r.t density, temperature and relative BIM composition as well.A certain attention is paid to the so-called critical regime when target electron stopping equals the target ion contribution. \\[4pt] [1] B.Tashev et al, Phys.Plasmas 15,102701(2008) and NIMA 606,218(2009) [Preview Abstract] |
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UP9.00153: Development of 2D Ionization Chamber for Particle Therapy Chi-Li Ho Recently rapid growth in particle therapy has imposed new detector developments for quality assurance (QA) purposes. Relatively high dose deposition from proton or carbon ion beam requires stringent performance from detectors. Cyclotron, synchrotron, or future laser driven accelerators represent DC and pulse sources which need different timing designs in electronics. Dose and position measurements of particle beams can serve clinical, beam diagnosis and QA purposes. We categorize the parameters of detectors - electronics, DAQ, and simulation--to serve for later optimizations. Gaseous detector is preferred because of its radiation hardness, and operation at ionization mode provides stable condition. We present our progresses in 2D ionization chamber development. Basic studies are carried out on 1D ionization chamber, which consists of 16 6-mm-wide strips. Results of measuring electron and proton beams are compared with MC simulations. 2D array ionization chamber design is optimized and based on 1D prototype measurements. [Preview Abstract] |
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UP9.00154: Kinetic Description of the Longitudinal Dynamics of Intense Charged Particle Beams with Strong Self-Fields* Ronald C. Davidson, Edward A. Startsev, Hong Qin A kinetic model is developed that describes self-consistently the longitudinal dynamics of a long, coasting beam propagating in straight (linear) geometry in the z-direction in the smooth-focusing approximation. Making use of the three-dimensional Vlasov-Maxwell equations, and integrating over the phase space $(x_\bot ,p_\bot )$ transverse to beam propagation, a closed system of equations is obtained for the nonlinear evolution of the longitudinal distribution function $F_b (z,p_z ,t)$ and average axial electric field $\left\langle {E_z^s } \right\rangle (z,t)$. The primary assumptions in the present analysis are that the dependence on axial momentum $p_z $ of the distribution function $f_b (x,p,t)$ is factorable, and that the transverse beam dynamics remains relatively quiescent (absence of transverse instability or beam mismatch). The analysis is carried out assuming that axial spatial variations are weak over a length scale comparable to the conducting wall radius $r_w $. A closed expression for the average longitudinal electric field expressed in terms of geometric factors, the line density $\lambda _b $, and its derivatives $\partial \lambda _b /\partial z$, is presented for the class of bell-shaped density profiles. The general formalism described here is valid for the entire range of beam intensities. The properties of the solitary-wave structures are also investigated. *Research supported by the U. S. Department of Energy. [Preview Abstract] |
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UP9.00155: Collective Focusing of Intense Ion Beam Pulses for High Energy Density Physics Applications Mikhail A. Dorf, Igor D. Kaganovich, Edward A. Startsev, Ronald C. Davidson The collective focusing concept in which a weak magnetic lens provides strong focusing of an intense ion beam pulse carrying an equal amount of neutralizing electron background is investigated by making use of advanced particle-in-cell (PIC) simulations and reduced analytical models. The original analysis by Robertson [Phys. Rev. Lett.\textbf{ 48}, 149 (1982)] is extended to the parameter regimes of particular importance for several high energy density physics applications. The present paper investigates: (a) the effects of a moderately strong magnetic field,\textit{ $\omega $}$_{ce}\ge $\textit{$\omega $}$_{pe}$; (b) suppression of the applied magnetic field due to the presence of the beam self-fields generated when $r_{b}\ge $\textit{c/$\omega $}$_{pe}$; and (c) the influence of a finite-radius conducting wall around the beam cross section. Here,$ r_{b}$ is the beam radius, $c$ is the speed of light, and\textit{ $\omega $}$_{ce}$ and\textit{ $\omega $}$_{pe}$ are the electron cyclotron and electron plasma frequencies, respectively. [Preview Abstract] |
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UP9.00156: Masking the Paul Trap Simulator Experiment (PTSX) Ion Source to Modify the Transverse Distribution Function and Study Beam Stability and Collective Oscillations E.P. Gilson, R.C. Davidson, M. Dorf, P.C. Efthimion, R. Majeski, E.A. Startsev, H. Wang A variety of masks were installed on the Paul Trap Simulator Experiment (PTSX) cesium ion source in order to perform experiments with modified transverse distribution functions. Masks were used to block injection of ions into the PTSX chamber, thereby creating injected transverse beam distributions that were either hollow, apertured and centered, apertured and off-center, or comprising five beamlets. Experiments were performed using either trapped plasmas or the single-pass, streaming, mode of PTSX. The transverse streaming current profiles clearly demonstrated centroid oscillations. Further analysis of these profiles also shows the presence of certain collective beam modes, such as azimuthally symmetric radial modes. When these plasmas are trapped for thousands of lattice periods, the plasma quickly relaxes to a state with an elevated effective transverse temperature and is subsequently stable. Both sinusoidal and periodic step function waveforms were used and the resulting difference in the measured transverse profiles will be discussed. [Preview Abstract] |
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UP9.00157: Studies of Collective Dynamics and Excitations in Intense Charged Particle Beams and Barium Ion Source Optimization Using the Paul Trap Simulator Experiment Hua Wang, E.P. Gilson, R.C. Davidson, P.C. Efthimion, R. Majeski, L. D'Imperio The Paul Trap Simulator Experiment (PTSX) is a cylindrical Paul trap that simulates the nonlinear transverse dynamics of intense charged particle beam propagation through an equivalent kilometers- long magnetic alternating-gradient (AG) focusing system. Understanding the collective dynamics and excitations of intense charged particle beam propagation is of great importance for a wide variety of accelerator applications. Envelope equations which describe the evolution of a Kapchinskij-Vladimirskij (KV) beam envelope will be discussed and experimental measurements of collective excitations will be compared with the theoretical results. A barium ion source is being developed to replace the cesium ion source in order to use a laser-induced fluorescence (LIF) diagnostic to study the ion density transverse profile and ion velocity distribution function. A schematic design and the planned characteristic operating parameters of this new barium ion source will be presented. [Preview Abstract] |
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UP9.00158: Realistic Simulation of NDCX-II W.M. Sharp, A. Friedman, D.P. Grote, R.H. Cohen, S.M. Lund, M.A. Leitner, J.-L. Vay, W.L. Waldron The Virtual National Laboratory for Heavy-Ion Fusion Science is now constructing NDCX-II, an accelerator facility for studying ion-heated warm dense matter and aspects of ion-driven targets for inertial-fusion energy. Plans call for using twelve or more induction cells to accelerate 30-50 nC of Li$^{+}$ ions to 1.2-3 MeV. Plasma neutralization will enable compression of the beam to the sub-millimeter radius and sub-nanosecond duration needed for the desired target experiments. The initial NDXC-II physics design was developed using idealized analytic waveforms. Acceleration schedules were first worked out with a fast-running 1-D particle-in-cell code ASP (Acceleration Schedule Program), then 2-D and 3-D Warp simulations were used to verify the 1-D model, design transverse focusing, and establish tolerances for beam and lattice errors. As part of recent work to refine and validate this physics design, the idealized waveforms in the simulations have been replaced by experimentally measured ones. ASP and Warp results obtained with these realistic waveforms are compared with those from earlier simulations, and ongoing work to optimize the acceleration schedule is discussed. [Preview Abstract] |
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UP9.00159: Simulations of Ion Beam Heated Targets on NDCX II J.J. Barnard, A. Friedman, L.J. Perkins, F.M. Bieniosek, M.J. Hay, E. Henestroza, B.G. Logan , R.M. More, P.A. Ni, S.F. Ng, S.S. Yu, S.A. Veitzer The Neutralized Drift Compression Experiment II (NDCX II) is an induction accelerator now being constructed at LBNL and scheduled for project completion in 2012. The design calls for a $\sim $2 - 3 MeV, $\sim $30 A Li$^{+}$ ion beam, delivered in a bunch with sub ns pulse duration, and transverse dimension less than $\sim $ 1 mm. The purpose of NDCX II is to carry out experimental studies of material in the warm dense matter regime and ion beam and hydrodynamic coupling experiments relevant to heavy ion fusion (HIF). In preparation for NDCX-II, we have carried out hydro simulations of ion-beam-heated, porous and solid, metallic and non-metallic, targets. We have shown the sensitivity of observables on equations of state. Pulse formats include single pulses of fixed ion energy, and and single or double pulses with variable energy to create shocks and investigate ion-coupling efficiency. Comparisons are made with simulations of ion driven direct drive HIF capsules. [Preview Abstract] |
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UP9.00160: A high current, low emittance Li$^{+}$ alumino-silicate ion source and injector Prabir K. Roy, Joe W. Kwan, Peter A. Seidl, Wayne Greenway, Dave P. Grote, Jerry Kehl, Matthaeus Leitner, William Sharp, Jeff Takakuwa, Jean-Luc Vay, William Waldron, James K. Wu We will present the design of a Li$^{+}$ ion source and injector for the Neutralized Drift Compression Experiment-II (NDCX-II) for warm dense matter experiments. The injector has been designed to use a large diameter ($\approx $ 11 cm) Li$^{+}$-doped alumino-silicate source with an injected ion kinetic energy of 100 keV, pulse duration of 0.5$\mu $s, and beam current of 100mA. Using small prototype emitters, at a temperature of approximately 1275\r{ } C, the space charge limited Li$^{+}$ beam current density of J $\approx $ 1 mA/cm$^{2}$ was obtained for a 0.64 cm diameter emitting area. The lifetime of the ion source is $\ge $ 50 hours while pulsing the extraction voltage at 2 to 3 times per minute (a rate expected in NDCX-II). We are designing and fabricating a larger diameter source, in parallel with continuing R {\&} D effort to increase the life time of the ion source. [Preview Abstract] |
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UP9.00161: Planned Experiments on the Princeton Advanced Test Stand A. Stepanov, E.P. Gilson, L. Grisham, I. Kaganovich, R.C. Davidson The Princeton Advanced Test Stand (PATS) device is an experimental facility based on the STS-100 high voltage test stand transferred from LBNL. It consists of a multicusp RF ion source, a pulsed extraction system capable of forming high-perveance 100keV ion beams, and a large six-foot-long vacuum with convenient access for beam diagnostics. This results in a flexible system for studying high perveance ion beams relevant to NDCX-I/II, including experiments on beam neutralization by ferroelectric plasma sources (FEPS) being developed at PPPL. Research on PATS will concern the basic physics of beam-plasma interactions, such as the effects of volume neutralization on beam emittance, as well as optimizing technology of the FEPS. PATS combines the advantage of an ion beam source and a large-volume plasma source in a chamber with ample access for diagnostics, resulting in a robust setup for investigating and improving relevant aspects of neutralized drift. There are also plans for running the ion source with strongly electro-negative gases such as chlorine, making it possible to extract positive or negative ion beams. [Preview Abstract] |
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UP9.00162: Ponderomotive acceleration and the supra-bubble regime for electrons in tenuous plasmas V.I. Geyko, I.Y. Dodin, N.J. Fisch, G.M. Fraiman In the present work, we study electron acceleration via interaction with ultraintense laser pulses in tenuous plasma. For electrons injected inside a pulse with arbitrary momenta, we demonstrate different regimes of ponderomotive acceleration and show that plasma dispersion affects this process at densities $n/n_c>a_0^{-4}$, where $n_c$ is the critical plasma density, and $a_0=eA/mc^2$ is the normalized laser amplitude, which we assume much larger than one. For a cold electron beam, the so-called \textit{supra-bubble} acceleration is studied, when electrons are pushed by a moving ponderomotive potential \textit{ahead} of the wakefield potential. In this case, the maximum energy gain, $\gamma\propto a_0\gamma_g$, is attained when the particle Lorentz factor $\gamma$ is initially about $\gamma_g/a_0$, where $\gamma_g$ is the pulse group speed Lorentz factor. The supra-bubble acceleration scheme operates at $\gamma_g\geq a_0$ and yields energies comparable to those attained through conventional wakefield acceleration for the same plasma and laser parameters. [Preview Abstract] |
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UP9.00163: Enhanced slowing of electron beams in finite resistivity plasmas Carl Siemon, Vladimir Khudik, Gennady Shvets The physics of underdense relativistic electron beam propagation through a collisional background plasma in the presence of transverse Weibel Instability (WI) is described. Collisional simulation results are presented, which sharply contrast collisionless dynamics in that longitudinal beam slowing is dramatically enhanced. Collisional beam slowing can be thought of as a two step process for high beam energies: 1) At first, the beam filaments' merger during the nonlinear stage of the WI induces transverse thermalization of the beam (Weibel slowing); 2) Once filaments stop merging, the electric force induced by collisions acts on the warm beam, causing its velocity to decay. Weibel slowing is more efficient with collisions than without because collisions enable magnetic field diffusion, which expedites the filaments' merger. When filaments merge, beam density and thus magnetic field strength increase, enhancing transverse thermalization, which decreases longitudinal beam velocity. Models are presented that quantify Weibel slowing and the slowing of a warm beam in the presence of collisions. These models, along with numerical results, show that the WI facilitates beam slowing at first, but actually impairs it once filaments stop merging. [Preview Abstract] |
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UP9.00164: Electron transmission through a PET nanocapillary foil and change of sample properties Susanta Das, B.S. Dassanayake, J.A. Tanis, N. Stolterfoht Transmission and guiding of 500-1000 eV electrons through a PET nanocapillary foil has been studied. Transmitted electron spectra show significant energy losses which increase with foil tilt angle and energy, indicating that electrons suffer both elastic and inelastic collisions before being transmitted or lost within the foil. Despite energy losses they are found to be guided through the foil. The results suggest that electrons and ion guiding are different processes [1,2]. The asymmetry in the angular distributions of the transmitted electrons suggests a change in the properties of the sample during the measurements, which is attributed to the long period of bombardment by the intense electron beam. It is suggested that hydrocarbons, causing a partial blocking or an increase in conductivity, deposited on the surface or inside the sample and the various inelastic processes between the electron beam and sample are the likely causes of the observed changes. Ref.; 1. S. Das, Ph.D. thesis, WMU, 2009, 2. N. Stolterfoht et al., Phys. Rev. Lett. 88 (2002) 133201. [Preview Abstract] |
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