Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session GP8: Poster Session III: ITER, General Tokamak, Simulation and Modeling; MHD, Linear and Nonlinear Phenomena; Waves and Instabilities in Basic Plasmas |
Hide Abstracts |
Room: Grand Hall East |
|
GP8.00001: ITER, GENERAL TOKAMAK, SIMULATION AND MODELING |
|
GP8.00002: Revisiting the Design of a Fusion Development Facility V.S. Chan, R.D. Stambaugh, A.M. Garofalo, J.P. Smith, C.P.C. Wong A Fusion Development Facility (FDF) is proposed to make possible a DEMO of the ARIES-AT type as the next step after ITER. The mission of the FDF should be to carry forward advanced tokamak physics and enable development of fusion nuclear science and technology. We have added more realism to the initial FDF concept [1] including inner and outer gaps from the plasma to the first wall; an improved estimate of the inboard/outboard blanket/shield thickness to protect the magnets/insulators; control coil positions; and realistic divertor geometry. Optimizing the mix of heating and current drive power has high leverage on the operating power. We have also revisited the assumed impurity fraction and the density profile peakedness.\par \vskip8pt \noindent [1] R.D.\ Stambaugh, {\em et al.}, Bull.\ Am.\ Phys.\ Soc.\ {\bf 53}, 259 (2008). [Preview Abstract] |
|
GP8.00003: Fusion Nuclear Science Facility Design Points R.D. Stambaugh, V.S. Chan, A.M. Garofalo, J.P. Smith, C.P.C. Wong A Fusion Nuclear Science Facility (FNSF) is needed to fill the gaps between ITER and current experiments and a fusion demonstration power plant (DEMO). Characteristics should be neutron flux at the outboard midplane of 1-2 MW/m$^2$, continuous operation for periods up to two weeks, a goal of a duty factor of 0.3 on a year and fluences of 3-6 MW-yr/m$^2$ in ten years of operation to enable development of blankets suitable for tritium, electricity, and hydrogen production. Systems code analysis that implements constant physics and technology assumptions over a wide range of aspect ratio has been updated for more detailed design considerations and neutronics calculations. Conventional copper-coil tokamaks with inboard breeding/shielding and an OH solenoid optimize at high aspect ratio with devices between DIII-D and JET in size. Removal of the inboard breeding/shielding and OH coil results in the distinct ST branch with devices about half the size and $A < 1.8$. All these devices are generally $Q < 5$ but possible avenues to higher $Q$ will be discussed. [Preview Abstract] |
|
GP8.00004: Low Aspect Ratio Fusion Nuclear Science Facility Mission and Parameter Space Y.-K.M. Peng, A. Sontag, S. Diem, J. Canik, M. Cole, P.J. Fogarty, M. Kotschenreuther, P. Valanju A Fusion Nuclear Science Facility (FNSF) is needed to bridge critical knowledge gaps between ITER and current R{\&}D toward a fusion DEMO. The FNSF aims to subject, \textit{for the first time}, components that tame the plasma-material interface and harness fusion power to the rigors of a full fusion nuclear environment, continuously for up to 10$^{6}$ s. This would test and reveal new physical properties and issues of interest, encourage R{\&}D to innovate and improve, and iterate to arrive at the component design knowledge needed to begin power engineering and technology testing. Physical properties include, for example, tritium permeation through material layers of plasma facing components; tritium extraction from solid breeders; thresholds of measurement of tritium concentration in fusion chamber. These encompass wide ranges of time constants (10$^{3}$ -- 10$^{6}$ s) and fusion neutron fluxes (0.01 -- 1 MW/m$^{2})$. A set of aspect-ratio-dependent plasma physics and engineering assumptions are used to estimate the parameter space of the FNSF, covering the entire range of neutron flux. It is found that for plasmas with R = 1.2 m, B = 2.2 T, A = 1.5, and kappa = 3, the required plasma current ranges from 3.5 MA to 8.5 MA, q$_{cyl}$ = 8.5 to 3.5, and Q = 0.05 to 2.5. The relationship with design parameters of higher aspect ratios will also be discussed. [Preview Abstract] |
|
GP8.00005: To Demonstrate an Integrated Solution for Plasma-Material Interfaces Compatible with an Optimized Core Plasma Robert Goldston, Jeffrey Brooks, Amanda Hubbard, Anthony Leonard, Bruce Lipschultz, Rajesh Maingi, Michael Ulrickson, Dennis Whyte The plasma facing components in a Demo reactor will face much more extreme boundary plasma conditions and operating requirements than any present or planned experiment. These include 1) Power density a factor of four or more greater than in ITER, 2) Continuous operation resulting in annual energy and particle throughput 100-200 times larger than ITER, 3) Elevated surface operating temperature for efficient electricity production, 4) Tritium fuel cycle control for safety and breeding requirements, and 5) Steady state plasma confinement and control. Consistent with ReNeW Thrust 12, design options are being explored for a new moderate-scale facility to assess core-edge interaction issues and solutions. Key desired features include high power density, sufficient pulse length and duty cycle, elevated wall temperature, steady-state control of an optimized core plasma, and flexibility in changing boundary components as well as access for comprehensive measurements. [Preview Abstract] |
|
GP8.00006: Design studies of Vulcan, a compact steady-state tokamak for boundary studies using HTS magnets L. Bromberg, P.T. Bonoli, A.E. Hubbard, J. Schulz, D. Whyte Recent advances in high temperature superconductors (HTS) have been dramatic. We present investigation of a near-term tokamak design using present day HTS materials, conventional structures and manufacturing. The device is intermediate-scale ($\sim $ 1 m major radius, 7 T on axis), operating in H and limited D, for studies involving long-term operation with surface averaged power density $\sim $ 1 MW/m$^{2}$ and hot walls to study plasma-facing component issues and plasma sustainment under conditions relevant to fusion reactors. We will discuss operation time required under different plasma conditions to achieve steady state conditions. The design uses demountable joints at 40-50 K for easy access to the first wall and divertor, with capability for single-piece maintenance. Compact designs for He cooled first walls are described. Plasma heating and current drive options for this compact, high density plasma will be presented. Engineering/plasma tradeoffs will be described, including performance as a function of machine size and cost. [Preview Abstract] |
|
GP8.00007: Simulation experiment of interaction of plasma facing materials and transient heat loads in ITER divertor by use of magnetized coaxial plasma gun M. Nakatsuka, K. Ando, T. Higashi, Y. Kikuchi, N. Fukumoto, M. Nagata Interaction of plasma facing materials and transient head loads such as type I ELMs is one of the critical issues in ITER divertor. The heat load to the ITER divertor during type I ELMs is estimated to be 0.5-3 MJ/m$^{2}$ with a pulse length of 0.1-0.5 ms. We have developed a magnetized coaxial plasma gun (MCPG) for the simulation experiment of transient heat load during type I ELMs in ITER divertor. The MCPG has inner and outer electrodes made of stainless steel 304. In addition, the inner electrode is covered with molybdenum so as to suppress the release of impurities from the electrode during the discharge. The diameters of inner and outer electrodes are 0.06 m and 0.14 m, respectively. The power supply for the MCPG is a capacitor bank (7 kV, 1 mF, 25 kJ). The plasma velocity estimated by the time of flight measurement of the magnetic fields was about 50 km/s, corresponding to the ion energy of 15 eV (H) or 30 eV (D). The absorbed energy density of the plasma stream was measured a calorimeter made of graphite. It was found that the absorbed energy density was 0.9 MJ/m$^{2}$ with a pulse width of 0.5 ms at the distance of 100 mm from the inner electrode. In the conference, experimental results of plasma exposure on the plasma facing materials in ITER divertor will be shown. [Preview Abstract] |
|
GP8.00008: Models and Simulations of C60-Fullerene Plasma Jets for Disruption Mitigation and Magneto-Inertial Fusion Ioan-Niculae Bogatu, Sergei A. Galkin, Jin-Soo Kim We present the models and simulation results of C60-fullerene plasma jets proposed to be used for the disruption mitigation on ITER and for magneto-inertial fusion (MIF). The model describing the fast production of a large mass of C60 molecular gas in the pulsed power source by explosive sublimation of C60 micro-grains is detailed. Several aspects of the magnetic ``piston'' model and the 2D interchange (magnetic Rayleigh-Taylor) instability in the rail gun arc dynamics are described. A plasma jet adiabatic expansion model is used to investigate the in-flight three-body recombination during jet transport to the plasma boundary. Our LSP PIC code 3D simulations show that heavy C60 plasmoid penetrates deeply through a transverse magnetic barrier demonstrating self-polarization and magnetic field expulsion effects. The LSP code 3D simulation of two plasma jets head-on injection along a magnetic field lines for MIF are also discussed. [Preview Abstract] |
|
GP8.00009: The Motional Stark Effect Diagnostic for ITER Fred Levinton, Elizabeth Foley, Howard Yuh The United States has been tasked with the development and implementation of a Motional Stark Effect (MSE) system on ITER. In the harsh ITER environment, in order to transmit light to a detector, a shielded labyrinth is required to mitigate the effect of neutrons and radiation. This necessitates the use of several mirrors to relay the light to the vacuum interface outside the shielded region. However, the plasma facing mirror is susceptible to coating and erosion. This is problematic for any optical diagnostic and particularly polarimetry measurements such as MSE. Even thin coatings can change the phase and reflectivity of $s$ and $p$ polarized light when reflected from a mirror. This makes maintaining and tracking the MSE calibration very challenging. Our proposed approach to the MSE diagnostic is to implement a combination of conventional MSE polarimetry and a new approach to measure the magnitude of the magnetic field from the line shift of the Stark spectrum. The advantage of the latter approach is that the line shifts are independent of polarization. Results of this and conceptual designs of the proposed MSE system for ITER will be discussed. [Preview Abstract] |
|
GP8.00010: Design of a Prototype for the In Situ Calibration Source for the ECE Diagnostic on ITER P.E. Phillips, M. E. Austin, W.L. Rowan, J. Beno, A. Ouroua, R.F. Ellis A large area (200mm diameter) calibration source will be prototyped for ITER. The source will generate blackbody emission (emissivity $>$ 0.7) for frequencies greater than 120 GHz in the ITER vacuum environment. The device is a primary vacuum component (VQC 1B) and is subject to stringent vacuum requirements that will be tested in the case of this prototype. The source will operate at temperatures up to 800 $^{\circ}$C though it will not be actively heated during plasma operation. A major challenge is to assure high reliability both in maintenance of calibration and mechanical integrity. SiC has been selected as the active emissive surface. Prior to construction of the prototype, candidate-heating methods will be critically examined for reliability, efficiency, and ITER compatibility. Results of test of a resistively heated source will be presented. A progress report on the development of the prototype will also be presented. [Preview Abstract] |
|
GP8.00011: Performance of ITER H-mode discharges with various mixes of external heating computed using the MMM08 transport model in PTRANSP simulations A.H. Kritz, G. Bateman, F.D. Halpern, A.Y. Pankin, T. Rafiq, R.V. Budny, D.C. McCune The new MMM08 Multi-Mode transport model [1] is used in PTRANSP simulations to predict the time evolution of temperature and toroidal angular frequency profiles in ITER H-mode discharges. External heating and current drive sources include NBI, ECRH and ICRH, computed using NUBEAM, TORAY and TORIC respectively. Five mixes of heating (with associated current and torque drives, and fueling) are considered in order to examine various possibilities and contingencies for ITER. The evolution of the magnetic $q$ profile is computed using the NCLASS module for neoclassical resistivity and bootstrap current together with the KDSAW module for the effect of sawtooth crashes. The fusion power production and fusion $Q$, computed as a function of heating power mix and pedestal height, are compared with those obtained using the GLF23 model [2]. \newline [1] F.D. Halpern {\it et al.,} Phys. Plasmas 15 (2008) 065033; J.Weiland {\it et al.,} Nucl. Fusion 49 (2009) 965933; A.Kritz et al., EPS (2009) \newline [2] R.V. Budny, Nucl. Fusion 49 (2009) 085008 [Preview Abstract] |
|
GP8.00012: Quasilinear Relaxation of Alpha Particle Profiles in Burning Plasmas Katy Ghantous, N.N. Gorelenkov, M. Zarnstorff Using local quasilinear theory and analytic methods, we reconstruct the relaxed fast ion profile in burning plasmas and get information on the losses. With a local alpha drive and background dampings of TAE modes, we compute the critical alpha particle beta profile beyond which the modes destabilize. Previous local theory is improved by including more sophisticated damping and drive mechanisms such as the effect of alpha particle finite orbit width. Solving the equation for this threshold condition in the unstable regions, and taking into account conservation laws, we integrate a relaxed profile for the alpha particles and deduce losses if any. The theory allows to predict ITER scenarios with good alpha confinement where it is hazardous to operate in regimes with more than 5{\%} losses of energetic particles. [Preview Abstract] |
|
GP8.00013: Status of ITER ICH Matching System Design D. Swain, R. Goulding, D. Rasmussen Work on the design of the ITER ICH matching system is progressing. The system will deliver a total power of 20 MW for long-pulse ($>$ 3000 s) operation. The present matching system design has: hybrid combiner-splitter circuits for ELM resilience, active matching during a shot using a double-stub tuner circuit, decouplers between the eight inputs to each antenna, water-cooled matching components, and air-cooled matched transmission lines using turbulent cooling. The matching/decoupling system is evolving. While the main components are fairly well determined, the configuration of the decoupler connections and tuning of the decouplers is still under discussion. More detailed specifications and predicted performance of the rf system under different plasma conditions will be presented. [Preview Abstract] |
|
GP8.00014: ITER ECH Transmission System Test Stand and Prototype Development T.S. Bigelow, J.B. Caughman, D.A. Rasmussen, T.E. Whittle, M.A. Shapiro, J.R. Sirigiri, R.J. Temkin Progress in acquisition and testing of prototype components and developing a high power test stand will be described. The US ITER Project Office is responsible for providing the ECH transmission lines that are based on evacuated 63.5 mm diameter corrugated waveguide. Each line is designed to handle 170 GHz power at 1 MW and possibly up to 2 MW in the HE$_{11}$ mode. A total of 24 lines are planned. The ITER Organization has completed a conceptual design of the system. A number of prototype components have been procured primarily from industrial suppliers and testing of vacuum performance and mechanical alignment have been performed. A 140\r{ } miter bend was developed and tested at low power as an alternative to two adjacent 90 degree miter bends. A waveguide pumpout prototype and a compact waveguide switch have been built. Testing at high power is planned on a test stand being setup at ORNL. Work on installing a power supply and interim 400 kW 140 GHz gyrotron has progressed and procurement of a 170 GHz 0.5-1 MW gyrotron has begun. Low power testing is underway of a grating coupler to be used in a resonant ring for very high power testing of components at the 2 MW level or higher. [Preview Abstract] |
|
GP8.00015: Loss Estimate for ITER ECH Transmission Line M.A. Shapiro, E.J. Kowalski, J.R. Sirigiri, D.S. Tax, R.J. Temkin, T.S. Bigelow, D.A. Rasmussen The ITER ECH Transmission Lines (TL) are 63.5 mm diameter corrugated waveguides that will each carry 1 MW of power at 170 GHz. The current specifications require that the transmission efficiency of the HE$_{11}$ mode from the gyrotron to the plasma should exceed 83{\%}. The transmission losses on the ITER TL have been calculated for four possible cases corresponding to having HE$_{11}$ mode purity at the input of the TL of 100{\%}, 97{\%}, 90{\%} and 80{\%}. The losses due to coupling, Ohmic and mode conversion loss are evaluated. Estimates of the calorimetric loss on the line show that the output power is reduced by about 5 $\pm $1 {\%} due to Ohmic loss in each of the four cases. Estimates of the mode conversion loss show that the fraction of output power in the HE$_{11}$ mode is about 3{\%} smaller than the fraction of input power in the HE$_{11}$ mode. High output mode purity therefore can only be achieved with significantly higher input mode purity. We also present a new conservation theorem relating the field distribution offset and propagation angle due to any two linearly polarized LP$_{mn}$ modes ~propagating in the corrugated waveguide. [Preview Abstract] |
|
GP8.00016: The Ignitor-Columbus Path in Fusion Research Matteo Salvetti, Bruno Coppi The Ignitor and Columbus [1] machines are based on the use of normal conducting magnets to produce high magnetic fields and are designed to reach ignition by exploiting the favorable confinement characteristics of high plasma density regimes ($n_{0} \quad \cong $ 10$^{21}$ m$^{-3})$ [2]. The Columbus experiment is proposed as a parallel US project to the Ignitor program carried out in Italy. The possible discovery of new phenomena and the understanding of known ones, i.e. sawtooth oscillations, under fusion burning conditions will drive the design of future fusion reactors. Columbus is designed to reach ignition conditions in D-T plasmas where the $\alpha $-particles heating compensates for all energy losses and it takes advantage of the Ignitor R{\&}D effort and the technology acquired during the construction of its full size component prototypes. Columbus is geometrically self similar to Ignitor, the dimensions being increased by 25/22 ($R_{0} \quad \cong $ 1.5 m) and the volume by about 50{\%}. The toroidal magnetic field is decreased by the factor 12.6/13 and the average poloidal field produced by the plasma current is about equal to that of Ignitor for comparable values of the magnetic safety factor ($q_{a})$. The reference plasma current is $I_{p} \quad \cong $ 12.2 MA, the value that ITER would produce for the same $q_{a}$ but without reaching ignition. [1] B. Coppi and M. Salvetti, MIT-RLE report PTP (2003). [2] B. Coppi, MIT-RLE report PTP02/04 (2002), Presentation to the NRC. [Preview Abstract] |
|
GP8.00017: Improved Confinement Regimes for the Ignitor Experiment$^1$ A. Airoldi, G. Cenacchi, P. Detragiache, G. Clai, B. Coppi The Ignitor experiment$^2$ is designed to achieve ignition in well confined deuterium-tritium plasmas, with Ohmic heating only or small amounts of additional RF heating. Thanks to its unique features ($B_T \leq 13$ T, $I_p \leq 11$ MA, $\left\langle n\right\rangle \leq 5\times 10^{20} \rm m^{-3} $), Ignitor is the only device capable of accessing plasma parameters that are relevant to a net power producing D-T reactor. In fact, plasma regimes suitable to operate a reactor have yet to be produced. The Poloidal Field System in Ignitor makes it possible to operate with different configurations, from the reference ``extended limiter'' configuration to the configuration with double X-points just inside or just outside the first wall. X-point configurations are interesting as they may lead to a high confinement regime (H- mode) when the heating power is larger than a threshold value. H-mode operation in Ignitor has been assessed both with 0D calculations as well as 1 1/2D transport simulations. Quasi-stationary conditions can be attained when a process producing re-distribution of pressure and current profiles is active. The accessibility to improved confinement regimes with or without edge pedestal that have been observed on existing experiments is assessed.\\ $^1$Sponsored in part by ENEA of Italy and by the U.S. D.O.E.\\ $^2$B.Coppi, A.Airoldi, F.Bombarda, et al.,\textit{Nucl. Fusion} \textbf{41}, 1253 (2001) [Preview Abstract] |
|
GP8.00018: Design of MgB$_{2}$ Superconducting coils for the Ignitor Experiment* G. Grasso, R. Penco, S. Berta, B. Coppi, G. Giunchi A feasibility study for the adoption of MgB$_{2}$ superconducting cables for the largest (about 5 m in diameter) of the poloidal field coils of the Ignitor machine is being carried out. This initiative was prompted by the progress made in the fabrication of MgB$_{2}$ long cables, and related superconducting magnets of relatively large dimensions. These magnets will be cryocooled at the operating temperature of 10-15 K that is compatible with the He-gas cryogenic cooling system of Ignitor as well as with the projected superconducting current density of the MgB$_{2}$ material, at the magnetic field values $(\simeq 4-5$ T) in which these coils are designed to operate. The optimal cable configuration has been identified that can provide an efficient cooling of the MgB$_{2}$ conductors over times compatible with the machine duty cycles. MgB$_{2}$ superconductors hold the promise of becoming suitable for high field magnets by appropriate doping of the material and of replacing gradually the normal conducting coils adopted, by necessity, in high field experiments. Therefore, an appropriate R\&D program on the development of improved MgB$_{2}$ material and related superconducting cabling options has been undertaken, involving different institutions. [Preview Abstract] |
|
GP8.00019: Experiments with the Ignitor Pellet Injector (IPI) A. Frattolillo, S. Migliori, S. Podda, F. Bombarda, L.R. Baylor, J.B.O. Caughman, S.K. Combs, C. Foust, D. Fehling, J.M. McJill, S. Meitner, G. Roveta The four barrel, two-stage pneumatic injector for the Ignitor experiment (IPI), built by ENEA and ORNL, has been tested in the course of three esperimental campaigns. The optimal shaping of the propellant pressure pulse to improve pellet acceleration is provided by specially designed Pulse Shaping Valves. These have been modified and tested on a new facility that allows operating pellet injector components in conditions close to those at which they will have to operate on the IPI. Fast closing ($<$ 10 ms) valves drastically reduce the expansion volumes needed to remove the propellant gas at the end of the guiding tube. The four barrel (2.1, 2.2, 3.0 and 4.6 mm bores) pipe-gun cryostat is cooled down by a closed cycle refrigerator, and pellet diagnostics for measuring speed and mass of the pellets, as well as for capturing in-flight pictures of all four pellets were developed for this application. The final impact target is equipped with a shock accelerometer. The injector is designed to deliver pellets with velocities up to 4 km/s (2 km/s already achieved): the results of the latest experimental campaign will be reported. [Preview Abstract] |
|
GP8.00020: Ignitor Structural Analysis for Double X-points Configurations and Machine Cooling System A. Bianchi, P. Frosi, B. Coppi Recent activities carried out by ANSALDO for the Ignitor program include the structural analysis of the machine Load Assembly for two new scenarios that extend the operation of the machine to H- regimes, where the plasma equilibrium configuration is of the double X- point type and the out-of-plane forces increase. A complete structural analysis was carried out for three scenarios with $B_T = 13$ T and $I_p = 9$ and 10 MA and double X points just outside the plasma chamber, and $I_p = 9 $ MA and double X points just inside the plasma chamber. The lowered values of the plasma current can offset the increase of stress due to the less favorable X-point configuration relative to the ``extended limiter'' configuration. With the cryogenic plant involving gaseous helium and studied by LINDE KRYOTECHNIK AG, the toroidal field coils reach temperatures up to 230 K and the central solenoid coils approach 220 K after a pulse at the most extreme machine parameters, while the vertical field coils remain under 100 K. Several hours are needed to re-cool the machine down to 30 K before each pulse. New calculations are underway to verify the plant cooling performance at reduced parameters but with more stringent cooling times between shots. [Preview Abstract] |
|
GP8.00021: ICRH Component Prototypes for the Ignitor Expe-riment M. Sassi, S. Mantovani, P. Frosi, A. Cardinali, A. Bianchi The ICRH system for the Ignitor experiment is designed with a modular configuration to launch the power into the plasma through 4-strap antennas located in four of the 12 horizontal ports. Each module consists of 4 high power generators, operating in the frequency range 80-120 MHz, whose power is split over two ports (8 straps). A 30 $\Omega$ vacuum transmission line (VTL) transfers 0.4 MW of power per strap for a total power of 1.6 MW (at 115 MHz) per port in order to keep the maximum electric field below 5 kV/cm in the vacuum region of the straps and trasmission line. The RF configuration of the modules allows a full phase controls (toroidal and poloidal) of the straps though a PLL phase control. A detailed design of the ICRH antenna has been carried out, including the Faraday shield, the current straps, the vacuum transmission lines and the vacuum feed-through, and the antenna design has been integrated with the plasma chamber. An innovative plug-in system has been designed that should allow a simple installation of the antenna by remote handling. An improved port will be welded to the existing prototype of the D- shaped sector of the plasma chamber. A set of four strap will then be installed to test their coupling with the VTL coaxial cable. This facility will be used for the testing of welding procedures, vacuum tightness, plug-in system, and high voltage components. [Preview Abstract] |
|
GP8.00022: HBT-EP External Kink Mode Control Research Program: Recent Progress and Future Plans G.A. Navratil, B. DeBono, J.P. Levesque, M.E. Mauel, D.A. Maurer, T.S. Pedersen, N. Rath, D. Shiraki HBT-EP MHD mode control research is studying ITER relevant internal modular feedback control coil configurations and their impact on kink mode rigidity, advanced digital control algorithms, and the effects of plasma rotation on MHD mode stability. HBT-EP is installing a new segmented adjustable conducting wall with internal modular feedback control coils of varying toroidal angular coverage similar to that of the ITER `VAC02' internal coil design to test RWM interaction and multimode response to such highly localized control fields. In parallel with this hardware development, we have also extended our 3-D VALEN feedback modeling code to include the effects of multimode plasma response$. $This enhanced VALEN physics capability will allow detailed comparison and benchmarking of its new plasma model with our new wall, control and sensor coil geometry measurements. Multi-mode VALEN is now being tested and initial calculations of plasma stability for HBT-EP and NSTX will be presented along with details of the design and construction of our new wall-feedback coil system and future experimental plans. [Preview Abstract] |
|
GP8.00023: Design and Construction of New HBT-EP Passive Wall, Control and Sensor Coil System for Kink Rigidity Studies D.A. Maurer, B. DeBono, J.P. Levesque, M.E. Mauel, G.A. Navratil, T.S. Pedersen, N. Rath, D. Shiraki An important plasma response issue is to quantify the extent to which one can perform feedback on one unstable mode without destabilizing other stable MHD modes when they approach marginal stability as the plasma pressure is increased near the ideal wall limit. The HBT-EP experiment is investigating this effect of non-rigid, multimode plasma response during feedback control of the external kink using a new passive stabilizing wall geometry and new set of 120 modular feedback coils of varying toroidal angular coverage. Plasma multimode magnetic response will be measured and quantified using new pickup coil arrays containing 216 poloidal and radial magnetic sensors. This new passive wall-feedback coil system will allow the first detailed measurement of multi-mode, non-rigid kink mode response, first investigation of multi-mode feedback control of the kink mode, and allow the development of feedback strategies to mitigate sideband coupling to stable plasma modes thus limiting plasma non-rigid response and increasing feedback effectiveness. Design, construction, and installation details of the new wall and coil sets will be presented and discussed. Supported by U.S. DOE Grant DE-FG02-86ER53222. [Preview Abstract] |
|
GP8.00024: Evaluation of Non-axisymmetric Position Measurements of the HBT-EP Vacuum Chamber and Equilibrium Coils J.P. Levesque, B.A. Debono, M.E. Mauel, D.A. Maurer, G.A. Navratil, T.S. Pedersen, N. Rath, D. Shiraki The ability to do future multimode and non-rigid external kink studies on the HBT-EP tokamak will largely depend on alignment of the newly installed wall, sensors, and control coils. Also, small non-axisymmetric equilibrium field errors in tokamaks can have a dramatic impact on plasma performance [1], and reducing these field errors can improve plasma behavior. A measurement arm was built on a rotating axis in the center of HBT-EP in order to accurately locate and quantify misalignments of the equilibrium field coils, vacuum chamber, new wall segments, sensors, and control coils. The measurement arm is capable of determining locations of equilibrium coils and vacuum chamber ports to within 0.5 mm. Measurements are being used to correct small misalignments of magnetic coils and chamber components during installation of the new wall and magnetic feedback coils in HBT-EP. A ROMER coordinate measuring machine is used in conjunction with the measurement arm to verify coil alignment and to position the new wall segments in the vacuum vessel. Details of the measurement system and analysis of field errors produced by small misalignments of the equilibrium field coils will be presented. Supported by U.S. DOE Grant DE-FG02-86ER53222. \newline [1] J.K. Park \emph{et al}, Phys. Rev. Let. 99, 195003 (2007) [Preview Abstract] |
|
GP8.00025: Magnetics Design and Calibration for Multimode Plasma Response Measurements on HBT-EP D. Shiraki, B. Debono, J.P. Levesque, M.E. Mauel, D.A. Maurer, G.A. Navratil, T.S. Pedersen, N. Rath Accurate measurements of multimode plasma magnetic response are important for quantifying the effects of a variety of MHD phenomena. A new array of 216 poloidal and radial magnetic sensors has been designed, installed, and calibrated on the HBT- EP tokamak to measure multimode kink dynamics under feedback control. The new system provides double the toroidal resolution for real-time feedback over the previous system, as well as two complete poloidal and one complete toroidal array for high mode-number detection. A Monte Carlo simulation was used to determine the required accuracies, and a combination of bench and a new novel in-situ calibration were used to achieve this required performance. The in-situ calibration was carried out with four current ring sources which were built and installed in the vacuum chamber during an up to air period near the location of the plasma core, as well as the external vertical field, ohmic heating, and toroidal field coils. This calibration procedure is based on a linearized least squares algorithm to verify the location and orientation of each individual sensor. The new system will allow extensive multimode RWM studies, high mode-number feedback, and more accurate MHD spectroscopy detection of possible non-rigid, multimode kink dynamics. Supported by U.S. DOE Grant DE-FG02-86ER53222. [Preview Abstract] |
|
GP8.00026: Equilibrium Reconstruction and Ideal Stability Analysis of HBT-EP plasmas N. Rath, B. Debono, J.P. Levesque, M.E. Mauel, D.A. Maurer, G.A. Navratil, T.S. Pedersen, D. Shiraki Accurate knowledge of the safety factor profile is needed to evaluate MHD stability for resistive wall kink modes. In HBT-EP, we reconstruct the time evolution of the safety factor profile with the Tokamac code from equilibrium measurements of external magnetic fields and fluxes along with equilibrium coil currents. Additionally, internal magnetic field information is measured using movable edge magnetic probes to constrain the profile reconstruction. Recent installation of a new passive stabilizing wall for multimode kink studies has substantially increased the magnetic diagnostic measurement set available. Incorporation of this enhanced magnetic capability into the Tokamac code will be described. Eddy currents can have a large effect on the equilibrium of HBT-EP plasmas due to the new close fitting conducting wall used to stabilize the ideal kink. Eddy-currents in these wall segments will be modeled as a superposition of pre-calculated current eigenmodes whose weights are included in the equilibrium fitting procedure. Direct measurement of the eddy-currents from a known toroidal source current will be used to verify the eddy-current model. Ideal stability analysis of these equilibria using the DCON code will also be presented for a series of reconstructed equilibria. Supported by U.S. DOE Grant DE-FG02-86ER53222. [Preview Abstract] |
|
GP8.00027: MHD Mode Rotation and Amplitude Changes Induced by a Biased Electrode and Applied Resonant Magnetic Perturbations B. DeBono, J.P. Levesque, M.E. Mauel, D.A. Maurer, G.A. Navratil, T.S. Pedersen, N. Rath, D. Shiraki The effect of plasma rotation on the behavior of MHD modes is a topic of importance for both resistive wall and tearing mode stability. On HBT-EP, a biased molybdenum electrode inserted into the edge plasma is used to change the intrinsic ExB rotation of MHD activity of both kink and tearing mode fluctuations and magnetic torque is applied by currents in control coils to effect mode rotation and amplitude. Applied resonant magnetic fields have been observed to lead to characteristic mode amplitude modulations due to the applied slowing torque. For large applied bias voltage, kink fluctuations are observed to accelerate in the direction opposite to the naturally occurring mode rotation with frequencies up to two or three times the natural rotation rate. With the use of strong edge biasing, large (n,m)=(3,1) external kink modes are observed to change their rotation rate from +4 kHz to -30 kHz. At these velocities, the conducting wall segments behave like an ideal wall with a small amplitude rotating ideal kink mode still observable. Magnetic analysis of the behavior of the external kink under these fast rotating conditions will be discussed. Supported by U.S. DOE Grant DE-FG02-86ER53222. [Preview Abstract] |
|
GP8.00028: Plasma Rotation in Tokamaks for Steady State Conditions Pablo Martin, Enrique Castro New equations for tokamaks plasmas in steady state conditions have been found, which allow to find the plasma rotation velocities from the plasma geometry and some parameters such as ellipticity, triangularity, safety factor, shear and inductive electric field. The starting point of our treatment is the resistive MHD equilibrium equation. By additional operation with the del operator, a set of new differential equations has been found. These equations, together with the magnetic surface equations, allow determining the plasma velocities as a function of some experimental parameters, usually determined in most of the tokamaks. Particular cases, as circular geometry, can be treated in a simplified way. These results illustrate more realistic geometries with ellipticity and triangularity, which can only be solved numerically. The coordinates used in this word are those previously presented in several papers [1]. \\[4pt] [1] P. Martin, E. Castro and M. G. Haines, Phys Plasmas \textbf{12}, 102505 (2005) [Preview Abstract] |
|
GP8.00029: Parameter dependences of toroidal rotation and momentum transport in JT-60U Maiko Yoshida, Yutaka Kamada, Hidenobu Takenaga, Yoshiteru Sakamoto, Naoyuki Oyama, Hajime Urano Prediction and controllability of the toroidal rotation profile are open issues in ITER because parameter dependences of the ratio of the toroidal momentum diffusivity to the thermal diffusivity and the ratio of the convection velocity to the toroidal momentum diffusivity are not understood well. Parameter dependences of these ratios are essential to understand the mechanisms determining the rotation profile and the level of momentum transport. In this paper, dimensional and non-dimensional parameter dependences of the momentum transport coefficients and these ratios are investigated. It is found that the ratio of the toroidal momentum diffusivity to the thermal diffusivity increases with increasing the normalized poloidal Larmor radius. The ratio of the inward convection velocity to the toroidal momentum diffusivity decreases with increasing the normalized collisionality and decreasing the normalized poloidal Larmor radius. Relations between core and edge rotations are found and the physics mechanisms are discussed form the viewpoints of pinch effect and intrinsic rotation. [Preview Abstract] |
|
GP8.00030: Tokamak edge flows and their effect on the L-H transition power threshold A.Y. Aydemir At the edge collisional effects lead to a residual vertical electric field associated with the Pfirsch-Schl\"uter currents. Non-neutralized portion of this field, whose origin can be traced back to charge-dependent classical grad-B and curvature drifts, drives an ExB flow at the edge. Its poloidal component is in the direction of increasing major radius, regardless of the orientation of the fields and currents. The toroidal component is anti-symmetric about the mid-plane for an up-down symmetric system and reverses with the toroidal field. These flows have many features in common with the observed edge flows in tokamaks. A more careful analysis leads to a radial electric field that depends on the edge temperature gradient and shear. Without up-down symmetry, total contribution to the toroidal momentum and the edge $E_{\psi}$ clearly depends on the toroidal field direction. When the grad-B drift direction points towards the X- point, the net effect is positive; with toroidal field reversal, $E_{\psi}$ and the toroidal flow oppose the ambient flows and electric field due to, for example, the ion-orbit loss mechanism. The magnitude of this positive/negative contribution is also plasma-shape dependent. These features provide a compelling explanation for the grad-B drift-dependence of the L-H transition power threshold. We are continuing to study these effects using both initial value and free-boundary equilibrium codes. [Preview Abstract] |
|
GP8.00031: Modeling Flow Suppression of Error-field-induced Magnetic Islands in Tokamaks J.A. Breslau, W. Park Small deviations from axisymmetry in applied tokamak magnetic fields can induce island formation at magnetic surfaces whose rotational transform resonates with the perturbation. These islands have a braking effect on plasma rotation that can destabilize resistive wall modes. The IPEC code\footnote{J.K. Park, et al., \textit{Phys. Plasmas} \textbf{14}, 052110 (2007).} is useful for computing plasma response to harmonic perturbations in the infinite-conducting limit, assuming perfect shielding at the resonant surface, but cannot predict the nonlinear effects of finite-sized islands. Using the nonlinear extended MHD code M3D,\footnote{W. Park, \textit{et al}., \textit{Phys. Plasmas} \textbf{6}, 1796 (1999).} we explore the effects of a 2,1 perturbation on the nonlinear evolution of a family of equilibria with finite resistivity. Particular attention is paid to the effects of toroidal flow on suppressing island formation, making contact with the analytic theory of Fitzpatrick.\footnote{R. Fitzpatrick, \textit{Phys. Plasmas} \textbf{5}, 3325 (1998).} Island suppression is shown to depend strongly on the tearing mode stability properties of the equilibrium. [Preview Abstract] |
|
GP8.00032: Neoclassical toroidal plasma viscosity in the vicinity of the magnetic axis in tokamaks K.C. Shaing, Y. Sun, S.A. Sabbagh, M.S. Chu Error fields in tokamaks do not vanish on the magnetic axis. Thus, neoclassical toroidal plasma viscosity does not vanish on the axis either. Here, the theory for neoclassical toroidal plasma viscosity is extended to the region near the magnetic axis. Two isomorphic models are presented to investigate the neoclassical toroidal plasma viscosity in that region. The analytic expressions for the viscosity in the low collisionality regimes are calculated. The results can be compared to the experimentally measured toroidal flow damping rate in the near magnetic axis region. The isomorphic transformation developed here can also be used to transform all the analytic expressions of the neoclassical toroidal plasma viscosity calculated for tokamaks to any toroidal system with arbitrary equilibrium symmetry property. [Preview Abstract] |
|
GP8.00033: Bootstrap current and transport fluxes associated with bounce-transit and drift resonance in tokamaks B.C. Trans, K.C. Shaing, M.S. Chu, S.A. Sabbagh Bootstrap current and transport fluxes caused by the bounce-transit and drift resonance are calculated by solving parallel and toroidal momentum balance equations in tokamaks that have error fields and magnetohydrodynamic (MHD) activities. Because error fields are not localized around a surface where the safety factor is a rational number, the bootstrap current densities on either side of the rational surface do not cancel. Thus, there is a net contribution to the equilibrium bootstrap current. This may offer a possibility to control bootstrap current by error fields. Other transport quantities of interest such as parallel flow speed, radial electric field, and particle and heat flux are also calculated. [Preview Abstract] |
|
GP8.00034: Real-time MHD Mode Localization in ECE Measurements on ASDEX Upgrade N.K. Hicks, M. Garcia-Munoz, V. Igochine, M. Maraschek, M. Reich, J. Stober, W. Suttrop, W. Treutterer Electron cyclotron emission (ECE) measurements of the electron temperature ($T_{e})$ radial profile obtained with the 60-channel radiometer on the ASDEX Upgrade (AUG) tokamak are used to determine the locations of MHD modes that arise in the plasma. A neoclassical tearing mode (NTM) exhibits a phase reversal in $T_{e}$ fluctuations, and localizing this signature in the ECE $T_{e}$ profile identifies the radial position of the mode. This technique is part of the development at AUG of a real-time system for NTM suppression, as will be needed for successful operation of ITER. To improve the detection, ECE signals are correlated with magnetic signals. Results are presented on the performance of tracking NTMs in real-time during AUG discharges, and the prospects for the feasibility of this approach in closed-loop real-time NTM suppression are assessed. Building on the correlation approach for NTM tracking, a framework for correlation of ECE signals with other diagnostics is being developed, e.g. to localize Alfv\'{e}n eigenmodes. [Preview Abstract] |
|
GP8.00035: Extremum-Seeking Stabilization of Neoclassical Tearing Modes in Tokamak Plasmas William Wehner, Eugenio Schuster Current injection by Electron Cyclotron Current Drive (ECCD) has been proved experimentally as an effective method to stabilize neoclassical tearing modes (NTM). The effectiveness of this method is limited in practice by the uncertainties in the width of the island, the alignment between the island and the EC beam, and the EC power threshold for NTM stabilization. Heuristic search and suppress algorithms have been proposed and shown effective to improve the alignment of the EC beam with the island by just using an estimate of the island width. Making use of this estimate, a real-time, non-model-based, extremum-seeking, optimization algorithm is proposed in this work for EC beam steering and modulation in order to minimize the time (control energy) required for NTM stabilization. A model is proposed for the dynamic response of the island width to different parameters of the EC beam such as alignment, width, power, modulation duty-cycle and modulation phase. The efficiency of the proposed method is compared with traditional search and suppress algorithms. [Preview Abstract] |
|
GP8.00036: Simultaneous Control of Electron Density and Effective Atomic Number in Non-burning Tokamak Plasmas Daniel Boyer, Patrick Boyle, Eugenio Schuster, Glenn Bateman, Arnold Kritz The control of plasma kinetic profiles is one of the most fundamental problems in tokamaks. Precise regulation of electron density and effective atomic number, or alternatively hydrogen and impurity densities, is required during routine operation. A common means of actuation toward this end is the use of fast-acting gas valves for the injection of different species. However, non-linear coupling may lead to a closed-loop oscillatory behavior when there is a failure to properly account for the multi-input-multi-output nature of the problem. In this work, controllers are sought to regulate both the average value and spatial profiles of the electron density and effective atomic number. One-dimensional transport equations for these variables are represented in cylindrical coordinates by a set of partial differential equations (PDEs) and used for control synthesis based on nonlinear and backstepping techniques. Numerical simulations illustrating the effectiveness of the resulting control laws are presented. [Preview Abstract] |
|
GP8.00037: Estimation of Angular Momentum Transport Coefficients via Extended Kalman Filtering Theory Chao Xu, Eugenio Schuster The accuracy of first-principles predictive models for the evolution of plasma profiles is sometimes limited by the lack of understanding of the plasma transport phenomena. In this work we use the extended Kalman filtering theory to provide real-time estimates of poorly known or totally unknown angular momentum transport coefficients. These estimates are based solely on input-output diagnostic data and limited understanding of the transport physics. We first assume that the plasma dynamics can be governed by a tractable model obtained by first principles but the transport coefficients are considered unknown and to-be-estimated. The partial-differential-equation model is discretized both in space and time to obtain a finite-dimensional discrete-time state-space representation. The system states and to-be-estimated coefficients are then combined into an augmented state vector. The resulting nonlinear state-space model is used for the design of an extended Kalman filter that provides real-time estimations not only of the system states but also of the unknown transport coefficients. [Preview Abstract] |
|
GP8.00038: Remediating the Effects of Time Delays in Axisymmetric Plasma Controllers via Optimal Tuning and Predictor Augmentation Reza Arastoo, Eugenio Schuster, Michael Walker Introducing extra time delays into the axisymmetric control loops of certain superconducting tokamaks can have significant detrimental consequences. This study exposes and quantifies these effects by focusing on plasma current and radial position control in a vertically-stable circular plasma in KSTAR. Delays in the power supplies, data acquisition, and vessel structure are taken into account. Extremum-seeking-based optimal tuning of PID controllers is proposed as a possible method for remediating the negative effects of time delays. In addition, an augmentation of the control loop by the introduction of a predictor is proposed to improve the performance of the time-delayed closed-loop system. It is shown that the proposed predictor is robust against uncertainties in the values of the delays. The dual-locus technique is employed to assess stability of the closed-loop systems in the presence of time delays. [Preview Abstract] |
|
GP8.00039: Numerical Simulation on Applicability of Resonant Magnetic Perturbation to KSTAR Tokamak Doohyun Kim, Hyunsun Han, Ki Min Kim, Sang Hee Hong A numerical simulation is carried out to investigate the perturbed magnetic field configurations for a feasibility study on the resonant magnetic perturbation(RMP) to mitigate ELM damages to the divertor in KSTAR tokamak. The perturbed magnetic fields are described by vacuum superposition of equilibrium fields and fluctuating fields induced from the in- vessel control coils (IVCCs) will be installed in KSTAR. The equilibrium and induced fields are calculated using Grad- Shafranov equation and Biot-Savart law, respectively. For visualizing the magnetic field configurations, a field line tracing code has been developed using the 4th-order Runge-Kutta method. Magnetic field perturbations and island configurations can be found with this tracing code by describing poloidal positions of field lines as the increment of toroidal angle. And the Chirikov parameter is calculated to verify the generation of stochastic layer by overlap of magnetic islands. From this numerical work, it is confirmed that stochastic magnetic field lines are formed when the IVCC magnetic fields are generated, and the effect of RMP on KSTAR operation is discussed. [Preview Abstract] |
|
GP8.00040: Electron Cyclotron Emission Imaging on KSTAR T. Liang, C.W. Domier, N.C. Luhmann, Jr., B.J. Tobias, H.K. Park, G.S. Yun, W. Lee An innovative, four array electron cyclotron emission imaging (ECEI) system is under development to generate 2-D localized, time-resolved electron temperature T$_{e}$ images of the KSTAR plasma. The KSTAR system consists of two ECEI instruments on widely separated toroidal locations. Each instrument comprises a pair of imaging arrays which view the plasma through a single midplane port, with a quasi-optical beamsplitter to separate the high field and low field side signals.Translatable optics allow each array to independently focus on different portions of the KSTAR plasma, with each array forming 24 (v) $\times $ 8 (h) = 192 channel images of T$_{e}$ profiles and fluctuations. The ECEI instrument is scheduled for installation on the KSTAR tokamak in early 2010. ECEI will be employed in a wide range of physics studies, but will focus on unraveling the magnetic reconnection physics associated with the sawtooth oscillation. Technical capabilities of this diagnostic will be presented. Work supported by U.S. DOE Grant DE-FG02-99ER54531 and by POSTECH and KNRF. [Preview Abstract] |
|
GP8.00041: MHD Stability Study of Oblate FRCs G.A. Cone, R.D. Milroy, C.C. Kim The $n=1$ tilt, interchange, and shift modes of oblate FRC plasmas are simulated using the \texttt{NIMROD} code. The grid geometry approximates the shaped, close-fitting flux conserver used in the Swarthmore Spheromak eXperiment (SSX) oblate FRC study\footnote{M.~J.~Schaffer, M.~Brown, C.~Cothran, N.~Murphy, An oblate FRC concept for SSX, ICC Workshop, College Park, MD, Feb 2007}. The results validate the work by Belova et al\footnote{E.~V.~Belova, S.~C.~Jardin, H.~Ji, M.~Yamada, R.~Kulsrud, Numerical study of global stability of oblate field-reversed configurations, Phys. Plasmas, \textbf{8}(4), 1267 (2001)} which characterized important thresholds for these instabilities. The tilt mode changes from an internal mode to an external mode with decreasing FRC elongation, and in the oblate case it can be stabilized with a close-fitting conducting wall. By increasing the edge separatrix pressure for wall-supported FRCs, the growth rate of interchange mode decreases, and complete stabilization is achieved when the separatrix beta exceeds 30\%. Simulations of the dynamics of FRC formation from two counter-helicity spheromaks are beginning, and preliminary results will be presented. [Preview Abstract] |
|
GP8.00042: Physics-Based Preconditioning for a Radially Compressed FRC Model A.H. Glasser, V.S. Lukin SEL is a parallel code for solving initial-boundary value problems for coupled nonlinear fluid equations, using high-order spectral elements for discretization in 2 spatial dimensions, and a fully-implicit time step. Efficient parallel operation requires a scalable method for solving large, sparse matrices. In previous work, a framework for such a solver has been developed, separated into general-purpose modules for all applications and problem-specific code for each application. The heart of the method is physics-based preconditioning, which reduces the order and enhances the diagonal dominance of the linear systems to be solved. Approximations introduced at this stage are eliminated by matrix-free Newton-Krylov iteration on the full nonlinear system. The details of the approach depend on the coupling of the various physical variables. It has been successfully tested for ideal MHD waves in a doubly periodic plane. The purpose of this presentation is to describe the development and testing of physics-based preconditioning for a more interesting test case, modeling a radially compressed FRC with extend MHD. [Preview Abstract] |
|
GP8.00043: Perfecting Braginskii's eletron transport coefficients for high collisionality plasmas Jeong-Young Ji, E.D. Held It is known that Braginskii's transport coefficients for the heat flow and frictional force are in error up to 65\% for some finite values of $x=\Omega \tau$ and have significant error in the large-$x$ limit.~\footnote{E. M. Epperlein and M. G. Haines, Phys. Fluids {\bf 29}, 1029 (1986).} Here $\Omega$ is the electron-cyclotron frequency, and $\tau$ is the electron-ion collision time. In this work, we find fitting formulas which are practically exact (less than 1\% error) for the whole range of $x$ and $Z$, the ion charge, checked up to $Z$=100. The new fitting formulas are based on calculations with 160 moments (Laguerre polynomials)~\footnote{J.-Y. Ji and E. D. Held, Phys. Plasmas {\bf 13}, 102103 (2006); {\bf 15}, 102101 (2008).} for $x<100$ and on the asymptotic form for large $x$-values. [Preview Abstract] |
|
GP8.00044: Hybrid Kinetic-MHD Studies of ICC Devices using Lorentz PIC in Finite Elements Charlson C. Kim Progress on the implementation of a Lorentz force particle in cell (PIC) module for the NIMROD code wil be presented. This is an extension of the drift kinetic $\delta\!f$ PIC module already in place in NIMROD\footnote{C. C. Kim ``Impact of velocity space distribution on hybrid kinetic-magnetohydrodynamic simulation of the (1,1) mode'', Phys. Plasmas 15, 072507 (2008)}. Particular challenges of the implementation are PIC in high order finite element grids, resolving the finite Larmour radius (FLR) effect resulting from the Lorentz force (both spatially and temporally), and coupling to the resistive time scales of the MHD fluid variables. The Lorentz force PIC module will be used to study the effects of energetic ions on stability and confinement of innovative confinement concept (ICC) devices. FLR effects may also be important in the high energy tails of energetic particle distributions found in tokamaks. As an initial application, we use the the Lorentz PIC module as tracer particles to study trajectories in ICC devices and the resultant phase space distribution function. We also present simulations of FLR effects on tearing modes of reversed field pinch configurations. Comparisons with the drift kinetic model will also be presented where possible. [Preview Abstract] |
|
GP8.00045: A Tetrahedral Mesh 3-D Ideal MHD Equilibrium Code George J. Marklin, Chris Hansen A new code has been developed to solve the zero beta equilibrium equation mu0*J = curl(B) = Lambda*B on a tetrahedral mesh in an arbitrary 3-D geometry. The function Lambda(x) must be constant along field lines and on flux surfaces, but may be specified arbitrarily across surfaces, where surfaces exist. It must be spatially constant in ergodic regions where there are no surfaces. The solution is computed by alternately solving curl(B) = Lambda*B with Lambda specified, and then updating Lambda = Lambda(B) and iterating until converged. The first equation is easily solved using standard methods for computing Taylor states. The Lambda update is most easily accomplished by tracing field lines and attempting to find the q value, which is constant on a magnetic surface but generally does not exist for lines that do not lie on surfaces. Lambda may then be specified as a function of q where the q value exists, and constant where the q value does not exist. Examples will be presented of 3-D equilibria in HIT-SI for different ratios of spheromak current to injector current and injector current to injector flux. [Preview Abstract] |
|
GP8.00046: Plasma Solution Quality in Distorted, Body-Fitted Meshes in HiFi/SEL Weston Lowrie, Vyacheslav Lukin, Uri Shumlak Highly distorted meshes can yield significant errors in field solutions. Mesh metrics have been widely used to quantify distortion on computational meshes, although they rarely make strong correlations to solution accuracy. In this study several mesh quality metrics are compared to solution error norms for varying types and degrees of mesh deformation. The goal is to be able to better predict the solution error magnitudes based solely on mesh metrics. This work is done using the HiFi/SEL high-order finite (spectral) element code, which is a code that can incorporate many physics models, including the extended MHD plasma model. The code also makes use of a multiblock framework, where several structured logical blocks are coupled together into one computational domain. Each block uses a logical to physical grid transformation to deform the mesh into some physical shape of interest. This can sometimes yield highly distorted meshes, and it is important to know if a particular region of the domain is of acceptable quality. [Preview Abstract] |
|
GP8.00047: Non-reflecting boundary conditions for dissipative MHD E.T. Meier, A.H. Glasser, V.S. Lukin, U. Shumlak Non-reflecting boundary conditions (NRBCs) are frequently used to truncate computational domains without disruptive boundary effects. NRBC techniques are well established for hyperbolic problems, but not for mixed hyperbolic/parabolic equation systems like dissipative MHD. Practical MHD computation with C0-continuous spectral or finite element codes (the focus of this research) requires dissipation to ensure finite gradient length scales. For the Euler and ideal MHD equations, a hyperbolic-based NRBC has been implemented in SEL/HiFi. Simulation results and details of the implementation are presented. Progress toward an effective and stable NRBC for more complicated mixed hyperbolic/parabolic systems like disspative MHD and extended MHD is discussed. Derivations of well-posed NRBC for the Navier-Stokes equations (e.g. [1]) provide a basis for further development. \\[4pt] [1] J. Nordstrom and M. Svard, Well-posed boundary conditions for the Navier-Stokes equations, SIAM J. Numer. Anal., 43 (2005) 1231 [Preview Abstract] |
|
GP8.00048: Nimrod Simulations of FRC Formation with Rotating Magnetic Field Current Drive R.D. Milroy, C.C. Kim, C.R. Sovinec Three dimensional simulations of Field Reversed Configuration (FRC) formation and sustainment with Rotating Magnetic Field (RMF) current drive have been performed with the NIMROD code. The Hall term is a zeroth order effect with strong coupling between Fourier components, and recent enhancements to the NIMROD preconditioner allow much larger timesteps than was previously possible. A two-fluid option with a finite electron mass set equal to 1/100 of the ion mass is used. Boundary conditions to capture the effects of a finite length RMF antenna have been added, and simulations of FRC formation from a uniform background gas have been performed with parameters relevant to the TCSU experiment at the University of Washington. Results will be compared with experimental observations. The calculations indicate that the current drive extends considerably beyond the ends of the antenna, that the RMF appears to have a strong stabilizing effect on the configuration, and that an electron poloidal flow (poloidal current) provides the current drive on the inner field lines. [Preview Abstract] |
|
GP8.00049: Simulation and Visualization Progress of the PSI-Center Interfacing Group B.A. Nelson, C.C. Kim, R.D. Milroy, T.R. Jarboe The Interfacing Group of the Plasma Science and Innovation Center (PSI-Center - http://www.psicenter.org) performs simulations of collaborating Innovative Confinement Concept (ICC) experiments. Collaborators include the Bellan Plasma Group (Caltech), FRX-L (Los Alamos National Laboratory), HIT-SI (Univ of Wash - UW), LDX (M.I.T.), MST, Pegasus (Univ of Wisc-Madison), PHD (UW), SSPX (Lawrence Livermore National Laboratory), SSX (Swarthmore College), TCS (UW), and ZaP (UW). Extensive simulations of the Caltech experiment study the formation of a kink mode and its dependence on vessel size and current waveform. Simulations of translation and formation of field-reversed configurations (FRCs) continue. Initial Z-pinch sheared-flow stabilization studies are being performed. Output files from NIMROD and its nimplot postprocessor suite are interfaced to the powerful 3-D visualization program, VisIt (http://www.llnl.gov/visit). Results from these simulations, as well as an overview of the Interfacing Group status will be presented. [Preview Abstract] |
|
GP8.00050: Numerical Simulation of Non-inductive Startup and Flux Compression in the Pegasus Toroidal Experiment J.B. O'Bryan, C.R. Sovinec, D.J. Battaglia, T.M. Bird Nonlinear numerical computation is used to investigate DC helicity injection from washer-gun plasma sources in a spherical torus. The simulations model non-inductive startup in the Pegasus Toroidal Experiment (Univ. of Wisconsin), including relaxation of the current channels into a ``tokamak-like'' plasma and current amplification through flux compression. Our resistive MHD simulations with the NIMROD code (nimrodteam.org) use three-dimensional, anisotropic, temperature-dependent thermal conduction---corrected for regions of low-magnetization [Braginskii, Reviews of Plasma Physics, 1965]---and temperature-dependent resistivity. We investigate the effectiveness of the relaxation and flux compression, in terms of current drive, heating, and confinement. Preliminary results on current amplification are in rough agreement with the experiment and indicate significant plasma heating. [Preview Abstract] |
|
GP8.00051: Status of TRANSP and PTRANSP Douglas McCune, Robert Andre, Eliot Feibush, Marina Gorelenkova, K. Indireshkumar, Christiane Ludescher-Furth, Lew Randerson, Glenn Bateman, Arnold Kritz This poster describes the status of TRANSP and PTRANSP code development and run production operations. Production rates continue to climb as new users and tokamaks are added; statistics will be shown, including utilization of the recently added TRANSP MPI capability. New code features include greatly expanded flexibility in specification of transport models for density, temperature, and angular momentum profile prediction in PTRANSP runs. A new replay option enables retrieval of sources from TRANSP analysis runs for PTRANSP validation. TRANSP / PTRANSP free boundary MHD modeling options have been enhanced. New RF modeling capabilities, such as the ability to model ICRF with full toroidal mode spectrum, have been added. NUBEAM upgrades include improvements to deposition atomic physics and loss orbit distribution capture. Fusion Grid post-processing of TRANSP results has been improved using extraction of SWIM-SciDAC Plasma State files from TRANSP archives. Opportunities for PTRANSP support of SciDAC and FSP efforts will be presented. [Preview Abstract] |
|
GP8.00052: New RF heating and current drive codes in TRANSP K. Indireshkumar, D. McCune, J. Wright, R.W. Harvey, A.P. Smirnov Radio Frequency (RF) heating/current drive comprise an important aspect of present and future tokamaks. The PPPL tokamak transport code (TRANSP) has many RF modules currently available. This poster describes improvements to the RF modeling capabilities of TRANSP by incorporating the general ray tracing code (GENRAY) and upgrading the capabilities of the currently available TORIC code by including its parallel solver capabilities. Currently efforts are underway to incorporate the electron cyclotron (EC) heating/current drive aspects of GENRAY into transp. GENRAY has been built as a TRANSP library and a transp-based executable has been written. Routines from GENRAY have been adapted to produce the input files to GENRAY from the PPPL plasma state software and load the results into the plasma state. In regard to parallel TORIC, a transp executable has been developed and tested. Routines have been developed to broadcast the input data for parallel toric operation. Scaling studies of TORIC are underway on the PPPL Kruskal cluster. This poster will describe the status of GENRAY and Parallel TORIC in TRANSP along with results of scaling studies. [Preview Abstract] |
|
GP8.00053: Effect of anomalous transport on kinetic simulations of the H-mode pedestal G. Bateman, A.Y. Pankin, A.H. Kritz, T. Rafiq, G.Y. Park, S. Ku, C.S. Chang The MMM08 and MMM95 Multi-Mode transport models [1,2], are used to investigate the effect of anomalous transport in XGC0 gyrokinetic simulations [3] of tokamak H-mode pedestal growth. Transport models are implemented in XGC0 using the Framework for Modernization and Componentization of Fusion Modules (FMCFM). Anomalous transport is driven by steep temperature and density gradients and is suppressed by high values of flow shear in the pedestal. The radial electric field, used to calculate the flow shear rate, is computed self-consistently in the XGC0 code with the anomalous transport, Lagrangian charged particle dynamics and neutral particle effects. XGC0 simulations are used to provide insight into how thermal and particle transport, together with the sources of heat and charged particles, determine the shape and growth rate of the temperature and density profiles. [1] F.D. Halpern \textit{et al}., Phys. Plasmas 15 (2008) 065033; J.Weiland \textit{et al}., Nucl. Fusion 49 (2009) 965933; A.Kritz\textit{ et al}., EPS (2009) [2] G. Bateman, \textit{et al,} Phys. Plasmas \textbf{5} (1998) 1793 [3] C.S. Chang, S. Ku, H. Weitzner, Phys. Plasmas \textbf{11} (2004) 2649 [Preview Abstract] |
|
GP8.00054: Study of neoclassical effects on the pedestal structure in ELMy H-mode plasmas A.Y. Pankin, G. Bateman, A.H. Kritz, T. Rafiq, G.Y. Park, S. Ku, C.S. Chang, P.B. Snyder The neoclassical effects on the H-mode pedestal structure are investigated in this study. First principles' kinetic simulations of the neoclassical pedestal dynamics are combined with the MHD stability conditions for triggering ELM crashes that limit the pedestal width and height in H-mode plasmas. The neoclassical kinetic XGC0 code [1] is used to produce systematic scans over plasma parameters including plasma current, elongation, and triangularity. As plasma profiles evolve, the MHD stability limits of these profiles are analyzed with the ideal MHD stability ELITE code [2]. The scalings of the pedestal width and height are presented as a function of the scanned plasma parameters. Simulations with the XGC0 code, which include coupled ion-electron dynamics, yield predictions for both ion and electron pedestal profiles. Differences in the electron and ion pedestal scalings are investigated. \newline [1] C.S. Chang et al, Phys. Plasmas 11 (2004) 2649. \newline [2] P.B. Snyder et al, Phys. Plasmas, 9 (2002) 2037. [Preview Abstract] |
|
GP8.00055: Plans for kinetic particle-based predictive transport modeling of whole-volume tokamak plasma C.S. Chang, G. Park, S. Ku, H. Strauss, L. Sugiyama, G. Bateman, A. Kritz, D. Stotler, J. Cummings, S. Parker, S. Klasky Predictive transport modeling codes, such as PTRANSP, are based upon reduced transport equations, and require ad-hoc closures for most of the critical equilibrium physics phenomena (finite banana width effect, ExB shearing rate profile, orbit loss, MHD- effect, rf-driven transport, impurity transport, alpha particle transport, etc). SciDAC proto-FSP CPES is developing a kinetic particle-based transport modeling code XGC0, which utilizes the modern parallel computing environment ($\sim$1,000 processors) and which can evaluate these equilibrium kinetic physics phenomena at first principles level. The basic dynamics of ion, electron and impurity particles obey the Lagrangian guiding-center equation of motion in the self-consistent equilibrium electric field. The modeling region is the whole volume, from the magnetic axis to the material wall. The physics capability includes or will include radial anomalous transport coefficients, Ohmic heating, various auxiliary heatings, neutral kinetic transport with wall-recycling, impurity transport, MHD instabilities (in code integration), fusion reaction, and alpha particles. [Preview Abstract] |
|
GP8.00056: Transport analysis in toroidal helical plasmas using the integrated code: TASK3D A. Wakasa, A. Fukuyama, S. Murakami, C.D. Beidler, H. Maassberg, M. Yokoyama, M. Sato The integrated simulation code in helical plasmas, TASK3D, is being developed on the basis of an integrated modeling code for tokamak plasma, TASK. In helical systems, the neoclassical transport is one of the important issues in addition to the anomalous transport, because of strong temperature dependence of heat conductivity and an important role in determining the radial electric field. We have already constructed the neoclassical transport database in LHD, DGN/LHD. The mono-energetic diffusion coefficients are evaluated based on the Monte Carlo method by DCOM code and the mono-energetic diffusion coefficients database is constructed using a neural network technique. Also we apply GSRAKE code, which solves the ripple-averaged drift kinetic equation, to obtain transport coefficients in highly collisionless regime. We have newly incorporated the DGN/LHD module into TASK3D. We will present several results of transport simulation in typical LHD plasmas. [Preview Abstract] |
|
GP8.00057: Recent Advances in Simulation of Wave Interactions with Extended MHD Phenomena Donald Batchelor The Integrated Plasma Simulator (IPS), developed by the Simulation of Wave interactions with MHD (SWIM), project provides a framework within which some of the most advanced, massively-parallel fusion modeling codes can be interoperated to provide a detailed picture of the multi-physics processes involved in fusion experiments. The presentation will cover four topics: 1) recent improvements to the IPS, 2) application of the IPS for very high resolution simulations of ITER scenarios, 3) studies of resistive and ideal MHD stability in tokamak discharges using IPS facilities, and 4) the application of RF power in the electron cyclotron range of frequencies to control slowly growing MHD modes in tokamaks and initial evaluations of optimized location for RF power deposition. [Preview Abstract] |
|
GP8.00058: Modeling ECCD/MHD coupling using NIMROD, GENRAY, and the Integrated Plasma Simulator Thomas G. Jenkins, D.D. Schnack, C.R. Sovinec, C.C. Hegna, J.D. Callen, F. Ebrahimi, S.E. Kruger, J. Carlsson, E.D. Held, J.-Y. Ji, R.W. Harvey, A.P. Smirnov, W.R. Elwasif We summarize ongoing theoretical/numerical work relevant to the development of a self--consistent framework for the inclusion of RF effects in fluid simulations; specifically, we consider the stabilization of resistive tearing modes in tokamak geometry by electron cyclotron current drive. In the fluid equations, ad hoc models for the RF--induced currents have previously been shown to shrink or altogether suppress the nonlinearly saturated magnetic islands generated by tearing modes; progress toward a self--consistent model is reported. The interfacing of the NIMROD [1] code with the GENRAY/CQL3D [2] codes (which calculate RF propagation and energy/momentum deposition) via the Integrated Plasma Simulator (IPS) framework [3] is explained, RF-induced rational surface motion and the equilibration of RF--induced currents over plasma flux surfaces are investigated, and the efficient reduction of saturated island widths through time modulation and spatial localization of the ECCD is explored. [1] Sovinec {\it et al.}, JCP {\bf 195}, 355 (2004) [2]www.compxco.com [3] Both the IPS development and the research presented here are part of the SWIM project. Funded by U.S. DoE. [Preview Abstract] |
|
GP8.00059: Correlation Effects in ICRF Diffusion Coefficients Yu. Petrov, R.W. Harvey, E.F. Jaeger, L.A. Berry, D.B. Batchelor, P.T. Bonoli, J.C. Wright RF quasi-linear (QL) theory of wave-plasma interaction assumes at each resonant surface that a group of ions has no gyrophase-memory from the previous crossings. In contrast, the DC (Diffusion Coefficient) code obtains coefficients by direct numerical integration of the Lorentz force equation for ion motion in combined equilibrium and RF fields from the AORSA full-wave code, thus keeping gyrophase information. For a single toroidal mode, strong correlation effects are observed in form of peaks and dips in momentum space. Nevertheless, for a C-Mod minority ion ICRF heating test case, the radial profiles of power absorption calculated by the CQL3D Fokker-Planck code using DC results, agree well with those calculated by AORSA based on QL theory, at least at early times where the ion distribution is isotropic about the B-field. However, as the ion distribution develops anisotropy, differences in power absorption appear. We also examine effects of multiple ICRF toroidal modes in ITER. [Preview Abstract] |
|
GP8.00060: Equilibrium relations in a two-fluid and drift-kinetic theory of weakly collisional, magnetically confined plasmas J.J. Ramos A two-fluid system with finite-Larmor-radius drift-kinetic closures, applicable to weakly collisional, magnetically confined plasmas is presented. The ordering of the collisionality differs from more traditional approaches in that the ratio of mean free paths to equilibrium confinement lengths is assumed to be of the same order as the ratio of such macroscopic plasma lengths to the ion gyroradius, the inverse of which is adopted as the basic expansion parameter $\delta \ll 1$. The drift-kinetic equations are expressed in the moving reference frames of the macroscopic flows, which facilitates their coupling to and precise compatibility with the complementary two-fluid, extended-MHD equations, and the macroscopic flows are assumed of the order of the diamagnetic drift velocities. Different electron and ion equations are derived, based on the ordering of the small mass ratio as $\delta^2$. Considering axisymmetric equilibria with diamagnetic flows, several new results are obtained. These include specific relations for the particle and heat flows, the electric potential, the density and the pressure anisotropies, a precise evaluation of the ion gyroviscosity and a novel expression for the parallel collisional friction force. [Preview Abstract] |
|
GP8.00061: Neoclassical parallel closures for toroidal plasmas Mukta Sharma, E.D. Held, J.Y. Ji Closures for the parallel conductive heat fluxes and stresses are derived. A Chapman-Enskog-like approach is adopted and time-dependent effects are ordered small compared to parallel free streaming, collisional effects and particle trapping in magnetic wells. The distribution function is written as the sum of a dynamic Maxwellian and a kinetic distortion, $F$, expanded in Legendre polynomials. To lowest order, the magnetic moment and total energy of the particles are conserved. For an accurate treatment of collisional effects, a moment approach is applied to the full, albeit linearized, Coulomb collision operator. In contrast to previous derivations\footnote{E.D. Held, {\it et al}., Phys. Plasmas {\bf 10}, 3933 (2003).}, this work does not bounce-average when solving the lowest-order drift kinetic equation. In contrast, a Fast Fourier Transform algorithm is used to treat the one-dimensional spatial domain along the magnetic field and the drift kinetic equation is solved on a grid in the speed variable, $s=v/v_T$. This approach allows for parallel acceleration as well as examination of the closures in all collisionality regimes, i.e., Pfirsch-Schlueter, plateau and banana. The application of these closures in the NIMROD code is also discussed. [Preview Abstract] |
|
GP8.00062: Warping Simulation Time: Multi-Dimensional Asynchronous Hybrid Simulations Yuri Omelchenko, Homa Karimabadi Fusion and space plasmas are often characterized by the strong presence of ion kinetic effects and multiple temporal scales associated with plasma and magnetic field inhomogeneities. Traditional hybrid (electron fluid/particle ion) simulations of such systems face severe restrictions on the global timestep and sometimes even fail to resolve fine spatial structures. Recently we introduced a new approach to multi-scale modeling [1]: Discrete-Event Simulation (DES). Our new multi-dimensional code, HYPERS (HYbrid-Particle Event-Resolved Simulation) proceeds by adaptively selecting physically appropriate time increments for individual particles and local electromagnetic fields. As a result, numerical updates are always carried out when needed only. This enables fast, reliable and accurate simulations of energetic plasmas immersed in highly inhomogeneous magnetic fields. We report preliminary results from simulations of the interaction of streaming plasmas with dipole magnetic obstacles and discuss the applicability of this technique to other plasma configurations (magnetic confinement schemes) and theoretical approximations (MHD, two-fluid,Vlasov). \\[4pt] [1] Y.A. Omelchenko, and H. Karimabadi, J. Comp. Phys. 216, 153 (2006). [Preview Abstract] |
|
GP8.00063: Modeling of Anomalous Plasma Current Evolution in Tokamak Hybrid Operation Scenarios Hyunseok Kim, Wonjae Lee, Yong-su Na Tokamak experiments worldwide have discovered hybrid scenarios, which exhibit high confinement and stability simultaneously, where the q-profile is relatively flat at the centre with qmin ~ q0 ~ 1 in stationary conditions. However, the reasons for high fusion performance and for sustainment of flat q-profiles in hybrid scenarios are not clarified yet. In this work, a numerical modeling is carried out using ASTRA to simulate an anomalous current evolution, observed in some experiments and to figure out factors which affect the flat and stationary q- profile in hybrid scenarios. Firstly, the neoclassical resistivity is used for the simulation and compared with experiments. Then, the additional terms based on the ohm's law, like an anomalous parallel plasma resistivity, a hyper resistive term and etc., are introduced and their effects are investigated by comparing with experiments. Here, the parallel ohm's law for mean magnetic fields is adopted for current diffusion. The modeling results are expected to contribute to understand physical background of improvement of confinement and stability in hybrid operation scenarios. [Preview Abstract] |
|
GP8.00064: TGLF Transport Modeling With PTRANSP/GCNMP H.E. St. John, L.L. Lao, G.M. Staebler, J.E. Kinsey, P.B. Snyder, R.V. Budny, D. McCune We describe our effort in getting the turbulent TGLF confinement model operational with the PTRANSP transport code. Numerically intensive, TGLF requires parallel operation for determination of turbulent fluxes. This has driven our interface design to PTRANSP and motivated the purchase of a small parallel computing cluster for development. We note that larger clusters would be appropriate to exploit further parallel structure present in TGLF. A direct flux solver was developed to solve both the time-dependent and time-independent transport equations over a variable radial transport grid. A universal communication scheme based on netcdf data representation is under continuing development that will allow translation of PTRANSP, ONETWO, GCNMP, XPTOR, and GYRO code run time information from any of the codes to any other. Cross validation of results thus becomes much more feasible. Our initial efforts comparing XPTOR and PTRANSP/GCNMP calculations are discussed. Our initial foray into EPED1 type edge stability analysis is described. [Preview Abstract] |
|
GP8.00065: On the Development of a Scalable Implicit Stabilized Finite-Element Resistive MHD Solver L. Chac\'on, J.N. Shadid, R.P. Pawlowski We explore the development of a massively parallel, scalable fully-implicit stabilized unstructured finite element (FE) capability for low-Mach-number resistive MHD. We focus on the development of the stabilized FE formulation and the underlying fully-coupled preconditioned Newton-Krylov nonlinear iterative solver. To enable robust, scalable and efficient solution of the large-scale sparse linear systems generated by the Newton linearization, fully-coupled algebraic multilevel preconditioners are employed. These preconditioning methods are based on a variable-overlap additive one- level Schwarz preconditioner and a relatively new algebraic multilevel technique that employs a graph-based aggressive-coarsening aggregation method applied to the nonzero block structure of the Jacobian matrix. We will present verification results that demonstrate the expected order-of-accuracy, including prototype problems such as an MHD Faraday conduction pump, the hydromagnetic Rayleigh-Bernard instability, the magnetic island coalescence problem, and 3D tokamak calculations using Solove'ev equilibria. We will discuss initial encouraging results that explore the scaling of the solution methods on up to 4096 processors for problems with up to 64M unknowns on a CrayXT3/4, and will demonstrate a successful large-scale proof-of-capability calculation for 1 billion unknowns for the MHD Faraday pump problem on 24,000 cores. [Preview Abstract] |
|
GP8.00066: Nonlinear Simulations of Energetic Particle-induced Geodesic Acoustic Mode G.Y. Fu, R. Nazikian, R. Budny, M. Gorelenkova An Energetic Particle-induced Geodesic Acoustic Modes (EGAM) was previously shown to exist based on analytic theory and numerical simulation [1], in agreement with the recent experimental results of the beam-driven GAM-like n=0 mode in DIII-D [2]. In this work, nonlinear simulations of EGAM are carried out using a hybrid model in which the thermal plasma is modeled as a fluid whereas the energetic particle component is described by the drift-kinetic equation. For an analytic energetic particle distribution function, simulation results show initial saturation due to the flattening of particle distribution function in velocity space, followed by a bursting feature with frequency chirping. In order to model the DIII-D experiments more closely, realistic simulations have been carried out using the experimental neutral beam deposition profile from the TRANSP/NUBEAM code. Numerical results show mode bursting with clear radial propagation. The calculated mode frequency, mode radial extent, and density fluctuation level are consistent with the experimental observation in DIII-D [2]. \\[4pt] [1] G. Y. Fu, Phys. Rev. Letts. 101, 185001 (2008)\\[0pt] [2] R. Nazikian et al., Phys. Rev. Letts. 101, 185001 (2008). [Preview Abstract] |
|
GP8.00067: An Efficient Method for Verifying Gyrokinetic Microstability Codes R. Bravenec, J. Candy, W. Dorland, C. Holland Benchmarks for gyrokinetic microstability codes can be developed through successful ``apples-to-apples'' comparisons among them. Unlike previous efforts, we perform the comparisons for actual discharges, rendering the verification efforts relevant to existing experiments and future devices (ITER). The process requires i) assembling the experimental analyses at multiple times, radii, discharges, and devices, ii) creating the input files ensuring that the input parameters are faithfully translated code-to-code, iii) running the codes, and iv) comparing the results, all in an organized fashion. The purpose of this work is to automate this process as much as possible: At present, a python routine is used to generate and organize GYRO input files from TRANSP or ONETWO analyses. Another routine translates the GYRO input files into GS2 input files. (Translation software for other codes has not yet been written.) Other python codes submit the multiple GYRO and GS2 jobs, organize the results, and collect them into a table suitable for plotting. (These separate python routines could easily be consolidated.) An example of the process -- a linear comparison between GYRO and GS2 for a DIII-D discharge at multiple radii -- will be presented. [Preview Abstract] |
|
GP8.00068: The para-real algorithm applied to a simple drift-wave turbulence model for transport timescale studies Debasmita Samaddar, David E. Newman, Raul Sanchez We will present the results of applying the para-real algorithm\footnote{Martin J. Gander et al, Siam J. Sci. Comput. Vol.29, No.2, pp.556-578} to a fluid turbulence code. This new technique efficiently parallelizes the time domain and has been found to significantly reduce the computational wall time. If successfully applied, this will allow study of the turbulent transport dynamics on transport time scales - something that has heretofore been very difficult. We apply this algorithm to simulations of drift-wave turbulence\footnote{D.E. Newman et al, Phys. Fluids B 5, 1140 (1993)} in slab geometry as a function of the model and sheared flow. Preliminary science results as well as computational results will be presented. These include cases in which the relative dominance ExB nonlinearities will be tuned artificially and the transport exponents will be explored as the sheared flow changes. [Preview Abstract] |
|
GP8.00069: Benchmarking of the 2DX eigenvalue code D.A. Baver, J.R. Myra, M. Umansky The 2DX code is a linear eigenmode solver designed for toroidal plasma configurations with an x-point topology. Together with recent upgrades to BOUT, it is part of a project to provide validation and verification capability for large-scale turbulence codes. Comparing linear growth rates to a single simple linear code provides a reproducible result that can be used as a common standard. In addition to its topological capabilities, the 2DX code is distinct in its use of an equation language to input equations to be solved. This gives it the flexibility to handle nearly arbitrary systems of equations, giving it potential applications far beyond the scope of the current project. For purposes of the present project, however, editing the equation language is a potential source of error, so we will be presenting results from a standardized 6-field model instead. We will present 2DX results from both analytic and full geometry test cases. We will also present comparisons with BOUT. These tests will serve to build confidence in the 2DX code as a valuable tool for V{\&}V. Work supported by the U.S. DOE under grant DE-FG02-07ER84718. [Preview Abstract] |
|
GP8.00070: Progress on the Collisional-Radiative Modeling and the Analysis of Local Line Emission Profiles of Carbon Impurities from TJ-II Plasmas R. Florido, D. Suarez, R. Rodriguez, J.M. Gil, J.G. Rubiano, P. Martel, E. Minguez, B. Zurro, A. Baciero In a simple previous analysis the ratio of local emission profiles of carbon impurities (C$^{4+}$ at 227.1 nm and C$^{5+}$ at 529.0 nm) measured from TJ-II plasmas was found to have a significant dependence of neutral density profile. Hence, the aim of this work is to have a better theoretical understanding of this system and finally validate or discard the carbon spectral line ratio as a useful tool to estimate the neutral density profile in ECRH TJ-II plasmas. To assist in the theoretical modeling of this carbon system we use the collisional-radiative code ABAKO, which has been successfully applied to different ICF problems. Progress on the collisional-radiative modeling and simulation of MCF TJ-II plasmas are reported, paying particular attention to the influence of the charge-exchange recombination processes on the charge state distribution and carbon line ratio radial profiles. [Preview Abstract] |
|
GP8.00071: 3D-MAPTOR Code for computation of magnetic fields in tokamaks Esteban Ch\'{a}vez-Alarc\'{o}n, Julio Herrera-Vel\'{a}zquez A 3dimensional code has been developed in order to determine the magnetic field in tokamaks, starting from the assumption that the toroidal and vertical field coils are all circular, as well as the cross section of the plasma current distribution. It was earlier used to study the stochastization of the outer magnetic surfaces [1] and to reconstruct the evolution of the plasma column, using the experimental signals of tokamak discharges. These results were compared with tomographic reconsructions of the ISTTOK tokamak [2]. We present an upgrade of the code, in which rectangular toroidal field coils and D shaped plasma current cross sections can be included. The code is particularly useful to study the effect of the ripple along the toroidal coordinate.\\[4pt] [1] E. Ch\'{a}vez, et al., ``Stochastization of Magnetic Field Surfaces in Tokamaks by an Inner Coil'' in Plasma and Fusion Science, AIP Conference Proceedings Series 875 (2006) pp.347-349.\\[0pt] [2] B.B. Carvalho, et al., ``Real-time plasma control based on the ISTTOK tomography diagnostic'', Rev. Sci. Instrum. 79 (2008)10F329. [Preview Abstract] |
|
GP8.00072: Nonlinear simulation of Toroidicity-induced Alfv\'{e}n Eigenmode with source and sink Jianying Lang, Guo-Yong Fu, Yang Chen It has been known that in collisionless plasmas the nonlinear saturation of energetic particle-driven modes is caused by wave-particle trapping effect, which flattens the spatial distribution function of resonant particles and reduces the drive. However, when energetic particles are sufficiently collisional, much more complicated physics is involved during the nonlinear process. The nonlinear behavior of a single TAE is studied using the kinetic/MHD hybrid code m3d-K [Fu, Phys. Plasmas, 2006] in the presence of pitch-angle scattering, source, sink, and the slowing-down process. In the presence of only pitch-angle scattering, both steady state and pulsation behaviors are observed depending on the regime of collision rate. For steady state cases, the scaling of nonlinear saturation level with collision rate agrees with the analytical prediction [Berk, Phys. Plasmas, 1990]. Our preliminary simulations with source, sink, and slowing-down process have obtained nonlinear steady state, where the saturation level increases with slowing-down rate. Flattening in the spatial distribution function is observed during nonlinear saturation. Detailed variation of distribution function and parameter scaling will be further explored and compared to analytic theories. This work is supported by DOE Energetic Particle Simulation Center PEPSC. [Preview Abstract] |
|
GP8.00073: Edge Biasing of SINP-Tokamak Plasma in High-Q Regime Rabindranath Pal, Debjyoti Basu In high $q$ regime ($q_{edge}=5-7)$ of SINP-TOKAMAK [an iron-core device having major and minor radii of $30$ and \textit{7.5 cm}, respectively and B$_{toroidal}$ = \textit{1.2 Tesla}] fast edge biasing experiment is carried out introducing a Molybdenum electrode of \textit{5mm} in diameter, radially positioned at \textit{7.0 cm}. Biasing seems to cause a change in plasma current density profile forming a negative shear in the region \textit{6.4-6.9 cm} and it leads to better confinement and longer duration of plasma current as was observed\footnote{Ghosh J., Pal R., Chattopadhyay P. K. and Basu D. 2007 Nucl. Fusion \textbf{47} 331} also in very low q (VLQ) regimes of the same machine. The electrode current drawn in this regime is about \textit{5-10 amp}. Lowering of H$_{\alpha}$ signal and loop voltage is also observed indicating better confinement, independently confirmed by diamagnetic loop too. On applying bias, electron density and temperature profile develop sharper gradient near the edge. Interestingly, electrostatic and magnetic fluctuations, observed by inserting electric and magnetic probes in the edge plasma, are suppressed in the inner region (\textit{6.4-6.8 cm}) in the frequency range of \textit{30-70 kHz} by the effect of electrode biasing. [Preview Abstract] |
|
GP8.00074: MHD, LINEAR AND NONLINEAR PHENOMENA |
|
GP8.00075: Measurements of reduced parallel electron distributions using whistler wave absorption Derek Thuecks, Fred Skiff, Craig Kletzing, Stephen Vincena We present the first results of a diagnostic designed to measure the reduced parallel electron distribution using resonant absorption of whistler waves at the electron cyclotron frequency. According to warm-plasma theory, a whistler wave that is swept in frequency is Doppler-shifted into resonance with those parts of the electron phase space density with a velocity component parallel to $B_0$. In our experiments, whistler waves were launched and received by a pair of dipole antennas immersed in a cylindrical discharge plasma at two positions along the axial background magnetic field. The whistler wave frequency was swept from somewhat below and up to $|\omega_{ce}|$. The measured wave absorption through the plasma was proportional to the reduced parallel electron distribution function. The background theory and initial results from this diagnostic are presented here. These first results show that this diagnostic succeeds in measuring changes to the distribution function due to cooling of the plasma during the transition from the discharge to the afterglow. [Preview Abstract] |
|
GP8.00076: Measurements of Turbulent Transport of Fast Ions Shu Zhou, Heinrich Boehmer, William Heidbrink, Roger McWilliams, Troy Cater, Pavel Popovich, Shreekrishna Tripathi, Steve Vincena Due to gyroradius averaging and drift-orbit averaging, the transport of fast ions by microturbulence is often smaller than for thermal ions. In this experiment, spatial transport of lithium fast ions [1] with gyroradii of 5.9 cm is studied in the LArge Plasma Device (LAPD). The baseline condition is a uniform quiet plasma; in the comparison condition, the cylindrical plasma column is compressed so the helical fast-ion orbits pass through the turbulent edge region (broadband drift-wave fluctuations with density fluctuations of $\sim $20{\%}) The fast-ion energy and pitch are varied to study drift averaging. Initial observations indicate that changes in parallel energy are more evident than radial transport. Measurements of the fluctuations by triple, swept Langmuir, and B-dot probes are compared with two-fluid simulations by the BOUT code. Calculations of the expected fast-ion transport in the simulated turbulence will be compared with the experimental profiles. \\[4pt] [1] Y. Zhang et al. , Rev. Sci. Instrum. 78, 013302 (2007). [Preview Abstract] |
|
GP8.00077: Confinement regimes in simple magnetized toroidal plasmas Ivo Furno, Ambrogio Fasoli, Benoit Labit, Paolo Ricci, Christian Theiler, Barrett Rogers In the simple magnetized torus TORPEX (R = 1 m, a = 0.2 m), we explore experimentally the accessibility of a high confinement mode, which is predicted by theory for this configuration [P. Ricci, et al., Phys. Rev. Lett. 1\textbf{00}, 225002 (2008)]. We consider different gases (H$_{2}$, D, He, Ne, N$_{2})$ and we measure the dependence of the temperature and density gradients upon the ratio $\gamma $/v$^{\mbox{'}}_{ExB}$ ($\gamma $ is the interchange linear growth rate and v$^{\mbox{'}}_{ExB}$ is the velocity shear). We observe that $\gamma $/v$^{\mbox{'}}_{ExB}$ decreases from $\gamma $/v$^{\mbox{'}}_{ExB}\sim $6-7 for hydrogen to $\gamma $/v$^{\mbox{'}}_{ExB}\sim $0.3-0.4 for neon. Consistently with theory, temperature and density profiles steepen when $\gamma $/v$^{\mbox{'}}_{ExB}$ drops below unity. The perpendicular particle flux is estimated by particle balance and measured using a multi-pin flux probe. First simulation results are presented with a global fluid code that solves the drift-reduced Braginskii equations in the whole TORPEX domain. [Preview Abstract] |
|
GP8.00078: Interchange instabilities, turbulence and fast ion interactions in the TORPEX device A. Fasoli, A. Diallo, L. Federspiel, I. Furno, D. Iraji, E. Kung, B. Labit, S. Muller, G. Plyushchev, M. Podesta, F. Poli, P. Ricci, C. Theiler Electrostatic turbulence, related structures and their effect on transport are investigated on TORPEX simple magnetized plasmas using high resolution diagnostics and several control parameters, fluid models and numerical simulations. A critical pressure gradient to drive the interchange instability is experimentally identified, consistently with linear theory. Interchange modes nonlinearly develop blobs, radially propagating filaments of enhanced plasma pressure. Blob velocities and sizes are obtained from probe measurements using pattern recognition and are described by an analytical expression that includes ion polarization currents, parallel sheath currents and ion-neutral collisions. Limiter configurations with varying angles between field lines and the metal surface are explored. To complement probe data, a fast framing camera and a movable gas puff system are installed. Density and light fluctuations show similar signatures of drift-interchange activity. Further developments of optical diagnostics, including an image intensifier and LIF, will be discussed. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector in scenarios for which the development from linear waves into blobs is fully characterized. [Preview Abstract] |
|
GP8.00079: Langmuir wave linear evolution in inhomogeneous nonstationary anisotropic plasma I.Y. Dodin, V.I. Geyko, N.J. Fisch Equations describing the linear evolution of a nondissipative Langmuir wave in inhomogeneous nonstationary anisotropic plasma without magnetic field are derived in the geometrical optics approximation. A continuity equation is obtained for the wave action density, and the conditions for the action conservation are formulated. In homogeneous plasma, the wave field $\tilde{E}$ universally scales with the electron density $N$ as $\tilde{E} \propto N^{3/4}$, thereby resulting in a modified adiabatic index of the plasma $\gamma = 3/2$, whereas the wavevector evolution varies depending on the wave geometry. [Preview Abstract] |
|
GP8.00080: Numerical studies of electrostatic eigenmodes in a pair-ion plasma Suwon Cho It has been known that there exist various waves in a pair-ion plasma having ions of equal mass and opposite charge without electrons and that a branch in the intermediate frequency range is a backward wave. The dispersion relation of electrostatic eigenmodes is studied to analysis these experimental results. Including the non-uniform density profile in a bounded plasma, the wave equation is converted to a matrix eigenvalue equation via the finite difference method, which is then solved numerically. The shooting method is also used to confirm the results. Numerical results shows that there can be a backward wave in a bounded plasma, while it is absent in the corresponding infinite plasma. In addition, the eigenmodes in an ordinary electron-ion plasma are examined for comparison. [Preview Abstract] |
|
GP8.00081: Nonlinear Wave Driven Processes in Plasmas Robert Bingham, R.M.G.M. Trines, L.O. Silva, J.J. Santos, B. Brandao, J.T. Mendonca, P.K. Shukla Nonlinear wave driven processes in plasmas are normally described by either a monochromatic pump wave that couples to other monochromatic waves, or as a random phase wave coupling to other random phase waves. An alternative approach involves a random or broadband pump coupling to monochromatic and/or coherent structures in plasmas. This approach can be implemented through the wave kinetic model. In this model, the incoming pump wave is described by wither a bunch (for coherent waves) or a rea (for random phase waves) of quasi-particles. We will present a generalized statistical theory describing a range of phenomena that include generation of zonal flows from drift mode turbulence, photon acceleration of intense lasers and intense radio waves and photon shocks. Possible applications include a diagnostic of large amplitude plasma waves and studies of zonal flows in planetary atmospheres will also be discussed. [Preview Abstract] |
|
GP8.00082: Equal energy phase space trajectories in resonant wave interactions Oded Yaakobi, Lazar Friedland Adiabatic evolution of a nonlinear resonantly driven wave system and two/three coupled waves system generic to a variety of plasma physics problems is studied. The corresponding Hamiltonian, depending on the coupling, detuning and nonlinear frequency shift parameters has a variable number of fixed points. The system can be bistable due to repeated separatrix crossing in the phase space. It is analytically shown that the oscillation periods along trajectories corresponding to the same value of the Hamiltonian are equal, and the difference of the corresponding areas under them is obtained as a function of the system parameters. A scheme of simultaneous adiabatic change of system parameters is constructed, in such a way that any pair of trajectories that have equal energy at some time will continue to have the same energy anytime. These results are generalizations of a previous work [O. Polomarov and G. Shvets, Phys. Plasmas, 13, 054502 (2006)] for a single resonantly driven wave. [Preview Abstract] |
|
GP8.00083: Multidimensional autoresonant three-wave interactions Lazar Friedland, Oded Yaakobi The theory of autoresonant three-wave interactions is generalized to more than one space and/or time variation of the background medium. In the most general case, the three waves propagate in a four-dimensional (4D) slowly space-time varying background, with an embedded 3D linear resonance hypersurface, where the linear frequency and wave-vector matching conditions of the three waves are satisfied exactly. The autoresonance in the system is the result of weak nonlinear frequency shifts and nonuniformity in the problem and is manifested by satisfaction of the nonlinear resonance conditions in an extended region of space-time adjacent to the resonance surface despite the variation of the background. The threshold condition for autoresonance is found. Asymptotic description of the autoresonant waves far away from the resonance surface is obtained. The theory is illustrated and tested in 2D numerical simulations. An application to stimulated Raman scattering in a nonuniform, time-dependent plasma case is discussed. [Preview Abstract] |
|
GP8.00084: A Simple Analytic Double Layer Francis Perkins A simple model comprised of Debye-shielding electrons and inter-penetrating ion beams leads to formation of double layers. The key non dimensional parameter is the ion-beam energy to electron temperature ratio. The simple model permits an analytic solution which will be presented. [Preview Abstract] |
|
GP8.00085: Nonlinear traveling waves in energetic particle phase space Boris Breizman The near-threshold regimes of wave excitation by energetic particles reveal a rich family of nonlinear scenarios, including formation of nonlinear structures with time-dependent frequencies. Previous descriptions of such structures were limited to the case of small deviations of their frequencies from bulk plasma eigenfrequencies. However, in many frequency-sweeping events, the range of sweeping is comparable to the frequency itself. The need to interpret such dramatic phenomena requires a non-perturbative formalism, which this work presents. The underlying idea is that the time-dependent frequencies represent nonlinear traveling waves in fast-particle phase space. Such waves form due to initial instability and resonant particle trapping, and they then evolve slowly due to dissipation. The coherent bunches of trapped particles slow down considerably during this process, which results in significant frequency sweeping. An analytic solution of this type has been constructed for a simple one-dimensional model. This solution suggests an efficient numerical approach to modeling frequency-sweeping events in tokamaks. [Preview Abstract] |
|
GP8.00086: Interaction of Electron Holes with Ion Density Perturbations David L. Newman, Martin V. Goldman, Naresh Sen, Haihong Che Kinetic simulations show that electron phase space holes in a homogeneous ion background can persist for long times with little change in speed, shape, or size. Spatial perturbations in the ion density change this situation: Short-scale ion perturbations comparable in length to the size of the holes themselves can degrade the coherence of individual holes. Ion perturbations with much longer scale lengths, however, have a quite different effect, causing holes to accelerate toward density maxima with little degradation. This latter behavior may account for the correlation between the hole spatial location and the location of density maxima observed in two recent 2-D Vlasov simulation studies: In the first case, the perturbations are due to ion-Bernstein waves near double layers in Earth's auroral zone [N. Sen, Ph.D. Thesis, Univ. of Colorado, 2009]. In the second case the density maxima are due to lower-hybrid waves in Buneman-unstable current channels [D.~L.~Newman and M.~V.~Goldman, 2008 Fall AGU Meeting, SM31B-1735] inferred from the separatrix region in published PIC simulations of magnetic reconnection. The detailed nature of the interaction of holes and various scale density perturbations will be addressed using 1-D and 2-D Vlasov simulations. [Preview Abstract] |
|
GP8.00087: Parallel Electric Field in Nonlinear Magnetosonic Waves in Tow- and Three-Component Plasmas Seiichi Takahashi, Yukiharu Ohsawa With theory and electromagnetic particle simulations, we have studied the electric field parallel to the magnetic field, $E_ {\parallel}$, in nonlinear magnetosonic waves in an electron-ion (e-i) plasma and in an electron-positron-ion (e-p-i) plasma. Our theory for the e-i plasma shows that the integral of $E_{||}$ along the magnetic field, $F=-\int E_{\parallel}ds$, is given as $ eF \sim \epsilon \Gamma_e T_e$ in small-amplitude ($\epsilon \ll 1$) pulses in a warm plasma with electron temperature $T_e$, where $\Gamma_e$ is the specific heat ratio. In a cold plasma, it is given as $eF \sim \epsilon^2 m_i v_A^2$, where $v_A$ is the Alfven speed. For the e-p-i plasma, $F$ is large if the positron-to-ion density ratio $n_{p0}/n_{i0}$ is small. These theoretical predictions were verified with simulations. Furthermore, the relation $n_{e0} eF \sim \epsilon(\rho v_A^2+\Gamma_e p_{e0})(n_{i0}/n_{e0})$ is found to fit fairly well to the simulation results for shock waves with $\epsilon \sim O(1)$ in e-i and e-p-i plasmas, where $\rho$ is the mass density and $p_{e0}$ is the electron pressure. These results indicate that $E_{\parallel}$ can be strong in nonlinear magnetosonic waves. [Preview Abstract] |
|
GP8.00088: Second Harmonics of Reversed Shear TAE in Alcator C-Mod Geometry Eugene Chen, Herbert Berk, Boris Breizman, Linjin Zheng Experiments on Alcator C-Mod, operating with reversed magnetic shear, reveal Toroidal Alfven Eigenmodes (TAE) together with signals at twice the mode frequency. The double frequency signals can be viewed as second harmonic sidebands driven by quadratic non-linear terms in the MHD equations, in analogy with a corresponding theory for Alfven Cascades [1]. However, these nonlinear sidebands have not yet been quantified by any of \newline the existing codes. In this work, we extend AEGIS code [2] to capture nonlinear effects iteratively by treating the nonlinear terms as a driving source in the linear MHD solver.~ We first compute the TAE mode structure for realistic geometry and q-profile and then use it to find the spatial structure of the second harmonic density perturbation, which can be directly compared with PCI measurements at Alcator C-Mod. [1] H. Smith, B. N. Breizman, M. Lisak and D. Anderson, Physics of Plasmas 13 042504 (2006) [2] L. J. Zheng and M. Kotschenreuther, Journal of Computational Physics 211 (2006) 748-766 [Preview Abstract] |
|
GP8.00089: Studies of HF-induced Strong Langmuir Turbulence at the HAARP Ionospheric Observatory J.P. Sheerin, J.M. Gerres, M.R. Keith, N. Adham, A. Wittbrodt, B.J. Watkins, W.A. Bristow, P.A. Bernhardt, C.A. Selcher High power HF transmitters may induce a number of plasma instabilities in the interaction region of overdense ionospheric plasma. We report results from our recent experiments using over one gigawatt of HF power (ERP) to generate and study strong Langmuir turbulence (SLT) and particle acceleration at the HAARP Observatory, Gakona, Alaska. Among the effects observed and studied are: SLT spectra including the outshifted plasma line or free-mode, appearance of a short timescale ponderomotive overshoot effect, collapse, cascade and co-existing spectra, control of artificial field-aligned irregularities (AFAI), the aspect angle dependence of the plasma line spectra, and suprathermal electrons. Mapping the intensity of SLT versus pointing angle, we have discovered a number of regions of strong interaction displaced from the primary HF interaction region. Experimental results are compared to previous high latitude experiments and predictions from recent modeling efforts. [Preview Abstract] |
|
GP8.00090: Nonlinear vacuum polarization in intense blackbody radiation and its effects on the radiation spectrum Sheldon Wu, Frederic Hartemann, Craig Siders, Christopher Barty A study of thermally induced vacuum polarization stemming from the Euler-Heisenberg radiation correction to Maxwell equations is conducted. While nonlinear effects associated with interactions of electromagnetic pulse with a background photon gas had been previously calculated, we examine the possibility of nonlinear corrective terms to the blackbody radiation spectrum. Suitable conditions can be found in both astrophysical and laboratory environments. Inertial confined, ignited thermonuclear plasmas will produce intense blackbody radiation at temperatures in excess of 20 keV. In this theoretical investigation, our analysis shows that in an ideal incoherent blackbody the radiation spectrum is unaffected in the regime studied. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
|
GP8.00091: The refraction phenomena in the shock wave dispersion on plasma inhomogenities Anna Markhotok, Svetozar Popovic Recently we introduced a new mechanism of the shock wave (SW) dispersion on a boundary [1] and then employed it to control the structure of a SW front as it interacts with plasma. Now we consider same effect but under specific conditions on the interface with weak gradients at the interface, but stronger in the bulk. These conditions appear more realistic for most applications, and what is also important, the SW dispersion in this case becomes stronger. We derive all relations using the approach similar to [1]. Then the model applied to calculate the structure of the SW front dispersed on a plasma sphere numerically and compared with the existing experiments. The strength of the effect is demonstrated by comparing results of calculation for different conditions on the interface. Comparative calculations show significant difference in the shock front shapes and they are in complete agreement with the experiments. More results are underway which can contribute to understanding of basic phenomena in weakly ionized gases, combustion, and shock dynamics on the interfaces.\\[4pt] [1] A. Markhotok, S. Popovic, L. Vuskovic, Phys. of Plasmas J. \textbf{15}, 3 (2008). [Preview Abstract] |
|
GP8.00092: Stability Control for High-beta Plasmas on JT-60SA G. Matsunaga, M. Takechi, S. Sakurai, S. Ide, M. Matsukawa, N. Oyama, N. Aiba, G. Kurita, A. Ferro, E. Gaio, L. Novello, S. Sakasai, Y. Kamada JT-60SA is designed and under construction as fully superconducting tokamak under a combined project of the ITER satellite tokamak program of EU-JA (Broader Approach Activities) and the Japanese national program. One of the main purposes of JT-60SA is the steady-state high-beta operation above the ideal no-wall beta-limit with suppressing resistive wall modes (RWMs). In order to control the RWMs, the RWM control coils and error field correction coils (EFCCs) are to be installed. The current design of these coils composes of 18 sector coils (6 coils in toroidally and 3 coils in poloidally) so as to suppress n=1-3 RWMs and to compensate various error fields. The EFCCs can also be utilized to apply the resonant magnetic perturbation to ergodize the peripheral magnetic filed structure to mitigate and avoid the large edge localized modes. The design and analysis of these in-vessel tools for high-beta plasmas on JT-60SA will be presented. [Preview Abstract] |
|
GP8.00093: Investigation of equilibrium plasma beta limits in 3D magnetic topologies Mark Schlutt, C.C. Hegna, E. Held, S.E. Kruger A fluid model is used to investigate pressure-induced magnetic islands in 3D equilibria. We revisit previous analytic island calculations, allowing for finite parallel heat transport, to find an equation for equilibrium island widths. Finite parallel heat transport can alter the resistive interchange and bootstrap current contributions to magnetic island formation. However, the effect of Pfirsch-Schl\"uter currents driven by resonant components in $\frac{1}{B^{2}}$ on magnetic island formation is largely unaffected by transport processes. 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 magnetic perturbations. These perturbations alter the magnetic spectrum, producing magnetic islands and stochastic regions. Numerical simulations are performed that include a heating source and self-consistent anisotropic transport in different magnetic configurations. The support of pressure gradients in stochastic regions is investigated. The connection between flux surface destruction and the breaching of MHD stability boundaries is also considered. [Preview Abstract] |
|
GP8.00094: Three dimensional Simulations of Self-Organization in a Driven Dissipative Plasma System Dastgeer Shaikh, B. Dasgupta, Q. Hu, G.P. Zank We perform a fully self-consistent 3-D numerical simulation for a compressible, driven dissipative magneto-plasma driven by large-scale perturbations, that contain a fairly broader spectrum of characteristic modes, ranging from largest scales to intermediate scales and down to the smallest scales, where the energy of the system are dissipated by collisional (Ohmic) and viscous dissipations. Additionally, our simulation includes nonlinear interactions amongst a wide range of ?uctuations that are initialized with random spectral amplitudes, leading to the cascade of spectral energy in the inertial range spectrum, and takes into account large scale as well as small scale perturbation that may have been induced by the background plasma ?uctuations, also the non adiabatic exchange of energy leading to the migration of energy from the energy containing modes or randomly injected energy driven by perturbations and further dissipated by the smaller scales. Besides demonstrating the comparative decays of total energy and dissipation rate of energy, our results show the existence of a perpendicular component of current, thus clearly con?rming that the self-organized state is non-force free. [Preview Abstract] |
|
GP8.00095: Kinetic Effects of Energetic Particles on Nonlinear Resistive MHD Stability R. Takahashi, D.P. Brennan, C.C. Kim Kinetic effects of energetic particles can play a crucial role in the stability of the 2/1 tearing mode in tokamaks (JET, JT-60U, and DIII-D), where the fraction of energetic particle $\beta _{frac}=\beta _{h}$/$\beta _{ }$is high. Using model equilibria based on experimental reconstructions, it has been shown that energetic particles have significant damping and stabilizing effects at experimentally relevant $\beta _{N}{\rm s}{\rm g}\beta _{frac}$, and $S$, and weaker damping and stabilizing effects in the ideal unstable regime, and excite a real frequency of the 2/1 mode [1,2]. Initial nonlinear and two-fluid effects are now investigated. A qualitative extrapolation of these results is discussed for what to expect from energetic particle effects on resistive MHD modes in ITER.\\[4pt] [1] R. Takahashi, \textit{et al}., Phys. Rev. Lett. 102, 135001 (2009).\\[0pt] [2] R. Takahashi, \textit{et al}., Nucl. Fusion 49, 065032 (2009). [Preview Abstract] |
|
GP8.00096: 2D Langmuir Maps of Kinked UW-RWM Plasma David Hannum, Matt Brookhart, Cary Forest, Roch Kendrick, Gabriel Mengin, Carlos Paz-Soldan The rotating wall machine is a linear screw-pinch built to study the role of different wall boundary conditions on the Resistive Wall Mode (RWM). Its plasma is created by a hexagonal array of seven electrostatic guns. The guns can be biased to discharge up to 1 kA of current each, firing independently or in tandem. The 20 cm diameter, 1.2 m long plasma column is held in place by a 500 G (max) axial guide field. A singletip Langmuir probe inserted from the opposite end of the chamber yields measurements of $T_e, n_e$ and $V_p$ in $r$ and $z$. Though it is tied to the guns at the cathode end, the plasma column is free to slip over the anode end. At higher plasma currents, the kinking column oscillates past the probe tip. Ensemble averaging of the I-V curves is required to derive reliable Langmuir measurements for these plasmas. I will present 2D Langmuir profiles in configurations scanning the plasma current $I_p$, guide field $B_z$, and number of plasma sources (one gun / seven guns). [Preview Abstract] |
|
GP8.00097: Resistive Wall Mode Study in a Line-tied Screw Pinch Carlos Paz-Soldan, Matthew Brookhart, Gabriel Mengin, David Hannum, Roch Kendrick, Cary Forest Recent studies on the University of Wisconsin - Rotating Wall Machine (UW-RWM) are presented. The device has been constructed to test the hypothesis that the resistive wall mode (RWM) can be stabilized by two differentially rotating solid conducting walls. The RWM can rotate with (or lock to) either the stationary or the moving wall, but not both simultaneously. This allows stabilizing image currents to persist despite finite wall resistivity. This method has application to future breeder blanket designs, with the rotating wall replaced by flowing liquid metal. A rotating wall of 21cm diameter is under construction that will be capable of reaching 6000 rpm. Engineering design and performance of this wall will be presented. The timing and role of the MHD safety factor (q) on the RWM in the static wall case will also be presented. Recent spectroscopic studies of neutral and impurity interactions will be described. The UW-RWM studies the RWM through 120 radial, axial, and azimuthal flux loops in screw pinch geometry 1m long and 20cm across. Discharges up to 7kA can be maintained at flat top for 20ms or ramped by a pulse width modulation scheme. [Preview Abstract] |
|
GP8.00098: Studies of Line-tied Reconnection on the RWM Experiment M. Brookhart, C.B. Forest, D.A. Hannum, R. Kendrick, G. Mengin, C. Paz-Soldan An internal kink instability has been observed to grow and saturate in the Rotating Wall Machine Experiment. Detailed measurements show that an ideal, line-tied kink mode begins growing when the safety factor drops sufficiently below 1 inside the plasma; the saturated state corresponds to a rotating helical equilibrium. In addition to the ideal mode, reconnection events have been observed to periodically flatten the current profile and change the magnetic topology. The reconnection events strongly resemble the reconnection phenomena described in numerical simulations of a nearly identical geometry. Recently, the 2D equilibrium current profile has been measured using an axially and radially scanning magnetic probe so that better comparisons between experiment and theory can be carried out. The measurements show the current channel diffuses radially, inconsistent with Spitzer resistivity. To determine the effect of neutrals on conductivity, neutral fraction is being independently quantified via H$\alpha $ emission. Future work will involve the construction and installation of a 2D coil array to measure fluctuations in the current at the axial midpoint of the experiment in an effort to characterize the reconnection rate in this inherently 3D geometry. [Preview Abstract] |
|
GP8.00099: Toroidal Mode Coupling in Tokamaks with Anisotropic Viscosity S.E. Kruger, D.P. Brennan, E.D. Held, C.R. Sovinec, C.C. Hegna Most linear and nonlinear theories of toroidal mode coupling of tearing modes in tokamaks [1,2] have used isotropic viscosity. NIMROD's implementation of Braginskii's anisotropic parallel viscosity, however, enables an evaluation of the effect of stress anisotropy on mode coupling for a wide range of collisionality. Numerical simulations with the NIMROD code are used to compare the effect of anisotropy on mode coupling in conditions representing low collisionality. To simplify the analysis, we choose a high aspect ratio, circular cross-section equilibrium. We also present preliminary comparisons with a more complicated coupling of tearing modes with a 1/1 mode using a realistic free-boundary, high beta DIII-D equilibrium [3]. \\[4pt] [1] M.\ Persson and R.L.\ Dewar, Phys. Plasmas, \mbox{\bf 1} 1256 (1994).\\[0pt] [2] R.\ Fitzpatrick, Nucl. Fusion \mbox{\bf 33}, 1533 (1993).\\[0pt] [3] D.P.\ Brennan, S.E.~Kruger, and R.J. LaHaye, {\em to be submitted to Physics of Plasmas} [Preview Abstract] |
|
GP8.00100: Effects of Electron inertia on the Waves in Hall Magnetohydrodynamics B. Dasgupta, Dastgeer Shaikh, G.P. Zank Electromagnetic waves in Hall Magnetohydrodynamics (HMHD) exhibit a variety of complex and interesting features. Earlier works have shown the mode conversion of incompressible MHD shear Alfv\'en wave into a compressible, the fast wave becoming mostly electromagnetic and the slow wave becomes almost a fluid-dynamical wave. These features have been supported partially by numerical simulations of Shaikh \& Zank (ApJ,640,L195,2006). Also, recent numerical simulations on solar wind turbulence show some interesting features for the power spectra, like the predominance of Whistler cascades in the range kde $<$ 1 , where k is the wave number and de is the electron inertial length. In this work, we first consider the linear dispersion relation for a complete two-fluid plasma with a finite electron mass. We analyze the different branches of the dispersion relation for different ranges of kde, i.e., kde $>$ 1 and kde $<$ 1, in addition to the ranges kdi $>$ 1 and kdi $<$ 1. The wave characteristics,revealing many significant features, are discussed. [Preview Abstract] |
|
GP8.00101: Does a contact discontinuity exist within the Hall-MHD model? Eliezer Hameiri The stability analysis of a z-pinch equilibrium state with a surface discontinuity, carried out within the Hall-MHD model (1), indicates that one jump condition across the discontinuity is missing. Indeed, Ref. (1) resorts to an extraneous condition based on mathematical convenience which seems not justifiable physically, with the result of the analysis being very different from equilibrium with continuous profiles albeit with sharp gradients. We attempt to generate the missing jump condition using a variational analysis for the equilibrium state. Surprisingly, the allowed equilibria appears so restricted, that they are practically irrelevant for fusion-related configurations. For example, the total plasma current must vanish. We give further indications for the correctness of this conclusion. (1) U. Schaper, J. Plasma Phys. 30, 169 (1983). [Preview Abstract] |
|
GP8.00102: Implicit Time Integration Schemes for a Finite Element Two-Fluid Plasma Code Bhuvana Srinivasan, Uri Shumlak The two-fluid model consists of the complete Euler equations for the ion and electron fluids and Maxwell's equations for the electric and magnetic fields. Two-fluid effects become significant when the characteristic spatial scales are on the order of the ion skin depth and the characteristic time scales are on the order of the inverse ion cyclotron frequency. In regimes where two-fluid physics is significant, it is necessary to account for the Hall term and the diamagnetic drift term that are missed in single-fluid MHD. WARPX is a finite element code based on the two-fluid plasma model which uses the Runge-Kutta discontinuous Galerkin method. The two-fluid plasma model has characteristic speeds ranging from the fluid sound speeds to the speed of light. As a result, the explicit time stepping schemes have a very restrictive time step governed by either the speed of light or the electron plasma frequency. This provides the motivation to study implicit time stepping schemes for the two-fluid plasma model where accuracy considerations alone determine the time-step. [Preview Abstract] |
|
GP8.00103: WAVES AND INSTABILITIES IN BASIC PLASMAS |
|
GP8.00104: Linear and nonlinear waves with orbital angular momentum in magnetized plasma Shahid Ali, Padma Kant Shukla, Jos\'e Tito Mendonca Here we discuss the concept of orbital angular momentum (OAM) for electromagnetic waves in a magnetized plasma. Nonlinear effects of photons with spin and OAM will be considered. In particular, we examine the case of parametric interactions between circularly polarized electromagnetic waves and Langmuir and ion acoustic waves, including the ponderomotive force of light with OAM in magnetized plasma (Shukla \& Stenflo, PRA). This will be a generalization of recent results published in PRL by J.T. Mendonca and B. Thide. We also examine the influence of OAM on the magnetic field generation by the inverse Faraday effect. [Preview Abstract] |
|
GP8.00105: Wave propagatiom and magnetic field generation in Rydberg plasmas Hugo Tercas, Nitin Shukla, P.K. Shukla, J. Loureiro, J.T. Mendonca We call Rydberg plasma the weakly ionized gas produced in magneto-optical traps. In such a plasma, the neutral atoms can be excited in Rydberg states. Wave propagation in Rydberg plasmas, and the mutual influence of plasma dispersion and atomic dispersion is considered. New dispersion relations are established, showing new instability regimes and new cut-off frequencies. It is also shown that the ponderomotive force of a large-amplitude electromagnetic wave in Rydberg plasmas can generate quasi-stationary magnetic fields. The present result can account for the origin of seed magnetic fields in ultracold Rydberg plasmas when they are irradiated by the high-frequency electromagnetic wave. [Preview Abstract] |
|
GP8.00106: Stability Analysis of Low Frequency Electrostatic Modes in a Large Scale Helicon Plasma in the Presence of Sheared Flows M. Gilmore, S. Xie, L. Yan, C. Watts, A.G. Lynn, D.E. Newman, D. Sammadar Experiments in the linear HelCat device, which investigate the nonlinear dynamics of edge fluctuations in the presence of varying flows affected by plasma biasing, show increasingly chaotic behavior as the bias is increased. These experiments have been conducted in a weakly turbulent regime near marginal stability. It is found that as bias is increased, flow shear penetrates into the plasma core from the edge, drift waves (DW's) suppress, and Kelvin-Helmholtz (KH) modes are excited. It is postulated that a 3-dynamical variables-type interaction between the DW, KH and the \textit{local} flow leads to chaotic behavior. Building on a linear theory by Light, \textit{et al} [Phys Plasmas \textbf{8}(10), 4675, 2001], a resonant interaction between the mode and the local flow, where mode conversion can occur, is investigated. We report on the linear stability, as well as progress on modelling the plasma with a 2D nonlinear fluid code. [Preview Abstract] |
|
GP8.00107: Nonlinear Dynamics under Applied Electrical Field at Magnetized Laboratory Plasma Edge Shuangwei Xie, Mark Gilmore, Christopher Watts, Lincan Yan Experiments conducted on a linear helicon plasma (HelCat) device shows evidence of drift wave instability fluctuations, which are suppressed when an increased positive DC electric potential is applied perpendicular to the magnetic field. Simultaneously, a new K-H instability appears, and deterministic chaos also can be observed during this transaction. Measurements show both axial and azimuthal flow speed, as well as Reynolds stress change under the effect of E$\times $B flow shear caused by this external disturbance. When neutral gas pressure is increased during the process, the suppression, requires a higher DC bias, and the K-H transition is not observed. From axial flow measurements, a possible mechanism is suggested from the reduced flow speed which may caused by increased collisions between charged particles and neutrals. Two simple models are presented to predict neutral change with increased gas pressure. [Preview Abstract] |
|
GP8.00108: Parallel shear and turbulence Tiffany Hayes, Mark Gilmore, Christopher Watts, Shuangwei Xie, Lincan Yan Instabilities may be caused in plasma due to (shear) flow. These flows can be transverse or parallel to the magnetic field. Past work has generally focussed on controlling and understanding the processes that occur from (shear) flow transverse to the magnetic field. At UNM experimental work is being performed in the the HelCat device (Helicon Cathode) to control the parallel flow in order to study and understand the processes that arise from this situation. It is also our aim to be able to control the transverse flow simulatneously, but independently of the parallel flow. By inserting a system of biased rings and grids into the plasma we are able to modify the flows, and hence the turbulence. Flows are measured using a seven-tip Mach probe. Results of our ability to control the flows independently are presented. [Preview Abstract] |
|
GP8.00109: The HelCat Helicon-Cathode Device at UNM Bricette Cyrin, Christopher Watts, Mark Gilmore, Tiffany Hayes, Ralph Kelly, Christopher Leach, Alan Lynn, Andrew Sanchez, Shuangwei Xie, Lincan Yan, Yue Zhang The HelCat helicon-cathode device is a dual-source linear plasma device for investigating a wide variety of basic plasma phenomena. ~HelCat is 4 m long, 50 cm diameter, with axial magnetic field $<$ 2.2 kG. An RF helicon source is at one end of the device, and a thermionic BaO-Ni cathode is at the other end. Current research topics include the relationship of turbulence to sheared plasma flows, deterministic chaos, Alfv\'{e}n wave propagation and damping, and merging plasma interaction. We present an overview of the ongoing research, and focus on recent results of merging helicon and cathode plasma. We will present some really cool movies. [Preview Abstract] |
|
GP8.00110: Spectral diagnostics for the HelCat helicon/cathode linear plasma device Christopher Leach, Jaksa Osinski, Edl Schamiloglu, Christopher Watts Several new optical spectroscopy diagnostics installed on the HelCat (Helicon-Cathode) device expand the measurement capabilities of the plasma characteristics to include ion temperature, neutral characteristics, as well as alternative and convenient ways to determine drift velocities, temperatures, densities, and species recognition. These are in addition to the numerous probe diagnostics that provide density, potential, temperature, saturation current, and other data for HelCat. A highly directive lens system allow analysis of specific regions of the plasma by a survey spectrometer and a 4m McPherson spectrometer inputted by a compact and finely tunable optical slit input apparatus that allows adjustment along three separate axes. We present an overview of the system design and initial data. [Preview Abstract] |
|
GP8.00111: Effects of Biasing and Boundary Conditions on Convective Blobs in Magnetized Laboratory Plasmas L. Yan, M. Gilmore, C. Watts, T.A. Carter Intermittent convective plasma transport across magnetic field lines (``blobs'') has been one of the most important issues in fusion-related edge plasma physics, and is thought to play a key role in cross-field transport in the tokamak scrape-off layer. Fundamental experiments on blobs are being conducted in both the linear LAPD and HelCat devices. HelCat is a 4 m long, 50 cm diameter device with B $<$ 0.22 T and both RF helicon and cathode sources. Blobs are always observed in LAPD and HelCat cathode plasmas. However, blobs are seen in HelCat helicon plasmas only under certain conditions. Biased electrodes (HelCat), and biasing of the cathode with respect to the wall (LAPD), are utilized to affect the sheared ExB flow profiles. Fluctuations, flux, and flows are monitored with probes. Measurements with biasing in both devices show that flow shear is modified at the edge and blob characteristics change. In HelCat, flows and blobs also change with axial and radial boundary conditions. [Preview Abstract] |
|
GP8.00112: Observation of Lower-Hybrid Cavitons in Laboratory Plasma Takao Tanikawa, Taisei Motomura, Shunjiro Shinohara Lower-hybrid (LH) cavitons are localized wave activities trapped inside density depletions whose frequencies are in the range of the lower-hybrid frequency. In order to investigate the relationship between LH cavitons and lower-hybrid solitary structures (LHSS's) observed in space plasmas, we have constructed Tokai Helicon Device (THD) which is a specially designed rf plasma device and utilizes helicon-type discharge with a flat segmented multi-loop antenna [1]. Plasma is produced repetitively ($\sim $10 Hz) with a typical discharge pulse duration of $\sim $10 ms. A second rf pulse of short duration (a few $\mu $s) is injected into an afterglow plasma using the same antenna as used for plasma production. A large amplitude LH wave can be excited at the resonance layer; then density depletion can appear if the condition is right. It can evolve to become an LH caviton. The process has been under investigation. The possibility of particle acceleration due to LH cavitons will be discussed. The characteristics of plasma produced in THD will also be presented. [1] T. Tanikawa \textit{et al}., Bull. Ameri. Phys. Soc. \textbf{51} (7), 164 (2006); T. Tanikawa and S. Shinohara, \textit{ibid}. \textbf{53} (14), 174 (2008). [Preview Abstract] |
|
GP8.00113: Wave Characteristics of Large-Diameter, High-Density Helicon Plasma with Short Axial Length II Taisei Motomura, Shunjiro Shinohara, Takao Tanikawa, Konstantin P. Shamrai We have demonstrated that a large-diameter, high-density ($\ge $ 10$^{12}$ cm$^{-3})$ helicon plasma can be produced in a low aspect ratio (the ratio of the axial length $L$ to the diameter $R$; in our case, $R$ =73.8 cm and 5.5 cm $\le \quad L \quad \le $ 35 cm) device using a flat spiral antenna (4-turn, 43 cm in diameter) installed just outside the quartz-glass window at one end of the device [1]. As a first step to understand the role of helicon waves in the discharge process, helicon wave characteristics in plasma has been investigated in detail [2]. It has been found that discrete axial eigenmodes, whose characteristics depend on the plasma density profile and the axial boundary conditions, exist in the excited helicon wave. The effects of the background magnetic field profile and the rf input power on the excited wave have also been examined in detail. [1] T. Motomura \textit{et al}., J. Plasma Fusion Res. Ser., in press. [2] T. Motomura \textit{et al}., Bull. Ameri. Phys. Soc. \textbf{53} (14), 175 (2008). [Preview Abstract] |
|
GP8.00114: Adiabatic invariance for eigenmodes and continuum modes in nonuniform plasmas Makoto Hirota, Shinji Tokuda Adiabatic invariance of wave action is investigated for general eigenmodes and continuum modes by exploiting the variational principle for linearized dynamical systems. This theory applies to various dissipationless plasma models and serves to explain quasi-linear evolution of modal behavior, for which the standard eikonal limit is not always suitable. Given a sufficiently slow evolution of the background fields, the wave action (or the action variable) attributed to each mode is conserved as long as the corresponding discrete or continuous spectrum is isolated from other spectra and zero frequency. The resonant coupling allows exchange of wave action among these modes. The invariance of wave action is demonstrated for the case of Alfv\'en and sound resonances in the MHD model, which leads to a new wave-quantum interpretation of the continuum damping and the resonant growth. The sign of the wave action generally determines whether the resonant instability occurs or not. [Preview Abstract] |
|
GP8.00115: EMIC Wave Characteristics and Their Effects on the Lifetime of Energetic Electrons in Earth's Inner Radiation Belt X. Shao, K. Papadopoulos, A.S. Sharma, A. Karavaev The stably trapped electrons with energies ($>$ 100 keV) in inner radiation belt have lifetimes of years and can have serious effects on spacecrafts and satellites. One possible way of mitigating these hazards is to reduce electron life times through pitch angle scattering by waves. At frequencies close to the ion gyro-frequencies, the Electromagnetic Ion-Cyclotron (EMIC) waves can have wavelengths short enough to gyro-resonate with energetic electrons, which can lead to significant changes in lifetimes of electrons in inner belt. We investigated the lifetime of inner belt energetic electrons subject to pitch angle scattering with EMIC waves by calculating the diffusion coefficient. For several hundred Watts of broadband EMIC waves in the shell volume enclosed by magnetic field lines at L = 2.0 with width dL = 0.1, the lifetime of 1 MeV electrons can be reduced to a few months. This is a considerable reduction and has important consequences, including remediation of artificially enhanced energetic electron fluxes. The key issues to be investigated further are: the validity of the model of excitation and propagation of EMIC wave in a multi-ion plasma with non-uniform ambient magnetic field, the energy conversion between EMIC waves and multi-ion plasma. These are addressed by the comparing models and laboratory experiments. The work was sponsored by ONR MURI grant. [Preview Abstract] |
|
GP8.00116: Pitch Angle Scattering of Energetic Particles by Waves Generated from a Rotating Magnetic Field Source A. Karavaev, X. Shao, A.S. Sharma, K. Papadopoulos, N.A. Gumerov, A. Gigliotti, W. Gekelman Injection of whistler waves into Earth's inner radiation belt to enhance precipitation of energetic electrons has been an active research area. Most mechanisms of pitch angle scattering of energetic particles are based on gyro-resonant wave-particle interaction. Recent experiments and simulations show that Rotating Magnetic Field (RMF) antennas in plasmas can be efficient radiation sources of MHD and whistler waves. In experiments conducted in the Large Plasma Device at UCLA, poly-phased current loops drove the RMF antenna. These experiments and simulations show that 75-85{\%} of the radiation generated is in guided propagation. The waves have non-local magnetic field gradients in the transverse direction and these provide ways to break the adiabatic invariants of electrons and precipitate them via a non-resonant scattering. Here we present simulations of non-resonant pitch angle scattering of particles by waves generated by RMF sources. EMHD simulations are used to model whistlers and the resultant EM fields are used in particle tracing codes to study pitch angle scattering. The simulations are conducted for a wide range of wave magnetic fields, including fields much larger than the ambient magnetic field in space plasma environments. The work was sponsored by ONR MURI grant. [Preview Abstract] |
|
GP8.00117: Experiments on the scattering of fast electrons by Alfv\'{e}n waves Alex Gigliotti, Walter Gekelman, Patrick Pribyl, Yuhou Wang, Alexey Karavaev, Xi Shao, Dennis Papadopoulos High energy ions and electrons with anisotropic distribution functions can be trapped in the earth's radiation belts for months, which pose a danger for satellites. A electron population with large $v_{\perp}$ and was introduced in the afterglow of the background LAPD plasma ($n_e =2.0e^{11} cm^{-3}$, $T_e = .25 eV$) .Microwaves at 2.45 GHz were launched into the plasma at an axial and radial location where $f_{micro} = f_{UH}$. A small (2 cm dia) electron hot spot was generated. Shear Alfv\'{e}n waves (f = 192 kHz) launched by an orthogonal loop antenna, with magnitude $B_{wave} \sim 2 G$ destroyed the hot spot within 10 cycles. The hot spot returns when the Alfven wave is shut off. Space-time evolution of the plasma density, flow and electron are presented. The mechanism of the electron de-trapping is under investigation. Planned experiments involve the interaction of fast ion rings with shear Alfv\'{e}n waves, and whistler waves with the anisotropic electrons. [Preview Abstract] |
|
GP8.00118: Direct Measurement of the Impedance of a Dipole Antenna Patrick Pribyl, Walter Gekelman, Alex Gigliotti The impedance of a dipole antenna radiating whistler waves has been determined by measuring the current and voltage directly at the antenna tips. The experiment was done at the Large Plasma Device at UCLA. The antenna was immersed in a magneto-plasma at the background magnetic field and plasma density, were both varied. The impedance was measured as a function of $f_{pe}/f_{ce}$. The standard technique is to measure forward and reflected power at the driven end of the probe shaft, and backing out transmission line properties and system reflections and resonances. Doing this accurately involves detailed knowledge of the circuit and transmission line properties associated with the antenna. The alternate technique we present is advantageous in that we can determine the impedance directly, accurate to zeroth order with transmission line effects only entering as corrections. Data of the radiation impedance from the lower hybrid frequency to higher than fce/2 will be presented. [Preview Abstract] |
|
GP8.00119: Non-localized waves in a magnetized argon plasma column A. DuBois, A. Eadon, E. Thomas The Auburn Linear EXperiment for Instability Studies (ALEXIS), a 170 cm long, 10 cm diameter plasma column, has been used for studies of sheared flows in plasmas. These flows play an important role in plasma stability and have led to observations of spatially localized ion cyclotron waves. In new experiments reported in this presentation, observations have been made of non-localized, low frequency waves in Argon plasmas. These waves are observed to extend over the entire radius of the column and vary with neutral gas pressure and magnetic field strength. Probe measurements of plasma parameters, such as plasma potential, density, and electron temperature, in various magnetic field strengths, will be presented. Initial data suggests these low frequency waves are driven by gradients in the density profile. [Preview Abstract] |
|
GP8.00120: Effect of sheared flows on different gas species in a magnetized plasma column Ashley Eadon, Ami DuBois, Edward Thomas Transverse and parallel sheared flows are important topics in both space and fusion plasmas, and have been the subjects of extensive study. In an effort to understand the plasma response to these flows across a large parameter space, a series of coordinated multi-scale experiments is being performed on three different devices that span the space (Space Plasma Simulation Chamber, SPSC) to fusion (Compact Toroidal Hybrid, CTH stellarator) regimes. The Auburn Linear EXperiment for Instability Studies (ALEXIS) is a 170 cm long, 10 cm diameter linear magnetized, rf generated plasma column, and acts as the ``bridge'' between the space and fusion devices. Multiple gas species (He, N, Ar) are being evaluated to determine the ideal combination of gas, magnetic field strength, and imposed electric potential structure to best study the plasma response to sheared flows of many different scale sizes. Measurements will be presented on the correlation between different low frequency wave features and the electric field structure in the plasma. [Preview Abstract] |
|
GP8.00121: Measurements of electric field induced fluctuations 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. In fusion plasmas strongly sheared flows at the plasma edge can lead to a reduction of core fluctuations and enhanced core plasma parameters. The effects of sheared flows on plasmas in the Compact Toroidal 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$^{-3})$, are studied using a biased electrode inserted into the edge plasma to alter the radial profile of the electric potential. The enhancement or suppression of fluctuations associated with the application of different applied potentials and electric fields is measured with a triple probe. Results will be presented for different biased limiters -- one with a single-tip and another with two axially-separated, individually biased electrodes. [Preview Abstract] |
|
GP8.00122: Laboratory Studies of Electromagnetic Velocity Shear-Driven Instabilities Erik Tejero, William Amatucci, Gurudas Ganguli, Edward Thomas Observations of low frequency, electromagnetic ion cyclotron waves have been made in many regions of the space environment. Many theoretical mechanisms have been presented to account for these waves and the resulting transversely accelerated ions. Sheared flows produced by localized electric fields coupled with a perpendicular magnetic field are a potentially important energy source that can create waves of this type. \textit{In situ} observations of sheared plasma flows collocated with electromagnetic wave activity have led to a laboratory effort to investigate the impact of electromagnetic, velocity shear-driven instabilities on the near-Earth space plasma dynamics. Under scaled ionospheric conditions in the NRL Space Physics Simulation Chamber, the transition from electrostatic to electromagnetic ion cyclotron wave propagation is being investigated. Results from experiments to characterize instability threshold and wave dispersion characteristics will be presented. Work supported by the Office of Naval Research. [Preview Abstract] |
|
GP8.00123: Experimental Investigation of Whistler Wave Propagation in the NRL SPSC David Blackwell, William Amatucci, Gurudas Ganguli, George Gatling, Christopher Compton, Christopher Cothran, Erik Tejero, David Walker Recent results of whistler wave propagation experiments in the Space Physics Simulation Chamber facility at the Naval Research Lab are presented. The full spectrum of whistler waves up to the electron cyclotron frequency cutoff is explored. Changing antenna near and far field radiation patterns are compared to varying transmitter power and plasma parameters. Transitions between helicon-like perpendicular long wavelength modes and short wavelength parallel whistler modes are clearly evident. These results are compared with transmitted antenna power measurements vs. frequency for recommendations of optimum operating conditions and antenna design. The background spectrum of whistlers excited by a filament plasma source and the interaction of driven whistlers with this spectrum is also explored. The waves are driven and detected using balanced dipole and loop antennas which measure the amplitude and phase of the wave in two dimensions (\textit{r} and \textit{z}). The magnetic field is varied from a few Gauss to 200 Gauss, with the density variable over three decades from 10$^{7}$-10$^{10}$cm$^{-3}$. [Preview Abstract] |
|
GP8.00124: Laboratory Investigation of Whistler and Lower Hybrid Wave Propagation B. Amatucci, D. Blackwell, G. Ganguli, G. Gatling, C.D. Cothran, D. Walker An experimental investigation of the generation and propagation of waves on the whistler/lower hybrid branch is underway in the NRL Space Physics Simulation Chamber. The plasma conditions have been scaled to the inner magnetosphere. These studies are carried out in both homogeneous plasma and plasma containing density structuring. In homogeneous plasma, resonance cone propagation of the waves is observed, consistent with theoretical predictions. In plasma containing a density depletion layer, wave ducting within the layer has been observed. Experimental results on the observed whistler/lower hybrid wave propagation characteristics will be presented. [Preview Abstract] |
|
GP8.00125: Determining plasma potential from rf measurements using an impedance probe David Walker, Richard Fernsler, David Blackwell, William Amatucci By using rf techniques with plasma probes in laboratory experiments we have demonstrated the existence of collisionless resistance in the sheath of a spherical probe, shown that this leads to a method of finding the electron sheath density profile, and proposed a method of measuring electron temperature using the rf results.\footnote{Walker, D.N., R.F. Fernsler, D.D. Blackwell, W.E. Amatucci, \textit{Phys of Plasmas}, \textbf{15}, 123506 (2008)} Most recently we are able to determine plasma potential and the electron distribution function from the rf measurements, the latter requiring only a first derivative of the inverse ac resistance with respect to bias. The technique has general application to diverse areas of plasma investigations in the laboratory, or in space plasma measurement application. It can be used with \textit{in situ} instrumentation itself and can be extended to provide an estimate of the sheath structure for arbitrarily shaped surfaces. Because the magnitude of the applied signal used is much smaller in magnitude than typical applied dc potentials, it is transparent to the existing plasma/probe interface. [Preview Abstract] |
|
GP8.00126: Particle-in-Cell simulation of energetic particles driven instabilities Yang Chen, Scott E. Parker, Jianying Lang, Guoyong Fu We present simulations of the evolution of energetic particles driven modes with the gyrokinetic turbulence code GEM\footnote{Y.~Chen and S.~E.~Parker, J. Comp. Phys. {\bf 220}, 839 (2007)}, except that kinetic electrons are replaced by a mass-less fluid model. PIC simulations of energetic particles use either the conventional full-f method or the $\delta\! f$ method. The latter is adequate for low-amplitude fluctuation amplitudes. The collisional $\delta \! f$-method\footnote{Y.~Chen and R.~White, Phys. Plasmas {\bf 4}, 3591 (1997)} is used to systematically account for collisions and particle source and sink. Steady state saturation amplitudes are benchmarked with predictions of analytic theory. We also employ full-f simulations\footnote{Y.~Todo {\it et.~al}, Phys. Plasmas 10, 2888 (2003)} to study bursty events in which the instabilities reach large amplitudes and cause macroscopic redistribution or loss of the particles. With full-f it is easy to retain all the nonlinear effects and treat accurately discontinuities in the distribution function at phase-space boundaries. Whereas the energetic particle current is neglegible in the Ampere's law in $\delta \!f$ simulations, it is important in full-f simulations. Thermal ion kinetic effects are observed to be important. [Preview Abstract] |
|
GP8.00127: Geodesic Acoustic Modes Induced by Energetic Particles Tianchun Zhou, Herbert Berk A global geodesic acoustic mode driven by energetic particles (EGAM) has been observed in JET[1, 2] and DIII D[3, 4]. The mode is to be treated fully kinetically. The descriptions of the background electrons and ions are based on standard high and low bounce frequency expansion respectively with respect to the mode frequency. However, the energetic ions must be treated without any expansion of ratio between their bounce frequency and the mode frequency since they are comparable. Under electrostatic perturbation, we construct a quadratic form for the wave amplitude, from which an integro-differential equation is derived. In the limit where the drift orbit width is small comparison with the mode width, a differential equation for perturbed electrostatic field is obtained. Solution is obtained both analytically and numerically. We find that beam counterinjection enhances the instability of the mode. Landau damping due to thermal species is investigated. [Preview Abstract] |
|
GP8.00128: Anomalous Resistivity Generated By Ion Acoustic Instabilities in Weakly Collisional Plasmas C. Black, A. Bhattacharjee, K. Germaschewski, C.-S. Ng The anomalous resistivity associated with the current-driven ion-acoustic instability has been proposed as a mechanism for magnetic reconnection, and other forms of transport and dissipation. Recently, it has been shown that the underlying eigenmode spectrum of weakly collisional plasmas in the limit of small collisions is fundamentally different from that of collisionless plasmas [C. S. Ng, A. Bhattacharjee, and F. Skiff, Phys. Rev. Lett. 83, 1974 (1999)]. This raises the question of how quasilinear predictions of anomalous resistivity derived from the Vlasov equation differ from those obtained from a weakly collisional theory, even in the limit of zero collisions. We have reformulated traditional quasilinear theory for the Vlasov equation by including the Lenard-Bernstein collision operator. We compare the predictions of quasilinear theory with simulation results obtained from a code which integrates the kinetic Lenard- Bernstein equation coupled to the Poisson equation. We compare our results with classical estimates given by Sagdeev in the absence of collisions, and extend these estimates to the collisional regime. Comparisons will be made also with other numerical studies in the collisionless regime. [Preview Abstract] |
|
GP8.00129: Study of Toroidicity-Induced Alfv\'{e}n Eigenmodes on the Madison Symmetric Torus J.J. Koliner, C.B. Forest, S. Oliva, J.K. Anderson, J.S. Sarff, A.R. Almagri, D. Spong Alfv\'{e}n waves are likely of fundamental importance in the reversed-field pinch (RFP). The large magnetic fluctuations are expected to inject energy into Alfv\'{e}n modes, and their subsequent cascade to shorter wavelengths may drive ion heating. A new effort is in progress to understand toroidicity-induced Alfv\'{e}n eigenmodes (TAE's) through their structure, driving terms and damping mechanisms on the MST. Coupling of multiple eigenmodes can introduce undamped TAE's with frequencies from hundreds of kHz up to the cyclotron frequency at over 2 MHz. These modes can also become unstable by inverse Landau damping due to fast ions, a condition pertinent to neutral beam injection heating on MST and fusion alpha particles in future RFP devices. Frequencies of weakly damped modes have been calculated by solving a 3D partial differential equation that describes shear Alfv\'{e}n dynamics numerically based on MST equilibrium conditions. To excite the calculated modes, a single strap poloidal antenna connected to a 1 kW broadband amplifier will be employed. An array of 64 toroidally distributed magnetic pickup coils will be utilized synchronously to resolve power spectra and mode numbers in the relevant range of frequencies. [Preview Abstract] |
|
GP8.00130: Waves for Alpha-Channeling in Mirror Machines Andrey Zhmoginov, Nathaniel Fisch Alpha-channeling is a technique for extracting energy from alpha particles and using it to sustain fusion reaction by means of using waves to control the particle dynamics. The rough optimization of the energy transfer efficiency with respect to rf region parameters performed earlier for a mirror machine suggested that up to $60\%$ of alpha particle energy can be redirected to fuel ions. However, the limitations on the wave parameters imposed by plasmas were not previously considered. Using kinetic plasma dispersion relation and assuming quasi-longitudinal, or quasi-transverse wave propagation, we systematically search for modes which can be used to extract alpha particle energy in mirror plasmas. This method is further used to identify such modes in several device designs including the LAPD experiment. Since the damping rate of the suitable modes on fuel ions is small compared to the damping rate on electrons, coupling the identified modes to the waves in the plug is shown to be a mechanism which can potentially increase the effective reactivity of tandem mirrors. [Preview Abstract] |
|
GP8.00131: Stimulated Raman scattering of a laser pulse in magnetized plasma Sukhdeep Kaur, A.K. Sharma A Gaussian laser beam propagating through a magnetized plasma pushes the plasma radially outward, creating a plasma channel. The channel guides and self focuses the laser. However, the channel is susceptible to Stimulated Raman back Scattering. In this process laser couples to a space charge mode to produce the frequency downshifted electromagnetic side band and the laser exert a ponderomotive force on the electrons, driving the space charge mode. The nonlocal effects reduce the growth rate or the Raman process. [Preview Abstract] |
|
GP8.00132: Electric Field Measurements in a Helicon Source Based on Stark Splitting of Neutral Helium Lines Stephanie Sears, Saied Houshmandyar, Jerry Carr, Matthew Galante, Katherine Tallaksen, Saikat Chakraborty Thakur, Earl Scime For laser injection along the axial magnetic field, splitting of the 587.56 nm line of neutral helium has been observed during laser-induced-fluorescence experiments in a helicon plasma source. In a radially uniform region of magnetic field, the line splitting is localized to within $\pm $ 1 cm of the axis of the device. The spatial localization and independence of magnitude of the splitting on the source magnetic field strength rule out Zeeman splitting as an explanation for the observations. When compared to previous measurements of Stark splitting of the 587.56 nm He I line [\textit{Ritschl,} 1934], the observed line splitting suggests electric field strengths on the order of kV/cm. Here we report measurements of the observed line splitting (and absolute shifts) as a function of neutral pressure, plasma density, plasma density profile, and magnetic field strength. Similar measurements are presented for the nearby, but weaker, He I line at 587.60. This work was supported by NSF award PHY-0611571. [Preview Abstract] |
|
GP8.00133: Cavity Ring Down Spectroscopy measurements of Ar-II velocity distribution functions Saikat Chakraborty Thakur, Jerry Carr Jr., Earl Scime The WVU helicon source group routinely uses laser induced florescence (LIF) to measure the ion velocity distribution function (ivdf) of argon plasmas. We are constructing a continuous wave Cavity Ring Down Spectroscopy (cw-CRDS) system that will provide an alternate approach to measuring the ivdf. CRDS is a proven, ultra-sensitive, laser absorption technique that will provide sufficient resolution to measure the Doppler broadened absorption line of Argon and other plasma species. We will use a CW tunable diode laser at 668.43 nm, pumping the Ar-II metastable 3d$^{4}$F$_{7/2}$ level to the 4p$^{4}$D$_{5/2}$ level, to gather data from our steady state, high density, helicon plasma source CHEWIE (the Compact HElicon Waves and Instabilities Experiment). The cw-CRDS data will then be compared to LIF measurements. Once optimized for the Ar-II transition, we will use a similar experimental configuration to complete the difficult measurement of the ivdf for He-II transition at 1012.36 nm. This report will present completed design schematics for the CRDS system as well as initial data gathered through the early stages of operation. [Preview Abstract] |
|
GP8.00134: Comparison of Particle in Cell Simulations of Double Layers in a Xe-Ar Helicon Plasma with Experimental Results Alexander Hansen, I.A. Biloiu, E.E. Scime, A. Meige Recent experiments in plasma sheaths at grounded boundaries confirmed theoretical predictions that in a multi-ion species plasma presheath, ions do not accelerate up to their own Bohm, i.e., sound, speed in the presheath but instead accelerate up to a common bulk sound speed at the sheath-presheath boundary. Those ion speed measurements were obtained in argon plasmas doped with lighter helium and in xenon plasmas doped with lighter argon. Since the double layer (DL) that forms in expanding helicon source plasmas is essentially a plasma sheath in the plasma volume instead of at the plasma boundary, it was expected that similar effects would be observed in the DL presheath. However, in preliminary experiments on the HELIX device at West Virginia University [Biliou, Ph.D. thesis, WVU], we have found that addition of a small amount of argon in xenon plasma DL experiments suppresses formation of the DL in xenon plasmas. In order to understand this observation, we have performed simulations using a particle-in-cell code with Monte Carlo collisions, [Meige {\it et al}., {\it PoP} (2005)] which we have modified to handle the presence of two ion species in the plasma. We present results from the simulation for unmixed argon and xenon plasmas, for mixed argon and xenon ions, and compare the results with those from the experiment. [Preview Abstract] |
|
GP8.00135: Time Resolved Studies of Ion Beam Formation in Pulsed Helicon Argon Plasma Jerry Carr Jr., Saikat Chakraborty Thakur, Alex Hansen, Saeid Houshmandyar, Stephanie Sears, Earl Scime Previous laser induced fluorescence (LIF) measurements of the parallel ion velocity distribution function (ivdf) in expanding, steady-state, argon helicon plasma demonstrated the existence of an accelerated population of ions co-existing with a stationary ion background. Downstream of the expansion region, a clear ion beam (evidence of a double layer) is seen only for source frequencies greater than a critical frequency. To investigate the ion beam formation, the plasma is pulsed at 5 Hz with a 50{\%} duty cycle. The parallel ivdf, obtained with high time resolution LIF, and electrostatic fluctuations are measured both upstream and downstream at several plasma frequencies. At lower frequencies, large electrostatic instabilities appear and the ion beam vanishes. Below the critical frequency, large electrostatic instabilities are observed after the ion beam begins to appear in the LIF measurements. The beam vanishes and the electrostatic fluctuations decay later in the discharge. Above the critical frequency, we observe a less energetic beam and no electrostatic fluctuations. [Preview Abstract] |
|
GP8.00136: Propagation of Alfv\'{e}n Waves from Kinetic to Inertial Regimes in a Helicon Plasma Source Saeid Houshmandyar, Earl Scime Ion heating by Alfv\'{e}n waves is an active area of research in both laboratory and space plasma physics. As new instruments have enabled more detailed exploration of the solar corona (e.g. \textit{Hinode} spacecraft, \textit{Science}, Dec. 2007), the evidence in support of significant role of Alfv\'{e}n waves in coronal ion heating continues to accumulate. Due to their high density, helicon sources can be employed for coronal plasma studies (e.g., ${\beta _{HELIX} } \mathord{\left/ {\vphantom {{\beta _{HELIX} } {\beta _{Corona} \cong 0.1-1}}} \right. \kern-\nulldelimiterspace} {\beta _{Corona} \cong 0.1-1})$. However, the non-uniform electron density and temperature profiles of helicon sources ($\bar {\beta }=\beta _{HELIX} {m_i } \mathord{\left/ {\vphantom {{m_i } {m_e =\left( {{\mbox{v}_{th-e} } \mathord{\left/ {\vphantom {{\mbox{v}_{th-e} } {\mbox{v}_A }}} \right. \kern-\nulldelimiterspace} {\mbox{v}_A }} \right)^2}}} \right. \kern-\nulldelimiterspace} {m_e =\left( {{\mbox{v}_{th-e} } \mathord{\left/ {\vphantom {{\mbox{v}_{th-e} } {\mbox{v}_A }}} \right. \kern-\nulldelimiterspace} {\mbox{v}_A }} \right)^2}=0.1-20)$ lead to kinetic and inertial regimes in Alfv\'{e}n wave propagation. As showed by Vincena \textit{et al}. (PRL, \textbf{93}, 105003, 2004), the accumulation of magnetic field energy at the kinetic-inertial boundary results from a turning point in the perpendicular group velocity. Here we present observation of Alfv\'{e}n waves in HELIX (Hot hELIcon eXperiment) launched by a dipole antenna whose axis is aligned along the background magnetic field. The waves are excited at sub-cyclotronic frequencies within the high density region of a helium plasma. Profile measurements of the wave amplitude show evidence of ducting of the waves along the high density core of the plasma as well as wave energy accumulation at the kinetic-inertial boundary. Furthermore, phase and group velocities measurements are also compared to Alfv\'{e}n wave dispersion models. [Preview Abstract] |
|
GP8.00137: Kinetic correlations in driftwave fluctuations Fred Skiff, Vikram Patel, Shunjiro Shinohara We report measurements of kinetic fluctuations in dissipative driftwave fluctuations in a uniformly magnetized plasma cylinder. A singly ionized CW Argon plasma produced by an inductive plasma source is probed using two independent diode-laser based laser-induced fluorescence systems operating on transitions from separate metastable states (transitions at 668nm and 1047nm). The ions are weakly collisional in the $n$= 10$^{9}$ cm$^{-3}$, \textit{Te}=2 eV,\textit{ Ti}=0.1 eV plasma with a neutral pressure of 2x10$^{-4}$ Torr. Earlier measurements have been made using a single laser beam and were therefore limited to correlations at the same ion velocity. Here we look at the use of two laser beams, as well as the fast wavelength scanning of a single laser, to detect correlations between ions at different velocities parallel to the confining magnetic field. Tests are performed to explore correlations due to optical pumping (which are clearly observed at high laser power between the metastable magnetic sublevels). Of particular interest is the correlation width in velocity space, which pertains to the effective degrees of freedom of the plasma. [Preview Abstract] |
|
GP8.00138: Frequency Domain Nonlinear Energy Transfer Study in a Laboratory Plasma M. Xu, G.R. Tynan, C. Holland, Z. Yan, S.H. Muller, J.H. Yu We report here the experimentally measured internal and kinetic nonlinear energy transfer rates in a linear plasma device CSDX (Controlled Shear Decorrelation Experiment) using a recently developed technique [1]. The results clearly show net kinetic energy transfer from turbulence to zonal flows, thus directly show the turbulence-driven mechanism of shear flows. In addition, it was found that the radial flux of vorticity is dominantly responsible for the redistribution of kinetic turbulent energy among different frequencies (or different scales). Combined with published results [2], we know that it plays dual roles: the spreading of momentum in configuration space and the spreading of energy among different scales. The bicoherence was also computed, which shows that the phase coherence between zonal flows and turbulence plays an important role in the nonlinear energy transfer from turbulence to zonal flows. We also report the results from linear eigenmode stability analysis based on the Hasegawa-Wakatani model. Despite the use of flat profiles of electron temperature, ion viscosity, etc., it shows that the first two radial eigenmodes n=1 and n=2 are linearly unstable at the typical CSDX condition, suggesting that these two radial eigenmodes are the driving source for zonal flows. [1] M. Xu et al , Phys. Plasmas 16 042312 (2009). [2] Holland C et al, Phys. Rev. Lett. 96 195002 (2006) [Preview Abstract] |
|
GP8.00139: Search for chaos in the transition to drift wave turbulence M.J. Burin, J.A. Noguez, G.R. Tynan, S.H. Muller, M. Xu In the transition to fully developed turbulence one may encounter chaotic regimes, as has been demonstrated both theoretically and experimentally in a variety of flows, including within magnetized plasmas. Such regimes appear as windows of low-dimensional order, allowing for a better understanding of the underlying nonlinear dynamics, which may in turn allow for new avenues of control. We have recently begun a study of the turbulent transition in the Controlled Shear De-Correlation Experiment (CSDX), a helicon-powered magnetized plasma column at UCSD, in light of chaotic signatures such as the maximum Lyapunov exponent as derived from Langmuir probe time-series. We report our preliminary results on the role of chaos in the transition in general and with respect to the emergence of a self-organized shear layer (zonal flow) in particular. [Preview Abstract] |
|
GP8.00140: Temperature and Density Measurements in Low Density, Magnetized Plasmas Using a Multipass Thomson Scattering Cavity Derek Schaeffer, Carmen Constantin, Nathan Kugland, Erik Everson, Christoph Niemann, Chris Ebbers, Siegfried Glenzer We present experiments to study the temperature and density in low density, magnetized plasmas in UCLA's Large Plasma Device (density $\sim $10$^{12}$ cm$^{-3}$, temperature $\sim $5 eV) using a single-shot multipass Thomson scattering cavity with small scattering parameter alpha (non-collective regime). We frequency double the Phoenix laser (1064 nm, 10 J, 5 ns) to a 532 nm probe beam and send the beam $\sim $20 times through the target point in an image-relayed cavity, with the resulting mulitpassed beam yielding an effective probing energy of $\sim $100 J over $\sim $100 ns. The data will be compared to Langmuir probe measurements to characterize the ambient plasma and the setup will be used in future experiments to characterize exploding plasmas. [Preview Abstract] |
|
GP8.00141: Experimental and Numerical Study of Drift Alfv\'{e}n Waves in LAPD Brett Friedman, P. Popovich, T.A. Carter, D. Auerbach, D. Schaffner We present a study of drift Alfv\'{e}n waves in linear geometry using experiments in the Large Plasma Device (LAPD) at UCLA and simulations from the Boundary Turbulence code (BOUT). BOUT solves the 3D time evolution of plasma parameters and turbulence using Braginskii fluid equations. First, we present a verification study of linear drift Alfven wave physics in BOUT, which has been modified to simulate the cylindrical geometry of LAPD. Second, we present measurements of density and magnetic field fluctuations in the LAPD plasma and the correlation of these fluctuations as a function of plasma parameters, including strength of the background field and discharge current. We also compare the measurements to nonlinear BOUT calculations using experimental LAPD profiles. [Preview Abstract] |
|
GP8.00142: Drift wave turbulence simulations in LAPD P. Popovich, M. Umansky, T.A. Carter, D.W. Auerbach, B. Friedman, D. Schaffner, S. Vincena We present numerical simulations of turbulence in LAPD plasmas using the 3D electromagnetic code BOUT (BOUndary Turbulence). BOUT solves a system of fluid moment equations in a general toroidal equlibrium geometry near the plasma boundary. The underlying assumptions for the validity of the fluid model are well satisfied for drift waves in LAPD plasmas (typical plasma parameters $n_e \sim 1\times 10^{12}$cm$^{-3}$, $T_e \sim 10$eV, and $B \sim 1$kG), which makes BOUT a perfect tool for simulating LAPD. We have adapted BOUT for the cylindrical geometry of LAPD and have extended the model to include the background flows required for simulations of recent bias-driven rotation experiments. We have successfully verified the code for several linear instabilities, including resistive drift waves, Kelvin-Helmholtz and rotation-driven interchange. We will discuss first non-linear simulations and quasi-stationary solutions with self-consistent plasma flows and saturated density profiles. [Preview Abstract] |
|
GP8.00143: Resonant Drive and Suppression of Unstable Drift-Alfv\'en Waves Using Alfv\'enic Quasimodes D.W. Auerbach, S. Vincena, T.A. Carter, P. Popovich, B. Friedman, D. Schaffner We report here on the interaction between antenna-driven quasimodes and pressure-gradient driven drift-Alfven waves in the LAPD experiment at UCLA. A cylindrical low-density ($1 \times 10^{12} \ cm^{-3}$) depletion in the center of the higher density ($3 \times 10^{12} \ cm^{-3}$) bulk plasma is generated by selectively blocking primary electrons and therefore plasma producton. A broad frequency range (on the order of 10 kHz) of drift-Alfv\'en waves (DAW) form in the gradient region. Using a dual-strap Alfv\'en wave antenna, a low-frequency (1 kHz to 30 kHz) beat-wave quasimode between two co-propagating higher frequency Alfv\'en waves is driven into the cylindrical density gradient region. When the beat-frequency quasi-mode is at the spontaneous DAW frequency, we see resonant driving of the mode. More interestingly, when the quasi-mode is driven off-resonance, we observe strong suppression of the drift-Alfv\'en instability. By varying the beat-frequency, a range of suppression behavior is achieved. Both single coherent-mode fluctuations and broad-band turbulent fluctuations are suppressed. We provide experimental details of the observed quasimode-DAW interaction and discuss future plans for theory and simulation efforts to uncover the mechanism for the suppression. [Preview Abstract] |
|
GP8.00144: Investigation of flows in LAPD and their relation to edge turbulence and intermittency D. Schaffner, T.A. Carter, B. Friedman, S. Vincena, D.W. Auerbach, P. Popovich We report on measurements of spontaneous flows and turbulence in the Large Plasma Device (LAPD) at UCLA. Measurements of perpendicular and parallel flow using a six-sided Mach probe reveal edge-localized perpendicular flows. The source of this flow is under investigation and may be generated by boundary effects or turbulent processes. Particular cases where a plasma depletion zone is created, including inserting a blocking disk within the cathode region and forming a compressed column, are used to analyze the effects on plasma flows. Ultimately, the relationship between the flows, turbulence and intermittency---the formation of blobs---is sought. [Preview Abstract] |
|
GP8.00145: Studies of a plasma with a hot dense core in LAPD Bart Van Compernolle, Walter Gekelman, Patrick Pribyl, Chris Cooper Recently, considerable effort in the LArge Plasma Device at UCLA (LAPD) has gone into the study of large cathodes which would enable higher discharge currents and higher densities. The new cathode is made out of Lanthanum HexaBoride (LaB6). LaB6 has a low work function and has higher emissivity than Barium oxide coated cathodes. The operating temperature of LaB6 cathodes lies above 1600 degrees Celsius. Tests of this cathode in the Enormous Toroidal Plasma Device (ETPD) showed that densities in excess of $2 \ 10^{13} \mbox{cm}^{-3}$ and electron temperatures of 12 eV are feasible. Small LaB6 cathodes (3mm - 2cm) have been used before in LAPD in several experiments on heat transport and on magnetized flux ropes. The cathode presented in this paper has a 8 cm diameter, and can be positioned at different radial locations. The cathode will be pulsed into the standard background plasma ($n_e = 2 \ 10^{12} \mbox{ cm}^{-3}, .25 \leq T_e \leq 6 \mbox{ eV}, \mbox{ dia } = 60 \mbox{ cm}, \mbox{L} = 18 \mbox{ m}$) creating a plasma with a hot dense core. We present the characterization of the core plasma at different conditions. Studies of the heat transport and density spreading at the interface between the core plasma and background plasma will be done by use of a variety of probes (Langmuir, magnetic, Mach, emissive) as well as fast photography. [Preview Abstract] |
|
GP8.00146: Temporal sequences in the evolution of fluctuations on LMD-U H. Arakawa, S. Inagaki, K. Kamataki, M. Yagi, S. Sugita, S.-I. Itoh, Y. Nagashima, T. Yamada, N. Kasuya, A. Fujisawa, K. Itoh We investigate the dynamics of turbulence in linear magnetized plasma confined in LMD-U device. By use of poloidal array of probes and 2D-movable probe, spatio-temporal structure of drift wave fluctuations are studied. Abrupt changes of the density fluctuations are observed, and temporal evolution of Fourier-decomposed elements (both in poloidal direction and in frequency space) at the onset of transition is studied in detail. Primary components that lead the transition are identified. Then, delay times of higher harmonics after the sudden jump of primary components are measured using the conditional averaging and the cross correlation technique. This reveals the coupling between many Fourier components and yields the time for dynamical evolution in cascades. This experiment provides quantitative information of nonlinear interactions. [Preview Abstract] |
|
GP8.00147: Experimental Investigation of the ETG Nonlinear Saturation Mechanism Xiao Wei, Vladimir Sokolov, Amiya K. Sen We have produced and identified the slab electron temperature gradient (ETG) mode in the Columbia Linear Machine (CLM) [1]. Now we investigate the nonlinear saturation mechanism of the ETG modes. We study 3-wave coupling physics via the bi- coherence investigation and find signatures of the interaction between ETG modes and some low frequency modes. The saturation mechanism of the ETG modes is suspected to be related to the damping of low frequency modes. More detailed identification of the low frequency modes including their dispersion will be reported. \\[4pt] [1] X. Wei, V. Sokolov, A.K. Sen, APS DPP08 TP6.00131 [Preview Abstract] |
|
GP8.00148: Experimental Study of Local Anomalous Ion Thermal Transport with A Novel Time Dependent Energy Analyzer Vladimir Sokolov, Amiya K. Sen In the past we have reported measurements of global ion thermal transport due to ion temperature gradient (ITG) modes [1]. It is clearly more desirable to obtain local measurements of the same, which we now report for basic experiments in the Columbia Linear Machine. For local measurement of radial ion thermal transport we used a novel time dependent ion energy analyzer. For compensation of plasma potential fluctuations in energy analyzer measurements, we use floating potential fluctuation of Langmuir probe as a feedback signal with gain +1 and apply this voltage on energy selector grid. The simultaneous measurement of the ion current fluctuations of analyzer $ \tilde I_{IEA}(t) $ and the fluctuation of ion saturation current of Langmuir probe $ \tilde I_{SAT}(t)$ allow us to determine local fluctuations of ion temperature $ \tilde {T_i}(t)$. The local thermal flux is obtained from cross-correlation of ion temperature fluctuations and potential fluctuations. The radial profiles of the plasma density, ion temperature, and total thermal flux were obtained at different levels of ITG mode. The results indicate that the ion thermal transport is $ \Gamma \propto ( \partial T_I / \partial r )^{\alpha}, \alpha >1$. \\[4pt] [1] B. Song, J. Chen, and A.K. Sen, Phys. Rev. Lett. 70, 2407 (1993). [Preview Abstract] |
|
GP8.00149: Measurement of neutral flow velocity field associated with an anti-E$\times $B vortex Kohei Ogiwara, Mitsutoshi Aramaki, Shinji Yoshimura, Masayoshi Y. Tanaka Recently, a class of vortices, which rotate opposite to the direction of E$\times $B drift, have been observed in a magnetized plasma. It is considered that this vortex is driven by the force generated through the momentum transfer due to charge exchange collisions between ions and neutrals. To confirm the mechanism of the vortex formation, we have developed a high resolution LIF system and measured the neutral flow velocity field, in which the LIF system with an extended cavity diode laser is combined with saturated absorption spectroscopy to achieve high wavelength resolution ($\pm $5fm). The LIF spectra of argon metastable atoms have been measured by tuning the laser wavelength to 696.735 nm and detecting the fluorescence photons of 826.679 nm. The neutral flow velocity field has been visualized for the first time. It is found that the radial flow is directed to the center of vortex with its maximum velocity of about 40 m/s. The direction of the azimuthal rotation is the same as that of the ion flow. [Preview Abstract] |
|
GP8.00150: Coupling of axial plasma jets to compressional Alfven waves Stephen Vincena, Walter Gekelman The coupling of mass, energy, and momentum from a localized, dense, and rapidly expanding plasma into a large-scale magnetized background plasma is central to understanding many physical processes; these include galactic jets, coronal mass ejections, tokamak pellet fueling, high-altitude nuclear detonations, chemical releases in the ionosphere, and supernovae. The large-scale magnetized plasmas are capable of supporting Alfv\'{e}n waves, which mediate the flow of currents and associated changes of magnetic topology on the largest size scales of the external system. We present initial results from a laboratory experiment wherein a fast-moving, laser-produced plasma (LPP) is allowed to propagate along the magnetic field lines of a pre-existing plasma column (17m long by 60 cm diameter). The LPP is generated using a 1J, 8ns Nd:YAG laser fired at a graphite target. The laser is pulsed along with the background plasma at 1Hz. This work focuses on the coupling of the LPP to compressional Alfv\'{e}n waves in the background plasma. The experiments are conducted at UCLA's Basic Plasma Science Facility in the Large Plasma Device. [Preview Abstract] |
|
GP8.00151: Effect of two ion-species on the propagation of shear Alfven waves G.J. Morales, S.T. Vincena, J.E. Maggs An experimental and theoretical modeling study of the propagation properties of shear Alfven waves of small transverse scale in the presence of two ion-species is presented. In a two-ion plasma, depending on the mass of the heavier species, ion kinetic effects can become prominent, and significant parallel electric fields result in electron acceleration. Theory predicts the appearance of frequency propagation gaps, and spatial structures that mix the cone-propagation characteristics of Alfven waves with radially expanding ion Bernstein modes. The experiments are performed at the Basic Plasma Science Facility (BaPSF) at UCLA and consist of the spatial mapping of shear waves launched by a small disk exciter and by a large loop antenna. A variety of two ion-species combinations are explored by mixing the basic gases Helium, Neon, Argon and Hydrogen to generate highly reproducible plasmas. It is found that propagation bands below the ion cyclotron frequency of each of the two species can be identified, but there appears to be a large damping mechanism whose strength increases with the mass of the heavier species. For a 50-50 mix of Argon-Neon the damping is so severe that no signals can be observed in the Neon propagation band. [Preview Abstract] |
|
GP8.00152: On generation of Alfv{\' e}nic-like fluctuations by drift wave-zonal flow system in Large Plasma Device experiments C.E. Correa, W. Horton, G.D. Chagelishvili, V.S. Avsarkisov, J.G. Lominadze, J.-H. Kim, J.C. Perez, Troy Carter According to recent experiments, magnetically confined fusion plasmas with ``drift wave-zonal flow turbulence'' (DW-ZF) give rise to broadband electromagnetic waves. Sharapov et al (2008) report an abrupt change in the magnetic turbulence during L-H transitions in Joint European Torus [P. H. Rebut and B. E. Keen, Fusion Technol. {\bf 11}, 13 (1987)] plasmas. A broad spectrum of Alfv{\'e}nic-like fluctuations appear from $\textbf{E} \times \textbf{B}$ flow driven turbulence in the Large Plasma Device (LAPD) [W. Gekelman et al., Rev. Sci. Instrum. {\bf 62}, 2875 (1991)]. We present one possible theoretical explanation of electromagnetic fluctuations in DW-ZF systems in LAPD experiments. Fluctuation modes of spectrally stable sheared flows are non-normal. The linear operators of the fluctuations modal analysis are non-normal and the corresponding eigenmodes are not orthogonal. The non-normality results in linear transient growth with bursts of the perturbations and the mode coupling, which causes electromagnetic waves. The transient growth substantially exceeds the growth of the classical dissipative trapped-particle instability. [Preview Abstract] |
|
GP8.00153: A Krein-Like Theorem for the Linearized Vlasov-Poisson Equation George Hagstrom, Philip Morrison We consider the linearized Vlasov-Poisson equation in the Banach space with the norm $\|\{f_k\}\|=\sum_k k^2\|f_k\|_{W_{1,1}}$. We perturb the equations by changing the equillibrium solution $f_0$. We prove that that always exists an infinitesimal perturbation of $f_0'$ in the $W_{1,1}$ norm can create an instability at any solution of the equation $f_0'(v)=0$. If we restrict to dynamically accessible perturbations we instead recover a result similar to Krein's theorem for linear finite dimensional Hamiltonian systems. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700