Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session QI1: Wave and Particle Interactions |
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Chair: Tom Intrator, Los Alamos National Laboratory Room: Adam's Mark Hotel Plaza Ballroom ABC |
Thursday, October 27, 2005 9:30AM - 10:00AM |
QI1.00001: Current Drive by Electron Bernstein Waves Invited Speaker: In many conventional tokamaks, waves in the electron cyclotron range of frequencies (ECRF), the X mode or the O mode, have been successfully used for generating plasma current and for modifying the current profile. In the same range of frequencies the electron Bernstein waves (EBW) offer an intriguing alternative for generating plasma currents. EBWs are particularly suited for the high-$\beta$ plasmas encountered in spherical tori (ST) like NSTX. Unlike the X and O modes, EBWs have no density limits and are strongly absorbed by electrons in the Doppler-shifted vicinity of harmonics of the electron cyclotron resonance. We have been studying two different means of driving plasma currents by EBWs -- the Ohkawa scheme [1] and the Fisch-Boozer scheme [2]. The two schemes together provide enough flexibility to generate supplemental current for steady state operation and for modifying the current profile. We discuss the basic physics of the two schemes as applied to high-$\beta$ operations in NSTX. More detailed insight is obtained from computations using the code DKE which solves the drift kinetic equation with Fokker-Planck collision operator and quasilinear RF diffusion operator. We find that EBWs damp on the tail of the electron distribution function. Consequently, the EBW current drive efficiency is higher than for the X or O modes. Additionally, the core plasma is better suited for the Fisch-Boozer scheme while the Ohkawa scheme works best on the outboard plasma side. In NSTX, current can be driven in the core and in the outboard plasma region using a single source frequency corresponding to the fundamental electron cyclotron resonance in the center. These studies extend to traditional ECRF current drive and could have applications in conventional tokamaks including ITER. [1] T.\ Ohkawa, General Atomic Technical Report GA-A13847 (1976); and J.\ Decker, in {\it AIP Conf.\ Proc.\ 694}, New York (2003), p.\ 447. [2] N.\ J.\ Fisch and A.\ H.\ Boozer, {\it Phys.\ Rev.\ Lett.} {\bf 45}, 720 (1980). [Preview Abstract] |
Thursday, October 27, 2005 10:00AM - 10:30AM |
QI1.00002: Self-consistent full-wave and Fokker-Planck calculations for ion cyclotron heating in non-Maxwellian plasmas Invited Speaker: High-performance burning plasma devices such as ITER will contain significant concentrations of non-thermal plasma particles arising from fusion reactions, neutral beam injection, and wave-driven diffusion in velocity space. Initial studies in 1-D [1] and experimental results [2] show that non-thermal energetic ions can significantly alter wave propagation and absorption in the ion cyclotron range of frequencies. In addition, these ions can absorb power at high harmonics of the cyclotron frequency where conventional 2-D global-wave models are not valid. In this work, the all-orders, full-wave solver AORSA [3] is generalized to treat non-Maxwellian velocity distributions. Quasi-linear diffusion coefficients are derived directly from the global wave fields and used to calculate the energetic ion velocity distribution with the CQL3D Fokker-Planck code [4]. Alternately, the quasi-linear coefficients can be calculated numerically by integrating the Lorentz force equations along particle orbits. Self-consistency between the wave electric field and resonant ion distribution function is achieved by iterating between the full-wave and Fokker-Planck solutions.\newline [1] R. J. Dumont, C. K. Phillips and D. N. Smithe, Phys. Plasmas \textbf{12}, 042508 (2005).\newline [2] A. L. Rosenberg, J. E. Menard, J. R. Wilson, \textit{et al}., Phys. Plasmas \textbf{11}, 2441(2004).\newline [3] E. F. Jaeger, L. A. Berry, J. R. Myra, \textit{et al}., Phys. Rev. Lett. \textbf{90}, 195001-1 (2003).\newline [4] R. W. Harvey and M. G. McCoy, in \textit{Proceedings of the IAEA Technical Committee Meeting on Advances in Simulation and Modeling of Thermonuclear Plasmas} (IAEA, Montreal, 1992). [Preview Abstract] |
Thursday, October 27, 2005 10:30AM - 11:00AM |
QI1.00003: Oscillating Field Current Drive Experiments in the Madison Symmetric Torus Invited Speaker: Oscillating field current drive (OFCD), or AC magnetic helicity injection, is a technique proposed to drive a steady-state current in a toroidal plasma using sinusoidal toroidal and poloidal loop voltages. If the phase between the two loop voltage oscillations is nonzero, net magnetic helicity is injected into the plasma. We have extended the detailed dynamical picture of OFCD through 3D nonlinear MHD computation at a large Lundquist number of $5\cdot 10^5$, which elucidates the important role of current diffusion by magnetic fluctuations. In MST, we have applied OFCD at modest input power (several hundred kW) to demonstrate current drive of about 10{\%} of the total current. The OFCD-generated current is roughly consistent with theoretical expectations, based on either the 3D MHD computation or a 1D relaxed-state model. We investigate the dependence of the current drive on the phase $\delta $ between the two voltages. Maximal current is driven for $0<\delta <\pi /2$, and not for the phase of maximal helicity injection ($\pi /2)$. The difference might be attributed to the effect of OFCD on magnetic tearing fluctuations that during OFCD are observed to be smallest when the current drive is maximal. The penetration of the oscillatory field and the evolution of the current density profile during OFCD are shown through equilibrium reconstructions which use Motional Stark effect and laser Faraday rotation data for the internal magnetic field. The current profile oscillates during the OFCD cycle and becomes more centrally peaked over several cycles. Studies of the confinement properties are beginning, in particular using a new multipoint Thomson scattering system. [Preview Abstract] |
Thursday, October 27, 2005 11:00AM - 11:30AM |
QI1.00004: Fast Ion Confinement in the MST Reversed Field Pinch Invited Speaker: The influence of magnetic fluctuations and consequent magnetic stochasticity on transport and confinement of fast ions is important for fusion plasmas as well as for astrophysical situations. In fusion plasmas it affects the feasibility of neutral atom injection as a means to heat the plasma and will determine the confinement of fusion produced alpha-particles during magnetic reconnection. For astrophysical plasmas, energetic particle transport in stochastic fields can be important in thermal conduction in galaxy-cluster plasmas and cosmic ray propagation. We use the reversed field pinch laboratory plasma to measure transport of fast particles in a stochastic magnetic field. The standard RFP plasma contains a stochastic magnetic field which becomes particularly ergodic during reconnection events. We measure the confinement of fast ions generated by injection of energetic neutral atoms. The confinement is determined through detection of neutrons from the fusion reaction between the beam-produced deuterium ions and the background deuterium plasma. The results indicate that even for a level of magnetic fluctuations at which the magnetic field is stochastic the fast ions energy loss is consistent with the classical slowing down rate and their confinement time is at least~20~ms. This is much longer than expected (1~ms) from the simple picture of the ions streaming along the stochastic magnetic field. However, a several-fold increase in the magnetic stochasticity, as observed during reconnection events in MST, can significantly deteriorate the ion confinement These observation are in agreement with numerical simulation of ion trajectories as well as with analysis of the overlapping of islands in the ion guiding center trajectories. [Preview Abstract] |
Thursday, October 27, 2005 11:30AM - 12:00PM |
QI1.00005: Collective fast ion instability-induced losses in NSTX Invited Speaker: A wide variety of fast ion driven instabilities are excited during neutral beam injection (NBI) in the National Spherical Torus Experiment (NSTX) due to the large ratio of fast ion velocity to Alfv\'{e}n velocity, V$_{fast}$/V$_{Alfv\mbox{\'{e}}n}$, and high fast ion beta. The ratio V$_{fast}$/V$_{Alfv\mbox{\'{e}}n}$ in ITER and NSTX is comparable. The modes can be divided into three categories; chirping energetic particle modes (EPM) in the frequency range 0 to 120~kHz, the Toroidal Alfv\'{e}n Eigenmodes (TAE) with a frequency range of 50 kHz to 200~kHz and the Compressional and Global Alfv\'{e}n Eigenmodes (CAE and GAE, respectively) between 300~kHz and the ion cyclotron frequency. Fast ion driven modes are of particular interest because of their potential to cause substantial fast ion losses. In all regimes of NBI heated operation we see transient neutron rate drops, correlated with bursts of TAE or fishbone-like EPMs. The fast ion loss events are predominantly correlated with the EPMs, although losses are also seen with bursts of multiple, large amplitude TAE. The latter is of particular significance for ITER; the transport of fast ions from the expected resonance overlap in phase space of a ``sea'' of large amplitude TAE is the kind of physics expected in ITER. The internal structure and amplitude of the TAE and EPMs has been measured with Heterodyne reflectometry and soft x-ray cameras. The TAE bursts have internal amplitudes of \~{n}/n~$\le $~1{\%} and toroidal mode numbers 2~$<$~n~$<$~6. The EPMs are core localized, kink-like modes similar to the fishbones in conventional aspect ratio tokamaks. Unlike the fishbones, the EPMs can be present with q(0)~$>$~1 and can have a toroidal mode number n~$>$~1. The range of the frequency chirp can be quite large and the resonance can be through a fishbone-like precessional drift resonance, or through a bounce resonance. [Preview Abstract] |
Thursday, October 27, 2005 12:00PM - 12:30PM |
QI1.00006: Interpretation of Core Localized Alfven Eigenmodes in DIII-D and JET Reversed Magnetic Shear Plasmas Invited Speaker: Newly developed core localized fluctuation diagnostics have revealed a surprisingly large multiplicity of high-n Alfven eigenmodes in reversed shear regimes in DIII-D [1], where toroidal mode numbers from n=8 to 40 are observed, and in JET [2] where mode numbers up to n=20 are observed. These modes, found near the zero shear surface, are reversed shear Alfven eigenmodes with interesting ``twists". (a)~Their range of frequency is affected by plasma compressibility [3]. (b)~Plasma rotation, especially in DIII-D, leads to large Doppler shifts (up to 1.5~MHz is measured whereas Alfven Cascades oscillate locally at $\sim$100~kHz), leading to observed frequency down-chirps that are actually generated by frequency up-chirps in the rotating plasma frame at the zero shear surface. (c)~The high-n property of the mode leads to the need for inclusion of ion diamagnetic drift terms for the frequency identification. At large n the instability drive comes from both injected neutral beams and thermal ions, particularly in high performance discharges. Addition of these new features was required in the NOVA end NOVA-K codes in order to account for the observed frequencies and instability drive. These observations, together with the recognition of the importance of the background plasma in driving Alfvenic instability, may lead to a significant re-evaluation of the role of internally generated Alfven waves in thermal transport and fast ion confinement in burning plasma experiments with reversed shear.\par \vspace{0.5em} [1]~R.~Nazikian, et al., Proc.\ 20th IAEA Fusion Energy Conf.\ 2004, IAEA-CN-116/EX/5-1.\par [2]~S.E. Sharapov, et al., Phys.\ Rev.\ Lett.\ ${\bf 93}$, 165001 (2005).\par [3]~M.S. Chu, et al., Phys.\ Fluids B ${\bf 4}$, 3713 (1992); H.L. Berk, et al., Proc.\ 20th IAEA Fusion Energy Conf.\ 2004, IAEA-TH/5-Ra. [Preview Abstract] |
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