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 TI3: RF in Tokamaks and ITER |
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Chair: Paul Bonoli, Massachusetts Institute of Technology Room: Centennial II |
Thursday, November 5, 2009 9:30AM - 10:00AM |
TI3.00001: Iterated Finite Orbit Monte Carlo Simulation with Full Wave Fields for Tokamak ICRF Wave Heating Experiments Invited Speaker: A predictive understanding of ICRF wave heating is important for tokamak experiments and ITER. Finite-orbit effects due to drift motion of non-Maxwellian species may significantly modify the ICRF wave propagation and absorption in the plasma. To self-consistently obtain wave heating and fast-ion velocity space information, the 5D finite-orbit Monte-Carlo plasma distribution solver ORBIT-RF is integrated with the 2D full-wave code AORSA. To evaluate the finite-orbit effects on wave heating, the ORBIT-RF wave absorption model has been validated against linear full-wave zero orbit-width predictions from CQL3D/AORSA and measurements in Alcator C-Mod and DIII-D experiments for various ion cyclotron harmonics. Systematic comparison performed with an initial Maxwellian distribution largely reproduces linear absorption directly evaluated by AORSA dielectric tensor. An inward shift of ORBIT-RF absorption peak for high harmonics compared with that of AORSA has been observed. This inward shift may be indicative of finite orbit effects, which can produce a noticeable change in the radial absorption profile. DIII-D simulations based on combined ray-tracing zero orbit-width calculations using GENRAY/CQL3D show discrepancies with the measured FIDA spectroscopic data. To assess the finite-orbit effects on this difference, the non-Maxwellian plasma distribution evolution calculated by ORBIT-RF is iterated with wave fields computed from AORSA including quasilinear and collisional orbit diffusion. To accurately start the ORBIT-RF/AORSA iterations, beam-ion distributions computed from PTRANSP from experimental profiles are used as initial conditions. Comparison of ORBIT-RF/AORSA results against FIDA measurements of fast-ion distribution and results from CQL3D/AORSA with zero orbit assumption will be presented. [Preview Abstract] |
Thursday, November 5, 2009 10:00AM - 10:30AM |
TI3.00002: Advances in High-Harmonic Fast Wave Physics in the National Spherical Torus Experiment Invited Speaker: Improved core high-harmonic fast wave (HHFW) heating, particularly at longer wavelengths and during low-density start-up and current ramp-up, has now been obtained by lowering the edge density with lithium conditioning, thereby moving the propagation onset away from the vessel wall. Significant core electron heating of deuterium neutral beam injection (NBI) fuelled H-modes has been observed for the first time over a range of launched wavelengths. The observed broadening of the electron heating profile in H-mode relative to L-mode plasmas is consistent with simulations obtained with ray tracing and full wave models. Newly taken camera images indicate that fast wave interactions can deposit considerable RF energy on the outboard divertor plate, especially at longer wavelengths that begin to propagate closer to the vessel walls. Edge power loss can also arise from HHFW-generated parametric decay instabilities that drive ions in the edge onto direct loss orbits that intersect the wall, and may be the cause for an observed drag on edge toroidal rotation in combined HHFW and NBI discharges. Fast-Ion D-alpha emission clearly shows fast-ion profile broadening in the plasma core that is much greater than predicted by Fokker-Planck modeling when HHFW power is applied to NBI-fuelled plasmas, pointing to the need for a full-orbit treatment in the simulation. Large ELMs have been observed immediately following the termination of RF power, whether the power turn off is programmed or due to antenna arcing. RF power has been successfully applied during large ELMs by setting the source reflection coefficient trip levels to relatively high values -- an approach potentially important for ITER ICRF heating. Plans for an HHFW ELM-resilience upgrade will be presented. [Preview Abstract] |
Thursday, November 5, 2009 10:30AM - 11:00AM |
TI3.00003: Plasma wave simulation based on a versatile FEM solver on Alcator C-Mod Invited Speaker: A new efficient full wave simulation code of the lower hybrid (LH) wave was developed using the finite element method (FEM). A dielectric tensor consisting of the cold plasma contribution and the electron Landau damping (ELD) was used. The non-trivial problem of introducing non-local hot plasma effects into an FEM solver was addressed by iteratively solving the coupled problem of the Maxwell's equations with the convolution integral. With this approach, the EM problem is numerically sparse, and the computational requirements are reduced significantly compared to spectral domain solvers [1]. The simulation of an Alcator C-Mod scale plasma has been done on a desktop computer, suggesting the possibility of an ITER scale plasma simulation. The algorithm was implemented using a general purpose FEM software, COMSOL Multiphysics, and the simulation results of a Maxwellian tokamak plasma showed good agreement with ray tracing calculations in the strong single pass absorption regime. Integration of a Fokker-Planck calculation for a more realistic non-Maxwellian plasma is underway and initial results show reasonable shift of the power absorption towards the plasma edge [2]. Importantly, the FEM approach allows seamless handling of the core, SOL, and antenna regions. This flexibility has been exploited to address issues of antenna-plasma coupling in the LH and ICRF frequency ranges. Techniques to use the FEM package for this purpose were validated by solving the LH grill antenna coupling problem whose analytic solution is known. The code has been applied to a new Alcator C-Mod ICRF antenna to assess the antenna near field pattern [3]. \\[4pt] [1] J. C. Wright, et. al., Comput. Phys. 4, 545 (2008) \\[0pt] [2] O. Meneghini, et. al., this conference \\[0pt] [3] M. Garrett, et. al., this conference [Preview Abstract] |
Thursday, November 5, 2009 11:00AM - 11:30AM |
TI3.00004: Understanding and Predicting the Dynamics of Tokamak Discharges during Startup and Rampdown Invited Speaker: Understanding the dynamics of plasma startup and termination is important for present tokamaks and for predictive modeling of future burning plasma devices such as ITER. We report on experiments in the DIII-D tokamak that explore the plasma startup and rampdown phases, and on the benchmarking of transport models. Key issues have been examined such as plasma initiation and burnthrough with limited inductive voltage and achieving flattop and maximum burn within the technical limits of coil systems and their actuators while maintaining the desired $q$ profile. Successful rampdown requires scenarios consistent with technical limits, including controlled H-L transitions, while avoiding vertical instabilities, additional Ohmic transformer flux consumption, and density limit disruptions. The ITER baseline startup and rampdown scenarios have been demonstrated in DIII-D by scaling the dynamic phases by the ratio of the resistive current penetration time between the devices while maintaining the same $I/aB$ and scaled shape within the phase. Discharges were typically initiated with an inductive electric field typical of ITER, 0.3~V/m, most with 2nd harmonic electron cyclotron (EC) assist. A fast framing camera was used during breakdown and burnthrough to study the formation physics. An improved ``large aperture'' ITER startup scenario was developed and aperture reduction in rampdown was found to be essential to avoid instabilities. Extrapolation of these scenarios to burning plasma devices requires model validation. Current evolution using neoclassical conductivity in the Corsica code agrees with rampup experiments, but prediction of the temperature and internal inductance evolution using the Coppi-Tang model for electron energy transport is not yet accurate enough to allow extrapolation to future devices. [Preview Abstract] |
Thursday, November 5, 2009 11:30AM - 12:00PM |
TI3.00005: Kinetic theory of geodesic acoustic modes Invited Speaker: Geodesic Acoustic Modes (GAM are linear eigen-modes of poloidal plasma rotation supported by plasma compressibility in toroidal geometry. GAMs are linearly coupled to drift-waves via toroidal side-bands of plasma pressure, can be nonlinearly driven by Reynolds stress from small-scale fluctuations (similar to Zonal Flows) and therefore expected to play an important role in dynamics of drift-wave turbulence. GAMs have also been prominently featured due to their inherent relation to Beta Alfven Eigen-modes (BAE), particularly in plasmas with highly energetic particles Multiple GAM and BEA modes were observed in high-temperature tokamak plasmas and are currently subject of active experimental and theoretical studies. This talk will describe the currents status of GAM/BEA theory. New results will be presented emphasizing the relation of GAM/BAE modes with neoclassical rotation in a tokamak. It is shown that the GAM intrinsically involve anisotropic perturbations of plasma pressure (corresponding to parallel viscosity). Moreover, the GAMs and standard equilibrium (neoclassical) plasma rotation represent two limit cases of poloidal plasma rotation: high frequency rotational mode (GAM) and the low frequency over damped (damping is larger than the real part of the frequency) mode of the neoclassical equilibrium rotation. Most importantly, new regimes of global GAM/BAE modes will be reported. These regimes occurs as a result of the parallel kinetic response of electrons which has not been included previously. It is shown that in certain regimes (corresponding to global modes), the electron response becomes strongly electromagnetic and GAM/BAE modes have significant electromagnetic component in the side-bands (thus it is not of the Alfven type) . Resulting modifications in the mode dispersion and mode damping will be presented and potential consequences for GAM/BAE excitation and electron transport will be discussed. [Preview Abstract] |
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