Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session JI2: Fast Ion Physics, RF Theory |
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Chair: Tobin Munsat, University of Colorado Room: Ballroom DE |
Tuesday, October 30, 2012 2:00PM - 2:30PM |
JI2.00001: Simulation and Theory of Long Range Frequency Sweeping of TAE Modes Invited Speaker: Ge Wang Toroidal Alfven eigenmode (TAE) excited by energetic particles is extensively observed in magnetic fusion. Frequently, the wave frequency is found to sweep in the TAE gap and even penetrate into the continuum. To achieve more realism than a chirping model based on the bump-on-tail instability, TAE wave equation reduces to a Volterra integral equation which couples to currents from energetic particles. The code enables tracking of the chirping signals arising from the resonant interaction in phase space. A down-chirping signal produced by clumps eventually enters the Alfven continuum, whereupon the mode amplitude and chirping rate both increase more rapidly. In contrast, the up-chirping signal never penetrates the continuum. An adiabatic theory (ADT) quantitatively produces the simulation's down-chirping dynamics including an explosive response in the continuum. ADT for the up-chirping signal reproduces the simulation until its frequency approaches the upper continuum. Two extrinsic dissipation models are employed that generally give similar results, though for frequencies near the upper gap/continuum boundary, there is a dependence on the dissipation models. The hole smoothly vanishes as it goes into the continuum in ADT for both models. However, simulations show that the hole suddenly disintegrates before reaching the upper continuum for one case and smoothly decays for the other. The discrepancy for the hole's decay is apparently explained from the calculation of an adiabatic validity parameter that implies that the hole's disintegration takes place when the adiabatic condition breaks down as the upper continuum is approached. Ongoing improvements to TAE modeling takes into account the spatial profile variation of mode structure. The most significant disparity between newest Hamiltonian and an older simpler one occurs near continuum tips where there is the possibility that topological change in the phase space may emerge. Two major qualitative results of this theory: chirp penetration into the lower continuum with enhancement of the field amplitude but with no chirp penetration into the upper continuum, has been observed in the MAST experiment. [Preview Abstract] |
Tuesday, October 30, 2012 2:30PM - 3:00PM |
JI2.00002: Interplay between coexisting MHD instabilities mediated by energetic ions in NSTX H-mode plasmas Invited Speaker: Alessandro Bortolon In next-step fusion devices different types of MHD instabilities are likely to coexist and possibly interact. This work addresses an interesting interplay between low frequency (5-20 kHz) and ion cyclotron frequency (1-2 MHz) modes, commonly observed in the early phase of beam heated NSTX H-modes (P$_{NB}$=4-6MW). The first have been characterized as peripheral kinks, with low toroidal number n=1-3. The second are clusters of Compressional Alfv\'{e}n Eigenmodes (CAE), with larger n=9-15, propagating in direction of the beams. Despite the temporal and spatial scale separation, the destabilization of CAEs strongly correlates with the presence of low frequency kinks. The kink onset is accompanied by a collapse of the core fast ion density, measured by the Fast-Ion D-Alpha diagnostic (FIDA), while data from neutron and fast-ion loss detectors indicate modest increase of fast ion losses. This suggests that the kink activity may cause fast ion redistribution in phase space, which may in turn affect CAE stability. Based on an ideal MHD description of the peripheral kink structure, validated against experimental data, full-orbit simulations with the SPIRAL code have been used to assess its effect on the fast ion distribution function. Results confirm that in presence of the kink the beam ions are redistributed from core to periphery, with peak density reduced by 20{\%} and total losses increasing by less than 5{\%}, in agreement with observations from fast ion diagnostics. SPIRAL simulations also show that an enhanced pitch angle scattering associated with the kink tends to populate velocity space regions of more parallel pitch (0.5$<$V$_{\vert \vert }$/V$<$1), where direct resonances of observed CAE modes are expected, providing an additional drive for the destabilization of the CAE. The results underline the role of the fast ion distribution as coupling element between different MHD instabilities. [Preview Abstract] |
Tuesday, October 30, 2012 3:00PM - 3:30PM |
JI2.00003: M3D-K Simulations of Beam-Driven Alfven Modes in DIIID Invited Speaker: Guoyong Fu Multiple beam-driven Reversed Shear Alfven eigenmodes (RSAEs) and Toroidal Alfven Eigenmodes (TAE) were observed in the DIII-D discharge ({\#}142111) [1]. Extensive hybrid simulations with the global kinetic/MHD hybrid code M3D-K [2] have been carried out to investigate these beam-driven Alfven eigenmodes using experimental parameters and profiles from this discharge. The purpose of this work is for code verification and validation as well as for physics understanding needed for predicting energetic particle-driven instabilities and energetic particle transport in burning plasmas. We first benchmark M3D-K code with the linear ideal MHD stability code NOVA. The M3D-K results agree well with those of NOVA with respect to mode structure and mode frequency in the MHD limit. With energetic beam ions, the simulations results show that the destabilized n=3 mode transit from RSAEs to TAEs as the minimum of the safety factor drops in agreement with the measured frequency sweeping. The calculated 2D mode structure in poloidal cross-section exhibits a twisting feature or radial phase shearing consistent with the Electron Cyclotron Emission Imaging (ECEI) data [1]. An analytic theory has been developed to explain the radial phase shearing observed in the simulations and experiments. It is found that both the fast ion drive and background damping can cause radial phase shearing. The direction of phase shearing changes when the toroidal magnetic field is reversed whereas the shearing direction is independent of plasma current direction. This symmetry agrees with the experimental observation from ECEI. Finally, nonlinear simulations of the beam-driven modes with particle collision and particle source and sink have been carried out and results show that other Alfven modes becomes destabilized after the initial saturation of the n=3 RSAE mode. The details of the linear and nonlinear simulation results will be presented. \\[4pt] [1] B. J. Tobias \textit{et al.}, Phys. Rev. Lett. \textbf{106}, 075003 (2011).\\[0pt] [2] G.-Y. Fu et al., Phys. Plasmas \textbf{13}, 052517 (2006). [Preview Abstract] |
Tuesday, October 30, 2012 3:30PM - 4:00PM |
JI2.00004: Fast Ion Confinement and Stability of an NBI-heated RFP Invited Speaker: Jay Anderson Energetic ions are fundamentally important for both fusion and astrophysical plasmas. While well-studied in tokamak and stellarator plasmas, relatively little is known in RFP plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in MST with an intense 25 keV, 1 MW hydrogen neutral beam injector (300 MW/m$^{2}$ at injection port). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3-5{\%} D$_{2})$ both demonstrate that the fast ion population is consistent with classical slowing of the fast ions, zero cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops during the 20 ms (several times the bulk plasma confinement time) NB injection. TRANSP simulations predict a super-Alfvenic ion density of up to 15{\%} of the electron density with both a significant velocity space gradient and a sharp radial density gradient. There are several effects on the background plasma including enhanced toroidal rotation, electron heating and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 50{\%} reduction in the magnetic fluctuation amplitude is observed during NBI. Simultaneously, beam driven instabilities are observed for the first time in the RFP. Repetitive $\sim $50$\mu $s bursts of m=1 modes have scaling signatures of both Alfvenic and continuum energetic particle modes. The dominant modes are n=4 (EP-like) and n=5 (AE-like), which nonlinearly couple to an n=1 mode. Modeling for TAE modes in the RFP is performed using AE3D, but the mode features are not fully consistent with the code predictions. [Preview Abstract] |
Tuesday, October 30, 2012 4:00PM - 4:30PM |
JI2.00005: Energetic Ion Transport and Neutral Beam Current Drive Reduction Due to Microturbulence in Tokamaks Invited Speaker: D.C. Pace Energetic ion transport due to microturbulence can significantly broaden the fast ion pressure profile with consequences for energetic particle driven instabilities and non-inductive current drive from neutral beam injection. Experiments on the DIII-D tokamak reveal the incremental beam ion transport driven by microturbulence, particularly in ITER-relevant scenarios where off-axis current drive is used by tilting one beamline for injection at a normalized minor radius of $r/a=0.5$. Anomalous fast ion transport in MHD-quiescent plasmas is investigated using spectroscopic measurements that probe the beam ion distribution, providing radial profiles of beam ion density. These measurements are compared to synthetic diagnostic simulations in which the classical (i.e., non-turbulent) beam ion distribution is calculated by the Monte Carlo neutral beam code NUBEAM. Differences between these two profiles indicate the presence of non-classical transport effects. These plasmas are further modeled by the new energy and pitch angle dependent anomalous diffusivity feature of NUBEAM, with the turbulent diffusivities calculated self-consistently using the gyrokinetic code TGYRO/TGLF. It is also possible to define analytic expressions for the anomalous diffusivity based on existing theoretical models. Microturbulence-induced beam ion transport decreases neutral beam current drive (NBCD) efficiency, especially for off-axis beam injection where the ions are deposited in regions with stronger turbulence. Instances of NBCD up to 30\% below expected values based on neoclassical predictions occur in conditions where strong microturbulence driven thermal transport is inferred. The ability to model and predict the impact of microturbulence in off-axis NBCD scenarios will be presented. [Preview Abstract] |
Tuesday, October 30, 2012 4:30PM - 5:00PM |
JI2.00006: Scattering of waves by blobs in tokamak plasmas: venturing beyond geometric optics Invited Speaker: Abhay Ram The edge region of tokamak plasmas is replete with density fluctuations that manifest themselves as blobs. Radio frequency (RF) waves, commonly used for heating and for current profile control, have to propagate from the excitation structures to the core of the plasma through this active region. The blobs modify the propagation properties of the waves through reflection, refraction, and diffraction. There are two basic approaches to studying the effects of density fluctuations on RF waves. The first is the geometric optics approach in which the effect of fluctuations is to refract the RF beam or rays. There are two subsequent consequences--diffusion in real space leading to a spatial deflection of the rays, and diffusion in wave vector space leading to the broadening of the launched spectrum. The geometric optics approach is limited to small density fluctuations of order 10\% or less. However, in experiments, larger amplitude fluctuations are generally observed. The second approach to scattering is through a full wave analysis which extends the range of validity well beyond that of geometric optics. Such an analysis is theoretically and computationally much more challenging. The full-wave description includes reflection, refraction, and diffraction of the RF waves by blobs. A full-wave model for the scattering by a density blob has been developed for RF waves in any frequency range. A detailed analysis provides insight into the scattering by blobs and shows that power from the incident RF wave can couple to other plasma waves and to surface waves. The scattering depends on the ratio of the RF wavelength to the spatial scale of the fluctuations. When the ratio is large, as would be the case for electron cyclotron waves, there is an enhancement of the electric fields near the edge of the blobs. Additionally, wave fields propagating along the ambient magnetic field are excited which take power away from the primary wave propagating to the core. When the ratio is of order one or less, as in the case of lower hybrid and ion cyclotron waves, diffractive scattering is prominent and the spectrum of the waves is broadened. The theoretical model for the scattering, along with numerical results and experimental consequences, will be discussed. [Preview Abstract] |
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