Abstract
Recent progress in the analysis of the low and high frequency beam ion driven instabilities in the National Spherical Tokamak Experiment (NSTX) [S. Kaye et al., Fusion Technol. 36, 16 (1999)] plasma is reported. The low Alfvén speed with respect to the beam ion injection velocity in NSTX offers a window in the plasma parameter space to study instabilities driven by super-Alfvénic fusion alphas, which are expected in the International Tokamak Experimental Reactor—ITER [D. J. Campbell, Phys. Plasmas 8, 2041 (2001)]. Low frequency magnetic field activities identified as an instability of toroidicity-induced Alfvén eigenmodes (TAEs) have been observed in NSTX and analyzed with the linear hybrid kinetic magnetohydrodynamic stability code NOVA-K [C. Z. Cheng, Phys. Rep. 1, 211 (1992)]. The comparison between the TAE analysis and observations in NSTX and DIII–D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] similarity experiments confirms that the toroidal mode number of the most unstable TAE modes scales with q−2 and is independent of plasma major radius, where q is the safety factor. This scaling helps validate the predictive capability of the NOVA-K code for studying TAE stability in future burning plasma devices. The subion cyclotron frequency magnetic activities in NSTX are identified as compressional and global shear Alfvén eigenmodes (AEs) (CAEs and GAEs). CAE and GAE instabilities are driven by beam ions via the Doppler shifted cyclotron resonance by the velocity space bump-on-tail distribution function in the perpendicular velocity. Results of the GAE/CAE theoretical and numerical analysis are presented.
Highlights
Collective instabilities associated with superthermal ions in plasmas have been of interest to fusion researchers during the last four decadessee for instance Refs. 1 and 2, and references therein
To simulate GAE/CAEs in realistic NSTX plasma conditions we have developed a nonlinear hybrid kineticMHD simulation code, HYM,32,33 which is capable of simulating the mode structure, saturation, and energetic particle transport
A detail study shows that the unstable low-n TAEs are reproduced for NSTX, whereas for DIII–D the modeling predicts nϭ2 – 7 TAEs to be unstable
Summary
Collective instabilities associated with superthermal ions in plasmas have been of interest to fusion researchers during the last four decadessee for instance Refs. 1 and 2, and references therein. The plasma is heated with a deuterium beam with power of Pbϭ1.5– 3 MW and an injection energy of usually Eb0ϭ80 keV With such a low magnetic field, the typical ratio of beam ion velocity to the Alfven velocity satisfies 1Ͻb0 /AՇ3 ͑in ITER 1Ͻ␣0 /AϽ2, where ␣0 is the birth velocity of alpha particle. The machine size scaling of the most unstable mode numbers was verified in the specially designed similarity experiments on NSTX and DIII–D,23 in which similar plasma parameters were established with the exception of their major radii and the safety factors.. Some new features of the high frequency magnetic fluctuation spectrum were observed in NSTX, which suggests new instabilities associated with the shear Alfven branch, the so-called global Alfven eigenmodesGAEs. Similar to the compressional AEsCAEs, GAEs can be destabilized by beam ions via the Doppler shifted cyclotron resonance. III the experimental observations, development of analytical theory, and numerical tools for the analysis of these new instabilities are presented
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