Beam ion driven instabilities in the National Spherical Tokamak Experiment

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.

Investigation Of A Transient Energetic Charge Exchange Fux Enhancement ('spike-on-tail') Observed In Neutral-beam-heated H-mode Discharges In The National Spherical Torus Experiment

“…Using an explicit form of given in [58] and equations (7), (8), in the case of the l = 1 resonance we obtain the following estimate: where . We find that the magnitude used in our calculation (for which at ) is quite reasonable when in the instability region, as estimated in section 4.…”

“…Such beam-driven Alfvén eigenmodes [6][7][8][9] are destabilized when the fast-ion velocity v b is larger than the Alfvén speed v A . The modes can be divided into three categories; chirping Energetic Particle Modes (EPM) in the frequency range 0 -120 kHz, the Toroidal Alfvén Eigenmodes (TAE) with a frequency range of 50 -200 kHz and the…”

Study of Aspect Ratio Effects on Kinetic MHD Instabilities in NSTX and DIII-D

“…In practice, however, there are differences in the q profile between the two devices. In the similarity experiments, TAEs are observed with n = 1 2 in NSTX, whereas n = 2 7 in DIII-D. We have analyzed these experiments employing the NOVA and NOVA-K codes and recovered the observed scaling [6,7]. …”

Compressional Alfvén eigenmode structure in spherical tokamaks

“…These perturbations were found in a series of papers [16,17,18,19,20,14,15] to be CAEs and in some cases global Alfvén eigenmodes (GAEs) [21] driven by the energetic ions. For the instabilities identified as CAEs the instability frequency correlates with the evolution of the Alfvén velocity [12] and the perturbed magnetic field polarization is elliptical with a significant component parallel to the equilibrium field [13,14,19].…”