Study of Thermonuclear Alfvén Instabilities in Next Step Burning Plasma Experiments
Authors: N.N. Gorelenkov, H.L. Berk, R. Budny, C.Z. Cheng, G.-Y. Fu, W.W. Heidbrink, G. Kramer, D. Meade, and R. Nazikian
Date of PPPL Report: July 2002
Published in: Nuclear Fusion 43 (July 2003) 594-605.
A study is presented for the stability of alpha-particle driven shear Alfvén Eigenmodes (AE) for the normal parameters of the three major burning plasma proposals, ITER, FIRE and IGNITOR. A study of the JET plasma, where fusion alphas were generated in tritium experiments, is also included to attempt experimental validation of the numerical predictions. An analytic assessment of Toroidal AE (TAE) stability is first presented, where the alpha particle beta due to the fusion reaction rate and electron drag is simply and accurately estimated in 7-20keV plasma temperature regime. In this assessment the hot particle drive is balanced against ion-Landau damping of the background deuterons and electron collision effects and stability boundaries are determined. Then two numerical studies of AE instability are presented. In one the High-n stability code HINST is used. This code is capable of predicting instabilities of low and moderately high frequency Alfvén modes. HINST computes the non-perturbative solution of the Alfvén eigenmodes including effects of ion finite Larmor radius, orbit width, trapped electrons etc. The stability calculations are repeated using the global code NOVAK. We show that for these tokamaks the spectrum of the least stable AE modes are TAE that appear at medium-/high-n numbers. In HINST TAEs are locally unstable due to the alphas pressure gradient in all the devices under the consideration except IGNITOR. However, NOVAK calculations show that the global mode structure enhances the damping mechanisms and produces stability in all configurations considered here. A serious question remains whether the perturbation theory used in NOVAK overestimates the stability predictions, so that it is premature to conclude that the nominal operation of all three proposals are stable to AEs. In addition NBI ions produce a strong stabilizing effect for JET. However, in ITER the beam energies needed to penetrate to the core must be high so that a diamagnetic drift frequency comparable to that of the alpha particles is produced by the beam ions which induces a destabilizing effect.