Radiative Damping of Low Shear Toroidal Alfvén Eigenmodes

Paper i proceeding, 2010

An important issue for alpha-particle transport due to Alfvén instabilities in burning ITER plasmas [1] is the number of unstable Alfvén eigenmodes (AEs) and their properties at high plasma pressure. Multiple low shear toroidal Alfv´en eigenmodes (LSTAEs) [2, 3, 4] are of major concern for ITER scenarios, where extended regions of low magnetic shear may exist in e.g. sawtoothing plasmas or hybrid regimes [1, 5]. In contrast to the conventional TAE, which has a single eigenfrequency per TAE gap, the number of LSTAEs per gap is given by l ~= Є/S [4], and can be quite large in a low-shear region. Moreover, LSTAEs are less affected by high plasma pressure than TAEs: in fact, the very discovery of LSTAEs was galvanised by DT experiments on TFTR, where unstable AEs were detected at very high plasma pressure [2]. More recently, LSTAEs were recognized as important features of sawtoothing tokamak plasmas. On JET, core localized ”tornado” modes (LSTAEs inside the q = 1 radius) were found to precede monster sawtooth crashes [6], and on Alcator C-Mod, frequency chirping AEs associated with very low shear were observed during the sawtooth cycle [7]. In this contribution, we develop a theory of LSTAEs by incorporating non-ideal effects associated with finite ion Larmor gyroradius into the MHD model [4]. The radiative damping for LSTAEs at the top and bottom of the TAE gap is calculated in the limit of high mode number and first order gyroradius, using the characteristic LSTAE ordering d/dr << m/r. It is shown that radiative damping of LSTAEs is much smaller than that of TAEs with d/dr >> m/r. We also estimate the LSTAE drive due to ICRH-accelerated fast ions [8] and obtain the mode excitation threshold for ICRH-driven TAEs.