Excitation of Alfvénic modes in Ohmic tokamak discharges

Magnetohydrodynamic (MHD) mode activity in the Alfven frequency range has been detected in the absence of ´
energetic ions during discharges in several conventional tokamaks and spherical tokamaks, including the Tokamak
Fusion Test Reactor (TFTR) and the Mega-Amp Spherical Tokamak (MAST). In TFTR the dominant toroidal mode
number n was found to be zero; this is also the case in MAST discharges for which mode number information
is available. The observed properties of these modes are shown to be consistent with global Alfven eigenmodes ´
(GAEs). Although they appear to have little or no effect on plasma performance in present-day devices, the fact
that they are frequently observed in MAST Ohmic discharges suggests that they could be used as a diagnostic
of plasma equilibrium parameters. In principle, they could also provide the basis for a plasma heating scheme.
A possible mechanism for the excitation of the Alfven eigenmodes in the absence of fast ions is suggested by ´
two-fluid simulations of various tokamaks, in which high-frequency mode activity is found to be correlated with
relatively long-timescale MHD events in the plasma, such as internal reconnection events (IREs) or edge localized
modes (ELMs). A simple analytical model describing the excitation of Alfvenic modes by either IREs or ELMs is ´
proposed. The coupling of low- and high-frequency MHD is predicted to be strongest for radially-extended modes:
this is consistent with the low mode numbers of the activity observed in TFTR and MAST