Fast electron driven Alfvén eigenmodes in the current rise in Alcator C-MOD

Abstract

The injection of 0.3–1 MW of lower hybrid current drive (LHCD) from the very start of the plasma drives a suprathermal electron tail that excites a series of bursting high frequency (100–700 kHz) instabilities in the current rise in Alcator C-Mod. These high frequency bursting modes have relatively small amplitudemeasured with poloidal field pick-up coils on outboard limiters. They are observed very early in the current rise with one to three bands of frequencies. The frequency at each burst corresponds very closely to the centre of the gap frequency for toroidal Alfvén eigenmodes (TAEs), ω_TAE=v_A/2qR, for integer and half integerqvalues ranging typically from 11 down to 5.5. Measured average hard x-ray photon energies in the range 20–35 keV are in reasonable agreement with the theoretically calculated fast electron energy that matches the precession drift resonance condition for exciting Alfvén eigenmodes.