Fast Electron Driven Modes in the Current Rise in Alcator C-Mod

The injection of 0.3 -0.4 MW of Lower Hybrid Current Drive (LHCD) with 90° phasing from the very start of the plasma drives a suprathermal electron tail that excites a series of bursting high frequency (200 -700 kHz) instabilities in the current rise in Alcator C-Mod.These high frequency bursting modes have relatively small amplitude ( B θ ≤ 3 × 10 -6 T) measured with poloidal field pick-up coils on outboard limiters.They are observed very early in the current rise with several bands of frequencies that increase with decreasing density.The frequency at each burst corresponds very closely to the center of the gap frequency for toroidal Alfvén eigenmodes (TAEs), v /2 TAE A qR ω = , for integer and half integer q values from 11 down to 5.5.Measured average hard x ray photon energies in the range of 25 -35 keV are in reasonable agreement with the theoretically calculated fast electron energy that matches the precession drift resonance condition for exciting Alfvén eigenmodes.