Fast excitation of EGAM by NBI

Abstract

A new mechanism for the fast excitation of the energetic geodesic acoustic mode (EGAM) is proposed to explain the recent experiment in DIII-D (Nazikian et al 2008 Phys. Rev. Lett. 101 185001), where the mode turns on in less than a millisecond after the turn-on of the neutral beam injection. The existence of loss boundary in pitch angle when beam particles are injected counter to the plasma current is crucial to the formation of negative energy EGAM mode. The resonance of this negative energy wave with energetic particles, whose distribution decreases with energy, destabilizes the mode. We find that when there is a loss region, the onset time of instability can be significantly shorter than it would be if the injected particles had no loss region.