Observation of Global Alfven Eigenmode Avalanche Events on The National Spherical Torus Experiment
Authors: E.D. Fredrickson, N.N. Gorelenkov, E. Belova, N.A. Crocker, S. Kubota, G.J. Kramer, B. LeBlanc, R.E. Bell, M. Podesta, H. Yuh, and F. Levinton
Abstract:
Instabilities excited by the fast-ion population on NSTX [M. Ono, et al., Nucl.
Fusion 40 557 (2000)] extend from low frequency Energetic Particle Modes (EPMs) at 10's
of kHz, through Toroidal Alfven Eigenmodes [TAE] in the range of 50kHz to 150kHz to
Global and Compressional Alfven Eigenmodes [GAE and CAE] in the frequency range of
0.3MHz to 2.5MHz, or roughly 0.1ωci to 0.7ωci. The GAE instabilities exhibit complex
non-linear behavior, including onset of strong growth above an amplitude threshold. This is
conjectured to occur when resonance regions in phase space start to overlap, resulting in
enhanced rapid growth and redistribution of energetic particles, a process referred to as an
'avalanche' [Berk, et al., Nucl. Fusion 35 (1995) 1661]. The GAE are suppressed following
the avalanche, suggesting depletion of the fast ion population resonantly driving the modes,
and in some instances the GAE bursts appear to trigger lower frequency TAE avalanches or
Energetic Particle Modes, suggesting some significant redistribution of fast ions in phase
space has occurred. These are the first observations of avalanching behavior for an
instability driven through the Doppler-shifted cyclotron resonance. This paper also
provides internal measurements of GAE mode structure showing that the mode amplitude
peaks towards the plasma core.
__________________________________________________
Submitted to: Nuclear Fusion (November 2011)
__________________________________________________
Download PPPL-4702 (pdf 1.94 MBB 21 pp)
__________________________________________________