PPPL-5383
Destabilization of counter-propagating Alfvénic instabilities by
off-axis, co-current neutral beam injection
Authors: M. Podestŕ, E. D. Fredrickson and M.
Gorelenkova
Abstract: Injection of high-energy neutrals (NBI) is a
common tool to heat the plasma and drive current non-inductively in
fusion devices. Once neutrals ionize, the resulting energetic
particles (EP) can drive instabilities that are detrimental for the
performance and the predictability of plasma discharges. A broad NBI
deposition profile, e.g. by off-axis injection, is often assumed to
limit those undesired effects by reducing the radial gradient of the
EP density, thus reducing the drive for instabilities. However, this
work presents new evidence that off-axis NBI can also lead to
undesired effects such as the destabilization of Alfvénic
instabilities. Time-dependent analysis with the TRANSP code
indicates that instabilities are driven by a combination of radial
and energy gradients of the EP distribution. The mechanisms for
wave-particle interaction revealed by the EP phase space resolved
analysis are the basis to identify strategies to mitigate or
suppress the observed instabilities.