Status of Recent Experimental and Analytical Investigation of MHD-induced Energetic Ion Redistribution or Loss in the National Spherical Torus Experiment
Authors: S.S. Medley, R. Andre, R.E. Bell, D.S. Darrow, E.D. Fredrickson, N.N. Gorelenkov, S.M. Kaye, F.M. Levinton, B.P. LeBlanc, J.E. Menard, D. Stutman, A.L. Roquemore, K. Tritz, H. Yuh, and the NSTX Team
Abstract:
The National Spherical Torus Experiment (NSTX) is particularly well suited to investigate fast-ion driven instabilities because large values of the dimensionless parameters vfast /vAlfvén and βfast(0)/ βtot(0) required to drive such instabilities occur routinely in neutral beam heated plasmas. The instabilities can be divided into three categories; chirping Energetic Particle Modes (EPM) in the frequency range 0 – 120 kHz, the Toroidal Alfvén Eigenmodes (TAE) with a frequency range of 50 – 200 kHz and the Global and Compressional Alfvén Eigenmodes (GAE and CAE, respectively) between 300 kHz and the ion cyclotron frequency. These modes are of particular interest because of their potential to cause substantial fast ion redistribution or loss. Both the volume-integrated neutron and the line-integrated charge exchange neutral particle diagnostics show signal depletion due to fast-ion driven instabilities, but cannot distinguish between fast-ion redistribution or loss. Two recently implemented diagnostics on NSTX, the Motional Stark Effect (MSE) and scintillator Fast Lost Ion Probe (sFLIP), facilitate separation of redistribution and loss effects. In discharges with significant MHD activity, outward redistribution of the core-peaked energetic beam ions modifies the beam-driven current profile and hence the core q-profile. MSE-constrained q-profiles are being used to assess this effect. sFLIP measures the pitch and energy of fast ions that are ejected from the plasma and intercept the wall-mounted probe thus identifying fast-ion loss. For certain Hmode discharges where NPA measurements of the NB energetic ion spectra exhibit MHD2 induced fast-ion depletion, the sFLIP data confirm the existence of an ion loss that occurs primarily for passing particles near the NB full injection energy. Observations and TRANSP simulations representing fast ion instability-induced redistribution or loss phenomena in NSTX are presented.
_____________________________________________________________________________
Download PPPL-4235 April 2007 (pdf 7.1 MB).