Investigation Of A Transient Energetic Charge Exchange Fux Enhancement (ʻspike-on-tailʼ) Observed In Neutral-beam-heated H-mode Discharges In The National Spherical Torus Experiment
Authors: S.S. Medley et. al.
Abstract:
In the National Spherical Torus Experiment (NSTX), a large increase in the charge
exchange neutral flux localized at the Neutral Beam (NB) injection full energy is measured
by the E||B (superimposed parallel electric and magnetic fields) Neutral Particle Analyzer
(NPA). Termed the High-Energy Feature (HEF), it appears on the NB-injected energetic
ion spectrum only in discharges where tearing or kink-type modes (f < 50 kHz) are absent,
Toroidal Alfvén Eigenmode (TAE) activity (f ~ 50 - 150 kHz) is weak and Global Alfvén
Eigenmode (GAE) activity (f ~ 400 – 1000 kHz) is robust. Compressional Alfvén
eigenmode (CAE) activity (f > 1000 kHz) is usually sporadic or absent during the HEF
event. The HEF exhibits growth times of Δt ~ 20 - 80 ms, durations of ~ 100 – 600 ms and
peak-to-base flux ratios up to H = Fmax /Fmin ~ 10. In infrequent cases, a slowing down
distribution below the HEF energy can develop that continues to evolve over periods > 100
ms, a time scale long compared with the typical fast ion equilibration times. HEFs are
Transient energetic charge exchange flux enhancement (ʻspike-on-tailʼ)
2
observed only in H-mode (not L-mode) discharges with injected power Pb ≥ 4 MW and in
the pitch range χ = vll /v ~ 0.7 – 0.9; i.e. only for passing particles. Increases of ~ 10 - 30
% in the measured neutron yield and total stored energy that are observed to coincide with
the feature appear to be driven by concomitant broadening of measured Te(r), Ti(r) and
ne(r) profiles and not the HEF itself. While the HEF has minimal impact on plasma
performance, it nevertheless poses a challenging wave-particle interaction phenomenon to
understand. Candidate mechanisms for HEF formation are developed based on
quasilinear theory of wave-particle interaction. The only mechanism found to lead to the
large NPA flux ratios, H = Fmax /Fmin , observed in NSTX is the quasilinear evolution of the
energetic ion distribution, Fb(E,χ,r), in phase space and the concomitant loss of some
particles, which occurs due to the cyclotron interaction of the particles with destabilized
modes having sufficiently high frequencies, F ~ 700 - 1000 kHz, in the plasma frame that
are tentatively identified as Global Alfvén Eigenmodes.
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Submitted to: Nuclear Fusion (June 2011)
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