Alfvén eigenmode and energetic particle research in JT-60U

Abstract

Recent results on investigations of Alfvén eigenmodes, fast ion confinement and fast ion diagnostics in JT-60U are presented. It was found that toroidicity induced Alfvén eigenmodes (TAEs) were stable in negative shear discharges with a large density gradient at the internal transport barrier (ITB). If the density gradient was small at the ITB, multiple TAEs appeared around the q = 2 surface (pitch minimum) and showed a large frequency chirping (∆f ≈ 80 kHz). In low-q positive shear discharges, the location of the TAEs changed from outside to inside the q = 1 surface, owing to a temporal change of the q profile. A significant depression of the megaelectronvolt ion population was observed only with high-n (n up to 14) multiple TAEs inside the q = 1 surface. Non-circular triangularity induced Alfvén eigenmodes were observed for the first time. Considerable depression of the triton burnup was observed in negative shear discharges. Orbit following Monte Carlo simulations indicated that ripple loss was responsible for the enhanced triton losses. The fast ion stored energies in ICRF heated negative shear discharges were comparable to those of positive shear plasmas. Tail ion temperatures in high (second to fourth) harmonic ICRF heating experiments were first analysed with an MeV neutral particle analyser. The behaviour of MeV ions produced by ICRF heating was studied with gamma ray diagnostics. A scintillating fibre detector system for detecting the 14 MeV neutron emission was developed for the triton burnup studies. Ion cyclotron emission measurements discriminating between parallel and perpendicular components of the electric field were carried out for the first time.

Compatibility of internal transport barrier with steady-state operation in the high bootstrap fraction regime on DIII-D

“…In addition to the slowing down effect of the density discussed here, recent analysis with the Nova code [28] indicates that the Alfvén continuum in the core of these plasmas during the high density phase is relatively closed, which may also contribute to the reduced AE activity and resulting fast ion transport. These results are similar to the observations in [29,30] that AE modes are stable in negative shear discharges with large density gradient at the ITB, although the regime under consideration here is significantly different, with a q-profile that is flatter in the core and overall significantly higher [see figure 3(c)]. …”

Observation of Reversed Shear Alfvén Eigenmodes between Sawtooth Crashes in the Alcator C-Mod Tokamak

“…RSAEs are known to exist near the minimum of the q profile (q min ) in a reversed shear equilibrium and are driven unstable by the spatial gradient of the fast ion density from neutral beams, fusion born alpha particles, or in the case of Alcator C-Mod, an ICRH driven minority ion species [11,12]. RSAEs and other Alfvénic modes have been used for ''MHD spectroscopy''; that is, the inference of equilibrium parameters such as q min or q on axis (q 0 ) from frequency spectra [12,13].…”

“…RSAEs and other Alfvénic modes have been used for ''MHD spectroscopy''; that is, the inference of equilibrium parameters such as q min or q on axis (q 0 ) from frequency spectra [12,13]. The characteristic frequency sweep of the RSAEs has been well qualified in previous experimental [11,12], computational [14], and theoretical [15,16] studies. An approximate dispersion relation for the RSAEs can be found in Ref.…”

Measurements of the eigenfunction of reversed shear Alfvén eigenmodes that sweep downward in frequency

“…In the vast majority of published cases, [1][2][3][4][5][8][9][10][11][12][13][14][15][16][17][18][19][20][21] the modes first appear when q min ¼ m=n, then the mode frequency sweeps upward in frequency as the q profile evolves. Very rarely, 8,22,23 a mode will sweep downward to the minimum frequency, then sweep upward in the normal fashion.…”