Detection and investigation of chirping Alfven Eigenmodes with Heavy Ion Beam Probe in the TJ-II stellarator

Detection and investigation of chirping Alfven Eigenmodes with Heavy Ion Beam Probe in the TJ-II stellarator

Detection and investigation of chirping Alfven Eigenmodes with Heavy Ion Beam Probe in the TJ-II stellarator 150 150 UKAEA Opendata
UKAEA-CCFE-PR(18)73

Detection and investigation of chirping Alfven Eigenmodes with Heavy Ion Beam Probe in the TJ-II stellarator

Alfven Eigenmodes were studied in low magnetic shear flexible heliac TJ-II (B0=0.95 T, <R>=1.5 m, <a>=0.22 m) NBI heated plasmas (PNBI <1.1 MW, ENBI=32 keV) by Heavy Ion Beam Probe (HIBP) [1, 2, 3]. The L-mode hydrogen plasma was investigated at various magnetic configurations with rotational transform t(a)/2pi=1/q ~ 1.5 - 1.6. Co-, counter and balanced beam injection were explored. HIBP is capable to measure simultaneously the oscillations of the plasma electric potential, density and poloidal magnetic field. Earlier studies have shown the chirping modes with 250 kHz<fAE<380 kHz at the low density ne = (0.3 – 1.5)×1019 m-3 NBI heated plasmas with or without auxiliary ECR [3, 4]. Here we report the observation of the various types of the chirping modes, differs by frequency, radial location, amplitude and shape of the frequency dependence on time. Remarkably, the same mode may evolve, changing its type in the same frequency range in the single discharge. HIBP shows that location of the specific AE chirping may vary from ρ = 0 up to ρ = 0.8. The mode amplitude in potential varies from a few Volts up to 100 V. AEs are strongly affected by plasma current Ipl, e.g. caused by Ohmic heating coils. A variation of Ipl may even suppress the modes. Dual HIBP [5], consisting of the two HIBPs, separated at ¼ of the torus shows the high coherence between the plasma potential and density oscillations during the period of the frequency burst. The radial scan of the HIBP sample volume indicates the radial evolution of the potential and density perturbation during a single frequency burst of the chirping mode. Mean frequency of the chirping modes basically follow the model, based on the Alfven scaling and including actual iota evolution caused by Ipl changes [6].

Collection:
Journals
Journal:
Nuclear Fusion
Publisher:
IOP (Institute of Physics)