Suppression of fast-ion-driven MHD instabilities by ECH/ECCD on Heliotron J

Abstract

Experiments of suppressing fast-ion-driven MHD instabilities such as energetic particle modes (EPMs) and global Alfvén eigenmodes (GAEs) have been made by using a second harmonic X-mode electron cyclotron heating (ECH) and current drive (ECCD) in the helical-axis heliotron device, Heliotron J. ECCD experiments show that the GAE destabilized by fast ions of neutral beam injection (NBI) with the observed frequency around 140 kHz are fully stabilized, and the EPMs with the observed frequency around 90 kHz are suppressed when the EC-driven plasma current flowing in the counter direction reaches approximately 0.7 kA. The low magnetic shear under the vacuum condition is modified into positive magnetic shear when counter-ECCD is applied, and the amplitude of GAEs and EPMs decreases with an increase of the EC-driven plasma current. These results indicate that magnetic shear is a key role in controlling GAEs as well as EPMs. The comparison of the calculation of shear Alfvén spectra with experimental results shows that the increasing continuum damping rate with an increase in local magnetic shear by EC-driven current is important for both EPMs and GAEs. Moreover, the increase in plasma current lead to the inward movement of GAEs. This effect would also contribute to suppression of GAEs because the continuum damping rate increases more and more toward core. Steady ECH is also found experimentally to be effective to control the amplitude of both GAEs and EPMs. The amplitude of EPMs, and especially for GAEs decreases with an increase in the ECH power under fixed density conditions.

Modeling of the ECCD injection effect on the Heliotron J and LHD plasma stability

“…That is to say, the simulations must reproduce the mode number, radial location and frequency range of the n = 1 EPM and n = 2 GAE measured in the experiment. The measured instabilities are triggered between the middle-outer plasma region, in the range of frequencies between 80 to 100 kHz for the n/m = 1/2 EPM and between 140 to 160 kHz for the 2/4 GAE [25]. First, the growth rate and frequency of the n = 1 and 2 instabilities are calculated using different EP density profiles (fixed β f 0 = 0.008 and T f 0 = 14 keV, figure 3).…”

Tomographic Inversion Technique Using Orthogonal Basis Patterns

“…The objective geometry is of the Heliotron J device having a non-circular poloidal cross section. The Heliotron J is a medium sized helical axis Heliotron with L/M = 1/4 [3,4]. It is noted that the analysis using Phillip-Tikhonov regularization [5] has been already executed [6].…”

Development of a correlation ECE radiometer for electron temperature fluctuation measurements in Heliotron J

“…The first measurements with the new radial CECE diagnostic were made in a plasma with both ECRH and NBI heating. Energetic particle modes (EPM) are excited by fast ions with NBI heating in Heliotron J, and these modes are stabilized by using counter-ECCD [20][21]. During this experiment, the CECE-RF channel was set to measure 68.3 GHz, and it was correlated over 90 ms with the 12 GHz CECE-IF channel, corresponding to a spectral separation of 0.3 GHz and a sensitivity limit of 0.41%.…”