Study of ion-gyroscale fluctuations in low-density L-mode plasmas heated by NBI on KSTAR

Abstract

Broadband density fluctuations with peak frequency ranging from 150 to 400 kHz were measured using a multichannel microwave imaging reflectometer in core region of the low-density L-mode plasmas heated by neutral beam injection on KSTAR. These fluctuations have been studied by comparing the dominant mode scales estimated from the measurement with those predicted from linear gyrokinetic simulation. The measured poloidal wavenumbers are qualitatively comparable to those of the ‘fastest growing modes’ from simulations, whereas they are larger than those of the ‘transport-dominant modes’ by about a factor of three. The agreement on wavenumbers between the measurement and linear simulation (for the fastest growing modes) is probably due to sufficiently weak flow shear compared to the maximum linear growth rate. Meanwhile, the transport-dominant modes seem to be related to the fluctuations in lower frequencies (∼80–150 kHz) observed in some of the measurement.

Development of a collective scattering system and its application to the measurement of multiscale fluctuations in KSTAR plasmas

“…In the KSTAR tokamak, large-scale magnetohydrodynamical (MHD) instabilities [1][2][3][4][5][6][7][8][9][10] and ion gyroscale turbulence [11][12][13][14][15][16][17][18] have been measured and then studied primarily with advanced imaging diagnostics as well as conventional diagnostics. The advanced diagnostics such as the electron cyclotron emission (ECE) imaging system [19], microwave imaging reflectometer (MIR) [16,20], and beam emission spectroscopy (BES) [21,22] can measure the turbulence with long wavelength or low wavenumber of less than 3 cm −1 .…”

Quasi-coherent fluctuation measurement with the upgraded microwave imaging reflectometer in KSTAR

“…The spatial and temporal scales of the ion-gyroscale turbulence for the L-mode plasmas heated by NBI were estimated from the time-delayed cross-correlation between signals of the multiple poloidal channels, and compared to the parameters associated with the ion-gyroscale turbulence such as ion temperature gradient mode and trapped electron mode (TEM) [3]. The poloidal wavenumbers of the most unstable modes for the L-mode plasmas were deduced from peak frequencies (determined from cross-coherence spectra) and effective poloidal velocities of fluctuations, and they were compared to those predicted from linear and nonlinear gyrokinetic simulations [4,5]. Recently, characteristics of quasi-coherent modes (QCMs) known as a kind of TEM [6][7][8][9][10][11][12][13][14] were investigated for the ohmically heated or electron cyclotron resonant heating (ECH) assisted L-mode plasmas [15].…”