The combined kinetic effects of the ion temperature gradient and the velocity shear of a plasma flow parallel to the magnetic field on the drift-Alfven instabilities

Long-pulse plasma operation at high normalized beta, β N , above the n = 1 ideal MHD no-wall stability limit in KSTAR is presently limited by tearing instabilities rather than resistive wall modes. H-mode plasma operation during the recent KSTAR device campaign produced discharges having strong m/n = 2/1 tearing instabilities at β N lower than the ideal MHD no-wall beta limit. The unstable tearing mode consequently reduced plasma confinement and toroidal plasma rotation significantly. The experiment confirmed that an extended duration of electron cyclotron heating (ECH) at the initial phase of the discharge plays a critical role in mode destabilization. To study destabilizing mechanisms that affect the mode growth, the stability of the observed tearing modes from plasmas with significantly different β N is computed by using the resistive DCON code and the M3D-C 1 code employing different physics. The computed tearing stability index, ∆ ′ , differs between the mode that is destabilized by the early ECH at lower β N , and the mode that is destabilized at higher β N with observed mode triggering activity. Equilibrium reconstructions that include constraints from internal profile diagnostics are used as input for reliable computation of stability. The modified Rutherford equation (MRE) describing the evolution of the neoclassical tearing mode (NTM) island width has been constructed for KSTAR plasmas by using plasma parameters computed by the TRANSP code. In preparation for long-pulse plasma operation at higher beta utilizing increased plasma heating power, a resistive wall mode (RWM) active feedback control algorithm that includes magnetic sensor compensation of the prompt applied field and the field from the induced current on the passive conductors has been completed and enabled on KSTAR. Use of multiple toroidal sensor arrays is enabled for increased control performance by including the effect of varied mode helicities in the outboard region where the mode measurement is made. This analysis on beta-limiting instabilities and active mode control provides the required foundation for high confinement plasma operation on KSTAR without disruption.

Feasibility study of direct spectra measurements of Thomson scattered photons for fusion-grade plasmas is performed based on a forward model of the KSTAR Thomson scattering system. Expected spectra in the forward model are calculated based on Selden function including the relativistic polarization correction. Noise in the signal is modeled with photon noise and Gaussian electrical noise. Electron temperature and density are inferred using Bayesian probability theory. Based on bias error, full width at half maximum and entropy of posterior distributions, spectral measurements are found to be feasible. Comparisons between spectrometer-based and polychromator-based Thomson scattering systems are performed with varying quantum efficiency and electrical noise levels. K: Analysis and statistical methods; Data processing methods; Plasma diagnosticscharged-particle spectroscopy; Spectrometers 1Corresponding author.

The 14th experimental campaign from the Korea Superconducting Tokamak Advanced Research (KSTAR) device has passed since the first experimental campaign was carried out in 2008. The basic diagnostic systems such as magnetic diagnostics, interferometer, inspection illuminator, visible spectrometer, ECE radiometer have been used for the first plasma experiment in KSTAR. Currently more than 50 diagnostic systems have been continuously installed including improved basic diagnostics and advanced imaging diagnostics in KSTAR. A recent progress and future plan of diagnostics for KSTAR are briefly discussed.