Linear Stability and the Eigenmode Structure of Multiple Modes Induced by a Current Density Perturbation in Sawtoothing KSTAR Plasmas

Abstract

Multiple mode structures (m/n=2/2, 3/3, etc.) in addition to the m/n=1/1 mode in the core of the sawtoothing plasma have been studied on the KSTAR by a systematic scan of the electron cyclotron heating/current drive (ECH/CD) from the center to q~1 surface and using 2D Electron Cyclotron Emission Imaging (ECEI) diagnostic [1]. As the ECH/CD position was changed from the center to q~1 surface, the measured mode structure changed from m/n=2/2 to m/n=4/4 or higher right after the sawtooth crash. During the ramp-up period, the higher m/n modes transform into the lower m/n modes and eventually merge into a single m/n=1/1 mode prior to crash. The dynamics of multiple mode structures were compared with a reduced MHD simulation with an empirical source term for the radially localized current drive on a flat q-profile with q0 ~ 1 [2]. In order to model the formation and the time evolution of the multiple mode structures, the growth rates of all possible modes for sets of core equilibrium conditions and the modified current density profile has been investigated in detail using M3D-C1 code in cylindrical geometry. A series of radial safety factor profiles for each time stage of a sawtooth period has been compared for two cases to address the issue of the central q value in sawtooth physics; 1) q0 stays below 1 after crash (i.e. from 0.75 to 0.8)[3,4] and 2) q0 is above 1 after the crash and drops to q0 ~0.75 before crash. Then a current density perturbation due to ECCD has been added from the center to q~1 surface onto the two cases. The calculated result suggests that the double tearing can be a possible mechanism for the formation of multiple mode structures if q0 is above 1 right after the sawtooth crash. *Work supported by NRF of Korea (grant No. NRF-2014M1A7A1A03029865)[1] G. H. Choe et al, Nuclear Fusion 55 013015 (2015)[2] Andreas Bierwage et al, Nuclear Fusion 55 013016 (2015)[3] F. M. Levinton et al, Physical Review Letters 63 2060 (1989)[4] H. Soltwisch et al, Review of Scientific Instrument 59 1599 (1988)