S tability and Variation of Plasma Parameters in the L-2M Stellarator by Exciting Induction Current in ECR Heated Plasma

Variation of parameters of ECR heated p lasma was s tudied in the L-2M stellarator under conditions that t he basic magnetic configuration was modified by exciting induction current. Experiments were carried out at ECRH power of ~200 kW (~1 MW m -3 ) and average plasma density of ~2·10 19 m -3 . The direction of the current was chosen such that the total rotational transform decreased, and its value was large enough (up to 17 kA) for rotational transform to change sign in the inner layers of the plasma column. Computer modeling predicts the formation of a multi-axis magnetic structure in the inner layers of the plasma c olumn. Magnetic probe measurements show the presence of bursts in signals of Pfirsh-Schluter currents and variations of the spectrum and mode of MHD oscillations of the plasma column. The appearance of the n = 0 mode at currents above 10 kA is correlated to the appearance of the region where ι/2π = 0. It i s s hown that, in the presence of the induction current, the e lectron temperature in the region r/a ≤ 0.6 is lower by a factor of 1.3, whereas a characteristic jump in the temperature in the e dge plasma remains; t he density g radient at the plasma e dge decreases. The behavior of turbulent density fluctuations was s tudied b y using diagnostics of scattering o f probing radiation. It i s found that the probability density functions for increments of density fluctuations have heavier tails in the presence of a multi-axis structure. The spectral characteristics of turbulent fluctuations in the e dge plasma and the poloidal plasma velocity were found to vary with radius. The experimental evidence suggests that the formation of the magnetic island structure in the c ore plasma leads to more intense transport i n both core a nd edge plasma, but t he change in transport i s not catastrophic.