Fluctuations of the Magnetic Field of Plasma Currents in Uragan-3M Stellarator

Abstract

At the torsatron-type stellarator Uragan-3M, frequency spectrum and spatial structures of the fluctuations of the poloidal component of the magnetic field in frequency range 0.3–52 kHz, which are caused by the plasma current, are studied with a set of magnetic sensors. The studies were carried out in the microwave heating regime in confining magnetic fields of different strength. The magnetic field fluctuations were studied in weakly and strongly collisional plasmas. Studies showed that the spectrum of the recorded magnetic field fluctuations consists of a relatively narrowband number of frequencies. The fluctuating magnetic fields are caused by plasma currents, which can be presented as a number of poloidal structures with wave numbers m = 0, 1, 2, and 3 (and, possibly, also 4). Each spatial structure of plasma currents (expect the one with m = 0) can be represented as a sum of two oscillation types: a rotating and a standing structure. The rotating structure moves in the poloidal direction with a rotation frequency close to the frequency of the standing structure. The rotation direction is not constant and it changes during the discharge. The standing structure does not change its position (phase) and oscillates at a fixed frequency. The conducted analysis showed that the observed fluctuations of the magnetic field are a result of plasma instabilities, which cause perturbations of the plasma pressure. These oscillations have the growth rate and damping rate of the order of the carrier frequency. The perturbation of the plasma density initiates the appearance of polarization removal currents whose magnetic field is recorded in the described experiments. Estimates show that the fluctuation level reaches 4 × 10^–3 relative to the average plasma pressure in the discharge. It was also shown that, in these experiments, microwave heating does no cause the observed instabilities. The time behavior of the discharge parameters and the energy of magnetic field fluctuations at the boundary of the confinement region allows us to conclude that at the quasi stationary stages of the discharge, the observed instabilities determine the main energy losses from the plasma volume.