Influence of external perturbations created by an ergodic magnetic limiter at the plasma edge of the TBR tokamak

Ergodic Magnetic Limiters (EML) have already been used in tokamaks to create small magnetic field perturbations, mainly at the boundary of the plasma column, which resulted in a significant improvement of the magnetic confinement. These external perturbations usually reduced the plasma-wall interaction and, in result, decreased both the particle and energy losses and the impurity influx from the vacuum vessel. In TBR tokamak, a set of four EML rings was constructed and installed around the tokamak vacuum chamber, yielding a dominant m/n = 7/2 magnetic perturbation within the plasma edge. In order to investigate the influence of this external perturbation, a modified triple electrostatic probe was used to measure the plasma density (ne) and the electron temperature (Te). Alternatively analyzing groups of successive discharges, with and without the activation of the EML coils, it was observed that the plasma density and the electron temperature, in a fixed position, decreased almost linearly as the EML current increased, in the range 0 to 300 A. Above that, up to 400 A, no further improvement was obtained. Also, adjusting the EML current in 300 A and measuring Te and ne along the radial direction, it was obtained a significant decrease for ≤ 7.5 cm, approximately. (author). 4 refs., 6 figs