ENHANCEMENT OF THE ISTTOK PLASMA-CONFINEMENT AND STABILITY BY NEGATIVE LIMITER BIASING

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma cur rent reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through time-reso lved measurements of the line-averaged plasma density, electron densit y profile, electron temperature and ohmic power. Gross particle confin ement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the H-alpha radiation l evel. Plasma stability modifications are analysed by measurements of t he plasma column transverse displacement, plasma poloidal rotation fre quency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is deter mined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and sta bility, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.