Factors Influencing the Period of Improved Stability in Zeta

Further experimental and theoretical studies have been made of the period of improved stability in Zeta. Measurements, by Thomson scattering, show that the electron temperature, Te , at the start of the period is about 100 eV. Studies have also been made of Te during this period. Detailed measurements have been made of the plasma parameters (Ne, Te) at the edge of the discharge. Measurements of the axial field at a pinch parameter of 1.8 show that this field is reversed by about 20% of the initial axial field B0 at the start of the period. This reversal disappears after 1 ms when the improved stability ceases. At a pinch parameter of 2.9 the reversal is 80% of B0 and the period is extended to 3 ms. The degree of reversal is constant round the major axis of the torus. A reversed axial electric field at the edge of the plasma is always observed during the period. The dependence of stability on this field has been studied further by modifying the machine so that the field can be controlled externally. The observations of the field configuration in the outer regions during the period of stability are consistent with the requirements of MHD theory. However, residual fluctuations are still observed and an analysis has been made of the possible instabilities which might cause these fluctuations. The MHD stability close to the magnetic axis for localized modes with vanishing pressure gradient has been studied and compared with experiment. Resistive instabilities, in particular the tearing mode and the possibility of stabilization by ion viscosity, have been analysed for the measured configurations. Estimates have been made of the growth times. The trapped particle modes are predicted not to be serious during this period and calculations have been made of the shear stabilization of drift modes. (author)