No abstract

A detailed description of the time behaviour of a hydrogen discharge in the ST-Tokamak is based on measured radial electron temperature and density profiles at 12 different times, together with measurements of the Ohmic-heating current and voltage, the temporal, spatial, and spectral distributions of hydrogen light, the ion temperatures, and impurity concentrations. Early in the discharge the electron temperature profiles show evidence of a skin effect that develops on a time-scale of several milliseconds into a peaked profile of about 2.2 keV maximum. Thereafter the peak temperature stops growing and develops into a flat plateau, the width of which appears to be determined by the Kruskal-Shafranov limit. The average particle confinement time scales with , and reaches a maximum of 13-14 ms. The power balance is dominated by electron loss and re-cycling, rather than ion loss or radiation. The recycling process at the aperture limiter appears to involve sufficiently energetic neutral atoms to provide a fairly flat radial source function for particles, and hence to influence directly the development of the radial distribution of power input and energy balance.

A study is made of the spectral distribution of the soft X-ray emission produced by the thermal part of the electron velocity distribution in the ST Tokamak. The slopes of the spectra are in good agreement with the prediction from laser measurement of electron temperature if radial profile effects are taken into account. The absolute intensity is a factor 5 to 100 larger than is expected for hydrogen bremsstrahlung. This enhancement can be quantitatively accounted for by recombination radiation from oxygen and heavy-metal impurities. The enhancement factor ζ was measured at different radii, in order to study the impurity distribution in the tokamak. These, as well as other experiments, in which high-Z gases (Xe, Kr, A) were pulsed into the discharge, indicated that, within the measuring accuracy, Zeff does not vary substantially with radius.