The effect of plasma surface interactions on plt plasma parameters

In the four years of operation of the PLT tokamak (1976–1979), plasma parameters and machine operating conditions have been importantly affected by changes in the choice of limiter material and by the mode of vacuum vessel conditioning. Tungsten, stainless steel, and graphite limiters have been used. The fractional power lost by radiation and the source distribution of the radiation vary strongly with limiter material, wall treatment, and gas programming. With tungsten limiters, radiation from partially ionized tungsten atoms can strongly limit the temperature in the center of the plasma; with graphite limiters, radiation is primarily from carbon and oxygen in the outer cold region and not so much from the center, since in the hot core these atoms are totally ionized. With both tungsten and stainless steel limiters intermediate states exist which have rather uniform radiation source distributions; the characteristics of these distributions are affected by gas programming and by wall conditioning. Under some conditions the measured energy losses to the limiters and by radiation and neutral particles cannot account for all of the input energy by as much as 30. When the injected gas is changed from D₂ to H₂, the time constant for disappearance of neutron emission can be slow (35 pulses for a reduction by a factor of three) or fast (4 pulses for a similar factor), depending on previous history of wall treatment.