Recycling and wall pumping in long duration discharges on TRIAM-1M

Recycling and wall pumping have been studied comparing low (≈10¹⁸ m^-3) and high (≈10¹⁹ m^-3) density long duration plasmas in TRIAM-1M. The recycling coefficient of each plasma increases with time. There exist two time constants in the temporal evolution of the recycling coefficient. One is a few seconds and the other is about 30 s. The wall pumping rates of low and high density plasmas are evaluated to be ≈1.5 × 10¹⁶ atoms m^-2 s^-1 and ≈4 × 10¹⁷ atoms m^-2 s^-1, respectively. The difference may be caused by the total number of diffused ions and the charge exchange neutral flux with an energy of less than 0.7 keV. In an ultra-long discharge (≈70 min), the recycling coefficient becomes 1 or more before again decreasing below 1, i.e. the wall repeats the process of being saturated and refreshed. In high power and high density experiments, wall saturation phenomena have been observed. The discharge duration limited by wall saturation decreases with an increase in density.