Hydrogen–deuterium changeover experiments in a plasma-wall interaction simulator

Abstract

Hydrogen recycling from walls and limiters of tokamaks constitutes the major source of plasma particles after the initial few milliseconds of a plasma pulse. The physical processes involved in recycling are thus very important to the control of plasma density and composition. Isotope changeover experiments in the laboratory show that thermally activated processes such as recombination and diffusion play significant roles in recycling. These processes are important not only during the plasma pulse but also continue after the plasma pulse ends, thus the length of time between pulses, the wall temperature, and the vacuum pumping rate all influence changeover dynamics. This paper shows that for type 304 stainless steel walls changeover can range from ≳95% to <50% complete in one 200 ms pulse depending upon these parameters. The initial part of changeover during a plasma pulse is very rapid and involves equilibrating the isotopic compositions of the plasma reservoir and the near-surface wall hydrogen reservoir involved in recycling. The rate limiting step then seems to become vacuum pump-out rates. Because one of the steps in gas evolution during and after the pulse is the recombination process, which is dependent on the square of the surface hydrogenic concentration, changeover can be accelerated if the hydrogenic concentration at the surface is kept high by repeated plasma pulses in the new isotope. The old isotope is evolved as HD.