Application of glow discharges for tritium removal from JT-60U vacuum vessel

As part of study to establish in-vessel tritium reduction methods in carbon plasma facing fusion devices such as ITER, measurement of tritium removal characteristics of glow discharge methods, usually used for wall conditioning, have been applied to and examined in the vacuum vessel of JT-60U. Release rates of hydrogen isotopes (tritium and deuterium) as well as hydrocarbons from the JT-60U vacuum vessel induced by glow discharge cleaning (GDC) with He and H2 were measured. Release characteristics of hydrogen isotopes could be classified into three release processes, and each process was well described by a simple exponential decrease with time. It was found that H2 GDC showed the superior hydrogen isotope release characteristics to the He GDC, probably because of chemical processes, such as isotope exchanges assisted by the chemical sputtering process between discharged hydrogen and hydrogen isotopes in the plasma facing carbon tiles were enhanced by the H2 glow discharge. Based on the release kinetics observed in the present work, it is estimated that it takes several days to reduce tritium (deuterium) inventory within the removal depth by the H2 GDC in JT-60U to a half by continuous H2 GDC at 573 K.