Analysis for hydrogen particle balance of plasma-wall system in the large helical device

The hydrogen particle balance of the plasma-wall system in the large helical device (LHD) is analyzed, using a zero dimensional model for plasma particles, neutrals in vessel and hydrogen inventory in wall. Based on the measurement of neutral gas pressure, plasma density and the pumping speed of the cryo-pumping system, it is found that the hydrogen retained in the wall desorbes with short and long time constant. The short term desorption is of order of 1021 atoms with a time constant of a few minutes, which is much smaller than the wall pumping for one shot, 1022 atoms. In a long time scale of about one experimental day, the wall absorbs significantly large amounts of hydrogen, up to 1024 atoms. One of the possible reasons for the large wall pumping is a carbon deposition layer on the first wall surface. The effect of hydrogen retention on density control is also discussed.