Comparison of boronized wall in LHD and JT-60U

Boronization has been performed in many fusion plasma devices as one of several effective wall conditioning techniques. In LHD and JT-60U, boronization is operated as a typical wall conditioning method. Using material probes stayed in LHD during an experimental campaign, characteristics of a boronized wall are discussed in each device. Due to the complex three dimensional structure of the vacuum chamber, the thickness of boron film on the material probes depends on the distance from the anodes for glow discharge in LHD. On the other hand, such dependence is not clear in JT-60U due to thick carbon layers deposited on the material probes. To consider long-term operations, a deposited thick carbon layer with high boron-oxide concentration cannot be eroded completely by glow discharges, this layer is expected to be a saturated capacity of oxygen gettering. To keep the capacity of oxygen gettering, an additional boron layer is needed on the thick deposited layer. Since thin deposited layer on boron-coated area of LHD is eroded by glow discharge and flesh boron layer appears, a capacity of oxygen gettering is kept during long-term experiments.