Helical divertor operation and erosion/deposition at target surfaces in LHD

Divertor footprints have been identified within a few mm in accuracy after 10 000 shots. This is the large merit of the large helical device divertor for erosion/deposition studies due to high reproducibility with an external superconducting coils system. Helical distribution of divertor erosion is compared with the prediction from magnetic field characteristics. The measured net erosion depth is found to be about a factor 3 less than the estimated one. Numerical simulations have revealed the net erosion to be very sensitive to deposition of C impurity in the plasma. Eroded carbon atoms are mainly redeposited near the divertor tiles, and partly deposited near the divertor strike point, forming a mixed layer with promptly deposited metals. Deposited metals accumulate locally at the edge of microscale open pores and around grains of graphite. This kind of metals sink possibly plays an important role as an impurity source after the tiles installation. This aspect of ‘microscopic-PSI study’ is very informative for understanding macroscopic-PSI.