Divertor transport study in the Large Helical Device

The transport properties of the edge region in LHD have been investigated in order to clarify the divertor/SOL function of the heliotron type device. The momentum loss, mainly through the friction of counter-flows induced by the ergodic field lines, breaks the pressure conservation along flux tubes. This results in no high recycling regime even at high density operation, n ̄ ∼ 7 × 1019 m−3. The momentum loss is found to be higher than the case of W7-AS. This is because of the higher ratio of perpendicular and parallel transport scale length, ∼ 10−4, and of the momentum loss due to the friction of counter-flows, which is more effective in the ergodic layer than the island divertor. In the heliotron configuration, a large temperature drop from LCFS to divertor, an order of magnitude, is easily realized due to the long connection length in the ergodic layer. This is certainly a favourable feature for future reactor in terms of reduction of heat power load at the divertor