Fusion Science and Technology / Volume 50 / Number 2 / August 2006 / Pages 192-200
Technical Paper / Stellarators
Self-sustained detachment, named the Serpens mode (Self-regulated plasma edge 'neath the last closed flux surface), has been observed in the Large Helical Device (LHD), which is equipped with an intrinsic open helical divertor, in spite of the low divertor neutral pressure of order 10-3 Pa. Other states of detachment are also observed; one is transient partial detachment, observed only in the gas puff port during massive gas puffing, and another is complete detachment, which takes place when the hot plasma boundary shrinks below the last closed flux surface. The Serpens mode is categorized as a subset of complete detachment, but with large fluctuations in the divertor flux and H signal, which result from a rotating radiation belt called the serpent. In this paper, these are compared with experimental observations of partial detachment and multifaceted asymmetric radiation from the edge (Marfe) in a modular stellarator, Wendelstein 7-AS (W7-AS). There are many similarities between transient partial detachment in LHD and partial detachment in W7-AS and, especially, between the Serpens mode in LHD and Marfe in W7-AS.