The plasma boundary in single helical axis RFP plasmas

Single helical axis states obtained in high current reversed field pinch plasmas display, aside from a dominant mode in the m = 1 spectrum, also a dominant m = 0 mode, with the same toroidal mode number as the m = 1 one. The two modes have a fixed phase relationship. The island chain created by the m = 0 mode across the reversal surface gives rise, at shallow reversal of the toroidal field, to an X-point structure which separates the last closed flux surface from the first wall, creating a divertor-like configuration. The plasma-wall interaction is found to be related to the connection length of the field lines intercepting the wall, which displays a pattern modulated by the dominant mode toroidal periodicity. This configuration, which occurs only for shallow toroidal field reversal, could be exploited to realize an island divertor in analogy to stellarators.