The plasma boundary in Single Helical Axis RFP plasmas

TL;DR

This paper investigates the plasma boundary in Single Helical Axis RFP plasmas, revealing a divertor-like configuration caused by a dominant m=0 mode that could be used for plasma control.

Contribution

It identifies a novel plasma boundary structure involving a fixed phase relationship between modes, enabling potential island divertor applications in RFPs.

Findings

Presence of a fixed phase relationship between m=0 and m=1 modes.

Formation of an X-point structure creating a divertor-like boundary.

Connection length patterns modulated by the dominant mode toroidal periodicity.

Abstract

Single Helical Axis (SHAx) states obtained in high current reversed field pinch (RFP) 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.