Equilibrium 𝛽limits in classical stellarators
Abstract
A numerical investigation is carried out to understand the equilibrium limit in a classical stellarator. The steppedpressure equilibrium code (Hudson et al., Phys. Plasmas, vol. 19 (11), 2012) is used in order to assess whether or not magnetic islands and stochastic fieldlines can emerge at high . Two modes of operation are considered: a zeronetcurrent stellarator and a fixediota stellarator. Despite the fact that relaxation is allowed (Taylor, Rev. Mod. Phys., vol. 58 (3), 1986, pp. 741763), the former is shown to maintain good flux surfaces up to the equilibrium limit predicted by idealmagnetohydrodynamics (MHD), above which a separatrix forms. The latter, which has no ideal equilibrium limit, is shown to develop regions of magnetic islands and chaos at sufficiently high , thereby providing a `nonideal limit'. Perhaps surprisingly, however, the value of at which the Shafranov shift of the axis reaches a fraction of the minor radius follows in all cases the scaling laws predicted by idealMHD. We compare our results to the HighBetaStellarator theory of Freidberg (Ideal MHD, 2014, Cambridge University Press) and derive a new prediction for the nonideal equilibrium limit above which chaos emerges.
 Publication:

Journal of Plasma Physics
 Pub Date:
 December 2017
 DOI:
 10.1017/S0022377817000861
 Bibcode:
 2017JPlPh..83f7101L
 Keywords:

 fusion plasma;
 plasma confinement