DCON for Stellarators*

DCON is a widely-used code for determining the ideal MHD stability of a tokamak.¹  We present a new version of DCON for application to stellarators.  DCON originally worked by integrating the Euler-Lagrange equation for minimizing the ideal potential energy functional delta-W, using the solution matrix U to construct a plasma response matrix P, then using P to determine stability to ideal modes.  Fixed-boundary instability is indicated by the vanishing of 1/(det P), a generalization of the Newcomb crossing criterion.  Free-boundary instability is determined by adding P at the plasma-vacuum interface to a vacuum response matrix computed by another code and determining whether any of the eigenvalues is negative.  Integration of U is numerically unstable, requiring occasional re-organization to maintain linear independence of the column vectors of U.  A new method has been developed to integrate P rather than U, a numerically stable matrix Riccati equation.  An interface has been developed to the VMEC stellarator equilibrium code.  DCON also computes the Mercier criterion for local ideal interchange stability.  Results will be presented for the Wendelstein 7-X stellarator.

1 A. H. Glasser, Phys. Plasmas 23, 072505 (2016).