Verifying PB3D: a new code for 3D ideal linear peeling-ballooning stability

Magnetic nuclear fusion devices are a promising candidate for the confinement of thermonuclear plasmas but various instabilities set important limits on their operation. Peeling-ballooning perturbations, which can be described appropriately using high-n linear ideal MHD stability theory, are two of them, where high-n indicates that the perturbations are localized along the magnetic field lines [1]. A new numerical code, called PB3D (Peeling Ballooning in 3D) was written to investigate the stability of these instabilities in a fast and reliable way, solving the generalized eigensystem presented first in [8]. The important new aspect of this theory is that it describes stability of full 3-D equilibrium configurations that are allowed to perturb the plasma edge, in contrast with previous treatments such as used in the ELITE [9] or MISHKA code [5] that both treat the stability of axisymmetric equilibria. 3D effects are important for numerous reasons: In tokamaks axisymmetry is often broken, either deliberately, such as when RMP techniques are used to suppress periodic plasma relaxations called ELMs, or due to imperfections in the axisymmetric design, such as the toroidal ripple introduced by discrete toroidal field coils. Stellarators devices, on the other hand, are inherently 3D and cannot be approximated using axisymmetric theory. In this work, the verification of PB3D with stability results for axisymmetric equilibria is presented, indicating that these are accurately reproduced, and non-intuitive first 3-D results are given.