Second ballooning stability in high-β, compact stellarators

Second ballooning stability is examined in quasipoloidally symmetric, compact stellarator configurations. These high-beta (volume-average beta>4%) free-boundary equilibria are calculated using a reference Quasi-Poloidal Stellarator (QPS) configuration. QPS plasmas have low-shear, stellarator-like rotational transform profile with \B\ that is approximately poloidally symmetric. The high-beta QPS equilibria are similar in their magnetic configuration to previously studied tokamak-stellarator hybrids which have a high-shear, tokamak-like rotational transform profile. Both types of configurations have strong magnetic wells and consequently high interchange stability beta limits. Free-boundary QPS equilibria have regions of second stability at high beta. For infinite-n ballooning modes in QPS plasmas, the boundary for first instability is <beta>similar to2% and the boundary for second stability is <beta>similar to6%. Finite-n ballooning mode calculations show higher beta limits, <beta>>5%. Increasing plasma current (for fixed plasma pressure) can lower the finite-n ballooning mode beta limit to <beta>=3% by reducing magnetic shear. QPS plasmas with Ohmic current profiles (peaked on-axis) have both a lower infinite-n ballooning beta-limit for the onset of first instability and a higher beta-limit for the onset of second stability relative to QPS plasma with bootstrap current profiles (peaked off-axis). QPS plasmas are stable to low-n ideal magnetohydrodynamic kink modes and vertical modes for values of beta in this range (<beta>similar to6%) due to the low level of plasma current in QPS relative to an equivalent tokamak. (C) 2004 American Institute of Physics.