Non-resonant resistive instabilities localized near the magnetic axis in heliotrons and torsatrons

Abstract

A new type of non-resonant or near-resonant resistive pressure driven mode localized near the magnetic axis is found in heliotron and torsatron configurations. This mode has a ballooning-like structure and becomes unstable in highly resistive plasmas (magnetic Reynolds number S ≲ 105) when the pressure profile is sufficiently peaked in the almost shear free region around the magnetic axis. As S increases, the radial mode structure becomes narrower and the peak position approaches the magnetic axis. The numerically obtained growth rates are proportional to S−0.33. When S ≳ 105, a transition occurs to an unstable resistive interchange mode localized at a resonant surface. A non-resonant resistive mode with properties similar to those of the above mentioned mode is also observed in straight cylindrical configurations. The dispersion relation for these configurations is derived analytically in the electrostatic approximation, which gives the growth rate proportional to S−1/3. The non-resonant resistive mode may suppress highly peaked pressure profiles when the resistive interchange mode is marginal.