Study of MHD Equilibrium and Stability for Heliotron/Torsatron with Helical Axis

Abstract

MHD equilibrium and stability of LHD (Large HelicalDevice) are studied when circular plane magnetic axis is distortedinto spatial helical one by introducing unbalance between two helicalcoil currents. The helical axis effect increases rotational transformwith enhancing magnetic hill at zero beta, where beta is the ratio ofplasma pressure to magnetic pressure. The significant effect ofhelical magnetic axis is reduction of average radius of the outermostflux surface or elimination of edge region with large magnetichill. For finite beta plasmas this reduction of average plasma radiushas substantial stabilizing effect on pressure driven MHD modesthrough enhancement of magnetic well produced by both Shafranov shiftand helical shift of magnetic axis. However, by comparing the helicalaxis configuration of LHD to planer axis configuration with the sameradius produced by using limiter, the change of equilibrium andstability due to the helical axis is found weak in the LHD case. Themerit of the helical axis configuration is controllability of averageplasma radius without limiter.