Simulation Study of Ballooning Modes in the Large Helical Device

The magnetohydrodynamic (MHD) simulation code MHD Infrastructure for Plasma Simulation (MIPS) was benchmarked on ballooning instability in the Large Helical Device (LHD) plasma. The results were compared to the results of linear analysis by using the CAS3D code. Both the linear growth rates and the spatial profiles were found to be in good agreement. An extended MHD model with finite ion Larmor radius effects was implemented into the MIPS code. Ballooning instabilities were investigated using the extended MHD model, and the results were compared with those using the MHD model. Ion diamagnetic drift was found to reduce the growth rate of the short-wavelength modes; hence, modes with a diamagnetic drift frequency comparable to the ideal MHD growth rate are the most unstable. The most unstable toroidal mode number of ballooning instability in the LHD is reduced to |n| ? 5 for hydrogen plasma with ion number density ni ? 1019 m?3.