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MHD equilibrium and stability of LHD (Large Helical Device) are studied when circular plane magnetic axis is distorted into spatial helical one by introducing unbalance between two helical coil currents. The helical axis effect increases rotational transform with enhancing magnetic hill at zero beta, where beta is the ratio of plasma pressure to magnetic pressure. The significant effect of helical magnetic axis is reduction of average radius of the outermost flux surface or elimination of edge region with large magnetic hill. For finite beta plasmas this reduction of average plasma radius has substantial stabilizing effect on pressure driven MHD modes through enhancement of magnetic well produced by both Shafranov shift and helical shift of magnetic axis. However, by comparing the helical axis configuration of LHD to planer axis configuration with the same radius produced by using limiter, the change of equilibrium and stability due to the helical axis is found weak in the LHD case. The merit of the helical axis configuration is controllability of average plasma radius without limiter.
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Matumoto et al. (1995) studied this question.
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