Positional stability of tokamak plasma in a ℓ=2, m=4 helical field

The decay index of the external magnetic field of JIPP T-II torus with ℓ=2, m=4 helical windings is investigated. The horizontal displacement of the plasma column is measured as a function of the vertical magnetic field. A following relationship δB_V/δΔ≃^ℓ_hB_t/2πR, where ^ℓ_h is the rotational transform of the helical field, B_t is the toroidal magnetic field and R is the major radius of the plasma column, is experimentally confirmed. Although the value of the decay index is negative, for example n≃-5, the plasma column is also positionally stable in the vertical direction. The results suggest that a concept of strong focusing may be applicable. (author)