Detection of the free boundary plasma shift in a toroidal helical plasma on Heliotron-E

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Abstract
This paper describes the first successful realization of the magnetic detection of the free boundary plasma shift for helical plasmas. A procedure for determining the equilibrium plasma displacement in a stellarator by means of external magnetic field measurements (psi loops and modulated Rogowski coils) is discussed. Recent experimental results, the normalized displacement Δb/ap as a function of volume averaged beta β, are discussed and compared with analytical and published numerical MHD computational results. The typical measured plasma boundary shift, Δb/ap, in the standard Heliotron-E configuration (Rp = 2.20 m, ap = 0.21 m, ι(0)/2π ~ 0.53, ι(ap)/2π ~ 2.8) is (8-10) × 10-3, when the volume averaged beta is 0.50%. The measured normalized plasma boundary shift is nearly proportional to the diamagnetic volume averaged beta, for values of beta up to 0.95%. The magnetically determined plasma boundary shift Δb is less than 3 mm. The experimental observations on shift (Δb/ap versus βdia) are compared with the analytically expected plasma boundary shift. The measured shift lies in the range between the expected upper limit (Δb/ap = β(0)/2βeq) and the lower limit (Δb/ap= β/2βeq), where βeq=[ι(ap)/2π]2(ap/Rp) ~ 0.77 for the standard configuration of Heliotron-E. It is found that the measured plasma boundary shift depends strongly on the initial vacuum magnetic configuration parameters such as the horizontal position of the magnetic axis and the rotational transform. When the vacuum magnetic axis is shifted inward towards the major axis, significant decreases of the normalized plasma shift (Δb/ap) and of the plasma induced vertical field are observed, which are interpreted as being due to a reduction of the Pfirsch-Schluter current