Measurements of Pfirsch-Schlüter current and pressure profile for the high density ECH plasmas in Heliotron DR

Abstract

The Pfirsch-Schlüter current and pressure profiles are estimated from magnetic measurements for high density electron cyclotron heating (ECH) plasmas (n̄e = (2−3) × 1013 cm−3, Te0 = 200-400 eV, ⟨β⟩ ≲ 0.5%) in Heliotron DR. The small vertical field (≲ 5 G) due to the Pfirsch-Schlüter current is picked up by two kinds of magnetic loops which are set outside the vacuum chamber above the magnetic axis. The field is measured successfully because of the smallness of the unidirectional toroidal current in the ‘currentless’ ECH plasma and the smallness of the vertical component of the vacuum heliotron field at the loop positions. The pressure profile is estimated by comparing these data with those from diamagnetic measurements. The profile factor s, defined by p = p0 [ 1 − (r/a)2 ]s, is about two in macroscopically stable plasmas. A fast loss of plasma energy from the centre to the periphery is observed during the onset of the MHD instability. This method of measuring the pressure profile shape is simple and useful for heliotron type devices.