Plasma current startup by lower hybrid waves in the JIPP T-IIU tokamak

Abstract

The paper describes the characteristic behaviour of lower hybrid current startup in JIPP T-IIU. The current startup is carried out by the injection of 800 MHz lower hybrid waves into cold and low density plasmas (Te = 10 − 20 eV, n̄e = (1−2) × 1012 cm−3 produced by electron cyclotron resonance or lower hybrid waves (LHW) only. The plasma current rises with a characteristic rise-time of τr (≳ 30-50 ms) and approaches a quasi-steady state value, Ipm (= 5-20 kA), whereupon 10-50 kW LHW power is injected into the torus, controlling the vertical field. The rise-time is inversely proportional to the bulk electron density, ne, and is comparable to the collision time of current carrying high energy electrons with the bulk plasmas. On the other hand, the current drive efficiency in the quasi-steady state is almost independent of n̄e, i.e. Ipm/PLH = 0.4−0.7 A·W−1 for n̄e = (0.8−4) × 1012 cm−3. The conversion efficiency of RF energy injected into the torus is typically 5% during the current rise phase and 10% in the most efficienct case. The effects of the initial injection of ECH power and the observed parametric instabilities on the current startup are investigated from the viewpoint of seed current generation. During the rapid current rise when an appreciably negative loop voltage is observed, the bulk electrons are heated up to 150 eV. Various heating mechanisms responsible for the bulk electron heating are discussed.