Lower Hybrid Current Start-up and Ramp-up in the JIPP T-IIU Tokamak

Abstract

In the JIPP T-IIU tokamak, a current startup experiment is carried out by injection of 800 MHz lower hybrid waves into a cold and low density plasma (T _e =10–20 eV, n _e =1–2 × 10¹² cm⁻³) produced by electron cyclotron resonance or lower hybrid waves only. When the position of a current channel is well-centered by controlling a vertical field, the plasma current rises up with a characteristic rise time (≥30–50 ms) and approaches a quasi-steady state value I _pm . The rise time is much longer than the resistive diffusion time of the bulk plasma (≤5–10 ms) and decreases with the increase in the bulk electron density n _e . The current drive efficiency I _pm /P _LH reaches about 0.5 A/W for n _e ≅ 0.8–4×10¹² cm⁻³. The effects of parametric decay instabilities and return current on the current startup are investigated. It has been shown that the low density discharge started and driven by rf only can effectively be evolved to the higher density and higher current discharge by both weak inductive electric field and gas puffing. The current rampup by lower hybrid waves in an ohmically heated plasma is also tried to study the current drive efficiency and the return current effect.