Recent progress on steady-state high-performance plasma research in the Hefei Tokamak-7

Abstract

High performance discharges under steady-state condition in the Hefei Tokamak-7 (HT-7) have been investigated. Lower hybrid current drive (LHCD) was used both for sustaining plasma current and current density profile control. The experiments demonstrated that features of ion Bernstein wave heating in controlling the electron pressure profile can be integrated into LHCD plasmas and the assist localization of LHCD. This local synergy effect was used to tailor the current density profile in a negative shear configuration and to control the internal transport barrier (ITB) under steady-state conditions. The high performance with a stationary ITB at the footprint of the minimum q and βNH89>2 was sustained for >220 τE or >20 τCR. The fraction of noninductive plasma current was larger than 80% in such discharges with considerable bootstrap current. The duration at H89>1.2 and βN∼1 has been extended to nearly 8 s, longer than 400 τE. More than 90% of the plasma current was sustained by LHCD and bootstrap current. High-plasma performance and sustaining time was limited by the magnetohydrodynamic instabilities and the recycling, which caused an uncontrollable rise of the electron density.