A megawatt-level 28 GHz heating system for the National Spherical Torus Experiment Upgrade

Abstract

The National Spherical Torus Experiment Upgrade (NSTX-U) will operate at axial toroidal fields of ≤ 1 T and plasma currents, I_p ≤ 2 MA. The development of non-inductive (NI) plasmas is a major long-term research goal for NSTX-U. Time dependent numerical simulations of 28 GHz electron cyclotron (EC) heating of low density NI start-up plasmas generated by Coaxial Helicity Injection (CHI) in NSTX-U predict a significant and rapid increase of the central electron temperature (T_e(0)) before the plasma becomes overdense. The increased T_e(0) will significantly reduce the I_p decay rate of CHI plasmas, allowing the coupling of fast wave heating and neutral beam injection. A megawatt-level, 28 GHz electron heating system is planned for heating NI start-up plasmas in NSTX-U. In addition to EC heating of CHI start-up discharges, this system will be used for electron Bernstein wave (EBW) plasma start-up, and eventually for EBW heating and current drive during the I_p flattop.