Physics Design of a 28 GHz Electron Heating System for the National Spherical Torus Experiment Upgrade

A megawatt-level, 28 GHz electron heating system is being designed to support non-inductive (NI) plasma current (I-p) start-up and local heating and current drive (CD) in H-mode discharges in the National Spherical Torus Experiment Upgrade (NSTX-U). The development of fully NI I-p start-up and ramp-up is an important goal of the NSTX-U research program. 28 GHz electron cyclotron (EC) heating is predicted to rapidly increase the central electron temperature (T-e(0)) of low density NI plasmas generated by Coaxial Helicity Injection (CHI). 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. Also 28 GHz electron Bernstein wave (EBW) heating and CD can be used during the I-p flat top in NSTX-U discharges when the plasma is overdense. Ray tracing and Fokker-Planck numerical simulation codes have been used to model EC and EBW heating and CD in NSTX-U. This paper presents a pre-conceptual design for the 28 GHz heating system and some of the results from the numerical simulations.