Development of fully non-inductive plasmas heated by medium and high-harmonic fast waves in the national spherical torus experiment upgrade

Abstract

Abstract.A major challenge for spherical tokamak development is to start-up and ramp-up the plasma current (I p ) without using a central solenoid. Experiments in the National Spherical Torus eXperiment (NSTX) demonstrated that 1.4 MW of 30 MHz high-harmonic fast wave (HHFW) power could generate an I p = 300 kA H-mode discharge with a non-inductive I p fraction, f NI ~ 0.7. The discharge had an axial toroidal magnetic field (B T (0)) of 0.55 T, the maximum B T (0) available on NSTX. NSTX has undergone a major upgrade (NSTX-U), that will eventually allow the generation of B T (0) ≤ 1 T and I p ≤ 2 MA plasmas. Full wave simulations of 30 MHz HHFW and medium harmonic fast wave (MHFW) heating in NSTX-U predict significantly reduced FW power loss in the plasma edge at the higher B T (0) achievable in NSTX-U. HHFW experiments will aim to generate stable, f NI ~ 1, I p = 300 kA H-mode plasmas and to ramp I p from 250 to 400 kA with FW power. Time-dependent TRANSP simulations are used to develop non-inductive I p ramp-up and sustainment using 30 MHz FW power. This paper presents results from these RF simulations and plans for developing non-inductive plasmas heated by FW power.