Plasma Curent Start-up Experiment using Waves in the Lower Hybrid Frequency Range in TST-2

Abstract

Noninductive plasma current (Ip) start‐up experiments using RF power in the lower hybrid frequency range are being conducted on the TST‐2 spherical tokamak. The lower hybrid wave (LHW) has demonstrated efficient current drive in conventional tokamaks. However, in spherical tokamak (ST) plasmas with very high dielectric constants (;ε∼ωpe2/Ωe2≫1), accessibility of the LHW to the plasma core is severely limited. Our approach is to keep the plasma density low (such that ε∼1) during Ip ramp‐up. Once Ip reaches a level sufficiently high for neutral beam current drive, plasma can be densified and transformed into an advanced tokamak plasma dominated by the self‐driven bootstrap current. Initial plasma start‐up experiments were performed on TST‐2 using a combline antenna which excites a traveling fast wave. After formation of toroidal flux surfaces, RF power and vertical field were ramped up to increase Ip. Up to 12 kA of Ip has been obtained by this method. Soft X‐ray measurements indicate that the electron temperature increases as Ip increases, and hard X‐ray spectroscopy indicates that energetic electrons build up as Ip is ramped up.