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

Noninductive plasma current (I_p) 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 (;[variant_greek_epsilon]∼ω_pe²/Ω_e²>1), accessibility of the LHW to the plasma core is severely limited. Our approach is to keep the plasma density low (such that [variant_greek_epsilon]∼1) during I_p ramp-up. Once I_p 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 I_p. Up to 12 kA of Ip has been obtained by this method. Soft X-ray measurements indicate that the electron temperature increases as I_p increases, and hard X-ray spectroscopy indicates that energetic electrons build up as I_p is ramped up. [ABSTRACT FROM AUTHOR]