Demonstration of 300 kA CHI-startup current, coupling to transformer drive and flux savings on NSTX

Abstract

Discharges formed by transient coaxial helicity injection (CHI) in the National Spherical Torus Experiment (NSTX) have attained peak currents of 300 kA for the first time. CHI-started discharges are coupled to induction, and ramped up to over 1 MA. Up to an additional 400 kA of toroidal current is produced, compared with discharges with the same inductive drive without CHI. These CHI-inductively coupled discharges demonstrate flux savings over standard NSTX inductive-only discharges, requiring significantly less transformer flux to reach 1 MA of toroidal current, as well as exhibiting higher elongation and lower internal inductance. These results indicate the potential for substantial current generation capability by CHI in NSTX and in future toroidal devices.

Characterization of disruption halo currents in the National Spherical Torus Experiment

“…These vessel sections are isolated from each other by ceramic insulators. This allows "coaxial helicity injection" (CHI) to be tested as a plasma startup method [80,81]. The regions between the inner and outer vessels (at both the top and bottom of the machine) are referred to as the CHI gap.…”

Recent progress of NSTX lithium program and opportunities for magnetic fusion research

“…In the presence of a toroidal magnetic field, a toroidal current is generated which is amplified by as much as ∼100 compared to the arc current. NSTX has generated toroidal plasma current of up to 300 kA by CHI [31]. After the applied CHI electrode voltage is turned off, the toroidal plasma current ring is pinched off Fig.…”

“…As shown in Fig. 7, the CHI aided start-up produced a significant saving of inductive flux (as much as a factor 50%) to achieve 1 MA discharges with low inductance and low impuritiy levels [31]. For further improvements, the plan is to investigate the effect of a metal electrode (molybdenum tiles on the inboard divertor which acts as the cathode during CHI) to further reduce impurity generation and increase the CHI driven current toward 0.5 MA.…”

Initial results from solenoid-free plasma start-up using Transient CHI on QUEST

“…On HIT-II about 100 kA of the initial CHI produced toroidal plasma current was ramped up to 180 kA using the central solenoid; this was about 40% higher than the plasma current that was generated by induction alone [10]. On NSTX about 200 kA of the initial CHI followed by inductive ramp up produced toroidal current of 1 MA using 40% less central solenoid inductive flux [14,15]. If electron cyclotron heating (ECH) is used to heat the CHI plasma, the capability of CHI for current startup is projected to be significantly enhanced.…”