Plasma Initiation by Local Helicity Injection (LHI) on the Pegasus-III Experiment

The Pegasus-III program is focused on developing the physics basis and predictive models for non-solenoidal tokamak startup. Local helicity injection (LHI) is a non-solenoidal tokamak startup technique utilizing localized current sources near the plasma edge. Electron beams injected into the vacuum magnetic field relax into a tokamak-like state and drive I_p up to limits determined by Taylor relaxation, power balance, or the helicity injection capability of the hardware. Several issues must be addressed before this technique is ready to deploy on a large facility, including: scaling to increased B_T; reliable operation with high LHI drive voltage; injector geometry optimization; gas control; reducing impurity production; and long pulse (> 100 ms) operation. The LHI system on Pegasus-III features 4 sources near the outboard midplane with a total I_inj ≤ 16 kA and V_inj ≤ 1 kV, and a prototype non-circular injector with I_inj ≤ 8 kA and V_inj ≤ 500 V. A new Stark broadening diagnostic for arc fueling optimization and a cathode spot detection system to minimize impurity injection and improve injector reliability were deployed. These systems facilitate investigation of outstanding LHI physics issues during the initial run campaign at B_T < 0.3 T.