Ohmic Coupling and Sustainment of LHI-Produced Plasmas in the Pegasus Experiment

Reducing or eliminating the Ohmic solenoid (OH) is likely necessary for the ST path and may benefit tokamaks in general, motivating the development of solenoid-free startup techniques. Viable methods will: initiate plasmas with readily conservable poloidal flux / I_p; are compatible with sustainment current drive (CD) techniques; and have attractive MHD stability. Local helicity injection (LHI), being developed on the Pegasus ST, is a promising solenoid-free startup technique. Experiments that tested the viability of LHI startup plasmas using Ohmic drive for sustainment are presented here. I­_p in LHI-initiated plasmas was coupled to and further grown under pure Ohmic drive, realizing OH flux savings and conserving poloidal flux across the handoff. Magnetic fluctuations associated with LHI reconnection activity decayed to OH-like levels within several ms after LHI termination, suggesting a rapid healing of flux surfaces. The typically hollow LHI current profiles appear to be MHD favorable, evidenced by a delayed onset of low m, n = 1 tearing modes with respect to purely Ohmic comparison discharges.