Local Helicity Injection Studies on the PEGASUS-III Experiment

Initiating high-I_p plasmas without a central solenoid benefits both the ST and AT concepts. The PEGASUS program improved the physics basis and predictive models for non-solenoidal startup using local helicity injection (LHI), demonstrating plasmas with I_p > 0.2 MA. Major upgrades have converted the PEGASUS facility into the solenoid-free PEGASUS-III ST. They include: increased B_T = 0.6 T; improved shape control; and retaining ultralow A ~ 1.2. This B_T supports a new mission to expand the breadth and range of solenoid-free startup research on the facility with multiple techniques. Initial LHI experiments will extend I_p and T_e scaling studies to B_T > 0.15 T and test projections of access to I_p = 0.3 MA using two pairs of compact injectors (I_inj = 4 kA, V_inj  ~ 1 kV, A_inj = 4 cm² each) on the low-field-side of the tokamak and new voltage-controlled power supplies. A port-mounted, non-circular injector with equivalent helicity drive to the LFS arrays will test the feasibility of a compact, active current source geometry technically attractive to NSTX-U and beyond. Other planned studies include: beam-driven instabilities and dynamo current drive; impurity assessment; fueling; and scenario development for RF heating and CD.