Integrated Studies of Solenoid-Free Tokamak Startup with Pegasus-III

Minimizing or completely eliminating the need for induction from a central solenoid during startup, ramp-up and sustainment of a tokamak plasma is a critical challenge in magnetic fusion. To address that challenge, the Pegasus-III facility is under construction to provide a dedicated US platform for non-solenoidal tokamak startup and sustainment studies. Its mission is to perform comparative studies of leading techniques for solenoid-free startup and provide guidance for 1 MA startup in NSTX-U and beyond. These include: local helicity injection (LHI); coaxial helicity injection (CHI); RF-aided and RF-only startup; and PF induction. Topics of study include current drive efficiency and confinement scalings at increasing $B_{T}=0.6$ T and $I_{p}=0.3$ MA, testing synergistic effects by combining techniques, and supporting technology development. The new facility features: a solenoid-free centerstack; an innovative 24-turn TF coil system with dynamic stress compensation; new divertor coils that also support CHI; new digital control systems for all magnet and HI power systems; and an initially-modest electron Bernstein wave (EBW) heating system. The science program will first establish high-$I_{p}$ LHI scenarios, followed by deployment and test of transient CHI, and eventually a modest sustained CHI system. Low-power EBW studies will be developed in the second and third year of operation. Expansion of the RF systems to provide RF-only initiation and sustainment is under evaluation.