Development of Non-Solenoidal Tokamak Startup on the <emph type="smallcap">Urania</emph> Experiment

Initiation of plasma current without a central solenoid is a critical scientific goal for the spherical tokamak (ST). Following several campaigns on the <emph type="smallcap">Pegasus</emph> Toroidal Experiment using local helicity injection (LHI), which achieved I_p > 0 . 2 MA without use of an Ohmic solenoid, the facility is being shut down to upgrade into the Unified Reduced A Non-Inductive Assessment (<emph type="smallcap">Urania</emph>) experiment. This enhancement to the <emph type="smallcap">Pegasus</emph> facility will remove the central solenoid entirely, increase the toroidal field a factor of 4, up to 0.6 T, and investigate a variety of non-inductive startup techniques, including LHI, transient and sustained coaxial helicity injection (T- and S-CHI), poloidal field induction, and EBW radiofrequency heating and possibly current drive. A next-generation LHI system is being designed and built for <emph type="smallcap">Urania</emph>. Unlike the previous circular plasma-cathode electron sources used for LHI, this injector has a curved-slot arc channel of 1 cm width and 16 cm in length that is mounted off of a re-entrant port. This geometry allows for large A_inj for helicity input, while keeping w_inj low for a high Taylor limit, and matches the plasma edge curvature. A prototype of this design is currently in fabrication for initial testing with I_inj from 8-16 kA, and V_inj up to 1 kV.

Work supported by US DOE Grants DE-FG02-96ER54375 and DE-SC0019008.