<emph type="smallcap">Urania:</emph> A Spherical Tokamak for Developing Non-Solenoidal Plasma Startup Techniques

Development of a routine non-solenoidal startup technique is a critical issue facing the spherical tokamak. A major upgrade to the <emph type="smallcap">Pegasus</emph>program is underway to develop and compare leading non-solenoidal startup techniques on a dedicated solenoid-free facility-the Unified Reduced A Non-Inductive Assessment (<emph type="smallcap">Urania</emph>) experiment. Facility upgrades for <emph type="smallcap">Urania</emph> include: increased B_T to 0.6 T, extended pulse duration (<= 100 ms), an enhanced poloidal field set for improved shape control, and an expanded diagnostic suite, while retaining an ultra-low aspect ratio (A 1 . 2) . Modelling and experiments have informed the design of a new local helicity injection (LHI) system capable of I_p <= 0 . 3 MA by leveraging the enhanced B_T to increase the Taylor limit early in the discharge. The new LHI system is designed to optimize helicity input and increase Taylor limit via a non-circular current source geometry. A prototype of this new source is in fabrication for testing on <emph type="smallcap">Pegasus</emph>. A novel system capable of transient and sustained coaxial helicity injection is in design. A modest power (200-400 kW) electron Bernstein wave radiofrequency heating and current drive system will be deployed.

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