Microwave current drive for STEP and MAST Upgrade

The UK’s Spherical Tokamak for Energy Production (STEP) reactor design program has recently taken the decision to use exclusively microwave-based heating and current drive (HCD) actuators its concepts. This is based on a detailed assessment considering all viable HCD concepts, covering grid plasma efficiency, physics applications, technology maturity, integration, maintenance, costs. Of two microwave techniques: Electron Cyclotron (EC) Bernstein Wave (EBW), EC was deemed lowest risk EBW retained as potential path more efficient, higher performing device. To assess ECCD GRAY beam tracing code employed perform scans of launcher position, toroidal poloidal launch angle, frequency over first 3 cyclotron harmonics. For EBW, GENRAY/CQL3D were used estimate CD demonstrating promising results. reduce uncertainties in present models coupling drive, MAST Upgrade will install dual (28, 34.8 GHz), 900kW, 5s gyrotrons from Kyoto Fusioneering, part enhancements package. be accompanied by flexible 2D steering system allow midplane coand counter-CD above co-direction off-axis CD. Coupling efficiency quantified measuring induced reflected (i.e. non-coupled) power plate inserted path. experiments also include driven solenoid-free start-up, increasing pulse length factor 10 compared previous experiments. presentation discuss STEP studies capabilities.