DIII-D Experimental Program on the Physics and Technology of Efficient RF Off-Axis Current Drive

DIII-D is engaged in an experimental effort to evaluate three technologies for efficient rf non-inductive current drive (CD) in the mid-radius region (ρ~0.5 – 0.85). CD in this region is needed for a steady-state tokamak reactor; its efficiency is a key economic factor. At DIII-D, current drive with nearly-vertically-launched Electron Cyclotron waves ("Top launch") has yielded twice the CD efficiency of the usual outside launch, in agreement with modeling. A pair of new top launchers is being tested in 2022 with up to 2.5 MW. The fast wave in the lower hybrid range of frequencies ("helicon"), was tested in DIII-D in 2021 at up to 0.3 MW coupled at 0.48 GHz with a comb-line traveling wave antenna; present efforts include upgrading the antenna feedthroughs and improving the transmission lines, which should permit doubling of the coupled power in 2022 and measurement of absorption and CD efficiencies. An innovative compact launcher for High-Field-Side launch of slow waves at 4.6 GHz is in final fabrication at MIT; installation in DIII-D is slated for FY23. A 2 MW power system with eight 4.6 GHz klystrons was installed and is being tested; the complete system will be ready for experiments in FY24.