First Results from the High Field Side Lower Hybrid Current Drive Experiment in DIII-D

For the first time in a tokamak, experiments on DIII-D have injected Lower Hybrid Current Drive (LHCD) RF power from the High Field Side (HFS) and detected an effect in the plasma.

The DIII-D HFS LHCD System is a novel approach for efficient RF noninductive off-axis current drive. The placement of the LHCD launcher at the centerpost improves wave accessibility and penetration, and opens the possibility for single-pass absorption [1]. The LHCD system is expected to provide efficient off-axis current drive with peak current density up to 0.4 MA/m² and 0.14 MA/MW coupled using n_|| = 2.7 [2]. LHCD waves launched using the unique launching geometry placed on the centerpost is predicted to greatly increase the ECCD efficiency at mid and large radius compared to existing RF techniques. The experiments use a prototype LHCD HFS launcher with additively manufactured (AM) GRCop-84.

The first HFS LHCD system was installed on DIII-D with eight 4.6 GHz klystrons, with the first RF launch into the plasma in 2025. The RF pulse length was extended to 2000 ms and 190 kW peak generated power was achieved for shorter pulses. Plasma experiments are in progress to test this new technique. The first measurements indicative of a plasma response to the injected LHCD RF are reported.

[1] P.T. Bonoli et al, Nucl. Fusion 58, 126032, (2018)

[2] S. J. Wukitch et al., EPJ Web Conf. 157, 02012, (2017).