Overview of DIII-D Helicon Program*

Experimental studies of the physics and technology of tokamak plasma heating and current drive with fast waves in the lower hybrid range of frequencies, referred to as "helicons", are ongoing at DIII-D, with power levels of up to 0.8 MW at 0.48 GHz from a klystron source applied to a 1.5-m-wide comb-line traveling wave antenna with 30 elements. Experiments in 2022-23 have focused on measuring the power deposition on electrons in the core of L- and H-mode plasmas preheated with neutral beams and ECH to increase helicon absorption. As designed, the antenna input impedance is "load resilient" to L-to-H transitions, to ELMs, and even to whether a plasma is present. Conditioning of the antenna was necessary for reliable power delivery; observed properties of this process are described. No deleterious effects such as significant impurity generation or a density rise have been observed to correlate with the antenna being powered. Accompanying posters in this session present details of the measurements, improvements in the technical aspects of the system, and of the development of new specific diagnostics to observe the effects of the helicon waves on the plasma in both the antenna near-field and core regions of the plasma.