High-Power Helicon System Upgrades and Repairs at DIII-D

Helicon plasma waves offer a promising route for plasma heating and current drive in fusion reactors. The DIII-D tokamak hosts a helicon system operating at 476 MHz, capable of outputting up to 1.2 MW.

In FY23, operational issues emerged, including damage to in-vessel thermocouples and RF probes around the antenna. Damaged thermocouples were removed and routed behind tiles was ensured for those remaining. All damaged RF probes were replaced and calibrated during the FY24 vent. Loose screws on the 150° inner conductor caused contact with the septum, rendering the 150° port inoperable. The bent bellows at the 150° port in-vessel inner conductor was removed, and the inner conductor was replaced during the FY24 maintenance vent.

During FY23, overheating incidents occurred in the elbow of the 210° port during prolonged high-power shots, though no visible damage was noted. To address this, elbows were copper-plated and diamond-coated to avoid multipactor effects in the future.

In the FY24 maintenance vent, the team removed the non-pressurized waveguide switch and implemented a patch panel to restore functionality of the Helicon feeding from both the 150° co-current and 210° counter-current directions.