DOE SBIR Phase I: Solid-State Klystron Driver for Lower Hybrid Current Drive

Launching radio frequency (RF) waves from the high-field side (HFS) of a tokamak has the potential to be an efficient off-axis current drive method while reducing the plasma-material interaction issues of the antenna. Researchers at the Plasma Science and Fusion Center (PSFC) at the Massachusetts Institute of Technology (MIT) have proposed to reuse existing equipment at MIT, including CPI klystrons, to demonstrate HFS RF launching at DIII-D. The current klystron driver is not scalable as the number of klystrons increases; is so large that it cannot be located close to the klystrons, which increases the risk of damaging klystrons during a fault; requires all klystrons be shut down in the event of a fault; and cannot operate for the full length of a DIII-D pulse.

EHT is developing the next-generation klystron driver for use by MIT for HFS RF launching experiments at DIII-D. The next-generation klystron driver will take advantage of the high frequency solid-state switching capabilities developed by EHT with support of the DOE SBIR program. The high frequency nature will allow for the development of a more compact system, which can be placed closer to the klystrons. This system will be designed so that there is one driver per klystron, which will allow the system to scale as more klystrons are added and experiments to continue in the event of a klystron fault.

EHT has previously developed a series-stack of solid-state switches and 800 V full-bridge rectifiers for driving plasma injectors. In the Phase I program, EHT will develop a high voltage full-bridge rectifier utilizing the EHT HV solid-state switch to convert the rectified three-phase high voltage to 50 kV pulse required for klystron driving to demonstrate the system at a breadboard-level. EHT will design the system and test each component prior to assembly. Finally, the full breadboard-level klystron driver will be tested. With the test results, EHT will prepare an initial design for implementation in a potential Phase II program. The goal of Phase II would be to develop a full prototype that can be tested with the MIT klystrons.

The fusion science community would be the initial beneficiary from the development of the next-generation klystron driver. This klystron driver is designed to drive the CPI klystrons at MIT. This new driver would allow the existing klystrons to be repurposed and used at DIII-D for HFS RF launching experiments that could validate the advantages of HFS RF launching for lower-hybrid current drive for efficient off-axis current drive as well as future RF experiments. Beyond fusion science, klystrons are used in particle accelerators, radar systems, and high-power transmitters. A more compact and responsive driver would have benefits in these applications as well.