Fast opening switch approach for high-voltage vacuum tube protection application

The operation of high-power, high-frequency vacuum tubes requires an appropriate protection method to avoid significant damages during arcing. Fast closing switches like spark gaps, thyratrons, ignitrons and semiconductors acting as charge-diverting bypass switches are the most commonly used protection method. These “crowbar” switches cause hard transient conditions for all subcomponents involved and usually result in a significant post-fault recovery period. The availability of fast high-voltage semiconductor devices, with flexible on/off control function, makes opening switch topologies possible and attractive to improve this situation. This paper describes a circuit topology to protect an Inductive Output Tube which is expected to operate within RF subsystems for accelerator applications. The topology is characterized by using a commercial available high voltage MOSFET switch with direct liquid cooling and completed with essential snubber extensions. The advantages of the opening switch approach are faster action, smaller fault energy, faster recovery, and more compact design. Initial test results of this topology are presented.