A cryogenic microwave switch

A cryogenic microwave switch has been developed which has an isolation greater than 70 db, a throughput of 0.99, and a rise time of a few nanoseconds. The switch consists of an H-plane T in which one arm contains a short and a sapphire tube containing He. The short is fixed so as to isolate the other two arms when the He is not excited. The high degree of isolation is a direct consequence of the superconducting properties of the waveguide. The tube of He is located a quarter guide wavelength from the T so that when a plasma discharge is created in it, the other two arms become tightly coupled. Plasma rise times of order several nanoseconds have been achieved by applying a high voltage dc pulse across the tube. The switch has been used to release microwave energy stored in a superconducting resonator, creating high power microwave pulses. Here we will discuss the design parameters of the switch and its observed performance.