A superconducting cavity coupler's function is to deliver RF power from the outside RF power source with minimal resistive losses to the superconducting cavity. At the same time, the coupler isolates the cavity vacuum from the outside environment and minimizes heat flow from the surroundings to the cryogenic temperature cavity. To prevent heat flow, the outer conductor of a coupler is made of stainless steel because of its low thermal conductivity. To decrease ohmic losses, the stainless steel is coated with a thin layer of copper. This coating is generally applied to the stainless steel using a galvanic or plasma-based process.
This approach has several drawbacks. First, the technology used to plate copper is not sufficiently developed to provide a reliable reproducible coating. For example, the copper coating often flakes or peels away from the stainless steel. Copper flaking is fatal for the superconducting cavity. In addition, the copper layer increases the thermal conductivity of the stainless steel outer conductor and increases the heat flow to the cavity. As a result, the cavity requires a more powerful cryo-plant to compensate, which reduces the efficiency of the system. Finally, the copper layer has a low residual-resistance ratio (RRR). It increases ohmic losses, deposits additional heat into the superconducting cavity, and reduces system efficiency.
An additional difficulty arises in protecting the ceramic surface of the dielectric RF window from charged particles emanating from the superconducting cavity. In the prior art, some waveguide couplers use a bent waveguide to remove the dielectric surface from line of sight of the superconducting cavity. This provides the dielectric surface with some protection from charged particles. However, this approach is not useable in a coaxial coupler.
Accordingly, methods and systems are required for superconducting cavity couplers that avoid these disadvantages.