Characterization of High-Power Superconducting Microwave Resonators

Abstract

A system for superconducting resonator characterization and intrinsic quality factor measurement is presented. High power (100s of watts continuous RF power and higher) and high Q (1E+9 and higher) superconducting resonators are two-port highly non-linear devices with specific features that make their accurate measurement and characterization very complex. To achieve fundamental mode electric field gradient of tens of megavolts such resonators are tested with high level of input power, with input port critically coupled to the source, and the output port undercoupled to the load and used as an electric field probe. With such configuration, direct measurements of resonator S-parameters using a VNA to determine external coupling become impossible, because reflection measurements from undercoupled port require very high power from the source to achieve operating condition, and high filed gradient. Measurements at low power level are not practical as well, because resonator loss and Q depends on the applied power due to non-linearity. Another significant complexity comes from the effect of resonator detuning due to mechanical vibrations – microphonics. This detuning is usually much larger that the resonator linewidth and therefore one cannot observe a smooth Lorentzian frequency response of such resonator using a VNA. In addition, resonance frequency depends significantly on the gradient of the electric field in the resonator due to Lorenz force detune and usually changes dynamically in the exponential manner while resonator is filled with energy. These last two features require additionally a separate phase locked loop to keep resonator tuned during measurements and adjustments of power. Finally, characterization involves microwave S-parameter calibration in liquid helium environment and high-power transmission lines. Such kind of complex characterization cannot be performed with any modern VNA alone and required us to design a new instrument: A superconducting vector cavity analyzer. Superconducting vector cavity analyzer allows to analyze cavity parameters such as quality factor, electric field gradient, frequency stability and others. The system allows to test up to four cavities during one cooldown in liquid helium, each cavity can contain up to four ports. The operating frequency range of the system is 0.1-10 GHz and available continuous wave maximum power is 500 W. This new instrument can do traceable uncertainty calculation of the measured quality factor and electric field gradient. The instrument is designed in a finished form of a relay rack with several modules: RF source; signal processing module; downconverter; high power auto-calibration module; switch module; high-power reflectometer; all controlled by a computer. A specially designed software performs all necessary error correction using custom short-open-load technique and absolute power calibration. The system has high level of automation and requires minimum effort for operations, essentially it represents a special type of high-power vector network analyzer designed to measure parameters of superconducting cavities.