A Superconducting Electron Speed Filter for Establishing High DC Voltages

A superconducting cavity resonator made of niobium is used as an electron speed filter. As only electrons with well-defined velocities can pass through the resonator via small apertures in the front and rear faces this device can be utilized for the calibration of high dc voltages Due to the high Q-factor of the superconducting resonator, the selectivity of the filtering system is increased by a factor of 25 as compared with the copper resonator used formerly, and field distortion due to the microwave coupling probes is decreased to less than 1 X 10-5 relative to the unperturbed field within the resonator. The relative uncertainty in the realization of calibration values between 40 and 100 kV is estimated to be 1.2 X 10-4 for an aperture diameter of 50 μm. When the difficulties in the electron beam adjustment are overcome, a relative uncertainty of a few parts in 105 should be realized, due to the use of smaller apertures.