The Variable Input Coupler for the Fermilab Vertical Cavity Test Facility

A variable input coupler has been designed for the Fermilab vertical cavity test facility (VCTF), a facility for testing bare 1.3 GHz 9-cell superconducting radiofrequency (SRF) cavities at 2 K, to provide some flexibility in the test stand measurements. The variable coupler allows the cavity to be critically coupled for all tests, including all TM010 passband modes, which will simplify or make possible the measurement of those modes with very low end-cell fields, e.g., pi/9 mode. The variable coupler assembly mounts to the standard input coupler port on the cavity, and uses a cryogenic motor submerged in superfluid helium to control the antenna position. The RF and mechanical design and RF test results are described.