First RF test at 4.2K of a 200MHz superconducting NB-CU cavity

A 200 MHz single cell elliptical Nb-Cu cavity has been fabricated and tested at 4.2 K and at 2.5 K in a vertical de-war fitted in a radiation shielded pit, 5 m deep and 2.5 m in diameter. The low field Q/sub 0/ reached 1.5 /spl times/ 10/sup 10/ at 4.2 K. Two multipacting barriers show up at E/sub acc/ = 3 MV/m and 1 MV/m. Helium processing is effective to reduce field emission and improve accelerating gradients by a factor of as much as 2. E/sub acc/ reached 11 MV/m at a Q/sub 0/ of 6 /spl times/ 10/sup 8/. The Q-drop is observed but shows a stronger field dependence as compared to expected Q-slope typical for Nb-Cu cavities.