A 200MHz single-cell superconducting Nb-Cu cavity has been fabricated and tested at 4.2 K and at 2.5 K. The low field Q_0 reached 1.5 * 10~(10) at 4.2 K. The accelerating gradient (E_(acc)) reached 11 MV/m at a Q_0 of 6 * 10~8. Two multipacting barriers show up at E_(acc) = 3 and 1 MV/m. Helium processing is effective in reducing field emission and improving accelerating gradients. The Q-drop is observed with a much stronger field dependence as compared to what is expected. Results on the radiation background near the cavity and the external magnetic field effect on the Q of a cold cavity are also reported.
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