Transient High-Field Behavior of Niobium Superconducting Cavities

Tests have been performed on the breakdown behavior of a TM010 mode, S-band niobium cavity at low temperatures. Unloaded Q´s of 9 × 107 at 4.2 K and of 7 × 109 at 1.35 K were measured during several tests performed using pulses long enough for the cavity to reach steady state. The breakdown field at 1.35 K was increased from 15 to 20 MV/m by processing the cavity at room temperature using 1 MW, 2.5 ps pulses. The response of the cavity at 4.2 K to 1 MW, 2.5¿s pulses was also tested in several cool-downs. In these tests the cavity was heavily overcoupled to lower its time constant to a value of 0.80 timnes the RF pulse length of 2.5 ¿s. This condition maximizes the energy transfer from the klystron source to the cavity. Measurements made during these experiments clearly indicated that fields of about 50 MV/m were being reached in the cavity without breakdown.