Beam Test of a Superconducting Cavity for the CESR Luminosity Upgrade

The prototype superconducting cavity system for CESR- Phase III was tested in CESR in August 1994. The performance of the system was very gratifying. The cavity operated gradients of 4.5-6 MV/m and accelerated beam currents up to 220 mA. This current is a factor of 3 above the world record 67 mA for SRF(1). The high circulating beam current did not increase the heat load or present any danger to the cavity. No instability attributable to the SRF cavity was encountered. A maximum of 155 kW of rf power was transferred to a 120 mA beam. The window was subjected to 125 kW reflected power and processed easily. In the travelling wave mode, vacuum bursts and arc trips prevented us from going above 165 kW. The maximum HOM power extracted was 2 kW. Beam stability studies were conducted for a variety of bunch configurations. In other tests a 120 mA beam was bumped horizontally and vertically by ∠10 mm. While supporting a 100 mA beam, the cavity was axially deformed with the tuner by 0.4 mm to sweep the HOM frequencies across dangerous revolution harmonics. In all such tests, no resonant excitation of HOMs or beam instabilities were observed, which confirms that the potentially dangerous modes were damped strongly enough to be rendered harmless. components needed for the CESR beam test. The cavity was first tested in the processing area with high power without beam. Once it operated CW at 6 MV/m it was installed in the CESR beam line, in the high bay area, west of the CLEO detector. The refrigeration system consisted of two units, nominally rated at 100 watt, feeding into a 1000 litre dewar. The cold gas from the cryostat was returned to the refrigerator. On one side, CESR dipole magnets were located < 1 m away from the SRF cavity; but on the detector side, the closest magnet was > 15 m away. Therefore most of the high current tests were carried out with a positron beam, so as not to irradiate the cavity region with too high a synchrotron radiation (SR) dose from the nearby magnet. Near the end of the test, however, a 57 mA electron beam was also run through the cavity, to evaluate how the cavity would perform in the presence of a severe SR dose. Most of the beam tests were conducted at 5.3 GeV, for which the total voltage required was 7-7.5 MV. Through most of the tests, the CESR NRF system of four 5-cell copper cavities provided 6 MV (gradient = 1 MV/m) and the SRF cavity provided about 1.5 MV.