DESY, Hamburg
The CW test of the 9 cell TESLA-type cavity in liquid helium bath at 2 K gives the cavity performance data for the cavity acceptance. The excitation of parasitic modes of the operating pass band is an error source for the cavity gradient and quality factor determination. The excitation of the parasitic modes in the operating pass band has been observed in 81 CW cold tests out of 170 and in 41 cavities under test out of 65 since 2006. The ramp up time of the parasitic mode depends strongly on the Qload of the input antenna. Some modes are exited in the combination with electron emission but others (7/9 Pi) have large amplitude without any attribute of field emission (x-ray or e- current at pick up). The growing up function of parasitic modes is different from the cavity response on the outside excitation but it describes a positive feed-back system. The relation of this effect to the field emission as well as to other test parameters has been investigated and is described in this paper.
DESY, Hamburg
TESLA 9-cell cavity was designed a decade ago for pulse operation at duty factor of a few percents. Recently, numerous coherent and synchrotron light sources projects base their driving superconducting linacs on this design assuming operation in a continuous wave (CW) mode at rather high gradients. We have performed CW tests of a standard 9-cell TESLA cavities installed in helium vessel and fully equipped with the standard TESLA-TTF auxiliaries, main coupler and both Higher Order Mode (HOM) couplers in the horizontal test cryostat to find out a limit in the CW operation. Tests details and results are presented and discussed.
DESY, Hamburg
INFN/LASA, Segrate (MI)
W.C. Heraeus GmbH COPY, Materials Technology Dept., Hanau
Few cavities of the 4th and 6th cavity generation treated accordingly the XFEL recipe have shown performance of ca. 15 MV/m caused by thermal break down without field emission. Effort to post purify some cavities with titanium, that was successfully applied for FLASH cavities, did not improve the performance. The T-map analysis detected the quench areas mainly close to the equator. Optical control by high resolution camera and non-destructive X-Ray radiography have been applied and allowed to monitor the defects in some cases with good correlation to T-map observation. In order to get more detailed information of defects some samples have been extracted from cavity and investigated by light microscope, SEM, EDX and Auger spectroscopy. The detected distinctions are discussed