Pande, Shivaji and Burt, Graeme (2023) STUDY OF VACUUM TRIPS IN DIAMOND LIGHT SOURCE SRF CAVITIES. PhD thesis, Lancaster University.
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Abstract
The CESR cavities operate at liquid helium temperature. The RF power supplied to the beam by each of the cavities is in excess of 200 kW at a total voltage in the range of 2.5 to 3.5 MV. The cavities suffer from vacuum trips with collapse of field in fraction of the filling time if operated above a certain voltage and cause the loss of stored beam affecting the reliability of the machine. Similar trips were observed at Cornell especially during the attempts to increase the travelling wave power which was limiting the beam current during the commissioning of the CESR ring. The multipactor in the CESR waveguide was studied at Cornell by R. L. Geng and P. Goudket concluded that the multipacting in the waveguide was the cause behind the trips. The studies at Cornell were mainly concentrated on Travelling Wave (TW) conditions and in the straight parts of the waveguide. As part of the efforts to supress the multipactor, they studied the single and multiple longitudinal groves of narrow width along the centre of the broad walls. These were observed experimentally to be not so effective as expected due to the lateral extension of the multipactor as the single grove covered a very narrow part of the broad wall. The effectiveness of multiple groves was worse as they were found to modify the surface E field. The most effective method of multipactor suppression proposed and used at Cornell was to use the low intensity longitudinal magnetic field along the straight waveguide. In the concluding part of his thesis, P. Goudket mentions that the use of 10G longitudinal magnetic field in straight part of the waveguide is sufficient to supress the multipactor there and is effective even in unclean waveguide surfaces. He mentions that the coil wrapped around the double E bend is enough to greatly supress the breakdown occurrences in the input coupler. Their observation suggested that the multipactor events occur away from the niobium parts of the coupler. At Diamond, the excitation of the anti-multipactor coils had no effect on the occurrence of the trips. Moreover, the vacuum trips observed at Diamond often were preceded by spikes on the waveguide e- pickup mounted just under the coupling iris. This pointed towards the existence of a discharge in the coupling region. Also, unlike the observation at Cornell, the trips observed at Diamond are voltage dependent and observed at voltage as low as 1 MV. The actual field in the coupling waveguide is SW in nature due to the step arising from the difference in heights of coupling waveguide and that on the vacuum side of the RF window even for matched operation. The operation of the CESR cavities at lower voltage makes it necessary to use the 3 stub tuners to lower their Qext to minimise the reflected power which enhances the SW between the cavity and the 3 stub tuner. As part of this thesis, detailed numerical modelling has been undertaken to study and understand the underlying phenomenon including the study of field emission and multipacting in the cavity, the coupling region and the waveguide in the presence of SW which was not covered in the studies at Cornell.