Effects of high solenoidal magnetic fields on rf accelerating cavities

Abstract

We have measured the effects of high (0-4.5 T) magnetic fields on the operating conditions of 805 MHz accelerating cavities, and discovered that the maximum accelerating gradient drops as a function of the axial magnetic field. While the maximum gradient of any cavity is governed by a number of factors including conditioning, surface topology and materials, we argue that J B forces within the emitters are the mechanism for enhanced breakdown in magnetic fields. The pattern of emitters changes over time and we show an example of a bright emitter which disappears during a breakdown event. We also present unique measurements of the distribution of enhancement factors, , of secondary emitters produced in breakdown events during conditioning. We believe these secondary emitters can also be breakdown triggers, and the secondary emitter spectrum helps to determine the maximum operating field.

Performance of a second generation X -band rf photoinjector

“…The system was then reramped with the solenoid on. Observations with this 11.424 GHz vacuum structure can be compared to 800 MHz gas filled structure results [43]. They are similar in that the conditioning was observed to be less stable and required longer recovery with a magnetic field present, but significantly different in that the breakdown threshold appeared independent of magnetic field.…”

Interim Design Report

“…There is empirical evidence that magnetic fields overlapping RF cavities, as present in the muon front-end, may induce breakdown in the cavities [273,274]. The performance of the muon front-end using a reduced field has been explored using ICOOL.…”

“…The presence of magnetic fields overlapping RF cavities has been identified as a technical risk that may reduce the capture efficiency of the muon front-end due to a reduction in the peak gradient that can be achieved in the RF cavities [273,274]. Three alternative lattices have been proposed for the cooling section to mitigate this risk.…”

Design for a Muon Front End with Magnetically Shielded RF Cavities

“…There is some evidence that the presence of these strong fields may limit the RF cavities by a factor of roughly 2, although experiments have been limited by a number of factors and further work is needed [2]. Several schemes have been investigated to work around this problem.…”