Initial studies of electromagnetic shower energy deposition in small-bore superconducting undulator structures in linac environments

One of the more promising technologies for developing minimal-length insertion devices for linac-driven, single-pass free electron lasers (FELs) operating in the X-ray range is based on the use of superconducting (SC) materials. Despite its advantage of minimal length, however, SC technology can present difficulties for insertion device design and operation. One critical problem, as observed, e.g., by Madey and co-workers in their initial (25 MeV) FEL experiments, was the frequent quenching induced by scattered electrons upstream of their (bifilar helical) SC device. In view of the short-term stability required of undulators for user facilities, we have initiated a systematic investigation of these earlier results to determine whether quenching could become similarly probable in SLACs (10–15 GeV) linac coherent light source (LCLS), a 1.5 Å FEL. Postulating that the onset and spread of normal zones are precipitated by directly-scattered or bremsstrahlung-propagated particle energy deposited into the SC material or into material contiguous with it, we have used the EGS4 particle-tracking code developed at SLAC to perform studies of scattered-particle energy deposition into SC structures with geometries comparable to a small-bore bifilar helical undulator. Preliminary numerical results for both the Madey and SLAC LCLS cases have been obtained.