Low energy dark current collimation system in single-pass linacs

Abstract

The dark current emitted from a surface of a radio frequency cavity may be a severe issue for the activation and the protection of the components of linear accelerators, if this current is lost in an uncontrolled manner. For a single-pass linac based on a photo-injector, we studied the possibility of using a collimator installed at low energy (below 10 MeV) to dump the maximum fraction of the dark current before it is transported along the linac. We developed and experimentally verified an emission and tracking model that we used to study and optimize the dark current mitigation at SwissFEL test facility. We optimized a collimator, which is expected to reduce by two orders of magnitude the transport of the dark current to the first compressor. We have also verified the effects of wakefield excited by the beam itself passing through the collimator at such a low energy, comparing the results of beam-based measurements with an analytical model.

Design, optimization and experimental characterization of RF injectors for high brightness electron beams and plasma acceleration

“…The emitters area 𝐴 e and the FN emission coefficient 𝛽 characterize the entire emission process and depend mainly on the cleanliness and roughness of the emitting surface. Those are unpredictable, so as a starting point for these two parameters, to make a preliminary DC estimation, we took 𝛽 = 70 and 𝐴 e = 0.01 μm 2 , evaluated at PSI for the SwissFEL Gun [16]. The PIC simulations are based on the technique described in [17] and include the solenoid model, which allows to estimate the energy spectrum and the DC beam transport along the photo-injector up to the diagnostics chamber.…”

Transverse beam stability with low-impedance collimators in the High-Luminosity Large Hadron Collider: Status and challenges

“…Collimators are widely used in particle accelerators. The systems find their usage from linear coherent light sources such as Linear Coherent Light Source [1], Swiss-FEL [2], or Next Generation Light Source [3] to high intensity circular colliders, both past, present, and proposed: Tevatron [4], Relativistic Heavy Ion Collider [5], Large Hadron Collider (LHC) [6], or Future Circular Collider (FCC) [7]. In superconducting colliders collimators play an essential role of protecting their superconducting magnets from quenches or damage in case of beam losses as well as controlling the beam halo [8].…”

Dark current studies of an L-band normal conducting RF gun

“…The second approach is to influence the dark current transmission via collimators and fast kickers before reaching the cryogenic system and undulators. Studies performed at SwissFEL pointed out that an optimized collimator at the low energy section can reduce the dark current transported to the first compressor by two orders of magnitude [22]. A dark current fast kicker system was installed at 0.45 m downstream of the cathode at FLASH [3,23] The dark current in the 2 nd bunch compressor was reduced by ~70% [24].…”