Abstract
The Linac Coherent Light Source II (LCLS-II) will be a hard X-ray Free Electron Laser whose linac can deliver a 1.2 MW CW electron beam with bunch rates up to 1 MHz. To efficiently generate such a high power beam, Super-Conducting Radio-Frequency (SCRF) cavities will be installed in the upstream portion of the existing 3 km Linac at the SLAC National Accelerator Laboratory. The 9-cell niobium cavities will be cooled at 2K inside 35 cryomodules, each containing a string of eight of those cavities followed by a quadrupole. The strong electromagnetic fields in the SCRF cavities will extract electrons from the cavity walls that may be accelerated. Most such dark current will be deposited locally, although some electrons may reach several neighboring cryomodules, gaining substantial energy before they hit a collimator or other aperture. The power deposited by the field emitted electrons and the associated showers may pose radiation and machine protection issues at the cryomodules and also in other areas of the accelerator. Simulation of these effects is therefore crucial for the design of the machine. The in-house code Track3P was used to simulate field emitted electrons from the LCLS-II cavities, and a sophisticated 3D model of the cryomodules including all cavities was written to transport radiation with the Fluka Monte Carlo code, which was linked to Track3P through custom-made routines. This setup was used to compute power deposition in components, prompt and residual radiation fields, and radioisotope inventories.
Highlights
The Linac Coherent Light Source II (LCLS-II) will be a hard X-ray Free Electron Laser whose linac can deliver a 1.2 MW CW electron beam with bunch rates up to 1 MHz
LCLS-II will add a 4.0 GeV, 1 MHz, CW Super-Conducting Radio-Frequency (SCRF) electron accelerator in the first 700 meters of the Linac, as well as additional transport beam lines and new, This is an Open Access article published by World Scientific Publishing Company
We expand the scope of a 2003 TESLA study,[3] simulating the surface emission of electrons within complex cavity shapes, their trajectory and energy variation in 3D electromagnetic fields, their transport between cryomodules including the effect of quadrupoles at different gradient settings, the subsequent re-acceleration in up to eight contiguous cryomodules, and the showers of lost electrons through a detailed model of the beam line, including damage estimates to components, inventories of activation products, and residual dose rates
Summary
Since 2009, the Linac Coherent Light Source (LCLS) hard X-ray Free Electron Laser has delivered electron pulses at 120 Hz by using the last third of the SLAC 2-mile linac, followed by a transfer line and up to 30 undulators, where the coherent radiation is generated. LCLS-II will add a 4.0 GeV, 1 MHz, CW SCRF electron accelerator in the first 700 meters of the Linac, as well as additional transport beam lines and new, This is an Open Access article published by World Scientific Publishing Company. Because the dark current energy is generally low compared to the beam, much of it will be swept out of the beam line by the quadrupole magnets at the end of each cryomodule, thereby irradiating the cryomodules and the tunnel
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