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Description
The Linac Coherent Light Source II (LCLS-II) at SLAC is a continuous-wave (CW) superconducting radiofrequency (SCRF) electron linac that accelerates electrons to about 4 GeV at MHz repetition rates, with a design beam power of 120 kW and a maximum credible beam power of 1.2 MW. Radiological commissioning advanced through staged beam destinations: halo collimators along the linac, the Beam Switch Yard Dump (BSYD) embedded in the 17 m iron muon shield wall between the linac and the undulators, and the electron beam dumps (EBDs) that follow the soft and hard X-ray undulators. At each stage, prompt and residual dose rates were measured, and passive shielding and active monitoring systems were validated.
All prompt dose rates at occupied service-area penetrations remained below the 5 μSv/h design limit at every beam power commissioned so far, up to about 19 kW. The highest readings stayed about a factor of two below that limit. FLUKA Monte Carlo calculations reproduce the measured prompt and residual fields, in most cases within a factor of two and at several locations within a few tens of percent. This shielding benchmark directly supports the radiation-protection framework for the High-Energy (HE) upgrade to 8 GeV, now being installed.
Beam-loss and radiation monitor commissioning produced several transferable findings. CIVIDEC B4 polycrystalline diamond Point Beam Loss Monitors (PBLMs) exhibited signal decay with an approximately 18-hour half-life after beam-off due to charge trapping, rendering them unsuitable for machine protection; single-crystal B1 replacements respond proportionally and promptly to beam loss with no extended decay. Thick targets attenuate the Cherenkov signal from quartz Long Beam-Loss Monitor (LBLM) fibers by more than an order of magnitude. Oxygen-15 activation of dump cooling water, read by a Residual Dose Monitor (RDM) at the heat exchanger, provides a real-time proxy for the power delivered to the dumps, the onset delay and the decay constant being set by the coolant transit time and by the short-lived activation products. Dark current from SCRF cryomodules produces a persistent radiation background, and characterizing how these fields propagate through the linac tunnel is essential for reliable interpretation of the fixed monitor baselines.
| Topics | T1. Source terms, new accelerator facilities and related topics |
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