Beam-loss detection for the high-rate superconducting upgrade to the SLAC Linac Coherent Light Source

The Linac Coherent Light Source (LCLS) x-ray free-electron laser is driven by the third kilometer of the 3-km SLAC linac, which accelerates electrons in normal-conducting copper cavities pulsed at 120 Hz. The first kilometer is being replaced by LCLS-II, a superconducting (SC) electron linac driven by continuous rf at 1.3 GHz and with a normal-conducting photocathode gun using continuous rf at a subharmonic, 186 MHz. Its 4-GeV, 120-kW beam has a 1-MHz maximum rate, with an upgrade to 8 GeV in planning. The beam from either linac can be switched pulse by pulse to either of two new undulators, to generate hard and soft x rays. Control of beam loss is critical for machine and personnel safety. Previous SLAC protection systems have depended on ionization chambers, including both local devices at expected loss sites and long gas-dielectric coaxial cables providing distributed coverage. These devices are unsuited to the SC-linac beam, because their ion collection time, over 1 ms, may allow the space charge of accumulated ions to null the electric field inside the detector, blinding it to an increase in loss. Instead, both the local and the distributed detectors have been replaced with faster devices. The full 4 km will be spanned by multiple radiation-hard optical fibers in lengths of up to 200 m, each coupled to a photomultiplier tube, to capture Cherenkov light from loss showers. These are supplemented by single-crystal diamond detectors at expected loss sites. Signals are integrated with a 500-ms time constant; the beam is stopped within $\mathrm{200}\mathrm{\text{ }}\mathrm{\text{ }}\ensuremath{\mu}\mathrm{s}$ if a threshold is exceeded. We report on our extensive tests of the detectors and the new signal processing.