Fig. 1. (a) Streamlined diagram of the complete LCLS-II setup, extending from the photoinjector to the near/far experiment halls, not depicted to scale. L, linac; BC, bunch compressor. The injector laser system is in sector 0. (b) Simplified diagram of the photocathode drive laser system. The laser, UV conversion unit, energy attenuator and conditioning system that adjusts the pulse size and duration are located in the laser room in the housing upstream of the accelerator. Diagnostics include power meters and cameras located in the laser room and on the gun tables and a cross-correlator in the laser room.

Fig. 2. The configuration of the LCLS-II laser-heater system, which includes a 1030 nm laser, chicane magnets, optical transition radiation (OTR) screen, energy collimator, pop-in YAG alignment screens (to align the laser and the electron beam) and undulators^[48,49].

Fig. 3. The calculated relationship between laser pulse energy and electron beam energy, specifically showcasing a 6 keV rms energy spread induced by the LH system in the case of a 100 pC bunch.

Fig. 4. (a) Generic layout for a central timing generator distributing timing information to receivers distributed along the beam line. (b) Interoperability of the LCLS and LCLS-II timing systems (shown in Figure 1).

Fig. 5. (a) UV pulse energy and (b) IR-UV conversion efficiency by optimizing the SHG beam size in the second SHG crystal. High repetition rates require finding a compromise among thermal stability, adequate spatial shape and conversion efficiency. Inset images show the transverse beam shapes from diameters 3.30 to 2.20 mm. IR-UV conversion efficiency across different SHG beam diameters in the second SHG crystal is at 928 kHz, with comparative data at 92.8 kHz for UV pulse energy and efficiency.

Fig. 6. The laser beam transport system for the LCLS-II. Inset: the laser beam profile is monitored by the virtual cathode camera.

Fig. 7. (a) Numerically generated temporal profile of the sum frequency pulse before applying a narrowband spectral filter (grey) and after (blue). (b) Experimental temporal profile at 256 nm collected with a cross-correlator with 70 fs, 1030 nm oscillator. (c) 256 nm spatial profile with an ellipticity of 0.63. (d) Simulated emittance comparison between temporal Gaussian pulse and shaped pulses with three different spectral filters, where DCNS with a 0.5 nm spectral filter demonstrates improved emittance at all electron bunch lengths^[58,69]. The charge used for optimization in part (d) is 100 pC.