Solenoid focusing is commonly used in accelerators for electron-beam containment and to compensate for space-charge induced emittance growth at low beam energies. However, misalignment between the solenoid field and the beam trajectory can result in degraded emittance compensation due to orbit kicks, dispersive effects, and non-linearities in the magnetic field profile. This paper presents a technique for beam-based measurement of solenoid misalignments, using expressions derived from a hard-edged solenoid linear transfer matrix with known beam parameters. The results of measurements conducted at the LCLS-II injector are presented as a validation of this method, along with simulation studies that estimate the impact of solenoid misalignments on emittance growth.

LCLS-II has been in user operations since 2023 and has ramped the beam rate up to 33 kHz to date. The LCLS-II photoinjector has demonstrated a low-emittance (half-micron) beam operating at a high rate. During the injector operation, we have also encountered several challenges, such as e-beam splitting and large dark current from the gun. The split e-beam compromises FEL lasing, while the large gun dark current generates substantial amounts of ions, which backbombard and damage the photocathode, as well as cause beam loss downstream, frequently damaging components. Recently, these problems have been successfully addressed. On one hand, a relatively larger laser size was implemented on the photocathode, which helped suppress space charge effects, resulting in the disappearance of the split beam. On the other hand, an over-inserted photocathode, recently installed on the LCLS-II gun, has reduced the gun's dark current by a factor of 1000. The gun dark current reduction has significantly extended the photocathode's operational lifetime, eliminated the QE and FEL intensity dependence on beam rate, and substantially reduced beam loss downstream. This paper presents detailed analyses and solutions for LCLS-II photoinjector operational challenges.

Shaping ultraviolet (UV) laser beams is critical for optimizing photoinjector performance for applications in free-electron lasers (FELs). It has been shown that a 50% truncated Gaussian beam can achieve the lowest emittance via space charge compensation at LCLS-I. However, conventional shaping techniques to prepare this beam are limited by significant power losses or are not adapted for UV light. Here we report a high-precision transverse-shaping technique based on custom fused-silica phase plates with >99 % transmission at 253 nm. This approach enables spatial beam profile tailoring and significantly enhances beam stability at the photocathode. Using IMPACT-T simulations, we predict a 33% (from 0.67um to 0.45um) reduction in normalized emittance for a 250 pC bunch at LCLS-I. Experimental implementation at FACET-II demonstrated a 37% emittance reduction (from 5.4um to 3.4um) at 1.6 nC. These results establish phase-plate beam shaping as a high-fidelity, low-loss approach for high-brightness photoinjectors. Implementation at LCLS-II which will enable stable operation at megahertz repetition rates is underway.