A Ytterbium-based photocathode gun drive laser is proposed for the Advanced Photon Source linac to replace the existing antiquated Nd:Glass laser. The proposed laser will readily operate at 30 Hz providing 0.3 mJ of 257-nm UV radiation per pulse yielding 1 nC from our copper cathode, s-band gun in support of user operations. In addition, the laser allows generation of lower-charge, low-emittance electron beams for high-brightness experiments in the APS Linac Extension Area. An advantage of updating the PC Gun drive-laser is that the configuration includes a downstream 3-m-long accelerating structure; this provides an additional 35-40 MeV of energy at the linac output over what is presently available from either of the two thermionic-cathode guns. Higher linac output energy will enhance stability for high-charge operation of the new storage-ring. We outline the laser physics requirements for our LCLS-I-style PC gun and summarize the expected beam performance.

We propose further investigations on the longitudinal-space-charge-impedance mechanism for inducing microbunching of relativistic electron beams within the Advanced Photon Source S-band linac. The microbunched content is evaluated by observing the coherent enhancements of optical transition radiation (COTR) generated as the beam transits a metal-vacuum interface. The facility also uniquely includes both thermionic cathode and photocathode rf guns as electron sources for comparisons of effects. Previously, we addressed mitigation of the COTR’s deleterious effects in the 2-D visible-light beam images at 325 MeV*. By extending our wavelength coverage into the NIR, we will access the much stronger enhancements predicted (>100)** and elucidate their spectral content. We will use an existing optical transport line for visible to NIR COTR (0.4 to 3.0 microns) from the diagnostics cube in the tunnel to an enclosed, external optics table. The inexpensive addition of a NIR-sensitive photodiode and integrating circuit with an existing digital oscilloscope in the optical setup would provide immediate extension of the detectors’ wavelength coverage and would enable the testing of the current model predictions for the microbunching instability into the NIR.

Maintaining beam transport efficiency in the APS linac requires several feedback mechanisms to control orbit, phase, and other parameters. Presently, we apply pre-computed matrices to sets of deviations from fixed setpoints, corresponding to proportional linear feedback. This approach works most of the time but is slow and can become unstable at low charge levels. We explore two alternative machine learning (ML) methods - adaptive Bayesian optimization (ABO, developed previously) and reinforcement learning (RL). To pre-train ML methods we use a differentiable linac simulation to generate a custom kernel and policy, respectively. All 3 methods are experimentally tested using a set of simulated disturbances, and performance in terms of charge stability and recovery speed analyzed. We find that both ABO and RL techniques are more flexible than standard feedback but behave quite differently if beam degradation is large. Overall, RL appears to be the more robust long-term method for rough correction, while ABO is best for fine tuning on recent history. Based on the above results we implemented a novel hybrid scheme that dynamically combines algorithm outputs using historical and expected performance. It also restricts parameter space to the most relevant region. Preliminary results show this to be both more stable and more accurate than the standard approach. We are now exploring strategies for dynamic retraining and other advanced capabilities.

A new storage ring based on a multi-bend achromat (MBA) lattice has been built at the Advanced Photon Source. Currently, the commissioning process is underway to bring beamlines back into operation. The APS linac consists of two S-band thermoionic cathode guns at the front end and thirteen S-band traveling-wave RF structures, all powered by five klystrons. A major upgrade is in progress to enhance the RF system in the APS linac. Specifically, the high power undulators and klystrons will be replaced with a newly designed solid-state switching modulator systems. Additionally, the RF control and diagnostic systems are being replaced by brand-new digital LLRF systems. As of now, one RF station has been successfully upgraded, commissioned, and it has been operating for half a year. Notably, the RF stability at this station shows significant improvement compared to other stations.