Two researchers work at an electron beamline for the HiRES apparatus.

Researchers adjust the HiRES apparatus.

From its origins as an R&D testbed to meet the challenges of LCLS-II and other photon sources of the future, APEX, the Advanced Photo-injector Experiment, also supports a user-science instrument: HiRES, the High Repetition-rate Electron Scattering apparatus for ultrafast electron diffraction (UED). This is expected to provide another way to address one of the grand challenges in the understanding of materials: following the dynamics of atoms and molecules at the atomic scale.

This capability is made possible by APEX, built to produce high-charge, picosecond-long electron pulses at MHz repetition rates to drive the next generation of light sources. This innovative electron source uses blends of selected semiconductor materials as the photocathode, driven by a femtosecond laser, together with high electric fields (tens of megavolts per meter) from continuously stored radio-frequency power.

APEX supplies HiRES with highly focused electron bunches, each containing up to 1 million electrons, at a rate of 1 million bunches per second.  The effect is like a fast camera shutter, enabling snapshots of samples as they change over femtoseconds—the timescale on which chemical bonds form and break. An initial laser pulse triggers a reaction in the sample that is followed an instant later by an electron pulse to produce an image of that reaction.

This could provide new insight in how to make materials with custom, controllable properties; improve the efficiency and output of chemical reactions; pinpoint defects and their effects; track electronic and superconducting properties in exotic materials; and detail chemical reactions in gases, liquids and biological samples that are difficult to study using more conventional, X-ray-based experiments.

Several scientists observe data on a computer monitor.

The HiRES team observes as live data comes in from the system.

The HiRES accelerator beamline is not only a UED beamline, but also an R&D testbed for developing novel beam diagnostics and control instruments, as well as AI/ML adaptive control algorithms. While today’s accelerators use closed-loop feedback systems to mitigate jitter, these systems still allow for a certain amount of fluctuation, which must be addressed to achieve optimal temporal resolution. The measurements of the beam arrival time are usually accomplished using methods that perturb or interrupt the beam and cannot be used for UED. A new approach is being developed that uses virtual diagnostics and machine-learning techniques, which allow for non-destructive monitoring of the beam dynamics and have shown promise for tackling control and diagnostics problems in accelerators.

Looking forward to the even greater demands of future photon sources such as LCLS-II HE (High Energy), we are now working on APEX2, which will also improve the performance of UED and ultrafast electron microscopy applications.