Many of the grand scientific challenges of the 21st century involve understanding and controlling matter and the flow of energy at the finest and therefore most fundamental relevant values of time (attoseconds), space (nanometers), and energy (milli-electron-volts). A prominent tool for addressing these questions is a source of coherent beams of x-rays made up of ultrafast pulses at a high repetition rate and with unprecedented average brightness.
We in ATAP, together with colleagues in the Engineering Division, are collaborating with SLAC National Accelerator Laboratory, Fermilab, Jefferson Laboratory, and Cornell University in the LCLS-II project based at SLAC. LCLS-II is a free electron laser (FEL) facility based on a high-repetition-rate electron gun and a superconducting linear accelerator feeding multiple FELs. Building this next-generation light source has required the collaborators to meet state-of-the-art challenges in accelerator physics and technology from end to end. “First light” was achieved July 17, 2020.
 Image courtesy SLAC |
LBNL has delivered on crucial responsibilities in the LCLS-II project, including two major deliverables: the injector source and the two FEL undulator arrays, one each for hard and soft X rays. The LCLS-II injector is based on APEX, the Advanced Photo-Injector Experiment, in which we are developing the extraordinarily demanding electron gun that will be needed by next-generation light sources.
LBNL also developed the hard X-ray and soft X-ray undulator arrays. We contribute as well to accelerator physics and technology studies in beam dynamics, FEL design, low-level RF, and management and integration of cryogenics systems.
Advanced Photo-injector Experiment 
APEX is dedicated to the development and test of a new concept high-repetition rate high-brightness electron injector optimized to operate at the performance required by a high-repetition-rate x-ray FEL. The baseline for the injector design for LCLS-II is the VHF gun of APEX. LBNL is responsible for the design, construction and commissioning of the injector.
APEX, circa Phase I in 2015, installed in the Beam Test Facility at LBNL’s Advanced Light Source. This view looks “upstream” and shows the two yellow cylinders of the two-slit emittance measurement system, part of the instrumentation that measures the beam parameters. The large silver cylinder is the face of the electron-gun housing.
The successful development of such an injector will dramatically impact not only the performance of future 4th generation light sources when high repetition rates (above 10 kHz) are required, but also high-repetition-rate ultrafast electron diffraction (more on this below) and microscopy, as well as inverse Compton scattering (ICS) applications.
The core of the system is a normal-conducting, continuous wave (CW) RF gun. The electrons are generated by laser-induced photoemission on high-quantum-efficiency (QE) photocathodes and accelerated by the cavity fields (~20 MV/m) to energies as high as ~750 keV. The gun cavity has been designed to resonate at about 186 MHz (the 7th subharmonic of 1.3 GHz). The low frequency and thus long wavelength offers three advantages. First, it makes for a large resonator, keeping the power density on the structure walls low enough to use conventional cooling techniques when running the cavity in CW mode. The large apertures in the cavity walls produce negligible distortion of the electromagnetic fields, and also allow good vacuum conductance, so it is easier to keep the high-QE semiconductor cathodes under high vacuum, minimizing contamination and damage for best QE lifetime. A final advantage is the maturity and reliability of the VHF RF and mechanical technology, a very important characteristic for a 24/7 user facility.
The APEX project is located in the ALS Beam Test Facility (BTF). The VHF gun has been fully commissioned and has demonstrated all major nominal parameters such as CW operation, accelerating field, beam energy, and vacuum performance. After meeting major milestones in a performance review conducted in March 2016 by an outside panel of experts, we are working on a final demonstration of APEX whose results will include reliability demonstration under continuous (cw) operation.
In 2015, a state-of-the-art polishing technique brought the anode and cathode areas of the VHF Gun in the APEX injector to a mirror finish. This polishing reduced “dark current” (emission when the high-repetition-rate gun should be off) by more than 3 orders of magnitude: from the already adequate 350 nanoamperes down to 0.1 nA.
The actual LCLS-II electron gun system incorporated much of the prototype’s design.
An APEX Spinoff: Ultrafast Electron Diffraction
Intended as an R&D testbed for the injectors of the next generation of light sources, APEX is also en route to becoming a user-science instrument in its own right through HiRES, the High Repetition-rate Electron Scattering apparatus for ultrafast electron diffraction. 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. Here, ATAP physicist and DOE Early Career Award recipient Daniele Filippetto, leader of the HiRES experiment, works on APEX. The work is being pursued through ATAP’s Berkeley Accelerator Controls and Instrumentation (BACI) Program.
To learn more about HiRES and ultrafast electron diffraction, see “Seeing Atoms and Molecules in Action with an Electron ‘Eye’,” an April 2015 feature by Glenn Roberts of LBNL Public Affairs, or “APEX-Enabled Scattering Experiment Expected to Become New Research Tool for Materials Science” in the February 2016 edition of the ATAP Newsletter.
FEL Undulators for LCLS-II
Undulator prototype
Shown at left is a prototype LCLS-II undulator installed in the magnetic measurement facility at LBNL. The main features are magnetic modules and a strongback structure to position them — and move them upon command — with micron accuracy in the face of tens of thousands of kg of magnetic repulsion. Each LCLS-II undulator array will consist of several individual sections like this one.
Undulators are precision series of powerful magnets, alternating in polarity along the beam path, that make the electron beam move up and down with a period of a few centimeters. This causes the beam to emit intense, highly directional photon beams with coherence properties that can be amplified by an FEL resonator (being designed and built by others in the overall project), resulting in intense coherent light for the users. The gap between the two rows of magnets (shown here at several cm) is adjustable, allowing the photon wavelength to be tuned without changing the electron beam parameters.
LBNL has some 35 years of expertise in state-of-the-art undulators, going back to the days when the Advanced Light Source was being designed (undulators were and are one of the defining features of the ALS) and the late Klaus Halbach was establishing an engineering foundation for undulator design here. Undulator design and technology has remained a core strength of LBNL Engineering, going hand in hand with FEL physics design in ATAP, and is a key part of the new Berkeley Center for Magnet Technology. Undulator development naturally became another major area in which LBNL contributes to LCLS-II.
