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, have been working for some years to define a next-generation light source that can meet these national needs. Currently we are collaborating with SLAC National Accelerator Laboratory, Fermilab, Jefferson Laboratory, and Cornell 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 will require the collaborators to meet state-of-the-art challenges in accelerator physics and technology from end to end.
The LCLS-II project achieved Department of Energy Critical Decision One (CD-1, approval of alternative selection and cost range) in August of 2014, after having received a CD-0 decision (statement of mission need) in 2013. The collaboration is currently preparing for a CD-2/3 review (approval of performance baseline and of start of construction) in late calendar 2015, in pursuit of a schedule that should provide first light to experiments in late 2019.
LBNL has 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 is also developing the hard- 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.
Phase I 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. In Phase I, installed and being commissioned, a complete suite of diagnostics allows for the full 6D phase-space characterization of the electron beam at the gun energy. Beams up to several hundreds of pC per bunch at MHz repetition rate will be characterized and compared with expected results from simulations. Testing of different photocathodes will continue also during Phases I and II.
The successful development of such an injector will dramatically impact not only the performance of future 4th generation light sources when high repetition rates (> 10 kHz) are required, but also high-repetition-rate ultrafast electron diffraction (UED) 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 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.
Phase I layout. In Phase I, scheduled to start in spring 2015 and now in procurement and installation, three pulsed 1.3 GHz accelerating sections (a modified version of the AWA cavities at Argonne National Laboratory) and one 1.3 GHz buncher cavity will be added to accelerate the beam up to about 30 MeV and compress it to the required lengths.
The APEX project is located in the ALS Beam Test Facility (BTF), and is organized in three phases. Phase 0 was dedicated to the development and characterization performance of the gun, and to the testing and characterization of different types of cathodes. It is essentially complete as of autumn 2014: 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.
Due to cost and shielding limitations, Phase II will operate in pulsed mode with a 10 Hz repetition rate rather than CW. The Phase II configuration allows us to reduce space charge forces to the level necessary to perform reliable emittance measurements, and to demonstrate the brightness performance of the gun.
FEL Undulators for LCLS-II
LBNL has a quarter century 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 naturally became another major area in which LBNL contributes to LCLS-II.
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Shown here 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. |
To Learn More…
Visit SLAC National Accelerator Laboratory’s LCLS-II site to learn more about the machine, the multi-institutional partnership that is designing and building it, and why users need its photon-beam qualities.
LBNL’s expertise in undulators has now been combined with other magnet-related areas of excellence in the Berkeley Center for Magnet Technology.
