The Linac Coherent Light Source (LCLS) at the Department of Energy (DOE)’s SLAC National Accelerator Laboratory is undergoing a high-energy upgrade known as LCLS-II-HE. A multidisciplinary team from the Accelerator Technology & Applied Physics (ATAP) and Engineering Divisions at Lawrence Berkeley National Laboratory (Berkeley Lab) is crucial to this upgrade. They will deliver nine new magnetic arrays, called undulators, and the components needed to retrofit 21 existing undulators for the LCLS-II soft X-ray line.

The high-energy upgrade to LCLS-II represents a significant advancement in X-ray technology. It will provide researchers with a high-resolution tool for investigating ultrafast, atomic-scale processes with unprecedented precision. This promises to further fundamental scientific research and help address important questions in energy storage, materials science, quantum physics, biology, and many other areas.

The LCLS-II generates both low-energy (“soft”) and high-energy (“hard”) X-rays from an electron beam, producing one million X-ray pulses per second—8,000 times more than its predecessor; it is the world’s most powerful X-ray free-electron laser. The facility achieved its “first light” in September 2023, culminating over a decade of collaborative efforts by scientists, engineers, technicians, and operations staff across the Department of Energy (DOE) and multiple institutional and private-sector partners. Berkeley Lab played a leading role in several critical components of the LCLS-II, which included overseeing the development, fabrication, and testing of undulators, creating a state-of-the-art injector that supplies electrons to the accelerator, and implementing a system for controlling the electron beam.

“Under Diego Arbelaez’s leadership, the engineering team is building upon the engineering excellence and robust manufacturing and QA processes established for the LCLS-II undulators and is on track to deliver successfully for LCLS-II-HE,” says the Engineering Division Director and the Lab’s Chief Engineer, Daniela Leitner.

Boosting X-ray science and technology

Once completed, LCLS-II-HE will provide double the peak X-ray energy and deliver a 3,000-fold increase in average X-ray brightness for its hard X-ray line. The Berkeley Lab team is collaborating with colleagues at SLAC to upgrade the soft X-ray undulator, enabling both X-ray lines to be used simultaneously with the new, higher-energy beam.

Undulators are a series of magnetic elements that create periodic alternating magnetic fields along the accelerator’s electron beamline. They are crucial components of the LCLS-II and are located in a section of the accelerator facility known as the “Undulator Hall.” This area features hundreds of meters of magnets with alternating polarity that cause the electrons to oscillate or “wiggle” back and forth, resulting in the emission of X-rays. Each electron that passes through amplifies these X-rays, which are synchronized to produce coherent, laser-like light.

According to Diego Arbelaez, a staff scientist in Berkeley Lab’s Engineering Division and the Lab’s senior team leader for LCLS-II-HE, the new and refurbished undulators for the soft X-ray line will ensure that as the maximum energy of the accelerator’s electron beam increases from 4 giga-electron volts (GeV) to 8 GeV—extending the photon energy range to at least 12.8 keV at up to 1 MHz repetition rates—the LCLS-II-HE retains its ability to continue generating lower-energy X-rays effectively.

“The soft X-ray line requires a longer oscillation period for the new undulators to continue producing X-rays at lower energies, specifically around 250 eV,” he explains. “To meet this requirement, we are constructing magnetic components for the undulators with oscillation periods of 56 mm, longer than the current 39 mm.”

However, increasing the oscillation period leads to higher magnetic fields, which he says “increase the mechanical forces acting on the undulator components, and so had to be factored into the updated design.”

New and improved

When evaluating the updated design, the researchers discovered they could reuse many mechanical components from the existing 21 undulators with only minor modifications. However, additional undulators were required to extend the oscillation period while maintaining the same number of oscillations. As a result, the team constructed nine new undulators in addition to the original 21.

“We gained valuable insights while working on LCLS-II,” says Kelly Hanzel, a mechanical engineer in the Engineering Division, who is responsible for managing external vendors and coordinating with technical leads and quality assurance engineers on LCLS-II-HE, “which we integrated into every stage of the process for the high-energy upgrade.”

Hanzel noted one such insight related to the magnetic modules used in LCLS-II’s soft and hard X-ray undulators, which had slightly different designs. “Feedback from SLAC indicated that the hard X-ray undulators were easier to adjust, so we implemented the original design of the hard X-ray undulators for both the new and retrofitted soft X-ray undulators for LCLS-II-HE.”

She continued, “We also included extra encoders to help SLAC track the undulator centerline, which was not part of the original design for LCLS-II.”

Furthermore, when constructing undulators, random fluctuations in the permanent magnet fields and mechanical and positional inaccuracies can lead to errors in the oscillating magnetic fields. These errors, says Arbelaez, must be corrected to meet the required magnetic field quality.

“We simplified the tuning process by optimizing the magnetic design to minimize saturation at the poles, where the magnetic field is strongest. This adjustment made tuning significantly easier while still meeting the design specifications.”

The team will deliver all 30 undulators to SLAC by spring 2025, with the LCLS-II-HE scheduled for completion in 2030. Experiments at the upgraded facility could begin as early as 2027.

Higher energy requires greater control

The Berkeley Accelerator Controls and Instrumentation (BACI) Program at ATAP and the Engineering Division also contribute to the LCLS-II-HE control systems. Collaborating with colleagues at SLAC, they upgraded hardware, including electronic control circuits called field-programmable gate arrays, firmware for low-level radiofrequency control, and superconducting radiofrequency resonance control designs.

BACI Program Head and Staff Scientist Qing Ji says that in addition to developing the electronics hardware and firmware, the team also created testing procedures “to ensure high reliability, performance, functionality, and compatibility within the LCLS-II-HE framework.” The team also supports features unique to the LCLS-II-HE, including beam synchronous acquisition, multi-chassis intercommunication, fail-safe remote firmware upgrades and management, adaptive feedforward control algorithms, and integration with the facility’s Experimental Physics and Industrial Control System.

Researchers from ATAP’s Advanced Modeling Program (AMP) are providing beam modeling and diagnostics. Ji Qiang, a senior scientist at AMP, leads this effort, which involves end-to-end simulations of electron beam generation, acceleration, and transport throughout the accelerator system.

“The Berkeley Lab team is doing an outstanding job delivering the essential magnetic systems for the LCLS-II-HE,” said ATAP Division Director Cameron Geddes. “This follows their significant contribution to LCLS-II, which achieved first light last year. The high-energy upgrade will ensure that the facility remains at the forefront of scientific research by advancing X-ray science and technology. The project further highlights the importance of multi-institutional collaboration and teamwork.”

Over the years, LCLS-II has engaged over 30 scientists, engineers, technicians, and operations staff. We thank all those who contributed to the project.

The Department of Energy’s Office of Science, Office of Basic Energy Sciences funds the research presented here.

For more information on ATAP News articles, contact caw@lbl.gov.