LCLS-II’s ‘First Light’ Revolutionizes X-ray Science

LCLS-II compressor skids. Credit: TJNAFThe LCLS-XFEL at SLAC National Accelerator Lab has launched its upgraded version, LCLS-II, generating its first X-rays. This advancement is set to revolutionize research, offering unparalleled capabilities for studying quantum materials with remarkable precision. Scientists worldwide are queued up to explore various applications, from improving computing and communication technologies using quantum materials to understanding fleeting chemical reactions for sustainable industries and clean energy. Additionally, the upgrade enables investigations into biological molecules' functions for pharmaceutical advancements and opens doors to entirely new scientific realms by studying the world's fastest timescales. 

“This achievement marks the culmination of over a decade of work,” said LCLS-II Project Director Greg Hays. “It shows that all the different elements of LCLS-II are working in harmony to produce X-ray laser light in an entirely new mode of operation.” 

Reaching “first light” is the result of a series of key milestones that started in 2010 with the vision of upgrading the original LCLS and blossomed into a multiyear billion-dollar project involving thousands of scientists, engineers and technicians across DOE, as well as numerous institutional partners. 


The superconducting linear accelerator (Linac) relies on cryogenic cooling to hit temperatures close to absolute zero, around 2.0 K. This temperature allows niobium, a vital material for RF (radio frequency) cavities, to lose all electrical resistance, significantly cutting down energy loss during acceleration and boosting efficiency. By keeping the cavities at 2.0 K, they operate below niobium's critical temperature, enabling superconductivity and high accelerating gradients with minimal energy loss, unlike copper-based accelerators. Liquid helium in cryogenic cooling maintains the cavities at this critical temperature for optimal performance.

At SLAC's LCLS-II, running both the superconducting and copper-based accelerators in tandem is groundbreaking. The copper accelerator operates at higher energies, complementing the superconducting one by providing beams with different features such as higher energy levels and intense X-ray pulses. Cryogenics is pivotal in enabling the superconducting accelerator for continuous-wave (CW) operation, vastly broadening the spectrum of possible experiments. 

Transfer lines and distribution boxes Credit: SLACCryomodules-The cryomodules used in the LCLS-II project are a marvel of engineering, designed with high-purity niobium for the superconducting RF (SRF) cavities. These superconducting cavities operate at remarkably low temperatures, around 2.0 K. The 1.3 GHz cryomodules house eight 9-cell SRF cavities cooled with superfluid helium at this critical temperature, enabling their efficient operation and exceptional performance. “The collaboration between Fermilab, Jefferson Lab and SLAC was pivotal in the successful design and construction of the cryomodules. Fermilab's expertise in engineering cryomodules, coupled with Jefferson Lab's contributions in manufacturing a significant portion of these modules, was essential in bringing LCLS-II to fruition," remarks LCLS-II-HE Project Technical Director Marc Ross. 

Cryoplant, Infrastructure and Process

The cryogenic system at LCLS-II operates through a complex infrastructure featuring two large helium cryoplants delivering substantial cooling power. Each cryoplant comprises a 4.5 K refrigerator coupled with a 2.0 K system capable to deliver 4.0 kW of isothermal refrigeration at 2.0 K. The 4.5 K refrigerators use oil-flooded screw compressors, liquid nitrogen pre-cooling and four stages of cryogenic turbo-expander to produce liquid helium. The 2.0 K system consists of a train of five cryogenic compressors allowing the cavities to operate at 31 mbar and 2.0 K. The supporting infrastructure includes a 1,900-square-meter building, housing the critical cryoplant’s gear and necessary systems. An electrical switchyard delivers the 10 MW required to power the screw compressors, and five cooling towers deliver the 1,200 m3/h required to evacuate the compression heat. 

Commissioning and Operation

The LCLS-II Linac's commissioning began with a systematic cooling-down process in March 2022, carefully considering cooling speed, gradient and cryoplant capacity. Remarkably, within just five days, the cool-down was achieved, a significant milestone in the project. Designed for year-round operation, the LCLS-II allows weekly maintenance called Planned Accelerator Maintenance (PAM), lasting 1 to 3 days for Linac maintenance, excluding cryoplant shutdowns or Linac warming. Until mid-2026, the cryoplant is expected to run continuously, pausing Linac operations for about a year to install 23 additional cryomodules for the LCLS-II HE (High Energy) Project. 

Beyond 2027, the two cryoplants will support the LCLS-II HE Linac. Major cryoplant maintenance will then happen every three years or longer. During maintenance, the Linac will be supported at reduced capacity by a single cryoplant, ensuring continual operation of the cryogenic components. 

Safety and Operation

Safety measures in the LCLS-II project focus on cryogenic operations, tackling risks like pressure hazards, cold gas exposure and potential oxygen deficiency. The engineering design meets ASME pressure codes, ensuring safety. All gas relief (vents and relief valves) are connected to exhaust pipes routed outside buildings to reduce oxygen deficiency hazard (ODH). All buildings are equipped with oxygen sensors connected to an oxygen deficiency monitor that will alarm if oxygen level decreases. SLAC maintains strict safety protocols for high voltage and industrial safety. Also automation and machine protection interlocks are widely implemented for efficient and safe operation. 

The launch of its upgraded version, LCLS-II, at SLAC National Accelerator Lab marks a groundbreaking advancement, generating its inaugural X-rays and promising a revolutionary leap in research with unparalleled capabilities for studying quantum materials with remarkable precision. Credit: SLACImpact-LCLS-II signifies a pivotal leap in X-ray science. It enables scientists to capture atomic-scale details of molecules, atoms and electrons in unprecedented ways. “This upgrade to the most powerful X-ray laser in existence keeps the United States at the forefront of X-ray science, providing a window into how our world works at the atomic level,” Secretary of Energy Jennifer M. Granholm notes. While Asmeret Asefaw Berhe from the DOE praises its potential impact on fundamental science, clean energy and national security through quantum information science. “I really look forward to the impact of LCLS-II and the user community on national science priorities,” she adds. 

The profound impact of LCLS-II spans diverse scientific domains. Its capabilities promise breakthroughs in quantum materials, ultrafast computing, sustainable manufacturing and materials science. By capturing atomic-scale snapshots of reactions and creating 'molecular movies,' LCLS-II will revolutionize understanding in biology, chemistry and physics, attracting researchers globally for a wide array of experiments in the near future. Director Mike Dunne emphasizes that this facility, offered at no cost to users, is set to drive a revolution across academic and industrial sectors, ushering in a wave of innovation and discoveries. 

“Experiments in each of these areas are set to begin in the coming weeks and months, attracting thousands of researchers from across the nation and around the world. LCLS-II is going to drive a revolution across many academic and industrial sectors. I look forward to the onslaught of new ideas – this is the essence of why national labs exist.”

Image 1: LCLS-II compressor skids. Credit: TJNAF

Image 2: Transfer lines and distribution boxes Credit: SLAC

Image 3: The launch of its upgraded version, LCLS-II, at SLAC National Accelerator Lab marks a groundbreaking advancement, generating its inaugural X-rays and promising a revolutionary leap in research with unparalleled capabilities for studying quantum materials with remarkable precision. Credit: SLAC

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