Berkeley Lab Delivers Injector That Will Drive X-Ray Laser Upgrade

Every powerful X-ray pulse produced for experiments at a next-generation laser project, now under construction, will start with a “spark” – a burst of electrons emitted when a pulse of ultraviolet light strikes a 1-millimeter-wide spot on a specially coated surface.

A team at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) designed and built a unique version of a device, called an injector gun, that can produce a steady stream of these electron bunches that will ultimately be used to produce brilliant X-ray laser pulses at a rapid-fire rate of up to 1 million per second.

The injector arrived Jan. 22 at SLAC National Accelerator Laboratory (SLAC) in Menlo Park, California, the site of the Linac Coherent Light Source II (LCLS-II), an X-ray free-electron laser project.

Getting up to speed
The injector will be one of the first operating pieces of the new X-ray laser. Initial testing of the injector will begin shortly after its installation.

The injector will feed electron bunches into a superconducting particle accelerator that must be supercooled to extremely low temperatures to conduct electricity with nearly zero loss. The accelerated electron bunches will then be used to produce X-ray laser pulses.

Scientists will employ the X-ray pulses to explore the interaction of light and matter in new ways, producing sequences of snapshots that can create atomic- and molecular-scale “movies,” for example, to illuminate chemical changes, magnetic effects, and other phenomena that occur in just quadrillionths (million-billionths) of a second.

This new laser will complement experiments at SLAC’s existing X-ray laser, which launched in 2009 and fires up to 120 X-ray pulses per second. That laser will also be upgraded as a part of the LCLS-II project.

The injector gun project teamed scientists from Berkeley Lab’s Accelerator Technology and Applied Physics Division with engineers and technologists from the Engineering Division in what Engineering Division Director Henrik von der Lippe described as “yet another success story from our longstanding partnership – (this was) a very challenging device to design and build.”

“The completion of the LCLS-II injector project is the culmination of more than three years of effort,” added Steve Virostek, a Berkeley Lab senior engineer who led the gun construction. The Berkeley Lab team included mechanical engineers, physicists, radio-frequency engineers, mechanical designers, fabrication shop personnel, and assembly technicians.

“Virtually everyone in the Lab’s main fabrication shop made vital contributions,” he added, in the areas of machining, welding, brazing, ultrahigh-vacuum cleaning, and precision measurements.

The injector source is one of Berkeley Lab’s major contributions to the LCLS-II project, and builds upon its expertise in similar electron gun designs, including the completion of a prototype gun. Almost a decade ago, Berkeley Lab researchers began building a prototype for the injector system in a beam-testing area at the Lab’s Advanced Light Source.

That successful effort, dubbed APEX (Advanced Photoinjector Experiment), produced a working injector that has since been repurposed for experiments that use its electron beam to study ultrafast processes at the atomic scale. Fernando Sannibale, Head of Accelerator Physics at the ALS, led the development of the prototype injector gun.

“This is a ringing affirmation of the importance of basic technology R&D,” said Wim Leemans, director of Berkeley Lab’s Accelerator Technology and Applied Physics Division. “We knew that the users at next-generation light sources would need photon beams with exquisite characteristics, which led to highly demanding electron-beam requirements. As LCLS-II was being defined, we had an excellent team already working on a source that could meet those requirements.”

The lessons learned with APEX inspired several design changes that are incorporated in the LCLS-II injector, such as an improved cooling system to prevent overheating and metal deformations, as well as innovative cleaning processes.

“We’re looking forward to continued collaboration with Berkeley Lab during commissioning of the gun,” said SLAC’s John Galayda, LCLS-II project director. “Though I am sure we will learn a lot during its first operation at SLAC, Berkeley Lab’s operating experience with APEX has put LCLS-II miles ahead on its way to achieving its performance and reliability objectives.”

Mike Dunne, LCLS director at SLAC, added, “The performance of the injector gun is a critical component that drives the overall operation of our X-ray laser facility, so we greatly look forward to seeing this system in operation at SLAC. The leap from 120 pulses per second to 1 million per second will be truly transformational for our science program.”