SLAC Today logo

From the LCLS Directorate: Commissioning Status

(Photo - Dale Knutson)
Associate Lab Director Dale Knutson. (Photo by Brad Plummer.)

The Linac Coherent Light Source is presently in its third phase of electron beam commissioning. The first phase, from April through August 2007, focused on the new electron injector and diagnostics needed to meet the LCLS beam requirements. The second phase of commissioning for this new generation of lightsource encompassed the full one-kilometer linac—the last third of the two-mile SLAC linac—that will carry the LCLS electron beam to the Undulator Hall. (See the LCLS interactive map for the layout.) The final commissioning phase began in November 2008, and is now focused on the new electron transport line from the linac through the undulators to the electron beam dump at the end of the line. Concurrently, the X-ray front-end systems, just downstream of the electron dump, are being installed and will be ready for commissioning in May 2009. A dedicated team at SLAC and our collaborators at Argonne National Laboratory and Lawrence Livermore National Laboratory deserve all of the credit for the very successful work to date.

During commissioning phase I, the first electrons from the new radio-frequency photocathode gun, which was designed and built at SLAC, were observed on April 5, 2007. The beam was quickly established to the nominal injector energy (250 MeV) in the SLAC linac. Over the next five months all injector systems were commissioned and the beam accelerated to as high as 16 GeV using the last third of the SLAC linac. This range of energy levels meets the minimum performance expected for user operations and the injector has performed extraordinarily well.

Phase II of commissioning, from December 2007 through August 2008, was focused on precise control of the radio-frequency klystrons that drive the linac, and demonstration of acceptable beam performance through the full linac, including tightly-packed electron bunches. Careful measurements of beam quality showed good performance and validated the accelerator design.

A laser heater system was fully installed on December 10, 2008, and was up and running successfully just two hours after installation was complete. The laser heater adds a beam "heating" effect that helps to stabilize the electron bunches. Though functioning well, the heater does not create all of the stability hoped; however, the small level of remaining instability appears to be low enough not to break up the beam significantly.

(Photo - Commemorative LCLS
The commissioning team earned the nickname "Two-shot" for their successful adjustments to beam steering December 13, 2008. (Photo by Shawne Workman.)

Phase III of commissioning is now well under way, focusing on the transport line that carries electrons from the linac through to the undulators, and the 132-meter permanent magnet undulator that will jog the electrons to generate X-rays. Beam was transported through the 340-meter linac-to-undulator transport line on December 13, 2008, in just ten minutes, and intentionally stopped on the insertable tune-up dump just in front of the undulator. After about one hour of preliminary measurements and small adjustments to the beam trajectory, the stopper was opened. The first shot of electrons was transported only about 25 meters into the undulator, where it was lost. But the saved trajectory information was then used to correct the steering, and the second shot was successfully transported all the way through the undulator to the main dump.

After this happy milestone event, the real work began.

The commissioning work since December has concentrated on checking the many new systems and their controls and interlocks, including setup of the collimation system to focus the electron beam, commissioning of the new wire-scanners used to quantify beam brightness, careful corrections of the beam trajectory, checkout of the new machine protection systems, and precision alignment of the undulators. A new method of beam-based alignment has now been successfully tested and is able to establish the required "few-micron" alignment of the beam through the undulator. At present only one undulator magnet is mounted on its girder. A test run of the beam through the undulator resulted in spontaneous X-rays. At least 18 undulators (of 33 possible) will be installed in early March, providing the first possibilities for free-electron laser light soon after.

(Photo - an LCLS undulator)
The first 3.4-meter long undulator segment installed on one of the 33 girders. (Photo courtesy of the LCLS commissioning team.)

The next phase of commissioning will start in May 2009 and will switch the focus from electrons to photons. By that time, the X-ray instruments for diagnostics and control will be installed in the Front End Enclosure, just beyond the electron dump, and will be ready for commissioning. The present measured electron beam brightness appears to be adequate to produce free-electron laser X-rays as soon as the Front End Enclosure is ready for beam.

Meanwhile, the X-ray transport systems are being extended into the Near Experimental Hall and the first LCLS X-ray experimental instruments are being installed. (See "Where the LCLS Ends: The AMO Instrument" for the first.) The Near and Far Halls are designed to contain six major X-ray instruments, optimized for a wide range of scientific experiments. This new machine will introduce a unique new tool for X-ray studies, with unprecedented peak brightness, 1-Angstrom spatial resolution, and femtosecond-scale temporal resolution. The world's first free-electron laser X-ray light is expected in the summer of 2009, and initial experiments will begin immediately to make use of this revolutionary new light source.

—Dale Knutson and the LCLS commissioning team
SLAC Today, March 2, 2009