Researchers take important step toward developing cavity-based X-ray laser technology

Researchers have introduced an necessary step within the growth of a next-gen know-how for making X-ray free-electron laser pulses brighter and extra secure. They used exactly aligned mirrors fabricated from high-quality artificial diamond to steer X-ray laser pulses round an oblong racetrack inside a vacuum chamber.

Setups like these are on the coronary heart of cavity-based X-ray free-electron lasers, or CBXFELs, which scientists are designing to make X-ray laser pulses brighter and cleaner—extra like common laser beams are right now.

“The profitable supply of a cavity-based X-ray free-electron laser will mark the beginning of a brand new technology of X-ray science by offering an enormous leap in beam efficiency,” stated Mike Dunne, director of the Linac Coherent Mild Supply (LCLS) X-ray laser on the Division of Vitality’s SLAC Nationwide Accelerator Laboratory, the place the work was carried out.

“There are nonetheless many challenges to beat earlier than we get there,” he stated. “However demonstration of this primary built-in step may be very encouraging, displaying that we now have the method and instruments wanted to maintain excessive cavity efficiency.”

The SLAC analysis staff described their work in a paper revealed in Nature Photonics. Early outcomes had been so encouraging, they stated, that the lab is already working with DOE’s Argonne Nationwide Laboratory, its longtime collaborator on the topic, to design and set up the subsequent, greater model of the experimental cavity system within the LCLS undulator tunnel.

Making X-ray laser pulses extra laser-like

Regardless of their identify, X-ray laser pulses should not but totally laser-like. They’re created by making accelerated electrons wiggle by way of units of magnets known as undulators. This forces them to offer off X-rays, that are formed into highly effective pulses for probing matter on the atomic scale. At LCLS, pulses arrive 120 occasions a second, a price that may quickly enhance to 1,000,000 occasions per second.

However due to the best way X-ray laser pulses are generated, they fluctuate in depth and comprise an unpredictable mixture of wavelengths. This creates what scientists name “noise,” which muddles their view of samples they're probing.

The introduction of a cavity has been proposed to beat this downside, adopting the method utilized by typical optical lasers. Cavities enhance the coherence of lasers by preferentially choosing gentle of a single wavelength whose peaks and troughs line up with one another. However the mirrors that bounce gentle round in common laser cavities received’t work for X-ray laser pulses—all you'll get can be a smoking gap in your mirror the place the X-rays drilled by way of.

The thought of utilizing crystals—and, extra lately, artificial diamond crystals—as mirrors to easy and assist amplify X-ray pulses inside a cavity has been round for a very long time, stated Diling Zhu, who led the experimental staff with fellow SLAC scientist Gabriel Marcus.

“The query was tips on how to produce diamond mirrors of excessive sufficient high quality and tips on how to line them up with sufficient precision to steer the X-rays across the cavity,” Zhu stated. “Ideally, in our case, the cavity would additionally match into the lengthy, slim tunnel that homes the LCLS undulators.”

Extra challenges and improvements embody discovering one of the best ways to take X-rays out of the cavity to allow them to be used for experiments and to optimally cool the mirrors, if wanted.

A barbell-like setup

The SLAC cavity venture began about 5 years in the past with a number of hallway conversations, Zhu stated. These led to a Laboratory Directed Analysis and Growth grant from the SLAC director to construct the setup used on this examine in an LCLS experimental hutch.

“What’s distinctive about this experiment is the big scale at which it was achieved. It’s practically 50 occasions greater than some other model I’ve seen revealed,” stated Rachel Margraf, a Stanford graduate pupil and one of many researchers who co-designed and carried out the experiment and analyzed the outcomes.

“The larger the cavity is,” she stated, “the tighter the alignment tolerances are, and the scientific neighborhood had been skeptical that these tolerances could possibly be achieved.”

The experimental setup consists of two boxy vacuum chambers that comprise the cavity elements. They’re related by two beam pipes, that are additionally saved underneath vacuum. From the facet, the entire thing resembles a 30-foot-long barbell.

Every cavity chamber homes two diamond mirrors, and every mirror is mounted on a set of 4 motors that exactly regulate its place and angle to the beam. The mirrors steer X-ray pulses by way of the beam pipes and from one mirror to the subsequent.

Crafting the proper diamond mirror

Synthesizing, choosing and shaping the diamond mirrors was a giant effort in itself.

The diamonds had been ready by Kenji Tamasaku, staff chief of the XFEL division on the RIKEN SPring-8 Heart in Japan and a world authority on diamonds for X-ray analysis, in collaboration with an trade companion.

Rising diamond crystals which might be pure sufficient for X-ray analysis is difficult, Tamasaku stated, as a result of they should be grown at excessive temperatures and pressures the place the slightest change in situations can disrupt the crystal progress.

The staff first used X-ray microscopes from SPring-8 and the Stanford Synchrotron Radiation Mild Supply (SSRL) at SLAC to fastidiously study every crystal and cherry-pick those with the fewest defects of their crystal construction. Then they recognized areas inside these crystals that had been defect-free for processing into mirrors.

“The standard of pure diamonds can't compete with that of diamonds used within the current examine,” Tamasaku stated.

Close to-perfect bits of diamond crystal had been reduce with lasers, first into slabs after which into S shapes about one-fifth of an inch lengthy that had been polished to a excessive shine, a course of first developed by specialists at Argonne. The mirrors have tags that may be clamped onto the experimental equipment with out placing pressure on the mirror itself.

Profitable outcomes

The aim of this experiment was to see how lengthy and the way effectively X-ray laser pulses might flow into contained in the cavity. LCLS laser pulses entered the setup 120 occasions per second by way of a precision diamond grating crafted on the Stanford campus. They hit every of the mirrors in flip, finishing as much as 60 laps—every about 46 ft lengthy—earlier than dissipating.

The researchers stated the round-trip journey effectivity throughout the cavity setup was greater than 96%—near the theoretical mirror efficiency restrict, and greater than ample to help a high-quality X-ray laser beam.

Finally, the objective is to retailer and flow into X-ray pulses within the cavity after which ship them by way of the undulator to accompany the electron beam because it wiggles by way of the magnets. Repeating this cycle 10 to 100 occasions ought to create X-ray laser beams which might be as coherent and secure as right now’s optical laser beams, Zhu stated. This can develop into potential with the completion of LCLS upgrades that considerably enhance the power and repetition price of its X-ray laser pulses.