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World’s brightest X-ray laser illuminates molecular secrets


LCLS quadrupole magnet Credit: SLAC

In September of 2009 experiments began with an incredible machine. It doesn’t get the press attention of the Large Hadron Collider, but its practical benefits could be equal or even greater in some ways. It’s billed as ‘the world’s most powerful X-ray laser’; the importance of that isn’t in bragging rights though. It means a capability that will have a major impact on science in the years ahead.

Eadweard Muybridge was an English photographer who was becoming known for his work in the 1870s. He was asked by Leland Stanford, the former governor of California, in 1877 to settle a question of whether or not all four hooves of a racehorse left the ground while running. He answered this question to the affirmative with a single photographic image. The following year he was able to capture motion of a horse, as it ran, through a succession of images using 24 equally spaced cameras along a track. If you compare a still-shot of the animal with the motion sequence you immediately realize how much more you can learn about it, and its behavior.

This is one of the values of the Linac Coherent Light Source instrument . Because of its ‘brightness’ and speed it can capture images of atoms and molecules in real motion in a way that wasn’t possible before. Two gentlemen, critical to its construction and operation, shared their knowledge and explanations about it.

Uwe Bergmann (UB) is Deputy Director of Science, Research and Development for the project.
John Galayda (JG) is Director of Construction for the project.

Galloping Sallie

Q. 'LCLS produces pulses of X-rays more than a billion times brighter than the most powerful existing sources.' That's a description from your website. What is the significance of that scale of intensity?

A. (UB) The significance is really how short the ultra-bright LCLS pulses are. The focused x-rays of an LCLS pulse are as powerful as all the sunlight hitting the earth focused to a square centimeter. The fact that the pulses are ultra-short allows us to study ultrafast processes and also helps us deal with one of the most difficult problems in x-ray science, namely radiation damage done to the sample.

One of the most common questions I’m asked is, “how do you image something that is sure to be destroyed by x-rays?” What distinguishes the LCLS from other x-ray sources is the fact that the pulses are short enough to outrun the radiation damage, allowing us to get atomic-scale information from an intact sample such as a virus, a cell or a protein.

Q. This instrument was built from the existing linear accelerator. Does this new project essentially retire the linear accelerator from its original purpose?

A. (UB) Yes. The existing accelerator has not been used for particle physics for some years now. Currently we are only using one kilometer of the available three kilometer long accelerator. Our plan is to use the second kilometer for our new LCLS II project, which will house more x-ray lasers. Eventually we hope to use all three kilometers for research.

Q. Were all of the technologies incorporated in the x-ray laser already tested and proven prior to construction, or was some of it just a matter of having faith in the theory and engineering?

A. (JG) The theory was tested at other facilities, but only for much longer-wavelength FELs. There were no proven, measured examples of the electron gun and the undulator that met LCLS specification, before the project started. Also we had theoretical predictions about how much “damage” to the key properties of the electron beam we should expect, as a result of accelerating electrons to the highest LCLS energy of 14 GeV. Theory said the laser would work. But the SLAC linac is the only linac in the world that can make 14 GeV electrons, so the first experimental tests had to be here. We had good diagnostics for measuring almost all the critical beam properties but one (the “slice” energy spread) and confirmed they were all within specification before we attempted to turn on the FEL. The FEL itself was the best diagnostic for this last parameter. So we did not get full experimental confirmation of this theory until we operated the LCLS as a laser. It turned out that the theory is quite accurate.

Q. What proved to be the toughest technological challenges?

A. (JG) The undulator system posed many technological challenges: controlling the magnetic steering fields to the necessary precision (errors less than the earth’s magnetic field), measuring and achieving near-perfection in the electron beam trajectory (our goal was to get the electron trajectory straight to a precision of a couple of microns over a path 10-20 meters long) were all challenges we worried about. I think it is fair to say that the fact that the performance of the electron gun, which exceeded design goals by a big margin, was a major breakthrough. Normally one thinks of technological challenges as the things that stopped progress for a while. We had many challenges that could have stopped progress but by hook, crook and luck, none of them did. The most significant technical challenge that threatened the schedule was the challenge of making an aluminum vacuum chamber for the undulator. The vacuum chamber had to be extremely clean, extremely narrow (5 mm high inside), extremely straight (200 microns over 3.5 meters) and very smooth inside. After several alternatives were explored and rejected, a solution was found by making an extruded chamber and then polishing the inside by pumping abrasive slurry through the chamber. This turned out to be an extremely economical, quick-to-build solution on top of all its other good properties.

Q. Experiments began in September 2009. What would you say were the biggest accomplishments since then?

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A. (UB) There have been several papers based on LCLS science published in the top scientific journals and it is a matter of taste at this point which one is the biggest accomplishment. All of the first experiments focused on proof-of-principle studies–to be expected given that the research was being conducted using a dramatically new tool. We have learned that several of the suggested concept's actually do work, which is why the science community is so excited. Some experiments are testing grounds to develop more sensitive spectroscopic tools. We also know now that the concept of `probe-before-destroy¢ works, which will open the door for the imaging of nanocrystals and nanostructures. Furthermore, single-shot imaging of individual viruses works, but further developments are needed before we embark on creating 3D images of the bio-world.

Q. I saw it explained in one of your videos that the power of the x-ray beam obliterates the target that it's imaging. If that's true then how do you capture multiple images in order to get the effect of motion?

A. (UB) It is only for certain samples that the LCLS pulse leads to destruction. In that case multiple identical copies are needed in order to create a movie. However, there are many solid-state systems, including superconductors, semiconductors and magnetic systems, which can handle the LCLS pulses. In most cases movies are produced in a stroboscopic way where a short pulse triggers a reaction and the LCLS pulse probes it. The movie is produced by repeating this many times under varying delay between 'pump' and 'probe'.

Q. Stanford University manages a project called ' Folding@home '. It uses distributed computing, such as Internet connected home PCs and PlayStation consoles, to simulate the protein molecule folding process. Does this machine allow you to image the real-life folding process in a way to determine the accuracy of the '@home' project's simulations?

A. (UB) Protein folding is one of the most important characteristics of biological systems. When you cook an egg, you denature the protein and that’s why it turns white—this process can never be reversed. We know that the folding and unfolding of proteins is critical for their function but we know very little about how this process works. The unique properties of the LCLS, in particular its beam coherence and the intensity of its ultrashort x-ray pulses, will enable researchers to watch these processes in action.

Q. What impact do you imagine this may have on medical science?

A. (UB) An enormous impact on the fundamental understanding of medical science is expected once the imaging of nanocrystals and single molecules is demonstrated. Understanding biological systems at atomic resolution will lead to a better understanding of the underlying mechanism of how life works and hence also to breakthroughs in medicine. Numerous studies using synchrotron radiation source such as the SSRL at SLAC have already made important contributions in the field of medical science. I think this field is really just in its infancy.

Q. Is x-ray imaging the primary purpose of the LCLS?

A. (UB) We don’t know yet which techniques will turn out to be the most important ones used at the LCLS. Keep in mind that spectroscopic techniques are also forms of imaging, as they probe, for example, where electrons are in a system. To a spectroscopist like myself, this kind of data is just as beautiful as a direct image. In the end we always want to create some form of an image that reflects a detailed aspect of nature.

Q. What are some of the major experiments planned for the next ten years?

A. (UB) There is a lot of excitement across the science community, particularly in the fields of material science, chemistry, biology and atomic, molecular and optical physics. Right now the competition for obtaining experimental time at the LCLS is so great that scientists have been reluctant to speak openly about their best ideas. Ask me again in our next interview?

 

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