“The side-to-side motion of electrons in a beam can be circular, elliptical, or linear, depending on the position of the Delta undulator’s magnet rows. These different motions then create circular, elliptical, or linear polarization in the light pulse.“
The 60-hour crunch: working with xray free electron lasers
Imagine your job (or dream job). Imagine that job only being able to be conducted in five places worldwide at the time of this post. Five countries, five sites. Now imagine you can only go for a few days out of the year to complete months or years worth of work.
You would imagine that there would be a sense of urgency in the way you conduct the job. You’d have to be as efficient as possible, because those precious 48 to 60 hours are the only chances you’ll get for at least a year, possibly more, to do this work. On top of that, because of the nature of this job, there’s probably a lot of competition to get to choose who comes to the site in a given year. You pour hours into creating the perfect proposal, a demonstration of why you should be granted the opportunity.
This is the nature of working with X-ray Free Electron Lasers (XFELs). As of 2022, there are five operating in the world, each involved in cutting edge chemical, physical, and biological research. They are truly sites in the world where boundaries are being pushed, and processes being made more efficient. It opens the doors to collaborations where previously there wouldn’t have been any, and so this field of science flourishes and has a strong backbone in the form of the XFEL.
In my four years, I’ve been involved in four experiments at the XFELs. This is quite numerous — during 2020 we had two runs, completely virtual. This was its own challenge, of course, and I had also just joined my laboratory group, so needless to say most of the time I had no idea what was going on. That’s okay, though. It certainly planted a seed of anticipation for my first trip in person to such an instrument.
In August of 2021 I went to Menlo Park, California, where the national lab SLAC is located (the acronym actually has a weird story — let me know if you want to hear it. The S is of dubious status right now, but the LAC stands for ‘Linear Accelerator’).
I’d never been so far from the Midwest before, and this marked the first year I ever rode a plane — at age 25, no less!
At SLAC there’s the Linac Coherent Light Source (LCLS), and this is where the X-ray laser is generated and used for experiments.
You may ask me: ‘Why not just make an X-ray laser in an academic lab?’ Some people kind of have, but not nearly to this extent. When I write a post on it, I’ll be sure to link it here. X-ray generation for the doctor’s office is very different than the X-ray generation of these XFELs. For lower energy X-rays, a small x-ray tube is fine. But for the high-energy, high-intensity physical and chemical applications, a lot longer of a 'tube' is needed. This involves the acceleration of electrons through either a ring (like the particle accelerators at CERN in Switzerland) or through a long linear path. (The ring ones are called ‘synchrotrons’, and there are far more of these than there are x-ray lasers. Cooler word, though.)
Here is an article that talks about the basics of X-ray radiation generation from the Australian Radiation Protection and Nuclear Safety Agency. Please ask if any questions arise!
Because SLAC incorporates a linear path (hence the ‘linear accelerator part of it’s name), it means that the electrons are pushed through a 2km-long tunnel, all the while creating high-energy radiation. The electrons are pushed with magnets, since electrons have a negative charge. They are pushed so fast that they begin to emit radiation in the X-ray region of the electromagnetic spectrum. They are also pulsed, and these pulses of light are very intense, able to destroy the molecules that come in contact with it. It is very dangerous to be in the room with this laser on [1].
Electromagnetic radiation spans from long-wavelength radio waves to x-rays and gamma rays on the short-wavelength/higher-energy range. The shorter the wavelength, the higher the energy associated with the radiation. UV radiation, for example, tans and burns our skin because of the high energy associated with it. But it does good too -- it creates the vitamin D in our skin that is essential for health [2].
X-rays have more energy in them than the wavelengths of light that are visible. They have unique properties, such as being able to pass through soft matter like your skin and muscles when getting an X-ray done at the doctor's office.
The scope of this operation is huge, and this size is required. It is no mystery now that you can’t just build this in your backyard, nor in an academic lab. Universities can have x-ray sources… but nothing like this. This laser is capable of creating ultra-intense, ultra-bright pulses of X-rays, which lends itself well to applications such as finding structures of proteins or dynamics of molecular motion.
In particular, my group is interested in ‘molecular movies’, and x-ray lasers can help provide some of the answers of how molecules move physically in response to light. We went to the LCLS to study vitamin B-12’s response to light, and overall it was very productive.
Yes, you can only go for a few hours out of the year, and getting in is highly competitive because of the demand for the cutting-edge technology. But if you plan those 60 hours well, you can have data to last an entire dissertation. I am currently working on analyzing data that was taken in 2017. So there is a lot of information that can be gathered, and the perk is that you get to travel!
Soon I will travel to Hamburg, Germany to do an experiment at the European XFEL. It’ll be my fifth XFEL experiment overall that I have been involved in, including collaborations. I'll keep this blog updated with the goings-on of the work and fun I'll have while there! And if you have any questions, please send an ask or message!
References
Overview of X-ray Lasers from LCLS
Sofferman, D. Journal of Chemical Physics, (2021), 154(9).
Images:
Locations of XFELs: Chemical and Engineering News, ACS
SLAC sign picture: taken by me during my visit in 2021
Overhead view of LCLS: LCLS website
View of x-ray tunnel: LCLS website
Electromagnetic spectrum: Encyclopaedia Britannica
