Tuesday, May 25, 2010

The brightest x-ray source in the Universe

What do you do with an aging linear accelerator? You use it to make an ultrafast free-electron hard x-ray laser, that’s what. And that laser is now the brightest x-ray source in the universe (so far as we know) and the only hard x-ray laser in the world. Where? At the SLAC National Accelerator Laboratory. At Stanford. It’s a national lab. Nice place, too.

Government labs love acronyms of acronyms, so the laser is called the Linac Coherent Light Source, or LCLS. Last month I was able to tour the new laser facility with the local IEEE Photonics Society chapter. It brought back flashbacks of grad school, but without the poverty. It’s a plush lab. For you laser geeks, here are some stats:

Pulse energy ~ 1 mJ
Pulse duration 5 to 200 fs
Peak power 10 GW
Average power 1 W
Repetition rate 120 Hz
Wavelength 0.1 to 10 nm
Electrical power bill per month: $ 1,000,000
Temperature control in beam delivery system: 0.01 degrees
Laser design: Self-amplified spontaneous emission (ASE)

The laser doesn’t sound that impressive, until you consider the infrastructure to build it. About 3 km of accelerator and beam delivery. A massive electrical bill to run the magnets and the air conditioning. A tunnel underground to keep the temperature stable (they used the same company to dig it that digs wine cellers in Napa Valley). The experiments are all done by remote control, just to ensure radiation safety. Not that they need to, with all the shielding and massive ground wires everywhere.

There is a great animation video that shows how it works, here. The electrons are accelerated to relativistic energies, then run through the free-electron laser, creating coherent hard x-rays. The rest is beam delivery. So, it’s just like your benchtop custom-built laser, but on a cosmic scale.

And what do they want to do with it? For one, it’s like a very fast x-ray camera that exists nowhere else on earth. It’s so fast, it can take diffraction images of crystals before the materials degrade from the x-rays. It can do pump-probe experiments on atoms and nuclei. It can reproduce the harsh environments in the universe by imparting a lot of energy into a very small volume. The SLAC SCLS aims a stream of x-rays at a small spot size, creating a huge intensity. (The new Livermore NIF laser also creates x-rays at the target, but they go in all directions. The NIF laser produces higher energy pulses, for fusion reaction.)

You can drive over the accelerator on Interstate 280, just near Sand Hill Road, near Stanford’s radio telescope and open cattle range. But you can’t just drive onto the SLAC national lab campus. There’s a guard. And anyway, the laser and beam delivery is all underground.

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