Back where I grew up, the old men had a saying: "It'll come in handy, even if I never use it." Ever since I had to store my own stuff, I have come to be more and more skeptical of the value of keeping old stuff. The good people at the Stanford Linear Accelerator (SLAC) have been listening to the same old men though, and some good stuff has come of it. Back in its day, SLAC, now called SLAC National Accelerator Laboratory, was king of the hill among particle accelerators. But, once past its prime, the question of what to do with all that expensive equipment was raised.

In a flash of inspiration, the SLAC researchers proposed turning it into a very bright X-ray laser that could be used by other researchers to study physics, chemistry, and biology questions. A shiny new acronym arose from the ashes of SLAC, as it became the Linac Coherent Light Source (LCLS), a type of free electron laser. Now, 18 years after it was first proposed, it has been switched on. And, much to everyone's surprise, it actually works really well.

Freeing electrons to make lasers

Let's start with free electron lasers. Here is the rule: charged particles radiate when they are accelerated. So, the antenna in your iPhone radiates by accelerating electrons along metallic strips (provided you're holding it right). Acceleration doesn't just include changes in total speed though, it also includes changing direction, so a simple current flowing around a loop will also radiate, since it's constantly changing direction.

A free electron laser takes advantage of this by sending a bunch of electrons between the poles of a series of magnets. The magnets do not change the absolute speed of the electrons, but instead push and pull the electrons around so that they undulate as they travel. (The series of magnets just so happens to be called an undulator.) As a result, the electrons radiate a continuous stream of radiation as they travel through the undulator.

This sounds pretty easy but, in fact, the free electron laser has a number of gotchas. For instance, all the electrons need to radiate in phase with each other, otherwise the light emitted by one electron gets absorbed by the others. In practice, this means that you need to bunch your electrons into groups that have the right spatial relationship with each other.

A second problem is that as the electrons radiate, they slow down—conservation of energy tells us that the energy that goes into light has to come from the electrons. So, at the start of the undulator, the electron bunch might be sitting on a crest of the wave associated with the light field. However, further down the undulator, the electrons will have lost ground compared to the light wave, and might no longer be sitting on the crest.

What this means is that the spacing of the magnets has to be very carefully managed so that, even though the electrons lose ground, they always travel between the poles of the magnet while sitting at the crest of the electromagnetic wave. This ensures that the light emitted from every part of the undulator is in phase, a characteristic of laser light.

Getting the SLAC out

To get to shorter wavelengths requires higher energy electrons that travel together to form a very dense and collimated clump. These bunches are very difficult to produce, which is where SLAC comes in. SLAC was a linear accelerator that was designed to send electrons to around 50GeV over a length of some 3.5km and then collide them with other particles for high energy physics experiments.

Because of this, it has a beautifully stable radio-frequency accelerator system, high quality vacuum equipment, and all the service tunnels ready built. In other words, most of the expensive bits of equipment and large infrastructure requirements for a short wavelength free electron laser were right there, gathering dust.

That is not to say that it was all roses and chocolate. The LCLS researchers didn't need or want 50GeV electrons, so they only use the last third of the beam line. Furthermore, the free electron laser needed really stable, highly intense, and well collimated electron bunches, so a new photoemitter that emitted shorter bunches was made, and a couple of sections that compressed the electron bunches were introduced into the accelerator path.

The basic set up was a photocathode—a metal surface that emits electrons when exposed to light—followed by a nine meter radio-frequency accelerator that pushed the electron bunch energy up to 250MeV. At this energy, the electrons have a fair bit more mass, so they have to work harder to push each other away. This allows the researchers to compress the electron bunch together.

At this point, the really serious acceleration gets under way, with 320m of hardware getting the electrons up to just north of 4GeV. This is followed by another beam compression section that bunches the electrons up as tightly as possible. Finally, another 0.5km of accelerator allows the researchers to tune the energy to between 3.5 and 15GeV.

These electron bunches are then put through a 130m long undulator section. The point of such a long undulator section is to avoid having to use mirrors to provide feedback as you would in a normal laser. The light field that is created at the start of the undulator starts off in competition with a whole bunch of other possible light fields. But the electrons have a small preference for one light field over all others, so more electrons radiate into that field than any of the others.

Now, something really cool happens. Because that field is a little more intense than all the others, even more electrons prefer to radiate into that field instead of the others. This stimulated emission process leads to a dominant light field with a particular wavelength, direction, and phase. However, this takes a certain amount of length to achieve—hence a normal laser has mirrors, and the LCLS has an undulator section that is a football pitch long.

Putting SLAC to work

Back at the start of the article, I said that everyone was surprised that it worked. It wasn't that people weren't sure it would work at all, but this is a hugely complicated machine. What was expected was that once it was turned on, it would take a certain amount of fiddling to get everything lined up correctly, and to get the beam energy exactly right—you know, all the details of real experiments. But, no, they turned on the electron beam, and it looked right. They then started inserting undulator sections and saw amplification. The whole thing worked the first time.

What do we get from LCLS? You get laser light at wavelengths between 2.2nm-0.6nm (visible light starts at 400nm) at energies of around 2.5mJ per pulse. The pulses themselves are very short, at around ~100fs in duration. This is energetic enough that, at the exit of the undulator, it will melt all materials. To cope with this, the researchers place the first optics and shutters some 50m downstream from the undulator.

They already have a switch yard to send the beam to different experimental stations. The beam is available for about 120 hours per week with a number of stations ready to go and users already queuing up to get time.

Nature Photonics, 2010, DOI: 10.1038/NPHOTON.2010.176

Listing image by Fermilab