Kyle Took a Bullet for Me: Free Electron Lasers, Part 1

One of the complaints that could be made about my blog-- if I had any readers to complain in the first place-- is that I don't blog about physics. Well, that's about to change*.

This past semester I took a course in laser sensor technology, with a focus on the industrial applications of lasers. The professor of the course, Dr. Matthew Reid, has developed technology that uses photonic radiation in the terahertz spectrum to scan pieces of wood and measure their density, moisture content, fibre structure, etc.. This info can then be used to determine the quality of that wood. His work has attracted attention from companies like Boeing, as well as local and provincial media. I only mention this to give background on why the course is being offered in the first place-- though being taught by a minor celebrity is worth mentioning as well.

Anyway, one of the course requirements involved writing a report and doing a presentation on some new laser technology which can be applied to industry. I gave my presentation on Wednesday the 9th, and I won't get my mark on the presentation and report until Monday the 14th. Still, I thought I'd blog about the technology that I looked at-- the free electron laser.

The difference between a conventional laser and a free electron laser, or FEL, boils down to how each device generates light. All lasers rely on a "gain medium" to produce light. In conventional lasers, the gain medium consists of some energized material that emits photons through a process called "stimulated emission." See the picture below(Source)

Light emitted in this manner is coherent, that is, all the photons have pretty much the same frequency(the photons can never all be at one and only one frequency-- this violates the laws of quantum mechanics-- but the spread or "bandwidth" is relatively narrow). This is what differentiates laser light from normal light.

FEL's also generate coherent light, but with a process completely different from stimulated emmission. First, some backgorund. The laws of electromagnetism say that whenever a charged particle, like an electron, is accelerated, it will emit a photon. One way to accelerate an electron is to use a magnetic field. Any electron travelling through a magnetic field B with some velocity v will undergo an acceleration a(using B to refer to Magnetic fields is just one of the many useful conventions we physicists have adopted). The magnitude of the acceleration-- and hence, the frequency and intensity of light emitted-- depends both on the velocity v and the magnetic field B, as well as their relative directions(for best results, v and B should be perpendicular; it doesn't work at all if v and B are parallel). In other words, if we can control the magnetic field strength and the speed electrons, we can determine what kind of light is emitted.

One can get high intensity light when the electron speed v is very, very close to the speed of light. Light produced this way is known as "synchrotron radiation", because it is most commonly generated by synchrotron accelerators that are used to push electrons to near-light speeds. To get even higher intensities, the ultrafast electrons are forced through an insertion device, which subjects the electrons to an oscillating magnetic field. In other words, the electrons are "wiggled" to produce light. See below for a picture(soruce)

The wiggly yellow line is the electron stream. The cones coming out of the wave's peaks and troughs are the emitted photons. As I mentioned before, the light produced this way is coherent, so it can be used to produce laser light.

So why use this process in the first place? There are a few reasons.

Remember when I said that the frequency and power of the light emitted this way depends on the electron velocity and magnetic field strength? Well, the magnetic field strength of the insertion device can be controlled by outside operator. This means that the frequency of the laser can be adjusted or "tuned," along an entire spectrum, depending on the laser design. The frequency of light emitted by conventional lasers, on the other hand, depends on the material used in the gain medium, and cannot be changed. This gives the FEL a huge advantage over conventional lasers.

Using this process also allows light to be generated at frequencies that could not be reached by conventional means. For example, x-ray laser light can only be produced by FELs. This type of lasing has been demonstrated at the Spring-8 accelerator in Japan(here's the link).

As well, the laser light is very intense. The Jefferson Lab FEL, for instance, produces light at an average intensity of 15000 joules per second (here's the link to Jefferson).

Finally, FEL's are able to generate very short width laser pulses at a very high rate. What does this mean? Well, many laser applications depend on the ability to generate very brief bursts of laser light with extreme intensity, as opposed to a constant stream of photons. By using pulses instead of a continuous photon stream, one can generate huge peak power(millions to billions of joules per second(!), depending on how short the pulses are). Conventional lasers need special devices(Q-switches, mode-lockers) to generate pulses, but FELs can do this on their own. And when I say they generate "very short width" pulses, I mean pulses less than 0.000000000001 seconds long! This amount of time is so short only an incredibly silly name can describe it: "subpicosecond". By comparison, Q-Switching a conventional laser can produce pulses on the order of "only" a nanosecond (0.000000001 s).

That's the basics of the FEL. I'll post up a blog entry on the FEL's applications sometime next whenever-the-hell-I-fell-like-it. For now, I'll close with a video of the Jefferson FEL burning a hole through a slab of plexiglas. Enjoy!