Because of a series of accidental turns in my career, I have more than a passing interest in plasma physics and in making those plasmas glow very bright. It's kind of odd, but plasmas were a really hot topic fifty or more years ago because they were shiny and new. Now it feels a bit back-to-the-future-ish to be taking an interest in a field that was until recently mostly populated by people nearing retirement.

There are a couple of reasons for the renewed interest, though: first, we have new ways of generating very hot plasmas. Lasers that emit very short pulses with very high intensity have given us new tools to explore the world of plasma physics. Concurrent to that, there seems to be renewed interest in doing experiments with soft X-Rays (light in the wavelength range of 1-50nm). These desires have come together in an unholy union to produce some of the biggest, baddest laser experiments you will ever meet.

Plasmas: How do they work?

A plasma is a very funny beast. At first blush, a plasma is similar to a gas: it is compressible, it flows, and it will even obey the ideal gas law to some extent. But a plasma is a mixture of charged particles: positively charged ions and negatively charged electrons floating around free as the birds. This means that when electric and magnetic fields are applied, a plasma behaves very differently from a gas. A plasma can be trapped and compressed by electromagnetic fields. Since we can generate vast electric and magnetic fields, very high temperatures and pressures can be reached.

Generating a plasma basically involves stripping an electron or two from their parent atoms. Simply apply an enormous electric field and you rip the electrons out of the atoms. As the electrons fly away, the atoms, which are now positively charged ions, are repelled by each other. The ions fly away from each other and mix with the electrons, creating a plasma.

Laser light is nothing more than an oscillating electric field, so a powerful laser can start a plasma. In fact, this is what the National Ignition Facility counts on to achieve fusion. But the process by which that plasma is generated tends to be self-defeating—once you start generating it, the properties of the plasma prevent more plasma from being produced. The basic physics behind this come down to the very small mass of the electron.

When the laser pulse hits a material, the electrons in the material start to oscillate back and forth, absorbing and reemitting the light. The more freedom the electrons have, the more vigorously they oscillate. This is why metals reflect light and dielectrics (where the electrons aren't as free to move about) only slow light.

Therein lies a hint about what's coming. Once you have a high enough density of free electrons, light is reflected by the free electrons. Envision an enormous laser pulse hitting a solid material. The leading edge of the laser pulse yanks a lot of electrons out of the atoms (say one or two per atom). These electrons now start to oscillate strongly in response to the incoming laser pulse. As a result, the rest of the laser pulse is reflected off the electron cloud and cannot ionize any more of the target material.

Ionizing little metallic wigs

To get around this problem, a group of researchers from Colorado State University and Heinrich-Heine University in Dusseldorf decided to turn their attention to the structure of the material target rather than the properties of the exciting laser pulse. They designed a target that consisted of fine nickel hairs just 55nm wide and up to five micrometers long. The spacing between the hairs was on the order of 130nm.

This turns nickel, which is quite a dense material, into a rather light material (about 12 percent of normal density). However, the hair structure has particular advantages in addition to low density. Their calculations showed that, as with a normal target, the leading edge of the laser pulse begins the ionization process. But the electrons don't travel very far from the wire, so the light pulse travels between the wires, penetrating deeply into the target. As it does so, it rips more and more electrons out of the nickel atoms. So the plasma takes much longer to become reflective, allowing the laser pulse to ionize right to the bottom of the wire.

Indeed, the researchers' experimental results indirectly confirm this—it's kind of hard to directly map the path the light takes in such an experiment—by examining the radiation produced by the plasma. They found that the typical target had so many electrons ripped away (26 in total) that the nickel atom was left with just two electrons (like helium). When they tried the same experiments with gold, they were able to remove 52 of gold's 79 electrons, which is a very large number.

The plasma densities, pressures, and temperatures were also close to those achieved in fusion experiments, which is several orders of magnitude better than we usually obtain from experiments like this. Though I should say that I doubt that these experiments can be translated to fusions research since there are likely to be some problems getting hydrogen or deuterium to grow in thin, long hairs.

The high density and temperature also make optical emission from the plasma much more efficient. The researchers showed that they could get about 50 times more X-Rays out of their plasmas than is possible with ordinary solid targets. Even better, the radiation was at shorter wavelengths than most other experiments achieve.

This is very important because soft X-ray imaging is a great tool for determining structure and composition of many materials. Not only that, but the short wavelength makes it possible to image fine features as well. Even though we've known about the possibilities of good X-Ray imaging for a long time, good X-Ray sources are usually facility-sized instruments, which means that you really have to want to use one before you will make the effort to build it.

You mentioned sharks lasers?

That said, these laser systems are bit facility-like themselves. The laser that the researchers used could produce two Joules of energy in a 60fs (a femtosecond is 1 x 10-15 s) pulse. At the surface of their target, they reached intensities of 5 x 1018 W/cm2, which is pretty intense—though nowhere near as intense as they could have made it. And the "low" intensity is important. These lasers are big machines that require constant tender loving care.

But it is possible to get the same intensity from a shorter pulse that is focused to a smaller area with much lower energy. My back-of-envelope calculation suggests that the same thing could be achieved with 20mJ pulses, which is something you can achieve with lasers that you can purchase today.

Those commercial laser systems are also not terribly friendly, but they are the sort of investment that an ordinary university department might consider buying as a matter of course. So this research may be a starting point for a new generation of higher efficiency X-Ray sources that just about any idiot could use.

Nature Photonics, 2013, DOI: 10.1038/nphoton.2013.217