You may have noticed there was a lot of coverage on lasers last week. Some of the headlines really caught my attention: “X-Ray Laser Turns Up the Heat to 3.6 Million Degrees” or “World’s Most Powerful X-Ray Laser Super-Heats Aluminum Foil to 3.6 Million Degrees”. Sounds like an impressive laser, right? I wondered what an X-ray laser was. And wouldn’t the laser vaporize the aluminum foil; aluminum melts at 660˚C.
So I decided to track down a copy of the paper. The news articles all refer to a study published in the Jan 25th issue of Nature; but, three separate studies on X-rays were published in that issue. Finding the right study was actually trickier than I thought it would be, especially since the authors weren’t always identified by name (only by research lab).
Turns out, the laser transforms the aluminum foil into plasma, a state of matter like gas.
Imagine you have a block of ice. It starts to melt when the sun heats it up. As the sun heats it up more, the water evaporates and becomes water vapour. If the water vapour gets heated enough, it will turn into plasma. Plasma is the 4th state of matter, which is similar to gas but consists of positively and negatively charged ions and free electrons.
In the study by Vinko et al. (2012), the solid aluminum foil is transformed in the plasma state by the heat of the X-ray laser. The X-ray can heat the aluminum foil to about 2 million degrees Celsius, which is an extremely large amount of heat considering that’s more than 10% of the Sun’s core temperature.
The X-ray laser used in Vinko et al. is officially known as an X-ray free-electron laser (FEL). Generally speaking, FELs generate X-rays by sending high speed electrons through a magnetic field. A beam of electrons is generated and accelerated close to the speed of light using a linear accelerator. The electron beam passes through an undulator, a series of alternately poled (North/South) magnets. The magnets cause the electrons to move in a wavy path and emit energy in the form of coherent X-rays. Coherent X-rays are in tuned with each other and result in a much stronger X-Ray. The effect is similar to what happens when everyone in a choir sings the same note in perfect tune, and strengthens the sound produced. This powerful X-ray can then be used to perform experiments such creating plasma materials or imaging proteins or viruses.
Although FELs have been around since the 1970s, what’s special about the FEL used by Vinko et al. (2012) is that it can be used to target specific ions emitted from the sample. The selective probe allows researchers to exert finer control when characterizing their samples. With this study, the team has shown the potential for the FELs to be used to understand the properties of materials and of living systems.
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Vinko, S., Ciricosta, O., Cho, B., Engelhorn, K., Chung, H., Brown, C., Burian, T., Chalupský, J., Falcone, R., Graves, C., Hájková, V., Higginbotham, A., Juha, L., Krzywinski, J., Lee, H., Messerschmidt, M., Murphy, C., Ping, Y., Scherz, A., Schlotter, W., Toleikis, S., Turner, J., Vysin, L., Wang, T., Wu, B., Zastrau, U., Zhu, D., Lee, R., Heimann, P., Nagler, B., & Wark, J. (2012). Creation and diagnosis of a solid-density plasma with an X-ray free-electron laser Nature DOI: 10.1038/nature10746
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