Bringing x-rays back to the lab

Synchrotron sources of x-ray radiation have become an invaluable part of materials science: spanning physics, chemistry, engineering and biology. The range of techniques available when using such high-intensity light is staggering. Unfortunately the cost of building and running such facilities is considerable, meaning that there is always significant competition to perform these experiments. A team of researchers, led by a group at Imperial College London has demonstrated a table-top x-ray source which may mean that some studies may be able to move away from synchrotrons and back into the lab [Kneip et al., Nat Phys, (2010) doi:10.1038/nphys1789].

 

This is not the first attempt to build such a device, and a number of previous sources have been built using similar techniques. The sources all make use of the wake generated when a laser is fired through a low density plasma. However the new system is 1 000 times brighter than previous comparable sources.

 

The group used the Hercules laser at the University of Michigan to fire short (30 fs) but intense laser pulses into jets of helium gas. The helium within the beam is ionized, forming a plasma. The interaction between the electrons within the plasma and the laser’s electric field causes the electrons to oscillate, analogous to their behavior inside a modern synchrotron. This oscillation of the charged particles emits electromagnetic radiation in the form of high energy x-rays. Finally, the charged electron beam is deflected away, leaving a beam of x-rays.

 

The x-ray beam has a broad spectrum with a peak in intensity at 10 keV (0.124 nm), in the so called hard x-ray regime. However, according to the first author of the paper, Dr. Stefan Kneip, “In principle, the x-ray energy can be tuned from soft [low energy] to hard [high energy] x-rays, simply by changing the laser and plasma conditions. We have already demonstrated this control at the few keV level”.  

 

The x-ray beam is extremely intense, coherent and stable compared to alternative lab based sources. Due to the manner in which the beam is generated, the beam is also short, existing as a sudden burst of radiation. The groups responsible are hopeful that systems such as theirs will be useful for time resolved studies, as well as phase contrast and lenseless imaging.

 

Unfortunately, systems such as the one developed by Imperial won’t be commercially available in the immediate future. Kneip told Materials Today that “high power lasers are currently quite difficult to use and expensive”. “However, laser technology is advancing rapidly, so we are optimistic that in a few years there will be reliable and easy to use x-ray sources available that exploit our findings”.