image: Simulations of the experiment showing the ion acceleration technique. The centre of the target becomes “relativistically transparent”, allowing the laser to penetrate. The ions in this region, both carbon and protons, are accelerated forwards due to extremely large accelerating fields set up in the transparent region. view more
Credit: by N.P. Dover, T. Ziegler et al.
Ion accelerators are key tools for applications as diverse as probing the fundamental nature of matter to treating cancer in hospitals. Standard accelerator technology has been continuously developed and optimised for over a century. However, there has recently been rapid growth in an entirely new method of particle acceleration, which uses powerful lasers to manipulate ionised matter, or plasma. These plasmas can provide particle accelerating electric fields millions of times larger than those possible in standard machines. This leads not only to a dramatic downsizing of the accelerator, but also inherently ultra-short beams that are less than a nanosecond long, much shorter than conventional techniques. These ultrashort beams enable a host of new applications, including studies of the FLASH effect, in which extremely high dose rates provided by the ultrashort beams result in enhanced sparing of healthy tissue when treating cancer with radiotherapy.
In a new paper published in Light: Science & Applications, an international team including Mamiko Nishiuchi from the National Institutes for Quantum Science and Technology, Japan, Tim Ziegler and Karl Zeil from Helmholtz-Zentrum Dresden–Rossendorf, Germany, Nicholas Dover from Imperial College London, UK, have together with co-authors made a significant leap forward in bringing these laser driven ion sources out of the lab and towards practical applications. They used two independent state-of-the-art high power lasers at Kansai Photon Science Institute in Japan and Helmholtz-Zentrum Dresden-Rossendorf in Germany to develop a new technique for accelerating ions with lasers.
They showed that the key to making copious high energy ions is by using targets which are “relativistically transparent”. Typically, plasma targets are used that are completely opaque to light, so the laser only interacts with the surface before bouncing off. However, for very intense lasers, the target electrons are accelerated to close to the speed of light, dramatically changing the refractive index and allowing the laser to enter the now transparent target. When this happens, the laser interacts with the entire target, and is strongly absorbed. This induces an extreme electric field which accelerates target ions in the laser direction. Although this regime has previously been investigated with very high energy, single shot lasers, the researchers have now shown that it can also be applied to high power femtosecond laser systems. This class of laser system is already commercially available and being rapidly developed to improve their stability and repetition rate, making them ideal for applications requiring high repetition rate ion pulses.
Furthermore, the researchers have investigated how to optimise ion acceleration by carefully choosing a target thickness matched to individual laser system parameters. This results in a target becoming transparent just at the most intense part of the laser pulse. By using two independent laser systems to replicate these results, the researchers demonstrated that the technique is robust and can be already applied to existing petawatt-class femtosecond laser facilities. This is ideal for delivering ultrashort pulse ion beams, which radiobiologists can use to unravel mysteries in high dose radiobiology.
Journal
Light Science & Applications