discovered_01_2015

FOCUS// THE HZDR RESEARCH MAGAZINE WWW.HZDR.DE 12 13 When a DRACO laser beam is shot onto a solid target, a two-micrometer titanium membrane, the electrons in the laser field are accelerated, generating strong electric fields on the surfaces of the target. Atoms from the membrane and impurities that are stuck to its surface – tiny amounts of hydrocarbons – are ionized and accelerated in these electric fields. Most of the ions are protons. Currently, these protons can achieve energy levels of 20 mega-electronvolts (MeV). Medical applications would require approximately 180 MeV. Tracking down a mysterious phenomenon One problem is the energy of the accelerated particles, another the stability: particle accelerators have to operate steadily and guarantee high reproducibility. ‘We have partially achieved that,’ says Josefine Metzkes. ‘Together with OncoRay colleagues, we have been able to radiate cells with many thousands of proton pulses under very constant conditions and examine the biological effect with great precision.’ While doing so, the group of researchers almost incidentally discovered an effect that had so far been unknown in the parameter range under investigation. When laser energy is continually increased in order to boost the energy of the protons, at a certain point their properties begin to change. ‘The proton beam virtually breaks up into filaments,’ Josefine Metzkes explains. ‘This is very significant for medical application, because we need homogeneous and predictable beams.’ It is as yet unclear what causes these instabilities or whether they depend on the energy of the laser. This is now to be studied using the DRACO laser, which is currently being revamped to increase its power by a factor of five. In the future, scientists will be able to accelerate protons at a laser power of one petawatt. By comparison, Germany’s power plants generate an overall output of about 194 gigawatts – the new laser system will be able to achieve a wattage that is 5,000 times stronger for a 30 femtosecond light pulse. Looking into the black box Scientists are not only interested in the protons, but also in the process of acceleration as such. It occurs on a length of just a few micrometers and within a few femtoseconds, that is, quadrillionths of a second. Josefine Metzkes is currently describing this process in her dissertation. She has examined exactly what happens at the target when the laser hits. ‘We want to find out whether and when a plasma builds on the membrane, in other words, when exactly the laser pulse starts to alter the target. The whole thing is a sort of a black box. But we must understand what is happening inside to be able to systematically vary the parameters of the laser.’ In order to shed light on the proverbial darkness, the scientists split off a portion of the DRACO pulse, change its frequency to generate blue light, and capture the reflection on the plasma with a camera. ‘To put it simply, imagine using a very fast flashlight to flash light onto a target and take a picture of it,’ Metzkes illustrates. This generates ring signatures that vary in size depending on the plasma conditions, thus providing information about changes in the target. For Josefine Metzkes, this means a few more pieces of the puzzle. PUBLICATIONS: J. Metzkes et al.: ‘Experimental observation of transverse modulations in laser-driven proton beams’, in New Journal of Physics 2014 (DOI: 10.1088/1367-2630/16/2/023008) K. Zeil, J. Metzkes et al.: ‘Direct observation of prompt pre-thermal laser ion sheath acceleration’, in Nature Communications 2012 (DOI: 10.1038/ncomms1883) RADIATION SOURCE: In this chamber, a high-intensity laser hits an electron beam, thus generating X-radiation. Photo: Frank Bierstedt Josefine Metzkes Josefine Metzkes is from Brandenburg and studied medical physics in Halle (Saale) and Toronto. Since 2008, she has been working in Dresden. She hit on the topic of her Master’s thesis, which is also the topic of her doctoral dissertation, while listening to the radio. ‘I just happened to hear a report about the installation of DRACO here at HZDR,’ she remembers. ‘And since I was about to finish my degree, I applied.’ Josefine Metzkes has accompanied the experiments on the DRACO laser from the start: from setting up the experimental chambers to adjusting the optics and analyzing the results. CONTACT _Institute of Radiation Physics at HZDR Josefine Metzkes j.metzkes@hzdr.de

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