Physicists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR; Dresden, Germany) are working on the DRACO laser to generate protons with very high energies. The scientists hope that their research will translate into the use of compact laser accelerators for cancer therapy with charged particles such as protons.

Experts have thought that commercially available laser systems would not be suitable for future cancer therapy applications because they have short laser pulses that achieve correspondingly low energy levels. But the intense, ultrashort light pulses of the DRACO high-power laser could do the trick. The system produces disks of light about 10 cm in diameter and as thin as a normal sheet of paper. If one of these disks is focused onto a thin metal foil, high electric and magnetic forces pull negatively charged electrons out of the foil. These electrons then accelerate positively charged protons away from the foil’s surface. The results indicate that the proton energies needed for cancer therapy could, in principle, be generated from such a short-pulse laser. This prospect motivated the Dresden researchers to study the particle acceleration process.

If the angle of the thin light disk is tilted slightly with respect to the axis of propagation, the electrons feel the rotation of the light disk and follow the direction in which the light hits the foil. Moreover, protons are accelerated along this direction as well and, in contrast to the electrons, maintain their direction. This phenomenon has enabled the Dresden physicists to also investigate the underlying acceleration process.

“During the first acceleration phase, the distance between the electrons and the foil is extremely small," explains doctoral candidate Karl Zeil. "Once the short laser pulse has pushed them through the foil, they immediately swing back again because the foil has a positive charge. That is one reason why we were very surprised to discover that not only the electrons follow the motion of the laser light, but also the protons exhibit this previously unknown directional dependence.”

Zeil also observed that the initial phase is decisive for the entire acceleration process. During the first 30 femtoseconds, which is equal to the length of the laser pulse, the acceleration is very efficient. The short and efficient acceleration phase is followed by a longer expansion phase, during which a uniform and symmetrical plasma cloud is formed. The protons, however, gain a great deal of energy during the first phase, which, in turn, makes them move so fast that they finally can reach higher energies than conventional models would predict.

Using simulations, the HZDR scientists are studying precisely how the fast electrons oscillate around the foil and thus accelerate the protons. “Experiments and simulations agree quite well with each other," Zeil says. "With the newly obtained data we can now extend the presently existing models. This essentially means that ultrashort pulsed lasers like our DRACO laser could potentially be capable of generating protons with sufficiently high energy so that they can be used in future cancer therapy."