Why don't electrons and electrons accelerate

Fast protons are necessary for cancer therapy, for example. It is not always possible to go to large accelerator facilities. Researchers have now set a new record for laser acceleration of these particles.

Pizza cone target; left: with "laser beam", right: X-ray

Dresden-Rossendorf - Fast particles play an important role in radiation therapy for tumors. In this way, rays can be used faster than protons to fight eye cancer. The generation of such high-energy particle beams is not only possible in large accelerator facilities. Laser light can also be used to accelerate protons and other charged particles extremely. For this purpose, the interaction of the laser with a material target, i.e. a material target object, is used. In the process, microscopic length scales generate very high field strengths.

In a recently presented experiment, a new record was achieved for this type of particle acceleration. A research team, in which several US universities and research institutions and the Research Center Dresden-Rossendorf (FZD) are involved, succeeded in generating a proton beam with an energy of 67 MeV (megaelectron volt). This corresponds to the energy that an electron or a proton would absorb if it were accelerated with a voltage of 67 million volts. For comparison: Synchrotron accelerators such as the BESSY in Berlin or PETRA III in Hamburg achieve energies that are around a hundred to a thousand times as large.

Particle acceleration with light is made possible by the high energy density of modern high-power short-pulse lasers. If a pulse from such a source hits matter, the electrons are accelerated to such an extent that they break away from their atomic cores. A plasma is created. The finite expansion of the light pulse now ensures that the electrons are not only accelerated transversely to the direction of propagation of the laser, as can also be observed with less intense light. Instead, after the pulse has traveled through the matter, a plasma wave of electrons swinging back occurs, which also contains a component in the direction of laser propagation.

In order to apply this effect to ions as well, foils with a special structure are used, at which the laser beam is aimed. In the case of the record experiment, a structure was chosen that the scientists call the pizza cone target due to its visual similarity to the Italian baking specialty. These targets consist of two foils that are separated by a kind of micro-cone. The laser beam generates a plasma on the first film, the electrons of which can pass through the film. On the other hand, the electrons emerge and create a strong electric field between the foil and the electron cloud, in which ions such as protons are accelerated. Due to the special structure of the target, the electrons are trapped like a trap. This leads to the high field strengths that make this record proton acceleration possible.

In order to be able to observe the interaction, the researchers used X-rays. Theoretical modeling using a computer simulation already describes the results well and will be able to help examine and interpret the measurement results more precisely. The next step is to measure the density of the proton beam, which is an essential parameter for cancer therapy in addition to the energy.

The advantage of laser systems for particle acceleration is their small size compared to the large particle accelerators. In principle, it is possible to set up a laser particle accelerator in a normal laboratory room, while large accelerators are hundreds of meters in size.