Lasers Heat Electrons to Several Billion Degrees

New high-power lasers can accelerate particles such as electrons and ions with intense, short pulses. This finding has attracted the interest of researchers, who are working out the details of the acceleration process that occurs when a laser beam impinges on a thin foil, accelerating ions from its rear surface to high energies.

The laser pulse heats the electrons in the foil to several billion degrees, causing them to gain energy. These electrons in turn give part of their energy to the ions, converting laser pulse energy to ion energy.

Physicists at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have developed a new theoretical model for predicting the density and temperature of these hot electrons that surpasses existing models in accurately describing experimental results and simulations.

Particle acceleration by short, intense pulses of light exhibits considerable advantages over conventional techniques: The distance needed for acceleration is much shorter, and the costs for such systems are potentially much lower.

When an intense laser pulse hits ion electron plasma (ions: orange, electrons: blue), electrons are heated to a couple of billion degrees. This initiates an explosive expansion of plasma ions, which are then accelerated to very high energies. The distribution of the electron temperature during radiation bombardment is depicted in the background. (Image: HZDR)

The potential of the new ion acceleration technology will be explored by a laser system that is currently under construction at the University Hospital in Dresden. It will be used jointly by the partners HZDR, the University Hospital and the Technical University of Dresden for cancer research and therapy.

For the first time, a prototype high-performance laser will be used in addition to a conventional ion accelerator for radiation tumor therapy, HZDR said.

High-power lasers such as the Draco laser at HZDR are about 10 to 100 times more intense than their predecessors, for which theoretical estimates of electron temperature and density were more or less accurate.

For the new generation of lasers, though, experimental findings considerably differ from predictions.

Thomas Kluge, a physicist in the Laser Particle Acceleration Div. at HZDR, and his colleagues have developed a new theoretical model of laser-electron interaction.

Hot electrons serve as intermediaries in laser ion acceleration by transferring energy from the laser to the ions. Hence, precise information on the energy of hot electrons is vital for future laser-driven cancer therapy facilities.

Existing models cannot accurately predict the properties of hot electrons specifically at very high intensity — as with the electrons generated by the high-power laser Draco and the petawatt laser Penelope, which is currently under construction at HZDR.

The Dresden researchers have developed an equation that allows precisely calculating the hot electron energy by taking into account the distribution of laser-accelerated electrons as well as their dynamics according to the theory of special relativity.

“These new insights surpass models that have been around for decades, thus permitting, on the one hand, an explanation of previous measurements while, on the other hand, allowing for predicting and optimizing future experiments with great precision,” noted Michael Bussmann, head of the HZDR’s junior research group Computational Radiation Physics.

The results were published in Physical Review Letters and are being applied to additional acceleration scenarios to permit the future use of laser accelerators in the medical field.

For more information, visit: www.hzdr.de