Fast gold

A new mechanism in laser-plasma acceleration has been discovered for heavy ions, which causes a significant increase in kinetic ion energy by means of the Coulomb explosion.

All of us are made of stardust - this poetic image contains a lot of (still) unknown, exciting physics that the poet might not really want to tell. Among the top 10 unsolved questions of physics, the question of the origin of the heavy elements - part of the stardust - is also ranked. So far, a deep insight into the interior of the heavy particles and their synthesis can only be gleaned if they collide at extremely high speeds and the resulting fragments of their atomic nuclei are analyzed. Not only nuclear physicists are interested in fast heavy ions but they are also in demand in materials research and medical research.

These ion beams are produced by particle accelerators, which are among the largest and most complex machines in the world. Of course, this also motivates the search for new technical concepts or their improvement. An alternative way to conventional accelerator technology is particle acceleration by laser generated plasma. This requires laser intensities in the so-called relativistic range, where an intense laser pulse accelerates electrons almost to the speed of light. The laser-plasma interaction is determined by relativistic effects of the electron-photon interaction. A single laser pulse generates enormously high, directed field strengths in the range of up to a few megavolts per micrometer in a spatially very limited plasma. In these fields, charged particles can be accelerated to high speeds over a relatively short distance, e.g. also gold ions.

Fig. 1 The laser pulse (1.3 J @ 35f) is focused on a 14nm thick gold foil. The picture shows the maximum ion energy as a function of its ionization level - as measured in the experiment (pink squares). The picture also shows the good agreement with our 2D-PIC simulation (black squares) - as well as a comparison between the prediction of the old theoretical model (black line) - and the new model we developed (blue line).

The challenge of heavy ion acceleration arises directly from a basic principle: ions are accelerated in proportion to their charge / mass Z / A), which leads to higher kinetic energies (~ MeV / u) for lighter elements, as it is difficult to obtain high levels of ionization in heavy ones Reach elements. Exactly this point was overcome by freestanding ultrathin gold foils: they delivered an unexpected high degree and specific ionization distribution for the heavy material (Z> 40 for gold), causing a tremendous, repulsive charge and acceleration of the heavy ions leads over a Coulomb explosion. Compared to previous experiments, we were able to generate kinetic energies of the gold ions at 1 MeV per nucleon with an order of lesser laser energy.

Previously common laser plasma acceleration models assume an averaged ionization, followed by a fixed spatially uniform electron density. Our theoretical analyzes of the experimental results (see picture) show a layerwise different ionization of the target foil, with atoms with the highest ionization at the edges of the foils. As a result, an extremely high space charge is generated there - which repulses the strongly positively charged, heavy ions - and additionally accelerates them.

Extrapolating our findings into the parameter range for a true collision experiment with fast heavy ions, femtosecond lasers with pulse energies of 100 J are needed.

Original publication

Coulomb driven energy boost of heavy ions for laser plasma acceleration

J. Braenzel, A. A. Andreev, K. Platonov, M. Klingsporn, L. Ehrentraut, W. Sandner, M. Schnürer

Physical Review Letters 114 (2015) 124801/1-5