Miniature particle accelerator: Micro-water droplets as source for laser ion acceleration

The recently published issue of Physical Review Letters (Vol. 103 Issue 13) selected a publication of the Max Born Institute for the cover image. In this publication (T.Sokollik et al., PRL 103, 135003 (2009)) proton acceleration on spherical isolated targets was investigated for the first time with the aid of "proton imaging". Already in a previous paper (S. Ter Avetisyan et al., PRL 2006), MBI researchers found that the interaction of an intense laser pulse with micro-water droplets produced quasi-monoenergetic protons. Simulations showed a spatial, asymmetric field structure resulting in directional ion emission. This could now be demonstrated with the help of "proton imaging" on micro-water droplets. An advantage of this target system is the high repetition rate with which the water droplets can be generated (MHz). However, it has also been found that the evaporating water in vacuum brings with it some disadvantages. The resulting surrounding plasma thus negatively influences the energy transfer between the laser pulse and the ions. To prevent this and to investigate further fundamental mechanisms on isolated targets is the subject of current investigations.