Collective Effects

The longterm goal of our experimental activities at the 'High-Field'-Beamline at CALA is the development of all ingredients to establish a new 2-step nuclear reaction mechanism called 'fission-fusion'. This draws on the unprecedented density of laser-accelerated ion bunches, in particular when focused laser intensities on target allow for entering the regime of 'radiation pressure acceleration' (RPA), theoretically predicted to dominate beyond 10²³ W/cm², with a potential onset already at much lower intensities. A crucial  property of this targeted new reaction scheme will be the achievable yield of exotic (i.e. extremely neutron-rich) new isotopic species to be generated in the fusion between two (neutron-rich) light fission fragments originating from the first stage of fission either in the projectile or target nuclei. This critically depends on the useful target thickness that can be exploited, which in turn is directly related to the stopping range of the impinging projectile species when hitting the second ('reaction') target. As the central idea behind the 'fission-fusion' scheme builds on the (so far only theoretically predicted) almost solid-state density of accelerated ion bunches, one may in turn ask if such an unprecedented property would not lead as well to novel effects in the interaction between projectile ions and the reaction target material. In particular the validity of the well-established description of the specific energy loss by the Bethe-Bloch equation could be questioned. As the stopping behaviour in the MeV/u range is dominated by electronic stopping, the picture should be tested where an accelerated ion pulse with (almost) solid-state density impinges onto the target foil. The first ions will interact with the electrons of the foil, literally pushing them aside like a snow-plough, such that subsequently arriving ions will see a locally reduced electron density and as such experience a reduced electronic stopping, observable as increased stopping range. This simplistic picture will have to prevail against emerging turbulent plasma instabilites (e.g. Rayleigh-Taylor instabilities) which will reduce such expected collective range enhancement effects. Beyond the mere interest in observing collective ion range enhancement effects for the first time, their importance for the fission-fusion process is obvious, as they would allow to use thicker reaction targets and as such would allow to increse the reaction yield achievable with this mechanism.

Another collective effect is predicted for the interaction of accelerated electron bunches impinging into a gaseous stopping medium. It has been caluclated that they could drive plasma wakes in the medium, which could collectively decelerate the energetic electrons, thus dissipating kinetic energy into the plasma wake without generating secondary radiation. Also this scheme needs experimental investigation.

At the 'High-Field'-Beamline of CALA we are presently working on ion beam range measurements using proton beams, stopped in stacks of nuclear track detectors. Schemes of detecting the Bragg peak position directly in a fluid stopping medium are under discussion.