Relativistic Electron Transport in Preformed Overdense Plasma


One of the central issues in the concept of fast ignition is the energy transport from the ignition pulse through the overdense plasma corona of the precompressed fuel core. The laser converts a fraction of its energy into relativistic electrons in the MeV range, which have to propagate over a distance of a few 100 μm of overdense plasma to finally deposit their energy in the core and ignite it. The electron propagation in this environment is expected to be affected by collective effects due to fields and currents induced by the electron beam itself. The aim of this experiment is to investigate the mechanisms of energy losses of the hot electrons in overdense plasma.

Experimental Setup

In our experiment using the ATLAS-10 facility (setup in figure 1), the hot electron production occurs in a long scale length plasma produced by a synchronised Nd:glass laser delivering 12J in 3ns at 2ω. Using thin plastic, aluminum or copper foils as targets, 1D hydrodynamic simulations using the MULTI-code show the creation of a long underdense plasma with scale lengths of a few 100 μm and a region of overdense plasma several 10 μm thick. Sideview interferograms obtained by using a Wollaston prism (figure 2) and a subsequent Abel-inversion give excellent agreement between theory and experiment for the preplasma creation (figure 3).
Interferogram Comparison MULTI with experiment
The electrons are accelerated to highly relativistic energies by self-focusing of the synchronized 1J/150fs Ti:sapphire laser pulse from ATLAS-10 in the underdense corona. First results using sideview-imaging of the interaction region at the target front surface show clearly the channel formation in the underdense plasma. Electrons with energies of more than 10 MeV penetrating through the target are diagnosed by a magnetic electron spectrometer, (figure 4).
Spektrum mit Fit


A detailed characterization of the electron spectrum under different experimental conditions is under way.

Other diagnostics are ion TOF-detectors and a Thomson parabola to investigate the spectrum of ions that are accelerated at the backside of the target. The electrons exiting the target at the backside build up a strong electric field that ionises atoms on the back surface and subsequently accelerates them. The spectrum of these ions is directly correlated to the electron spectrum at the back surface of the target.