Following a major experimental campaign at the Vulcan laser of the Rutherford Appleton Laboratory, a team of researchers of several European countries participating in the HiPER programme, including staff from CNR, Istituto Nazionale di Ottica (Pisa) has delivered a first set of results on the generation of dense plasma with parameters relevant to the physics of the fast ignition scheme of Inertial Confinement Fusion. The results were published in two articles recently appeared [B. Vauzour et al., Phys. Plasmas 18, 043108 (2011), L. Volpe et al., Phys. Plasmas 18, 012704 (2011)] on Physics of Plasmas, one of the reference Journals for the plasma physics community.
With the US National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory entering its crucial phase, the expected laboratory demonstration of laser-driven fusion is generating an intense activity world-wide to develop the next phase of exploitation of laser fusion for energy applications.
In view of this, a major effort is being dedicated to the development of suitable ignition schemes to increase the thermonuclear gain of laser fusion from the value of 10-20 expected from the present NIF schemes, to the value of >100 needed for industrial exploitation of laser fusion energy. In the fast ignition scheme, a larger quantity of nuclear fuel is compressed to pre-ignition conditions using a moderate laser energy. A high intensity laser pulse, focused on the target at the moment of maximum compression, generates a massive production of energetic electrons that propagate in the compressed material where they release energy. This process acts as a spark that ignites the nuclear fusion reactions that is then expected to burn the entire compressed pellet.
In Europe, the High Power Laser Energy Research Facility (HiPER) is facing these issues promoting a coordinated approach to R&D, numerical modeling and dedicated experimental campaign at European and international large laser facilities.
The HiPER experimental campaign on fast ignition relevant conditions was designed to investigate energy deposition of large currents of fast electron beams inside a compressed material. The fast electron currents generated in these conditions are well beyond the Alfven limit and the description of their propagation in compressed matter is poorly understood. The experimental data obtained in our experimental campaign are the first obtained in an European laser installation and among the first available world wide.
In the experiment, a set of four long laser pulses were used to symmetrically irradiate a cylindrical target to obtain a moderate, but significant compression. At the moment of maximum compression, a short, intense laser pulse is focused on the axis of the cylinder to generate fast electrons that propagate in the compressed material. A range of diagnostics, including proton radiography, X-ray imaging and spectroscopy and optical scattering measurements were used to assess the compression phase and the fast electron propagation phase.
The papers recently appeared on Physics of Plasmas are dedicated to the characterization of the compression phase carried out using the most advanced plasma diagnostic techniques including proton and X-ray radiography. Radiography is exploited to reconstruct the temporal evolution of the compression phase. Substantial agreement of experimental results with numerical hydrodynamic modeling demonstrates that a successful compression was indeed obtained in the experiment. These results show that the conditions required for fast electron transport studies relevant to ICF Fast Ignition studies were indeed achieved in the experiment.

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