La Gizzi

Recent results from experimental studies on laser?plasma coupling in a shock ignition relevant regime

Shock ignition (SI) is an appealing approach in the inertial confinement scenario for the ignition and burn of a pre-compressed fusion pellet. In this scheme, a strong converging shock is launched by laser irradiation at an intensity Iλ 2 >10 15 Wc m −2 µm 2 at the end of the compression phase. In this intensity regime, laser–plasma interactions are characterized by the onset of a variety of instabilities, including stimulated Raman scattering, Brillouin scattering and the two plasmon decay, accompanied by the generation of a population of fast electrons. The effect of the fast electrons on the efficiency of the shock wave production is investigated in a series of dedicated experiments at the Prague Asterix Laser Facility (PALS). We study the laser–plasma coupling in a SI relevant regime in a planar geometry by creating an extended preformed plasma with a laser beam at ∼7 × 10 13 Wc m −2 (250 ps, 1315 nm). A strong shock is launched by irradiation with a second laser beam at intensities in the range 10 15 –10 16 Wc m −2 (250 ps, 438 nm) at various delays with respect to the first beam. The pre-plasma is characterized using x-ray spectroscopy, ion diagnostics and interferometry. Spectroscopy and calorimetry of the backscattered radiation is performed in the spectral range 250–850 nm, including (3/2)ω, ω and ω/2 emission. The fast electron production is characterized through spectroscopy and imaging of the Kα emission. Information on the shock pressure is obtained using shock breakout chronometry and measurements of the craters produced by the shock in a massive target. Preliminary results show that the backscattered energy is in the range 3–15%, mainly due to backscattered light at the laser wavelength (438 nm), which increases with increasing the delay between the two laser beams. The values of the peak shock pressures inferred from the shock breakout times are lower than expected from 2D numerical simulations. The same simulations reveal that the 2D effects play a major role in these experiments, with the laser spot size comparable with the distance between critical and ablation layers.

The laser-matter interaction meets the high energy physics: Laser-plasma accelerators and bright X/gamma-ray sources

Laser matter interaction in the regime of super-intense and ultra-short laser pulses is discovering common interests and goals for plasma and elementary particles physics. Among them, the electron laser wakefield acceleration and the X/γ tunable sources, based on the Thomson scattering (TS) of optical photons on accelerated electrons, represent the most challenging applications. The activity of the Intense Laser Irradiation Laboratory in this field will be presented.

Laser wake field acceleration with controlled self-injection by sharp density transition

Laser Wake Field Acceleration of relativistic electron bunches is a promising method to produce a large amount of energetic particles with table top equipment. One of the possible methods to inject particles in the appropriate acceleration phase of the wake behind the pulse takes advantage of the partial longitudinal breaking of the wake crests across a density downramp. In this paper results of 2.5D PIC simulations, showing the production of an electron bunch with reduced energy spread, are reported. Also, a possible method to produce the required plasma density transition by laser explosion of a suitable couple of thin foils is discussed.

High brightness laser-plasma X-ray source at IFAM: Characterization and applications

A high brightness laser-plasma X-ray source has been set-up and is presently available at IFAM. A wide range of diagnostics has been set up to monitor the properties of the X-ray radiation and to control the main parameters including photon energy, flux intensity, and pulse duration. A beam extractor enables access to the X-ray radiation at atmospheric pressure. A simple, easy-to-use projection microscope has been built which is capable of single-shot micron resolution imaging with digital acquisition. Preliminary biomedical experiments show that the X-ray doses available on a single laser shot exposure of our source fully meet the conditions required for an important class of biological experiments based on X-ray induced DNA damage providing an ideal alternative to the long time exposures needed with X-ray tubes.

Experimental investigation of fast electron transport through Kα imaging and spectroscopy in relativistic laser–solid interactions

We report on experimental fast electron transport studies performed in the relativistic laser intensity interaction regime. The investigation has been carried out in the long-pulse (0.6u2009ps) regime relevant for the fast ignitor scheme in the inertial confinement fusion concept.Multilayer targets containing different materials were irradiated. Here we show the results concerning SiO2 or Al layers, respectively. The Kα radiation from a Cu tracer layer on the target rear side was found to be enhanced by a factor of about 8 with the irradiation of SiO2 targets with respect to the Al targets. The possible origin of this observation is discussed.

Spectroscopy of laser-plasma accelerated electrons: A novel concept based on Thomson scattering

The spectrum of relativistic electron bunches with large energy dispersion, like the ones usually generated with laser-plasma acceleration processes, is difficult to obtain with conventional methods. A novel spectroscopic concept, based on the analysis of the photons generated by Thomson scattering of a probe laser pulse by the electron bunch, is presented. The feasibility of a single-pulse spectrometer, using an energy-calibrated charge coupled device as detector, is investigated. Numerical simulations performed in conditions typical of a real experiment show the effectiveness and accuracy of the new method.

Study of second harmonic emissions for characterization of laser–plasma X-ray sources

An investigation of second harmonic (SH) and X-ray emissions from Al plasmas produced by 3-ns, 1.064-μm laser pulses at 10 14 W/cm 2 is reported. The SH and X-ray yields are strongly correlated as a function of the target position with respect to the laser beam focus. The SH originates from the underdense coronal plasma and has a filamentary source, while the X-ray source is uniform. The results suggest that, although the X-ray emission is significantly enhanced by the filamentation of the laser light in the corona, there is a smoothing effect in the energy transport process toward the overdense region.

High-order harmonic generation from a linear chain of ions

The high-order harmonic generation process due to the interaction of a multi-well quantum system with an intense laser field is examined. A plateau extension up to a photon energy of Ip + 8Up and the generation of attosecond pulses are demonstrated.

HiPER laser: From capsule design to the laser reference design

HiPER (High Power laser Energy Research) is the first European plan for international cooperation in developing inertial fusion energy. ICF activities are ongoing in a number of nations and the first ignition experiments are underway at the National Ignition Facility (NIF) in the USA. Although HiPER is still in the preparatory phase, it is appropriate for Europe to commence planning for future inertial fusion activities that leverage the demonstration of ignition. In this paper we shall detail some of the key points of the laser design and the way this design is connected to the capsule requirements.

On the effect of rear-surface dielectric coatings on laser-driven proton acceleration

Laser-driven ion acceleration has been experimentally investigated by irradiating, with tightly focused femtosecond laser pulses at 5×1019u2002W/cm2, thin metal foils, which have been back-coated with a μm thick dielectric layer. The observation we report shows the production of MeV proton bunches with an unexpected highly uniform spatial cross section.