Due to their particular properties, the beams of the multi-MeV protons generated during the interaction of ultraintense (I>1019 W/cm2) short pulses with thin solid targets are most suited for use as a particle probe in laser-plasma experiments. The recently developed proton imaging technique employs the beams in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in laser-plasma interaction experiments. In recent investigations carried out at the Rutherford Appleton Laboratory (RAL, UK), a wide range of laser-plasma interaction conditions of relevance for inertial confinement fusion (ICF)/fast ignition has been explored. Among the results obtained will be discussed: the electric field distribution in laser-produced long-scale plasmas of ICF interest; the measurement of highly transient electric fields related to the generation and dynamics of hot electron currents following ultra-intense laser irradiation of targets; the observation in underdense plasmas, after the ...

Electric field detection in laser-plasma interaction experiments via the proton imaging technique

Proton imaging is a recently proposed technique for diagnosis of dense plasmas, which favourably exploits the properties of protons produced by high-intensity laser-matter interaction. The technique allows the distribution of electric fields in plasmas and around laser-irradiated targets to be explored for the first time with high temporal and spatial resolution. This leads to the possibility of investigating as yet unexplored physical issues. In particular we will present measurements of transient electric fields in laser-plasmas and around laser-irradiated targets under various interaction conditions. Complex electric field structures have been observed in long-scale laser-produced plasmas, while global target charge-up and growth of electromagnetic instabilities have been detected following ultraintense interactions with solid targets.

Proton imaging: a diagnostic for inertial confinement fusion/fast ignitor studies

The optical reflectance of a strong shock front in water increases continuously with pressure above 100 GPa and saturates at ∼45% reflectance above 250 GPa. This is the first evidence of electronic conduction in high pressure water. In addition, the water Hugoniot equation of state up to 790 GPa (7.9 Mbar) is determined from shock velocity measurements made by detecting the Doppler shift of reflected light. From a fit to the reflectance data we find that an electronic mobility gap ∼2.5 eV controls thermal activation of electronic carriers at pressures in the range of 100–150 GPa. This suggests that electronic conduction contributes significantly to the total conductivity along the Neptune isentrope above 150 GPa.

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Electronic conduction in shock-compressed water

We propose a new approach that combines original techniques to achieve a very efficient laser beam homogenizer and shaper. The three retained concepts are microlens array, random phase plate and diffractive optical elements. The performance of usual homogenizers are improved in terms of efficiency, homogenization and shaping abilities. We report on the theory and the development of such diffractive beamshaper homogenizers. Finally, an application concerning laser to fibre coupling is described.

https://iopscience.iop.org/article/10.1088/1464-4258/1/3/310/pdf

Efficient beamshaper homogenizer design combining diffractive optical elements, microlens array and random phase plate

Design and fabrication of diffractive optical elements

The phase zone plate combines the simplicity of design and fabrication of the random phase plates with the advantage of near field target placement. The flexibility of design allows the cell structure to be easily modified to produce different focal spot shapes. The components are not specific to a particular near field intensity pattern provided several elements can be encompassed by the beam and therefore may be relevant to other laser applications requiring uniform beams on a target provided high spatial frequencies are not detrimental to the laser absorption process. >

C.N. Danson

Papers

A binary phase zone plate array for laser beam spatial intensity distribution conversion