Focusing intense and short laser pulses on solid target allows to generate dense plasmas, corresponding to high electronic density higher than 10²²cm^-3. Such plasmas are characterised by the couple electronic density - N_e, electronic temperature - T_e. One common method for determining this couple of parameters consists in the observation of a beam through the medium to study. In the specific case of dense plasmas, using a probe beam in the visible domain means having problems of reflexion and/or refraction of the beam by the plasma.

A beam of light can propagate in a plasma up to a density called critical density and defined in practical units as: N_cr[cm^-3]≈1.1x10²¹/λ²[µm]. For example, N_cr≈1.7x10²¹cm^-3 for λ=800nm. Above Ncr, the wave becomes evanescent.

Plasmas can exhibit sharp density gradient, and more and more when they are created by intense laser pulses with high temporal contrast. A refraction index gradient is associated to a density gradient, deviating the probe beam of its initial trajectory.

For probing  plasmas of electronic density close to solid density (10²²cm^-3), the optical diagnostic in the visible domain become inefficient.  One of the solutions for minimizing the reflection and refraction phenomena is to decrease the wavelength probe beam up to the XUV domain.
High Harmonic Generated (HHG) in gas jet has been chosen as the XUV probe to characterize dense and transient plasmas. Their unique properties: short duration, high brilliance, spectrum made of odd harmonics, have been exploited in various experimental geometries:


- Plasmas generated by intense irradiation of polypropylene thin foil target characterized by the transmission temporal variation of XUV probe beam with 100fs time resolution...

- Spectrally and temporally resolved reflectivity measurement for studying the temporal evolution of a dense plasma of interest for the Warm Dense Matter problematic...

- Set up of an interferometric diagnostic at 32nm to study dense plasmas, tested in "transmission" geometry through an aluminium plasma...