Extreme ultraviolet probing of laser irradiated solid targets

This thesis explores the applications of extreme ultra-violet (EUV) radiation to the probing of plasma materials and the importance of extreme ultra-violet radiation in radiative transfer in plasmas relevant to both ICF and astrophysics.

The complex refractive index of solid aluminium and the imaginary part of the refractive index of solid iron between 17 eV and 39 eV have been measured using EUV harmonics produced from an 800nm laser focused to 10^14Wcm^-2 in an argon gas jet impinging on a double slit interferometer.
There is potential for further work with this method to measure the complex refractive index of plasma material.

It is shown that the drop in opacity when bound-free photo-ionization is no longer energetically possible, enables measurements of the transmission of extreme ultra-violet (EUV) laser pulses at 13.9 nm to act as a signature of the heating of thin (50 nm) iron layers with 50 nm thick parylene-N (CH) overlay irradiated by 35 fs pulses at an irradiance of 3x�10^16 Wcm^-2. Comparing EUV transmission measurements at different times after irradiation to fluid code simulations shows that the target is instantaneously heated by hot electrons (with approximately 10% of the laser energy),followed by thermal conduction.

The diagnostic potential of extreme ultraviolet (EUV) coherent probing within a laser produced plasma with a simple transmission footprint is investigated. A fluid code is used to model the interaction of a 35fs, 2�x10^-14Wcm^2 800nm laser pulse with an 800nm thick aluminium target. A post processor is used to calculate the target’s refractive index and transmission to 45eV radiation. The effects of EUV radial phase variations at the rear of the target on the intensity distribution at a detector 1.5m from the target are studied. An irradiated aluminium target is found to
have little effect on the transmission of 45eV radiation. However, there can be significant phase differences in the probing beam in the radial direction due to heating beam footprint variations for example due to a non-uniform focal spot. These phase variations affect the subsequent propagation of the radiation, suggesting a simple diagnostic measuring the far-field footprint of coherent EUV radiation passing through an irradiated target is sensitive to radial variations of the target
heating.