Laser-plasma studies have been undertaken for 50 years using infra-red to ultra-violet lasers. We show that a new regime of laser-produced plasmas can be created with capillary discharge and free electron lasers operating in the extreme ultra-violet (EUV). For example, EUV radiation (wavelength < 50 nm) has a critical electron density above electron densities formed by ionization at solid material density and so potentially can penetrate to large depth into a solid density plasma. We explore here the importance of this penetration in ablating solid targets, in creating novel warm dense matter and in the diagnosis of plasmas.
Argon based capillary discharge lasers operate in the extreme ultra violet (EUV) at 46.9 nm with an output of up to 0.5 mJ energy per pulse and up to a 10 Hz repetition rate. Focussed irradiances of up to 1012 W cm-2 are achievable and can be used to generate plasma in the warm dense matter regime by irradiating solid material. To model the interaction between such an EUV laser and solid material, the 2D radiative-hydrodynamic code POLLUX has been modified to include absorption via direct photo-ionisation, a super-configuration model to describe the ionisation dependant electronic configurations and a calculation of plasma refractive indices for ray tracing of the incident EUV laser radiation. A simulation study is presented, demonstrating how capillary discharge lasers of 1.2ns pulse duration can be used to generate strongly coupled plasma at close to solid density with temperatures of a few eV and energy densities up to 1×105 J cm-3. Plasmas produced by EUV laser irradiation are shown to be useful for examining the equation-of-state properties of warm dense matter. One difficulty with this technique is the reduction of the strong temperature and density gradients which are produced during the interaction. Methods to inhibit and control these gradients will be examined.
A comprehensive simulation study is presented, examining the interaction of an EUV capillary discharge laser, operating at 46.9nm, within carbon at solid density. By incorporating a detailed model of photoionization, equation of state calculations, electronic term accounting and refractive index calculation into a pre-existing 2D radiative-hydrodynamic code POLLUX, target ablation and subsequent plasma expansion has been simulated for target material under intense (1011 W cm-2) EUV irradiation. Unique ablation based on direct photoionization by EUV photons creates solid density plasma with a temperature below 20eV. Plasma in this warm dense matter state is of particular interest to inertial con_nement fusion research. A reduction in focal spot size, due to a decrease in the di_raction limit, combined with increased target penetration allows for high-aspect ratio hole drilling and a signi_cant increase in the ejected target mass. This work outlines a comprehensive computational environment used to simulate the EUV/x-ray laser interaction within solid material and expanding plasma.
The potential for coherent extreme ultra-violet (EUV) light in probing laser-produced plasmas is investigated. New
results are presented to demonstrate that EUV radiation can be employed to measure heat penetration into solid targets
from electrons using the signature of a change of opacity due to heating. We examine, in particular, the effects of hot
electron heating of targets. In addition, phase variations after transmission through a laser-irradiated target change the
subsequent propagation of the radiation, suggesting a simple diagnostic measuring the far-field footprint of coherent
EUV radiation can be a useful measurement of the uniformity of target heating.
The evolution of the transmission of extreme ultra-violet (EUV) light from a germanium backlighter through heated thin iron targets has been measured at laser irradiances of about 8×1016 W cm-2. A rapid increase in transmission from 0 to 30% in 20 ps was observed. A two dimensional radiation hydrodynamics model was used to simulate the heating of the plasma and the transmission of EUV light as a function of time. The tamped iron targets were heated up to an average electron temperature of about 55 eV and a mass density of approximately 0.6 g cm-3. The transmission measurements are in reasonable agreement with modelling results. The experimental layout is similar to an X-ray laser experiment and therefore, for relatively low plasma temperatures, these kinds of experiments can be done in combination with X-ray laser experiments, giving transmission data for a range of wavelengths rather than a single X-ray laser wavelength.