The path to successful inertial confinement fusion (ICF) requires to observe and control the micro balloon deformations. This will be achieved using X-ray microscope among other diagnostics. A high resolution, high energy X-ray microscope involving state-of-the-art toroidal mirrors and multilayer coatings is described. Years of experiments and experience have led to a small-scale X-ray plasma imager that proves the feasibility of all the features required for a LMJ diagnostic: spatial resolution of 5μm, broad bandwidth, millimetric field of view (FOV). Using the feedback given by this diagnostic, a prototype for the Laser MegaJoule (LMJ) experiments has been designed. The experimental results of the first diagnostic and the concepts of the second are discussed.
Starting from FCI2 simulations showing good multi-keV conversion efficiencies of a preformed plasm from thin foils heated by two laser pulses, experiments have been performed with titanium and copper on the Omega laser facility at University of Rochester. The advantages of using this method are efficiencies close to gas targets due to the under-dense plasma created by the pre-pulse and X-ray emissions available at high photon energies that cannot be reached with gas targets. Optimum parameters (laser intensities, delay between the two pulses and thickness of the foil) for titanium and copper foils were estimated from simulations. An increase in the multi-keV conversion efficiency (above 4 keV) by a factor of 2, compared to the case without pre-pulse, is clearly shown on titanium targets. X-ray emission was measured by different diagnostics in good agreement and close to simulations results.
X-ray spectra of a few picosecond duration were emitted by aluminum, selenium and samarium thin foils irradiated with a 100 TW, 300 fs laser at 0.53 μm wavelength. They were measured in the 1600 eV range with high temporal and spectral resolution, using a high-speed streak camera coupled to a conical Bragg crystal. Gradients were limited by using thin foils (300 to 800 Å) deposited on a 50 μm gold pinhole. Frequency Domain Interferometry was set to measure the velocity of the critical density at the rear of the target and deduce the electron temperature. A few picosecond duration X-ray spectra have been measured. Sm spectra showed no spectral features in the measured wavelength range, providing a spectrally homogeneous backlighter for absorption spectroscopy. The duration of the emission was shorter when observed through a pinhole. 1-D hydrodynamic simulations coupled to an atomic collisional-radiative code have been used to simulate the X-ray emission of aluminum. The main features of the experimental time resolved spectra, obtained for the pinhole target have been well reproduced, for an initial temperature of 700 ± 100 eV.