K-alpha backlighter optimization using high-energy picosecond laser pulses in experiments at the PHELIX laser facility

Development of laser driven high-brightness fast x-ray sources is a key application of the PHELIX laser for experiments in combination with the heavy ion accelerator. X-ray diagnostics of high-energy density states of matter produced with intense heavy ion beams from GSI or FAIR will require multi-keV x-ray energies to penetrate the dense plasma [1]. Recent development of backlighter sources at GSI focused on high-intensity short-pulse driven sources due to their favourable scaling to higher photon energies. When light is focused at relativistic intensities onto matter a large fraction of the laser energy is converted into superthermal electrons which are accelerated to MeV energies. Interaction with the target material results in high-energy bremsstrahlung and characteristic line emission. Underdense plasma formed at the front surface is heated to keV temperatures, emitting Helium-alpha and Lyman alpha radiation from highly-charged ions. In recent experiments at PHELIX we optimized a Cu K-alpha backlighter source. Thin foils and wires with dimensions of few 10’s of microns were irradiated with laser pulses of energies up to 150 J and K-alpha and hard x-ray emission was determined quantitatively as function of the laser parameters pulse duration and focal spot size over a wide range of intensities exceeding 10W/cm. A variety of x-ray diagnostics was fielded in the experiments. An absolutely calibrated single-hit spectrometer gives a broad overview of the x-rays produced, from ~5 keV to over 20 keV. Fig. 1 shows a spectrum obtained from a 100 J laser shot. Up to 10 Cu K-alpha photons were generated per shot, corresponding to a conversion efficiency of laser energy into line radiation of approximately 10 which is typical for this class of experiments [2]. Besides strong K-alpha emission the spectra show Kbeta and He-alpha radiation. Advanced x-ray scattering techniques [3] require a high collection efficiency of the scattering spectrometer. Highly-oriented pyrolytic graphite (HOPG) provides an exceptionally high peak reflectivity [4]. At GSI we used a 70mm large curved HOPG crystal in a van-Hamos type spectrometer. Coupled with imaging plate detectors this constitutes a highly efficient spectrometer suitable as an x-ray scattering diagnostic. The resolution is limited to E/∆E~200 due to depth broadening and aberrations resulting from to the large mosaicity of the HOPG crystals. A much improved resolution is provided by a spherically bent Mica crystal spectrometer in Johannsen geometry. This spectrometer provides in addition 1-dimensional imaging. The spectrum of hard x-ray bremsstrahlung was assessed using an image plate based 8-channel detector with a set of lead filters of various thicknesses. A strong dependence of the bremsstrahlung spectrum on the focused intensity was observed. While in some applications this background is detrimental, it is also a powerful backlighter for dense high-Z targets. A fitting procedure based on genetic algorithm and using Monte-Carlo simulated bremsstrahlung emission and detection allowed us to deduce the hot-electron spectra.