Fig. 1. Schematic of experimental setup and target design. In the experiment, a solid layered target with a choice for the ablation layer is irradiated by an intense laser pulse, and diagnostics involving an x-ray time-resolved Cu K_α imager and time-integrated Cu K_α spectrometer, ESs, and a BSC are used simultaneously. The Cu K_α imager comprises a spherically bent quartz crystal combined with an x-ray streak camera synchronized with a frequency-tripled pickoff of the heating laser.

Fig. 2. (a) Example of x-ray streak camera (XRS) image measured in experiment. The white arrowed line in the image shows the laser profile, and the spatial scale is given for the detector plane at magnification M = 4 of the optical scheme. (b) Simulated K_α emission profiles obtained for different temperatures of Cu layer. The simulations were performed using the FLYCHK code with 30 keV electrons added at a level of 0.01% to stimulate the K_α excitation. The dashed gray region indicates the spectral window of the imager.

Fig. 3. (a) Hard x-ray spectroscopy setup inside vacuum chamber using bremsstrahlung cannon (BSC). (b) Materials and thicknesses of progressive filters for signals measured in imaging plates (IPs); not shown here are the PTFE (C₂F₄) filters used to stop electrons from penetrating the stack. (c) Measured dose as function of channel number, and comparison to dose fitted from model. (d) Comparison between x-ray spectrum f_ph(E) (black line) fitting signals in (b) and bremsstrahlung spectrum produced by 3D Maxwellian distribution of electrons f_e(E) (red line) propagating through target. The photon and electron distributions calculated by GEANT4 have temperatures of 30 and 36 keV, respectively.

Fig. 4. (a) Experimental arrangement of hot electron (HE) spectrometers. The angles 25° (ES1), 51° (ES2), and 31° (rear, ES3) are with respect to the target normal. (b) Typical electron spectrum measured by spectrometer ES3, and Maxwellian function fitting the experimental data. The experimental spectrum is not corrected for the electron propagation through the target.

Fig. 5. (a) Spectra of Cu K_α group recorded in shots irradiating Cu foil (top) and composite target with Al flash + CH ablative layer (bottom). The absence of the K-shell emission from highly ionized Cu indicates a relatively low temperature of the buried diagnostic layer. (b) Temporally and spatially integrated Cu K_α1 signal collected on IP and emitted from different types of composite targets. For Ti and Ni ablators, the K_α emission cannot be extracted unambiguously from the spectral background.

Fig. 6. (a) Electron temperatures measured by BSC diagnostics vs laser intensity. For better readability, a typical error bar is given for only one of the points. (b) and (d) Comparison of electron temperatures estimated from BSC x-ray spectra with those obtained from ES3 spectrometer before (b) and after (d) the GEANT4 correction accounting for transport into the target (d). The electron spectra were recorded behind the rear side of the target, and the dashed lines indicate equal values for the BSC and ES3 diagnostics. (c) Modification of energy distribution of HEs along their propagation into a multilayer target with Ni ablator, obtained from GEANT4 simulations. The violet curve is the 3D Maxwellian distribution of HEs with temperature 40 keV injected into the Ni ablator; the other curves are the distributions expected at different target layer interfaces. The red curve is the electron distribution at the rear side of the target, and it is fitted by an exponential function with a temperature of 68 keV (black dashed line) in the energy range of 150–300 keV. The experimental electron spectrum for shot 55 189 (black solid line) measured by the ES3 spectrometer is overplotted in the graph, suggesting that the measured spectrum with a temperature of Te′=68 keV corresponds to an input HE population with a temperature of T_e = 40 keV.

Fig. 7. (a) Examples of electron spectra for shots with Ni (high Z) and CH (low Z) ablation layers from front spectrometers. (b) Comparison of HE source populations registered by BSC and ES diagnostics. The dashed line corresponds to equal populations, and the ES data account for transmission through the target.

Fig. 8. (a) Calculated transmission of HEs emerging from rear surface of target as estimated by GEANT4 simulations. The values were calculated by injecting Maxwellian distributions of electrons with different input temperatures on the front surface of the target. Electrons were injected at 200 μm from the target surface with a spot size of 50 μm and a cone angle of 45°. The dashed rectangle shows the typical range of measured electron temperatures in the experiment. (b) Conversion efficiency of laser energy into HE energy obtained from analyzing BSC data. The dashed lines are to guide the eye and emphasize the trends for Al and parylene-C ablators, whereas the actual dependence can be different. For clarity, a typical error bar is given for only one of the data points.

Fig. 9. (a) Number of K_α1 photons (green) for different values of Cu layer (solid, dashed, and dotted lines) measured by K_α imager, compared to number of bremsstrahlung photons produced by collisions of HEs into target (blue line) and to the number of bremsstrahlung + recombination photons emitted by the plasma corona, calculated in 1D (black) and 3D geometry (red). The first two values were estimated by using GEANT4 simulations and the last one by post-processing the results of hydrodynamic simulations with the FLYCHK code. (b) Typical plasma conditions along density profile, derived from CHIC hydrodynamic simulations, and successively post-processed in the FLYCHK simulations. The plot corresponds to the conditions at the time of laser peak arrival on the Al ablator.

Fig. 10. (a) Dependence of HE population registered by rear electron spectrometer in range of 150–300 keV on K_α intensity. (b) Spatial FWHM profiles of K_α emission registered by K_α imager for different targets used in experiment.

Fig. 11. (a) Dependence of K_α emission duration on laser intensity for different target ablators. (b) Timing of HE generation as measured by K_α emission vs laser pulse profile observed in shots over CH, C, and Al ablators.