The X-ray source for Compton radiography of ICF experiments are generated by using intense picosecond lasers to irradiate wire targets [1]. The wire diameter must be designed thin enough, for example, about 10 μm, to comply a high spatial resolution. Therefore, the laser-target interception is low which limits the photon yield.
      To improve the laser-target interception without compromising on spatial resolution, we developed a technique of coded-source radiography based on laser-driven annular source. It involves utilizing a picosecond laser to irradiate a large-diameter, thin-walled tube target. This kind of large-diameter tube target by nature has an increased laser-target interception than that using a thin wire. The generated annular X-ray source has the same shape as the tube's cross-section. We use it to backlight the object to form an convolved image. After deconvolution, a high-resolution reconstructed image will be obtained.
      Simulation and experimental studies on this kind of improved radiography were conducted. First, the advantages in increased contrast and spatial resolution of the image relative to the one using classical backlighters were demonstrated through Monte Carlo models. Second, in the deconvolution process, a source with uniform intensity is critical since it affects the reconstruction accuracy. An example of annular source generation was demonstrated. In the formation of a uniformly distributed source, the effect of electron recirculation [2] plays an important role. To verify the scenario, a variety of codes including FLASH, EPOCH and Geant4 were used. Third, in our experiments of laser-driven X-ray source backlighting, the energy conversion efficiency from laser to X-rays of 50 - 200 keV (CE_50-200) was measured. It was significantly increased by 7 times through the coded-source radiography using a tube target with diameter of 100 μm, compared to that using a 20-μm-diameter wire target. Meanwhile, the spatial resolution reached  20 μm, consistent with the latter.
      We believe that this technique of annular coded-source radiography has great prospects for the high-power laser facilities equipped with multiple picosecond lasers with high F-number, such as NIF-ARC and SG-II UP. Because it can not only fully utilize the laser energy by using a tube target with ~ 100-μm diameter while maintaining a high spatial resolution, but also can employ multi-angle lasers to drive the annular source to easily achieve uniform intensity distribution.

1.  R. Tommasini, et al., "Short pulse, high resolution, backlighters for point projection high-energy radiography at the National Ignition Facility," Physics of Plasmas 24, 053104 (2017).
2. M. N. Quinn, et al., "Refluxing of fast electrons in solid targets irradiated by intense, picosecond laser pulses," Plasma Physics and Controlled Fusion 53, 025007 (2011).

ultra-intense laser-driven X-ray,high-brightness X-ray source,coded-source imaging,ICF diagnose,high-resolution imaging,effect of electron recirculation