# A Versatile 100 Hz Laser System with Few-Cycle and TeraWatt Pulses for Applications
## 6 Applications
### Imaging with Laser Induced X-Ray  
Currently, an experimental capacity is under construction, where an incoherent, point-like X-ray beam is to be generated on a tape target [30]. The major advantage of a point-like X-ray source is that it has extremely high spatial coherence compared to conventional sources, as well as very short light pulses [31; 32; 33]. Hence, the brightness of the laser-induced X-ray source is considerably higher than the conventional one [34]. It makes this source very promising even for conventional applications such as imaging metal barrels or engine blocks [35; 36], as the time necessary for scanning an object is considerably shorter than with conventional sources. In our system, the target chamber, including the tape target system, the focusing off-axis parabola, and the alignment objective are currently under testing. The image reconstruction algorithms have been developed and tested with a commercial X-ray tube (AXT-320 by YXLON, Hamburg, Germany), providing radiation in the 20 keV-320 keV range. The linear diode array (SEZ W2-480, by NTB, Dickel, Germany) consists of 5566 elements with a pixel pitch of 83 \(\upmu\)m, and has a 12 bit depth AD conversion. Because of the linear array, we assume fan-beam imaging geometry. Upon imaging, the objects are moved vertically, and rotated (see Figure 15a for the schematics of the system). The images are reconstructed with the filtered back-projection algorithm (see, e.g., [37]) in MATLAB (v.R2020b), then the 2D slices are combined and the resulted 3D volume is visualized with the 3D Slicer software (v.5.2.2). As a case study, we scanned a Rubik's cube with X-rays generated at 1.7 mA, 200 kV. The full rotation was made with a step of 0.2\({}^{\circ}\) in each height of 1 mm. Each recording took 0.02 s exposure time, while the total scanning process lasted for about 6 h. The tomographic reconstruction of the Rubik's cube (Figure 15b) provides an impression of the robustness of the image algorithm. To adopt the image acquisition system, Monte Carlo simulations were run to mimic the point-like x-ray source, using the Geant4-based, open-source simulation toolkit, GATE (v. 9.2) [38]. To investigate transmission radiography (and such CT) possibilities, we took the proportionally reduced experimental layout (Figure 16a). We assumed a monoenergetic X-ray source with a focal spot of 20 \(\upmu\)m and a cone-beam angle of 10 degrees. The source was positioned in a vacuum chamber 10 mm far from a 0.5 mm thick Kapton window. Outside the chamber, we assumed dry ambient air. As a test object, we defined a small hollow plastic cylinder with an inner radius of 1 mm, an outer radius of 1.67 mm, and a height of 3.33 mm. We set the source-to-object and the object-to-detector distance both to 10 mm. The detector was a flat-panel array with 100 \(\times\) 100 pixels 0.1 mm \(\times\) 0.1 mm in size, thus, the physical resolution was very close to the one mentioned in the above experiment (the cone beam geometry and the flat panel detector were assumed to only speed up the simulation process). Figure 16b-e show the radiographs with different energy and photon counts. It turned out that in this case, 5 keV was too low, while 20 keV was too high to achieve an acceptable image quality. It appears that 10 keV is a good choice for this object. Figure 15: Schematics of the X-ray CT system testing the data acquisition and mechanical control systems (**a**). A slice of a Rubik’s cube (**b**), as part of the full image reconstruction. As can be seen, the visibility of the cylinder decreases with the gradually increasing photon energies. One solution for this problem is propagation-based phase-contrast imaging [39; 40]. The small size of the laser-generated X-ray source enables the adaptation of the technique with moderate source-sample distances that can be conveniently built in an optical laboratory. This technique allows us to investigate the features of small and weakly absorbing samples that are hard to observe with conventional absorption-based methods.