# Characterization of an X-ray Source Generated by a Portable Low-Current X-Pinch
## 2 Materials and Methods  
A tabletop pulsed-power plasma generator able to produce a peak current of \(\sim\)45 kA, with a rise time (10-90%) of 50 ns, loaded with two thin wires of 5 \(\upmu\)m initial diameter, forming an X-pinch load, was used in the experiments. The distance between the anode and cathode was 15 mm. A detailed description of the pinch device, including a 3D design as well as the functional specifications, is given in [14], while details about the laser-probing plasma diagnostics--such as three-frame Mach-Zehnder interferometry and three-frame focusing shadowgraph layouts--can be found in [16]. Four different two-wire configurations were tested: (1) tungsten (W) wires loaded at an angle between the crossed wires of \(55^{\circ}\), (2) W wires loaded at an angle of 98\({}^{\circ}\), (3) gold (Au)-plated W wires at an angle of \(55^{\circ}\), and (4) Au-plated W wires loaded at an angle of 98\({}^{\circ}\). The chosen angles follow the suggestions of [21]. W is an ideal material for such studies, since it results in efficient pinching for the current of 45 kA [22], and yields a very small X-ray source size [10]. On the other hand, the Au coating thickness used on the plated W wires was 0.09 \(\upmu\)m for all of the experiments presented in this article. High-atomic-Z-number materials such as the Au coating are expected to decrease the rate of energy deposition within the core and, therefore, prohibit the formation of high-density plasma [23], which may lead to a modified emitted power spectrum and total X-ray yield. Figure 1 shows the (a) top and (b) front views of the vacuum chamber of the X-pinch device, along with the three diagnostics (i.e., point-projection radiography diagnostic, X-ray PIN diodes, and pinhole camera) connected to three of the nine ports of the chamber. An NW40 flange was used to connect two silicon P-type intrinsic N-type (PIN) semiconductor diode detectors onto one of the nine ports of the vacuum chamber for the time-resolved X-ray measurements. Figure 1c,d schematically present the point-projection radiography and the pinhole camera, respectively. For the time-resolved measurements, filtered photodiodes were used, mounted on SubMiniature version A (SMA) female connectors. A suitable biasing circuit [24] was used to supply each detector for the X-pinch experiments. The spectral sensitivity S(E)f of the detector in the presence of a filter is determined by the equation:  
\[\mathrm{S(E)_{f}=S(E)e^{-\mu(E)\tau}=S(E)T(E)} \tag{1}\]  
where S(E) is the intrinsic spectral sensitivity of the photodiode as a function of energy E, while T(E), \(\mu\)(E), and \(\tau\) are the filter transmission, the linear attenuation coefficient, and the thickness of the filter, respectively. The sensitivity curve of the used BPX65 PIN detectors over the photon energy ranges from 0.1 to 20 keV [24]. The developed experimental scheme of the point-projection radiography, presented in Figure 1c, implies a point source of photons, such as an X-pinch to project the shadow of the sample on a photographic film. In our case, the source was located at a distance of d\({}_{1}\) = 155 mm in front of the sample, and the photographic film was fixed behind the sample at a distance of d\({}_{2}\) = 82 mm, achieving a magnification of M = (d\({}_{1}\) + d\({}_{2}\))/d\({}_{1}\)\(\approx\) 1.53. Furthermore, an Al step wedge of six steps was used for the spectral analysis of the X-ray source. The finest step of the wedge had a thickness of 15 \(\upmu\)m, which was multiplied by 2 at every next step. The geometric resolution (or penumbra zone width) \(\Delta\mathrm{x}=\mathrm{S(d_{2}/d_{1})}\)\(\approx\) 0.53 S of a point-projection system is primarily determined by the size S of the photon source [25]. The pinhole camera provides time-integrated images of the X-ray source. The dimensions of the spot size of the X-ray source define its suitability for the point source projection radiography.