**7. Conclusion**

Laser-produced plasmas based on gas targets serve as versatile and nearly debris-free soft X-ray sources at a table-top size. In this chapter, a method has been shown by which the bril‐ liance of gas targets can be improved. To this end, a background pressure is applied to the gas jet that leads to a strong recompression of the target particles. For the example of an un‐ derexpanded supersonic nitrogen jet, the resulting barrel shock has been qualitatively vi‐ sualized by the Schlieren photography. The corresponding density distribution was obtained by a quantitative Hartmann-Shack measurement. Measured values of the shock lo‐ cation and particle densities are of the same order of magnitude as those of a first estimate that was partly based on correlations. The size of the resulting plasma is reduced by a factor of 0.71 and its shape becomes more uniform, thus improving the coherence properties of the source. At the same time, the number of photons per solid angle at *λ* = 2.88 nm is raised by a factor of 7.1. In this manner, the brilliance of the source is increased by a factor of 10.0 to *Br* = 3.15 × 1016 photons/(mm2 mrad2 s). Even greater increases may be obtained by using hydro‐ gen as the background gas since H2 shows a 13 times lower absorption of the generated Xrays compared to He [48]. However, for safety reasons H2 has not been employed here. A further increase in the plasma's brilliance is to be expected with increasing the stagnation and background pressure. An essential condition for achieving this is an improvement in the differential pumping system in order to lower the reabsorption of the soft X-rays by the background gas.
