Abstract
A method is presented for improving the brilliance of laser-produced soft x-ray sources that are based on pulsed gas jets as the targets. The conversion efficiency of laser energy into soft x-ray radiation is enhanced by locally increasing the particle density of the target species. This is achieved by applying a small background pressure to the supersonic flow emanating from a nozzle. In this manner, a supersonic jet with a so-called barrel shock system is formed. On passing the shocks, particles become locally concentrated, forming high-density regions that are used as the targets. An estimate of possible increases in particle densities is provided. The jet flow is then analyzed experimentally by Schlieren imaging, thus visualizing the spatial shock structure. Additionally, a quantitative measurement of the gas density is made using a Hartmann–Shack wavefront sensor. The beneficial effect of the applied background gas on plasma generation is clearly more prominent than its absorbing effect on the photons originating from the plasma. This is shown for a nitrogen target with helium as the background gas. A plasma, generated behind the barrel shock in the nitrogen jet, emits monochromatic photons at a wavelength of 2.88 nm. The peak brilliance of the source is increased by an order of magnitude, resulting in 3.15 × 1016 photons (mm2 mrad2 s)−1.
Highlights
A method is presented for improving the brilliance of laser-produced soft x-ray sources that are based on pulsed gas jets as the targets
The nozzle is opened for a period of 1 ms, generating an underexpanded supersonic jet that expands from stagnation pressures ps of up to 16 bar into vacuum, i.e. the background pressure pb is as low as 10−4 mbar
In the approach pursued in the present study, the background pressure pb is increased to several tens of mbar in order to generate a barrel shock in the supersonic jet
Summary
The setup of a standard soft x-ray source based on gas targets is used [4]. It basically consists of a piezo-electrically operated Proch–Trickl gas valve [12] mounted on a vacuum chamber, and a driving Nd:YAG laser (fundamental wavelength 1064 nm, pulse energy 800 mJ and pulse duration 6 ns). Plasma production takes place as soon as a critical power density of ≈1012 W cm−2 is reached (i.e. shortly before the focus) at a sufficiently large particle density [13] This initiates multiphoton ionization of the target gas followed by avalanche ionization, creating large numbers of free electrons. In the approach pursued in the present study, the background pressure pb is increased to several tens of mbar in order to generate a barrel shock in the supersonic jet For this purpose, helium is utilized as background gas due to its high transmittivity to photons generated by the plasma. Behind the shock system generated in the jet the gas density increases In this manner, regions involving high densities of the target gas are obtained at comparably large distances from the nozzle. Corresponding spectra can be found in figure 2, produced by
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