Abstract
Properties of gas clusters such as the size and number density when expanding into the vacuum after passing through a conical nozzle are analyzed for argon at an average density of 1020/cm3. Temporally and spatially resolved size and density distribution were measured from all-optical methods of Rayleigh scattering measurement and Nomarski interferometry using a CW laser. At the gas backing pressure of 80 bar, Ar clusters as large as 100 nm were obtained, which differs significantly from the size estimated by the conventional Hagena scaling law. The two independent methods of cluster characterization presented here would be useful to precisely determine the initial conditions in a variety of intense laser-cluster interaction driven applications such as neutron generation, thermonuclear fusion, efficient x-ray emission, and energetic ion acceleration.
Highlights
Properties of gas clusters such as the size and number density when expanding into the vacuum after passing through a conical nozzle are analyzed for argon at an average density of 1020/cm[3]
The cluster parameters are integrated over space and time and they are not quantified in absolute term
A rigorous tool to characterize the cluster parameters is required in precise determination of the initial conditions during laser-cluster interaction experiments
Summary
Clusters can be produced through a supersonic adiabatic expansion of gas with high Hagena parameter into the vacuum through a nozzle, where the collisional mean free path is much smaller than the nozzle outlet size[10,13]. Several experiments were performed to determine the constants a and b to match the Hagena scaling law in the given interval of Γ* (Table 1) As this semi-empirical law was derived without detail consideration of parameters such as the heat from condensation, any additional constraints of flows[14,19,25], or boundary layer effects[23], the real cluster size and density may largely deviate from the calculated results. In this range of Γ*, previous experimental works on cluster size determination[12,13,14] suggest that. As the cluster size (~ 10 nm) is much smaller than the wavelength of diode laser (635 nm), we are in the Rayleigh scattering regime as confirmed from our measurements (Fig. 1d,e)
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