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This paper studies the phenomenon of joint generation of terahertz (THz) and X-ray radiation in the argon nanocluster jet under the action of high-power femtosecond laser pulse in both the single-color and dual-color regimes. It was discovered that in a gas cluster beam the pulse duration affects the properties of THz and X-ray emission differently. For the same given total energy of optical pulse in the dual-color excitation regime of cluster medium, more than a five times increase of THz radiation power was observed in comparison with the single-color regime, while the conversion efficiency to the argon X-ray K-line reached 7 × 10 −6 and remained unchanged. The possibility of separation of contributions of different beam components into the THz signal was demonstrated experimentally, using contributions from clusters and nonclustered gas as an example. We suggest an interpretation of experimental results based on a theoretical model of cluster ionization that self-consistently predicts the level and dynamics of ionization and electron temperature in the clusters.Index Terms-Femtosecond laser pulses, gas cluster beam/jet, high-intensity radiation, nanoplasma, quadrupole radiation, terahertz (THz) and X-ray emission.

It was found that modification of the physical properties of the surface layer of a target makes it possible to control, effectively and within a wide range, the characteristics of a femtosecond laser plasma (temperature, degree of ionisation, emissivity, and nonlinear-optical susceptibility) at moderate intensities of 10 15^1 0 16 W cm À2 , which are optimal for promising laser-plasma femtosecond technologies. The targets used in an experimental investigation were periodic structures (gratings), ultrafine freely suspended films, and porous materials. Potential applications of such targets at intensities above 10 17 W cm À2 were considered. R V Volkov, V M Gordienko, M S Dzhidzhoev, M A Zhukov, P M Mikheev, A B Savel'ev, A A Shashkov Control of the properties and diagnostics of a dense femtosecond plasma formed from modified targets R V Volkov, V M Gordienko, M S Dzhidzhoev, M A Zhukov, P M Mikheev, A B Savel'ev, A A Shashkov

We report on effective x-ray generation under excitation of clusters of polyatomic CF 2 Cl 2 molecules by femtosecond laser radiation of moderate intensity I ≈ 5 × 10 15 W cm −2 (E ≈ 5 mJ). The maximal laser energy conversion efficiency to chlorine x-ray lines (E = 2.6-2.8 keV) reached 2 × 10 −5 when the CF 2 Cl 2 clusters were formed in the presence of the carrier gas helium. It was demonstrated that the spectrum of laser-induced x-rays can be used to detect mixed clusters, appearing in a 3% mixture of CF 2 Cl 2 with Ar. We proposed adding a light thermalizing gas to the mixture, i.e., using a ternary mixture CF 2 Cl 2 -Ar-He, to control the fraction of components that the mixed clusters are composed of. We have given the first demonstration of a dual-energy x-ray source with a tunable line amplitude that is controlled by the helium fraction in the mixture.