The interaction of a high-power laser with a low-density foam target can in some instances result in a significant enhancement in x-ray generation relative to that when the same laser is incident upon a homogenous solid. In this paper, we present x-ray emission studies from foam targets where the density is varied from under-dense to over-dense. The targets are irradiated with the first harmonic of Nd:Glass laser. The laser intensity on the target was approximately 2 × 1014 W/cm2 with the pulse duration of 500 ps. Mass-matched cellulose triacetate foam targets with densities of 2 mg/cc, 4 mg/cc, 7 mg/cc, and 20 mg/cc were used. The areal density presented by the targets on the laser beam axis was held constant at 0.2 mg/cm2 by varying the target thickness in inverse proportion to the density. The x-ray yield in the spectral range (5–8 keV) and (4.5–16 keV) was found to be enhanced by approximately 2.3 times in foam targets with the density of 2 mg/cc (under-dense) compared with foam targets with the density of 20 mg/cc (over-dense).
In this paper, enhancement in x-ray emission and reduction of kinetic energy of ions from low density foam plasma is demonstrated by performing experiment and hydrodynamic simulation. The plasma is produced by irradiation of solid gold and gold foam targets (densities 0.2 g/cc, 0.13 g/cc and 0.1 g/cc) at intensities in the range of 4×10 13 -1×10 14 W cm −2 . Time resolved x-ray emission is measured by an x-ray streak camera with 10 ps resolution. The x-ray flux measured by the streak camera from low density gold foam shows a 13% enhancement in comparison to solid gold in the spectral range >0.8 keV and above. Decrease in velocity of ions is observed in low density gold foam. In solid gold, thermal ions peak velocity is 31×10 4 m s −1 and spread in narrow energy width, however, in case of 0.1 g/cc, peak velocity reduces to 6×10 4 m s −1 towards target normal and emitted in broad energy range. Shadowgraphy results also provide evidence of narrower expansion of plasma from solid gold. However, total ion flux from low density gold foam is comparable to ion flux of solid gold indicating the process of volumetric absorption.
Optimization of a laser produced plasma (LPP) X-ray source has been performed by analyzing K-shell emission spectra of Al plasma at a laser intensity of 10 13-10 14 W/cm 2. The effect of varying the laser intensity on the emissivity of the K-shell resonance lines is studied and found to follow a power law, E α I α with α=2.2, 2.3, 2.4 for He β , He γ , He δ respectively. The emission of these resonance lines has been found to be heavily anisotropic. A Python language based code has been developed to generate an intensity profile of K-shell spectral lines from the raw data. In theoretical calculations, the temperature is estimated by taking the ratio of the Li-like satellite (1s 2 2p-1s2p3p) and the He β (1s 2-1s3p) resonance line and the ratio of the He-like satellite (1s2p-2p 2) and the Ly α (1s-2p) resonance line. To determine the plasma density, stark broadening of the Ly β spectral line is used. Simulation was carried out using the FLYCHK code to generate a synthetic emission spectrum. The results obtained by FLYCHK are T e =160 eV, T h =1 keV, f=0.008, n e =5 x 10 20 cm-3 and the analytical model resulted T e =260-419 eV and n e =3x10 20 cm-3 .
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