X-ray emission from laser-plasma interaction is an important x-ray source, and improving laser to x-ray conversion is imperative for various applications. The laser to x-ray conversion efficiency (CE) was simulated for gold targets with different initial densities. Using a 0.1 g/cm3 Au layer target, an x-ray conversion efficiency of 50.8% was obtained, which was 1.34 times of the 37.9% for the solid density target. It has been shown that the enhancement of the x-ray conversion efficiency is caused by the increase of absorption from the incident laser and reduction of ion kinetic energy due to the initial low density of the gold target.

As an important x-ray source, enhancement of x-ray emissions from laser-produced plasmas is significant for various applications. Due to less expanding kinetic loss, gold foam with low initial density can have an enhanced x-ray conversion efficiency compared with solid-density gold. However, low-Z impurities within gold foam targets will diminish the enhancement remarkably, and should be tightly controlled. This paper presents an experimental study of a high brightness laser plasma soft x-ray source, based on a 0.36 g cm −3 gold foam target with negligible impurities irradiated by nanosecond laser pulses with power density around 3 × 10 14 W cm −2 at the Shenguang II laser facility. A conversion efficiency, from multi-eV to multi-keV, of 51.2% is achieved in the x-ray emissions-about 21% relative enhancement compared with a solid-density gold target, and the highest conversion efficiency for Au foam planar targets yet. Good agreement has been achieved between the semi-analytical model prediction and the experimental results.

The enhancement of laser to x-ray conversion efficiencies using low density gold targets [W. L. Shang, J. M. Yang, and Y. S. Dong, Appl. Phys. Lett. 102, 094105 (2013)] is demonstrated. Laser to x-ray conversion efficiencies with 6.3% and 12% increases are achieved with target densities of 1 and 0.25 g/cm3, when compared with that of a solid gold target (19.3 g/cm3). Experimental data and numerical simulations are in good agreement. The enhancement is caused by larger x-ray emission zone lengths formed in low density targets, which is in agreement with the simulation results.

Foam gold was proposed to increase the laser to x-ray conversion efficiency due to its important applications. To understand the mechanism of x-ray enhancement, the detailed energy distributions and plasma profiles for laser-irradiated solid gold and foam gold targets were studied comparatively by hydrodynamic simulations using the code Multi-1D. It is confirmed that the radiation heat wave is subsonic for the normal solid gold target, while supersonic for the foam gold target. The shock wave, which is behind the supersonic radiation heat wave for the foam gold target, generates a plasma temperature gradient with high temperature near the shock wave front to produce an additional net outward radiation for enhancement of the x-ray emission. Much larger inward plasma velocity is also driven by the shock wave as an initial plasma velocity for the laser deposition and electron thermal conduct zone, which decreases the expanding plasma kinetic energy loss and helps to increase the x-ray radiation.