Investigation of fast-electron-induced $K$α x rays in laser-produced blow-off plasma

Refluxing of fast electrons generated by high-intensity, short-pulse lasers was investigated by measuring electron-induced $K$α x rays from a buried tracer layer. Using planar foils of Au/Cu/CH, the 150-J, 0.7-ps TITAN short-pulse laser was focused on the gold foil to generate fast electrons and the 3-ns, 300-J long pulse beam irradiated on the CH side to create expanding plasma as a conducting medium. By delaying the short-pulse beam timing from the long pulse laser irradiation, the plasma size was varied to change electron refluxing in the target rear. The total yields and two-dimensional images of 8.05-keV Cu-$K$α x ray were recorded with an x-ray spectrometer and two monochromatic crystal imagers. The measurements show that the integrated yields decrease by a factor of 10 from refluxing to the nonrefluxing limit. Similar radial profiles of the $K$α images in the rear were observed at all delays. Hybrid-particle-in-cell simulations using plasma profiles calculated by a radiation-hydrodynamic code HYDRA agree well with the measured $K$α yields. The simulations suggest that conducting plasma with the size of ∼300 μm in the laser direction and ∼600 μm in the lateral direction at the density of 2 × 10${}^{20}$ 1/cm${}^3$ is sufficiently large to prevent electrons from refluxing in the target. The parameters found in this study can be useful in designing experiments utilizing a $K$α x-ray source in refluxing regime or a tracer layer in nonrefluxing regime.

Figure 1

A schematic of the TITAN laser and diagnostic setup.

Figure 2

Measured $K$α images on the side and rear imagers (a),(c) no plasma (SP only) and (b),(d) with plasma (8-ns delay).

Figure 3

Measured escaping electron spectra for SP only (no drive) and delays of 1, 6, and 8 ns.

Figure 4

Contours of simulated 8.0-keV x-ray emission for (a) the solid Au/Cu/CH target and (b) the target with the blow-off plasma at 8-ns delay. Averaged lineouts in laser and lateral directions are obtained by averaging the density profiles within the vertical and horizontal rectangular boxes in white dotted lines.

Figure 5

Expanding plasma sizes in laser and lateral directions from the HYDRA simulations.

Figure 6

A comparison of measured and simulated $K$α yields.

Figure 7

simulated contours of 8.0-keV x-ray emission for (a) the solid Au/Cu/CH target and (b) the target with the blow-off plasma at 8-ns delay.

Figure 8

(a) A comparison of simulated $K$α profiles for various delays in log scale. (b) Normalized radial profiles of the $K$α emission for 0-, 1-, and 8-ns delay in linear scale.