Influence of pump pulse structure on a transient collisionally pumped $\mathrm{Ni}$-like $\mathrm{Ag}$ x-ray laser

Results of numerical simulations on a $\mathrm{Ni}$-like silver x-ray laser pumped by a single picosecond laser pulse are presented. Since the mechanisms responsible for the significant reduction in the pump energy are not well understood, the results of theoretical simulations with emphasis on the plasma kinetics and dynamics in a $\mathrm{Ni}$-like $\mathrm{Ag}$ x-ray laser are presented and referred to the experimental data. Special attention has been paid to the influence of the pump pulse shape and length on the gain and its duration. It was found that a low-level pulse pedestal being an integral part of the leading edge of the pump pulse is very beneficial to the pump energy reduction. The thermal cooling process has been identified as the mechanism strongly contributing to gain termination if a low-energy single-profile laser pulse with the width of a few picoseconds is used in the pump process.

Figure 1

Sketch of the pump pulse structure with three different forms of the background used in modeling—a Gaussian ramp, linear ramp, and flat-topped pulse. The picosecond laser pulse is described according to the notation applied to characterize the pump laser pulses.

Figure 2

Distribution of the local gain coefficient as a function of the distance from the target for a Gaussian pedestal and different times delayed with respect to the short pulse onset. The pump pulse is described in the notation of Fig. 1 by ($2\mathrm{ns}$, $2\mathrm{ns}$, $20\mathrm{mJ}$); ($3\mathrm{ps}$, $6\mathrm{ps}$, $2.6\mathrm{J}$).

Figure 3

The time history of the local gain coefficient for (a) different background pulse parameters combined with an intense component in the form ($3\mathrm{ps}$, $6\mathrm{ps}$, $2.6\mathrm{J}$) and (b) different intense components of the pump pulse preceded by the background pulse ($2\mathrm{ns}$, $2\mathrm{ns}$, $20\mathrm{mJ}$).

Figure 4

Distribution of the electron temperature vs the distance to the target for different times delayed relative to the short pulse onset. Pump pulse: ($2\mathrm{ns}$, $2\mathrm{ns}$, $20\mathrm{mJ}$); ($3\mathrm{ps}$, $6\mathrm{ps}$, $2.6\mathrm{J}$).

Figure 5

Distribution of the plasma density vs the distance to the target for different delays respective to the short pulse onset moment. ($2\mathrm{ns}$, $2\mathrm{ns}$, $20\mathrm{mJ}$); ($3\mathrm{ps}$, $6\mathrm{ps}$, $2.6\mathrm{J}$).

Figure 6

The time history of the local and ray-averaged (dots) gain coefficients as well as that of the averaged gain-length product. The pump pulse: ($2\mathrm{ns}$, $2\mathrm{ns}$, $20\mathrm{mJ}$); ($6\mathrm{ps}$, $6\mathrm{ps}$, $2.4\mathrm{J}$).

Figure 7

The time history of the energy deposition in the plasma under two different irradiation conditions. The group of the curves on the left side of the plot within the time gap between 0 and $12\mathrm{ps}$ corresponds to the pulse in form ($2\mathrm{ns}$, $2\mathrm{ns}$, $20\mathrm{mJ}$); ($1.5\mathrm{ps}$, $1\mathrm{ps}$, $3.2\mathrm{J}$) and the other one to the pulse described by ($2\mathrm{ns}$, $2\mathrm{ns}$, $20\mathrm{mJ}$); ($6\mathrm{ps}$, $6\mathrm{ps}$, $2.6\mathrm{J}$). $E_{pl}$ represents laser energy delivered (solid); $E_t$ is total energy absorbed (dashed); $E_{th}$ is thermal energy (chain); and $E_i$ is ionization energy (dash+double-dot).

Figure 8

The time history of the maximum ionization stage in the plasma irradiated with the pulse of different temporal forms.

Figure 9

Near-field patterns obtained by ray tracing: (a) $14\mathrm{ps}$ (maximum gain) and (b) $22\mathrm{ps}$ after the onset of the short component. The plots show only the upper half of the symmetric image.

Figure 10

The near-field image of the beam registered in the experiment with the pump pulse of $\sim 5\mathrm{ps}$ width and energy of $1.45\mathrm{J}$. The cross seen in the image center comes from the mesh supporting the Al filter. The vertical axis corresponds to the position of the target surface which was determined by separate shots with crossed wires positioned $3–4\mathrm{mm}$ from the target edge.