Plasma-kinetics perspective on a collisional Ni-like x-ray laser pumped by a single profiled laser pulse

Some aspects of plasma kinetics in a Ni-like Ag soft x-ray laser have been analyzed, starting from the real shape of the pump laser pulse measured with a third-order correlator. The role for pump energy reduction of a complex pump pulse structure at a low intensity level has been identified, and the modeling results on the gain lifetime correspond well with the length of the output x-ray pulse measured in the experiment.

Figure 1

The correlation trace of the amplified pump laser pulse measured with the OPA (third-order) correlator [9]. Time counting is related to the peak of the heating pulse.

Figure 2

(a) Modeled shapes of the profiled pump pulse and (b) zoomed pulse structures at the low-energy level. The shape of the pump pulse with an energy of 2.6 J corresponding well to the shape obtained from the third-order correlator measurement is traced with the solid line. Doted line sketches the pump pulse with “steep edges,” i.e., without any pedestal on the falling edge. The dashed line shows a pulse with a trapezoidal pedestal and a background level of ${10}^{-4}{I}_{\max }$. The dash-dotted line show nearly identic pulse but with the background at a level of one order of magnitude higher. The effect of the dashed and dotted pulse shapes is compared in the simulations.

Figure 3

The history of the energy partition (absorbed energy $E_{\mathrm{abs}}$, thermal energy $E_{\mathrm{therm}}$, ionization energy $E_{\mathrm{ion}}$, and kinetic energy $E_{\mathrm{kin}}$) in a plasma heated by pulses with two profiles: with steep edges (without pedestal sketched with the dotted lines) and including a trapezoidal pedestal with a very low background (dashed lines). The radiative energy has been neglected due to low level. Here “0” on the time axis corresponds to the peak of the heating pulse.

Figure 4

The modeled spatiotemporal evolution of the electron temperature in an Ag plasma containing abundance of Ni-like ions and created by pulses with two profiles: (a) with steep edges and (b) including a trapezoidal pedestal. Here “0” on the time axis corresponds to the peak of the heating pulse and the temperature values are given in electronvolts.

Figure 5

The modeled spatiotemporal evolution of the average ionization stage in an Ag plasma containing abundance of Ni-like ions and created by pulses with two profiles: (a) with steep edges and (b) including a trapezoidal pedestal. Here “0” on the time axis corresponds to the peak of the heating pulse and the ionization stage is given in arbitrary units.

Figure 6

The modeled spatiotemporal evolution of the electron density in an Ag plasma containing abundance of Ni-like ions and created by pulses with two profiles: (a) with steep edges and (b) including a trapezoidal pedestal. Here “0” on the time axis corresponds to the peak of the heating pulse and the density unit is $\left[{\mathrm{cm}}^{-3}\right]$.

Figure 7

The modeled spatiotemporal evolution of the local gain coefficient at 13.9 nm in a plasma with an abundance of Ni-like Ag ions produced by pulses with two profiles: (a) with steep edges and (b) including a trapezoidal pedestal. Here “0” on the time axis corresponds to the peak of the heating pulse. The gain coefficient is given in $\left[{\mathrm{cm}}^{-1}\right]$.

Figure 8

Modeled near-field spot of the x-ray beam at 13.9 nm obtained by pumping: (a) with steep edges and (b) including a trapezoidal pedestal. Please note that the maximum intensity in (a) is still six orders of magnitude lower than that in plot (b). The snapshots correspond to the situation 20 ps behind the gain peak (27 ps behind the peak of the heating pulse).

Figure 9

Modeled near-field spot of the x-ray beam at 13.9 nm obtained by pumping: (a) with steep edges and (b) including a trapezoidal pedestal. Similarity of the spot shapes is evident. The snapshots correspond to the situation 7 ps behind the peak of the heating pulse, i.e., at the moment of the gain peak.

Figure 10

The gain history of the maximum local gain for different pump pulse structures. Here “0” on the time axis corresponds to the peak of the heating pulse. The line styles used corresopond to the pulse shapes sketched in Fig. 2. Solid: the real measured pulse shape, dot: without any pedestal (corresponding to the traditional shape in double-pulse scheme), dashed: trapezoidal pedestal and very low background lasting 4 ns, dash-dotted: trapezoidal pedestal and stronger background lasting 4 ns.

Figure 11

Temporal shape of the output XRL pulse in far-field modeled with EHYBRID. Here “0” on the time axis corresponds to the peak of the heating pulse.