Ne-like Ge Research Articles

Ne-like Ge soft X-ray lasers driven by fundamental- and double-frequency lasers

In this study, experiments and simulations of Ne-like Ge soft X-ray lasers driven by nanosecond fundamental-frequency (1ω) and double-frequency (2ω) lasers are performed. The results show that 2ω lasers are also able to drive and produce Ne-like Ge soft X-ray lasers, even though the requirements for the driving conditions greatly increase compared to 1ω lasers. It is expected that the laser outputs at 23.2 and 23.6 nm will be stronger at higher power densities. The results provide a good foundation for executing experiments using Ne-like Ge soft X-ray lasers at existing large laser facilities, which cannot currently sustain 1ω drive lasers.

Collision strengths and effective collision strengths for Ne-like Ge

We have carried out a calculation of 241 levels for Ne-like Ge (Z = 36) to calculate electron impact collision strengths and effective collision strengths belonging to 1s22s22p5nl, 1s22s12p6nl (n = 3, 4, 5, 6; l = s, p, d, f, g, and h) configurations, which have been calculated by the fully relativistic flexible atomic code (FAC). Our calculations, based on the distorted-wave method with large configuration interactions, are included. Collision strengths have been generated over an electron energy range of (10–20 000 eV) and they have been listed at seven representative energies of 65.33, 1714.98, 2944.1, 4868.3, 7564.6, 11 040, and 15 221 eV in this work, and effective collision strengths data have been calculated from these at electron plasma temperatures 650, 850, 1050, 1250, 1450, 1650, and 1850 eV. Our results are compared with those available in the literature. It is found that the resonance effects are important in calculating the effective collision strengths.

Calculation of Atomic Data and Gain Coefficient for XUV & Soft X-Ray Laser Emission from Ge XXIII

Energy levels, transition probabilities, oscillator strengths, and collision strengths have been calculated for transitions in Ne-like Ge. The data refer to a 241 fine-structure levels belonging to the configurations 1s2 2s2 2p5nl, 1s2 2s1 2p6nl (n = 3, 4, 5, 6; l = s, p, d, f, g and h), which have been calculated by the fully relativistic flexible atomic code (FAC). These data are used in the determination of the reduced population for the 241 fine structure levels and gain coefficients over a wide range of electron densities (from 2 × 10+20 to 4 × 10+22 cm-3) and at various electron plasma temperatures (650, 850, 1050, 1250, 1450, 1650, 1850) eV by using the MATLAB R2013a Computer program for solving simultaneous coupled rate equations. The reduced population for the 241 fine structure levels the gain coefficients for those transitions with positive population inversion factor are determined and plotted against the electron density.

An experimental attempt to produce Thomson scattering with soft X-ray lasers

Thomson scattering is one of the important methods of diagnosing parameters of the hot and dense plasma state that receive widespread attention. But the probe light wavelength for Thomson scattering is more limited to the visible or ultraviolet light, so the electron density that is copable to be diagnosed is much lower than the critical density of the driving laser. In comparison, soft X-ray laser as a probe is hopeful for diagnosing higher density of plasma. The experimental attempt of soft X-ray laser Thomson scattering has been carried out using Ne-like Ge soft X-ray laser generated on the SG-II high-power laser device. According to the scattering case, incoherent scattering and coherent scattering experiments were carried out, but it is unable to obtain the significant scattering spectrum. Theoretical analysis shows that the main reason may be the focusing power density of Ne-like Ge soft X-ray laser being not enough, in the experiments. By optimizing the experimental conditions, it is expected to obtain coherent Thomson scattering in the future.

Output intensity of Ne-like Ge soft X-ray laser with double-pass amplification

Ne-like germanium soft X-ray lasers driven by nanosecond laser have very good applications, due to their stable output and strong intensity. The double-pass amplification mode, with a multi-layer reflective mirror added is investigated. The estimate and the experiment show that the output intensity of Ne-like germanium soft X-ray laser is enhanced by double-pass amplification. For 24mm length of double-pass amplification, the total output intensity is enhanced by 2.3 times as against that with no mirror added. Meanwhile, this mode results in quality decline of soft X-ray laser beam. So this mode is not entirely beneficial to the applied study.

Simulation of an X-ray laser in the transient gain-saturation regime

By using the TRANS code, we performed three-dimensional calculations of amplified spontaneous emission (ASE) in X-ray lasers on the 3p—3s transition in Ne-like Ge (λ = 19.6 nm) and Ti (λ = 2.6 nm) operating in the transient regime upon irradiation of a flat target by a high-power picosecond laser pulse focused into a line with one or two nanosecond prepulses. The hydrodynamics and population kinetics of the active medium of X-ray lasers were calculated by using the SS-9M code. The pulse duration, the gain, the spatial structure of the laser beam and the type of influence of a 'travelling' pump wave on the ASE brightness obtained in calculations are in agreement with the experimental data. The use of the 'travelling pump wave' leads not only to the increase in the ASE brightness but also considerably reduces its angular divergence.

Studies on Ne-like Ge X-ray laser driven by a 2ω nanosecond laser

Generally, it is more suitable for studies on Ne-like X-ray laser by using fundamental frequency laser as driving beam than by frequency doubling. However, many huge laser facilities built for ICF research are usually designed for doubling and trebling of laser frequency, and they are unfit for use as fundamental frequency laser. We simulated the Ne-like Ge X-ray laser making use of JB19 codes, respectively in the case of doubled and trebled frequency diving. The results indicated that the higher power output of Ne-like Ge X-ray laser is also obtained when driven by doubled frequency laser. However the required output of driving laser beam was 3.3 times that in the case of the fundamental. Any way it presents a direction of studying Ne-like X-ray laser with huge laser facilities in the future. A preliminary experimental study has proved this point.

Effect of ionization and recombination between excited states on gain of 19.6nm X-ray laser

As it is difficult to solve a large number of rate equations including doubly excited states of various ions, block matrix method is expected to dramatically reduce the machine time by dividing the large number of equations into pieces of small units, and solving them one by one, on condition that the ionization and recombination between excited states of ions can be ignored. In this paper, the influence of ionization and recombination between excited states on plasma status was studied, and detailed analysis of its mechanism was made. The effect on gain of Ne-like Ge 19.6nm X-ray laser respectively driven by 1.0ns pulse, 100ps pulse and 5ps pre-pulse and main pulse was studied. The simulation shows that for the 5ps driving pulse case, the ionization and recombination between excited states has relatively large influence, and it can not be ignored and that for the two quasi-stable state cases, in the gain region with electron density less than 5×1020cm－3, the FWHM temporal width of gain increases by～10%, and the FWHM spatial width of gain increases respectively by～13% and ～-23%, and the time the peak gain appears is delayed respectively by～1.4% and ～6.9%, which is tolerable, implying that the block matrix method could be applied in the qualitative modeling of QSS scheme X-ray laser. However, for accurate study, the ionization between the excited states could not be neglected.

Theoretical study of TCE Ne-like Ge 19.6nm X-ray laser

Strongly amplified X-ray laser observed in the recent transient collisional excitation (TCE) experiments pumped by a single short pulse suggests that with the help of low level prepulse producing lowly ionized preplasma it is also possible to obtain high gain in the TCE scheme. The TCE Ne-like Ge 19.6nm X-ray laser pumped by low level prepulse were studid with our code series. Time evolution of plasma status shows that the position of peak gain drifts outward and the gain region is widened as time goes on, which is similar to the QSS scheme.

Simulations of a Ne-like Ge X-ray laser and comparison with experiments in RFNC-VNIIEF

The results of 3D simulations of the laboratory Ne-like Ge x-ray laser in RFNC-VNIIEF are in good agreement with the experimental data. Calculations show that considerable rise of brightness and transverse coherence length can be achieved under half-cavity conditions.

Modeling RAL experiment to test our simulation

A code series for transient collisional excitation (TCE ) was newly developed based on the previous code series for QSS scheme, the TCE Ne-like Ge 19.6nm X-ray laser experiment done by RAL in 2000 was modeled to test our code series, a relatively good fit is obtained which provides convenience for our study of TCE scheme.

Theoretical predictions for the polarization of X-ray laser lines in the presence of a directed beam of hot electrons

The 19.6 nm laser line due to a 3p - 3s, J=0-1 transition in Ne-like Ge ions was found to be polarized parallel to the target plane with a degree of polarization of -53 % by Kawachi et al. (1995). They intepret it as a consequence of a an unbalanced population of the 3s magnetic sublevels. Kieffer et al. (1993) have shown that, during plasma heating, some electrons could be accelerated preferentially in the same direction as the pump laser. In the present work, we shall show that such anisotropic electrons can give a small difference in populations of the 3s sublevels. The 3s level spontaneous emission to the ground level is linearly polarized and can be reabsorbed by the Ne-like ion ground state which amplifies the population of the 3s levels, as well as it increases the different sublevel populations. The 3p-3s X-ray laser amplification depending on the 3s level population becomes polarized but with an opposite sign to the absorbed radiation from the ground level.

Extended lineidentification for Ne-like Ge XXIII

We report an investigation of the 2p53s, 3p and 3dconfigurations of Ne-like Ge XXIII, based on beam-foil spectroscopy.Several new 3s-3p and 3p-3d transitions have been observed and thedata are compared with earlier results.

Travelling wave chirped pulse amplified transient pumping for collisional excitation lasers

A ∼3 ps travelling wave chirped pulse amplified pulse at 6×10 14 W cm −2 superimposed on ∼300 ps background pulses is shown to be an efficient method to pump transient collisional excitation X-ray lasers in both Ni-like and Ne-like ions. Measurements of X-ray laser output as a function of plasma length are fitted with results of an amplified spontaneous emission model of the laser output taking account of travelling wave pumping effects. A small signal gain coefficient ∼42 cm −1 and a effective gain length product of ∼18 are measured for the Ni-like Sn laser at 120 Å. Simulations from a hydrodynamic and atomic physics code (EHYBRID) coupled to a ray trace code show that a spatially averaged small signal gain ∼65 cm −1 can be obtained in Ne-like Ge provided the optimum pumping pulse arrangement is used.

Simulation and optimization of the picosecond-laser-driven Ne-like Ge x-ray laser

We report numerical simulation of a collisionally pumped transient gain x-ray laser in Ne-like Ge ions. The laser system is calculated with a simplified atomic physics model and a self-similar expansion model. The validity of the solution is verified by a comparison of the calculated results for the Ne-like Ge J=0–1 x-ray lasers with the published results attained with sophisticated models. The effects of the opacity of the Ne-like ion 2–3 resonant lines and the pump laser’s pulse width on the dynamics of the transient gain are investigated through numerical calculations. It is found that the opacity affects the delay of the peak gain with respect to the peak of the pump pulse, the gain duration, and the peak gain coefficient as well. The driving laser conditions (laser wavelength, intensity, and pulse width) for the Ne-like Ge J=0–1 lasers are optimized. The results show that a moderate laser wavelength is good for a possible compromise between a high peak gain and a long gain duration.

Optimization of double drive pulse pumping in Ne-like Ge x-ray lasers

Pumping of the Ne-like Ge x-ray laser with two 100 ps duration pulses (a prepulse and main pulse) is investigated using a fluid and atomic physics code coupled to a 3D ray tracing postprocessor code. The modeling predicts the optimum ratio of the irradiance of the two pulses for the maximum x-ray laser output resulting from the balance between the relative lower electron density gradients and wider gain region which is produced with a larger prepulse and the higher peak gain coefficients produced with a small prepulse. With a longer pulse interval between prepulse and main pulse, a relatively lower optimum pulse ratio is found. The threshold irradiance of the main driving pulse with a prepulse required to make an order of magnitude enhancement of laser output compared to irradiation without a prepulse is also found at 3–4×1013 W/cm2 for Ne-like Ge.

Analysis of a picosecond-laser-driven Ne-like Ti x-ray laser

This paper models recent experiments in which a solid titanium target was illuminated by several joules of combined energy from a nanosecond laser pulse to create a preplasma followed by a picosecond laser pulse to drive the gain. Gains greater than 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are predicted for the Ne-like Ti 3p${\mathrm{}}^{1}$${\mathrm{S}}_{0}$\ensuremath{\rightarrow}3s${\mathrm{}}^{1}$${\mathrm{P}}_{1}$ transition at 326 \AA{}, which is driven by the monopole collisional excitation. High gain is also predicted for the 3d${\mathrm{}}^{1}$${\mathrm{P}}_{1}$\ensuremath{\rightarrow}3p${\mathrm{}}^{1}$${\mathrm{P}}_{1}$ transition at 301 \AA{}, which is driven by a combination of collisional excitation and self-photopumping. We also discuss the possibilities for driving a Ne-like Ge laser using this approach.

The effects of multi-pulse irradiation on X-ray laser media

Multipulse irradiation with 100 ps pulses of stripe Germanium targets is shown to enhance by up to several orders-of-magnitude the output of Ne-like Ge lasing on the J = 0–1 line at 196 Å compared to single pulse pumping. Various pre-pulse and multipulse configurations have been experimentally investigated for irradiances of ≈ 4 × 10 13W/cm 2 with a 1.06 μm wavelength pumping laser. The ionisation balance measured by a KeV crystal spectrometer (KAP crystal) has been found to not affect the X-ray laser output. Good agreement between the experimental results and a fluid code incorporating atomic physics, gain and X-ray beam ray tracing is obtained. The code results show that the enhanced X-ray laser output is produced by multipulse irradiation reducing the electron density gradients in the gain region and simultaneously increasing the gain region spatial size. These changes reduce the effect of refraction on the X-ray laser beam propagation.

A time-resolved spectroscopy study of the resonance-line emission in the Ge XXIII XUV laser

A comparison is presented of the temporally resolved resonance-line emission from the Ne-like Ge XUV laser (pumped with nanosecond pulses) with the predictions for the same emission from the hydro-atomic code EHYBRID. The specific lines chosen were the two 3s - 2p Ne-like lines at 10.01 and 9.762 Å, and the 3s - 2p F-like group of lines in the 9.4 - 9.6 Å region. Modification of the code to include 112 excited levels of the F-like ion facilitated a direct comparison between experiment and model of (i) the temporal variation of the emissions and (ii) the variation of the peak intensity ratios of the F-like to Ne-like emissions with irradiance on target. Agreement between experiment and model was generally good, with the trends of the F/Ne ratio variation with irradiance being clearly reproduced. The results show that the model is realistically simulating the hydrodynamics and atomic kinetics of the plasma. This provides further support that the model can simultaneously calculate the and the (sub-keV) transition intensities through population dynamics which are acceptably accurate.

X-ray laser beam propagation in multi-pulse pumping plasmas

The beam propagation of a Ne-like Ge X-ray laser at 19.6 nm produced by the double- and triple-pulse technique with a curved slab target has been examined using a ray-trace code in addition to a hydrodynamic code. The results show a significant gain increase in the triple-pulse pumping scheme. The predicted gains were in good agreement with the experimental data. The optimal design of experiments would require understanding the ray propagation in the gain region.