In this article, capillary discharge Ne-like argon 46.9nm soft X-ray laser has been firstly manifested with 4.8mm inner diameter alumina capillary for higher discharge currents. We have designed and installed capillary discharge setup for 4.8mm inner diameter alumina capillary to achieve intense 46.9nm laser. One dimensional Langragian Magneto-hydrodynamics (MHD) code was used to simulate the plasma conditions at the lasing time. The MHD code was used to perform the parametric studies of Z-pinch argon plasma, such as electron temperature, electron density and Ne-like argon ion density. The intensities of capillary discharge 46.9nm laser emitted from 4.8mm inner diameter alumina capillary were measured at 30, 36 and 40kA main discharge currents. According to the results, when the main current amplitude was increased from 30kA to 36kA and 40kA, the intensity of laser produced at optimum pressure increased up to 1.5 and 2 times, respectively. Moreover, we also studied the influence of predischarge current by increasing the predischarge current from 25 to 250A and investigated 140A as the best predischarge current for lasing. Hence, increasing the amplitude of main current using a comparatively larger inner diameter capillary is an effective way to improve intensity of capillary discharge 46.9nm soft X-ray laser. The maximum energy of 46.9nm laser was observed approximately 1.5mJ under best discharge conditions. The discussion has been made on the enhancement of 46.9nm laser intensity for higher main discharge currents and best predischarge current with experimental and simulated results. This is the first observation of 46.9nm laser with 4.8mm inner diameter alumina capillary.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
There are innumerable and irreplaceable applications of soft X-ray lasers in the world of scientific research , many researches have been done on them. Among all the compact schemes, soft X-ray lasers with capillary discharge pumping are the most capable and economical, which is encouraging for laboratory applications [2,3]. Capillary discharge soft X-ray lasers have many applications such as surface science, photochemistry, holography, dense plasma diagnostics , nanopatterning  and the ablation pattern of solids . Furthermore, the applications of soft X-ray laser can be further extended by increasing the intensity.
Therefore, the research to increase the intensity of soft X-ray laser needs to be done theoretically and experimentally. Rocca et al. described a capillary discharged Ne-like argon 46.9nm soft X-ray laser firstly in 1994 by using a 4.0mm inner diameter poly-acetyl capillary . The gain coefficient was calculated as 1.16 cm−1 for 4.0mm inner diameter capillary . However, the ablation of capillary wall was very severe, which may be the reason of decreasing main discharge current efficiency . Alumina capillary of 3.2mm inner diameter was used to minimize ablation due to higher heat resistance of alumina . Furthermore, it was proposed that laser pulse of energy up to 1 mJ, using a main current of 26 kA, can produce more than 5000 shots with this type of alumina capillary . Later on, alumina capillary of ~3mm inner diameter was used to enhance the intensity of soft X-ray laser, by Rocca’s team and many others [10–21]. In 2007, Kwek and Tan used 3mm inner diameter alumina capillary and measured value of gain coefficient was 1.2cm−1 for a main discharge current pulse 16 kA . In order to obtain high intensity soft X-ray laser, a higher amplitude main current pulse is essential to pinch the pre discharged plasma within the capillary. As a consequence of this, the laser amplification will reduce due to severe ablation of capillary wall. Due to severe ablation of the inner surface of the capillary, it can produce less laser shots with a main current of greater amplitude [2,3]. Therefore, inside the capillary main current density must be reduced to minimize the wall ablation. It can be accomplished by using alumina capillary of larger inner diameter.
Nevertheless, there are very few studies on alumina capillaries having inner diameter greater than 3.2mm. The highest gain coefficient obtained by Ben-kish et al. was 0.75 cm−1 for a main current of 40kA, by using a 16.5cm long alumina capillary having inner diameter of 4.2mm . By employing 3.3 and 4.0mm inner diameter capillaries, the possibilities to achieve soft X-ray laser for main current pulse of 20kA were analyzed . Ritucci et al. used 18.6cm long alumina capillaries having internal diameter of 2.4mm, 3.2mm and 4.0mm, and 3.2mm was declared as the best diameter, with the main current of 30kA .
The measured value of gain coefficient for 4.0mm inner diameter alumina capillary is relatively low whereas works in non- saturation region . Therefore, investigation on higher main currents with 4.8 mm or larger inner diameter was not a major concern. Every research group from all over the world used various amplitudes of the main current, which was directly influential on the Z-pinch plasma of gain medium as well as on the intensity of soft X-ray laser. Consequently, the main current is one of requisite parameters. We firstly generated a high-intensity capillary discharge 46.9nm laser by utilizing best predischarge currents along with higher main currents with 4.8 mm internal diameter alumina capillary. The laser intensity is higher than that of the extensively used 3.2mm inner diameter alumina capillary. Moreover, the alumina capillary with larger inner diameter utilized for high amplitude of main currents was especially important to achieve more intense 46.9nm laser. Therefore, it was required to research the influence of main current on the intensity of 46.9nm laser. However, to the best of our knowledge, there is no theoretical as well as experimental research for the influence of main current on the intensity of 46.9nm laser.
In this paper, we firstly simulated the plasma conditions at the lasing time by MHD code and measured the intensity of Ne-like argon 46.9nm laser utilizing alumina capillary of 4.8mm internal diameter for the main currents of 30kA, 36kAand 40kA. In order to obtain capillary discharge soft X-ray laser with higher intensity, the amplitude of main discharge current need to be increased. However, it will increase the ablation of capillary wall and then impede the laser amplification. So, the main discharge current density on the inner surface need to be reduced, which can be achieved with a larger inner diameter alumina capillary. Therefore, we used larger inner diameter alumina capillary which is beneficial to produce intense laser with higher main discharge currents. The estimated capillary life time is about 6000 shots. The optimum pressure, laser intensity and shape of 46.9nm laser spot produced by alumina capillary for different main currents were investigated theoretically and experimentally. American group estimated the maximum laser pulse energy about 1mJ by using 3.2mm alumina capillary for a main discharge current of 26kA, whereas the capillary lifetime was estimated about 5000 shots . The maximum laser pulse energy estimated in our experiment with 4.8mm inner diameter capillary was 1.5 mJ, whereas capillary lifetime was observed about 6000 shots. We increased the main current amplitude for the 4.8mm inner diameter alumina capillary from 30kA to 36kA and 40kA and measured the change in laser intensity. The theoretical results suggest that the significant enhancement in the laser intensity is due to increment in Ne-like Ar density.
Moreover, it is also observed that the laser intensity strongly depends upon the pre-discharge current. Japanese and Malaysia group studied the influence of predischarge current using a very low predischarge current of 10 to 20A for 3mm inner diameter capillary [10,16]. In this experiment, we increased predischarge current from 25 to 250A by using 4.8mm inner diameter capillary and found 140A as the best pre-discharge current to produce sufficient preionization before the arrival of main discharge current. It is investigated that high predischarge current is required to produce preionization in larger inner diameter capillary. After doing a series of experiments we successfully investigated the best predischarge current, optimum Ar initial filling pressure and main discharge current for newly employed 4.8mm inner diameter alumina capillary.
2. Description of capillary discharge soft X-ray laser system
At Harbin Institute of Technology China, the experiments were performed on the capillary discharge 46.9nm laser. A schematic diagram of capillary discharge soft X-ray laser system used in our experiment is shown in Fig. 1. A pre-pulse and main pulse system were used in the experiment to achieve lasing action. A water capacitor, 10-stage Marx generator and a main switch formed the main pulse generator. We vary the charging voltage of each capacitor of Marx generator and N2 pressure in the main switch in order to change the amplitude of main current. A change results in output voltage for the Marx generator by varying the charging voltage of each capacitor of the Marx generator. In , Abdullin et al. presented a comprehensive introduction of main current pulse generator.
Rogowski coil embedded in the discharge chamber was used to measure the main current waveform. To provide a pre-pulse through the argon filled capillary, the delay time of several microseconds was fixed before the injection of main discharge current through argon gas, a pre pulse generator is utilized. The pre-ionization of the gas by the pre-pulse current helps to produce uniform compression over the entire length of plasma column inside the alumina capillary throughout the propagation of main discharge current pulse. Production of capillary discharge 46.9nm laser without utilization of a pre-pulse is quite difficult task. X-ray diode (XRD) was used to study the temporal evolution of laser beam along the axis of alumina capillary. A gold coated cathode and steel mesh anode were the parts of X-ray diode. The signal from XRD and Rogowski coil was supervised by a (Tektronix 7104) digital oscilloscope having bandwidth of 1GHz. In order to research the spatial distribution characteristics of 46.9nm laser, the Ce: YAG fluorescent screen was used to detect the laser spot at 170cm from the capillary exit. The laser spot was captured by a high-resolution Nikon camera. The alumina capillary used was of the length 35cm and inner diameter of 4.8mm. Maximum main discharge current of 45kA was utilized to produce high intensity 46.9nm laser.
3. Theoretical simulation and experimental results analysis
Experiments were conducted to study the influence of main discharge current on the intensity of 46.9nm laser with 4.8mm alumina capillary whose length was 35cm. Before studying the effect of main current on the intensity of soft X-ray laser, it was necessary to deliberate various plasma parameters such as electron density, electron temperature and Ne-like argon density. Due to the pre-ionization of the argon in the experiment, the capillary is filled with homogeneous plasma with low ionization state. So, when simulating the Z-pinch process with MHD code, the inner diameter of the capillary is taken as the initial plasma diameter. In , Lan et al. presented comprehensive description on MHD equations which are used in this study. The ablation of wall material of alumina capillary was neglected during Z-pinch in MHD simulations.
Figures 2(a)-2(c) illustrate the simulated radial distributions of electron temperature at the lasing time at different initial pressures while the initial diameter of plasma is 4.8mm. The theoretical simulation results show that electron temperature increases as the amplitude of main current increases. The reason is that when the initial plasma diameter is constant and the main current amplitude is increased, the current density of plasma surface increased, so the magnetic pressure of the Z-pinch is increased and the Z-pinch processes become fast, which leads to the increase of the Z-pinch velocity and electron temperature when the plasma is pinched near the axis. The gain coefficient primarily depends upon the electron temperature and Ne-like argon density of plasma column . Therefore, increasing the amplitude of main current pulse may increase the gain coefficient of the laser. In addition, with the decrease of initial pressure, the electron temperature increases gradually. It is also noticed that the electron temperature increases as a function of radial distance. Figure 2 illustrates that the electron temperature with 40kA is higher than that with the 30kA and 36kA.
Figures 3(a)-3(c) depict the simulated electron density distributions in the radial direction at the lasing time at different initial pressures while the initial diameter of plasma is 4.8mm for the main currents of 30kA, 36kA and 40kA respectively. For each main current, as the argon initial pressure increased, the electron density also increased. This was due to an increase in the initial density of argon. The simulated electron density distributions have comparatively smooth region about 0.1-0.3mm. It is also observed that the electron density decreases significantly with radial distance. Furthermore, it can be seen that a concave electron density profile developed as we increased the main discharge current amplitude. The electron densities were in between ~2-3 × 1018 cm−3 in this relatively flat region for the main currents having amplitude 30, 36 and 40 kA at different initial argon pressures.
Figures 4(a)-4(c) illustrate the simulated radial Ne-like argon ions density distributions at the lasing time at different initial pressures while the amplitude of main discharge current is 30, 36 and 40kA, respectively. Theoretical results demonstrate that the values of Ne-like argon ion densities are high near the axis while densities decreased with radial distance. The Ne-like argon ion densities are much higher for the main current of 36 and 40kA. The pinching of preformed plasma becomes strong by increasing the main discharge current as a result more argon converts into Ar8+. It can be seen that at the same initial argon pressure, higher the amplitude of main discharge current, higher the Ne-like argon density. The Ne-like argon density is positively correlated with gain coefficient which is promising to obtain high intensity 46.9nm soft X-ray laser. Therefore, increasing the amplitude of main current pulse may be beneficial to increase the value of gain coefficient in order to achieve high intensity 46.9nm laser.
Figures 5(a)-5(c) show the dependence of laser intensity on the amplitude of pre-pulse currnet for three different main discharge currents. Without providing pre-pulse current the achievement of 46.9nm laser is quite difficult task. The experiment was performed with main discharge currents of 30, 36 and 40kA to investigate the influence of pre-pulse current on the intensity of 46.9nm laser. The pre-pulse current was increased from 25 to 250A. When the pre-pulse current is higher than 80A, the laser intensity amplitude is higher and shows a good reproducibility, whereas the intensity decreases and shows less reproducibility for pre-pulse of 80A. As the pre-pulse current goes above 140A laser intensity decreases significantly. Under high predischarge currents, the temperature of plasma increases rapidly due to high amplitude of pre-current pulse, volume of plasma ejecting out from the pinhole increases, and preformed Ar plasma density decreases, causing the laser intensity to decrease significantly. At the same time, it may affect the axial uniformity of plasma, resulting decrement in laser intensity. The laser intensity is low in the domain of low predischarge currents because low predischarge current is not capable of producing suffient preionization before the injection of main discharge current. The predischarge amplitude must be sufficiently high enough to produce significant axially uniform preionized in plasma column. This pre-pulse current dependency describes the importance of sufficient pre-pulse current to produce considerable preformed plasma before the injection of main discharge current. We observed that 140A is the best predischarge current for all main currents 30, 36 and 40kA to produce axially uniform preionization before the arrival of main discharge current.
Figures 6(a)-6(c) show the laser intensity as a function of intial Ar filling pressure and pre-discharge currents for three different main discharge currents. In this experiment we used three different predischarge currents and observed the laser intensity by changing initial filling pressure. It is observed that the laser intensity increases with increase in pre-discharge current and have maximum value for 140A. The predischarge current of 140A can produce higher abundance of argon ions without excessively high pre-ionization plasma temperature. The amplitude of laser becomes stable and increases significantly by increasing predischarge current. To obtain a stable laser pulse, it is observed that main discharge current must be initiated in axially uniform preformed plasma. When we increased predischarge current from 80 to 140A, the amplitude of laser pulse tends to be high and stable. As we increased predischarge current upto 250A the laser intensity decreased sharply. In case of 250A predischarge current, the plasma continuously heats up during the pre-ionization which increased the temperature simultaneously. However, due to the increase in temperature, the internal pressure of the plasma increased and ejected out from the small pinhole, causing a decrease in the plasma density in the capillary. Thus, the laser intensity is higher when the predischarge current is 140A because preformed plasma density is higher inside the capillary for this current. Therefore, the deviation of the plasma density from the optimum value may cause a decrease in the laser amplitude. We attributed this effect may be due to reduction of preformed plasma density, nonuniformity of preformed plasma column and absorption of laser plasma plume. In Fig. 6, the best pressure is almost same for different pre-discharge current, but is different for different main current. So, the best pressure for lasing mainly depends upon main discharge current and not affected by pre-discharge current.
Figure 7 depicts the intensity of 46.9nm laser at different initial pressures when capillary inner diameter is 4.8mm and the main current amplitude is 30, 36 and 40kA respectively. The predischarge current of 140A was kept constant while measuring the influence of main discharge current. It is interesting to find from experimental results that laser intensity exhibits a significant current dependent behavior. The Ne-like Ar density shows an obvious increase with the increase of main discharge current amplitude, which is consistent with the theoretical simulation results of Fig. 4. The value of gain coefficient depends upon the Ne-like Ar density which is favorable for high intensity 46.9nm laser. It can be seen that larger the amplitude of the main current higher the laser intensity, wider the lasing pressure range, higher the optimum pressure. When the main current amplitude is 30, 36 and 40kA, the optimum pressure of producing laser is 43, 53 and 61Pa respectively. When the main current amplitude is 30, 36 and 40kA the pressure range to produce laser is 20-65, 30-80 and 42-95Pa respectively. When the amplitude of main current is increased from 30 to 36 and 40kA, the intensity of the laser produced at the optimum pressure increases up to 1.5 and 2 times respectively. It was found that the Ne-like Ar density, laser intensity and laser pulse energy were strongly dependent on discharge currents. The results depict that a longer plasma column with higher axial uniformity can be produced for laser amplification with a Z-pinch effect, using best predischarge current and higher main discharge currents with 4.8mm inner diameter alumina capillary. For a 4.8mm inner diameter capillary, the radiation flux density and main discharge current density inside the capillary is lesser than 3.2mm inner diameter capillary. Consequently, the amount of ablated material is reduced, and utilizing single capillary more shots of laser can be attained. The reduction of ablation can improve the main current efficiency and impurity of gain medium, which is beneficial for the amplification of laser. In addition, contamination of the detection system and pressure sensor can be reduced. There was no major instability observed throughout the plasma Z-pinch process using a 4.8 mm diameter capillary because of intense laser pulses generation.
The peak intensity of capillary discharge soft X-ray laser is proportional to the main discharge current amplitude . Therefore, the intensity of 46.9nm laser can be further improved by using high amplitude main discharge current. For higher main currents, the utilization of alumina capillary with larger inner diameter is the best choice.
Figures 8(a) and 8(b) demonstrate the dependence of the laser intensity on the main discharge current at two different initial Ar filling pressure. We increased the amplitude of main discharge from 27 to 45kA by keeping pre-pulse current constant. The variation of the laser intensity as a function of peak discharge current is quite different for two different initial Ar pressure. The laser intensity is maximum in the domain of low main discharge current and decreases monotonically by increasing the main discharge current in case of 40Pa. For 40Pa pressure Ne-like argon (Ar8+) density can be obtained by low main discharge current, whereas Ne-like Ar density decreases by increasing main discharge current and Ar8+ converts into (Ar9+). Conversely, the laser intensity is low in the range of low main current and increases significantly by increasing the amplitude of main discharge current for initial Ar pressure of 60Pa, because high amplitude main current is required to pinch the high-pressure plasma in order to achieve population inversion. In case of 60Pa pressure laser intensity have low value in the region of low main discharge current because of low Ne-like argon (Ar8+) density, at low main discharge currents the electron temperature is small and most of Ar gas converts into (Ar7+), which is consistent with theoretical simulation results of Figs. 2(a)-2(c).
Without applying high amplitude main discharge current at 60Pa pressure hot and dense Ar plasma cannot be obtained which is necessary to achieve population inversion for lasing. The Ne-like argon ion density increases by increasing the amplitude of main discharge current as a result laser intensity increases. These results are consistent with the theoretical simulation results of Ne-like argon (Ar8+) density presented in Figs. 4(a)-4(c). It is obvious from above experimental results that for 60Pa initial filling pressure the laser intensity is very low in the domain of low main discharge current because low main discharge current is not capable of pinching the plasma column in order to produce sufficient Ne-like Ar.
The laser pulse energy was measured by a 1GHz bandwidth oscilloscope and XRD for main discharge currents ranging from 27 to 45kA. The purpose of these measurements was to estimate the laser pulse energy that can be achieved by 4.8mm inner diameter alumina capillary. The maximum laser pulse energy of 1.5mJ is obtained with main discharge current of 45kA and initial Ar filling pressure of 60Pa. The capillary life time was observed about 6000 shots, lasing was still observed after this number of shots but laser pulse energy dropped significantly due to ablation of capillary wall material. The comparison of laser pulse energy is presented in Table 1.
Our estimated results of maximum laser pulse energy are higher than the laser pulse energy reported by all other groups. The development of newly employed 4.8mm inner diameter capillary for higher main discharge currents with specified properties, significant enhancement in laser intensity and pulse energy thereby enabling many of technological applications of capillary discharge 46.9nm laser.
Figure 9 depicts the spatial distribution of the annular shape detected laser beam. To study the spatial distribution of 46.9nm laser, the Ce: YAG fluorescent screen is used to detect the laser spot at 170cm from the capillary exit. The laser spots captured at different main discharge currents are consistent with the experimental results of Fig. 7. It is also observed in this figure the output laser energy of the 40kA shot appears to be more intense than in the 30 and 36kA shots. According to simulated results obtained by MHD code the electron density increases with increase in main current amplitude. The refraction loss of the laser beam is positively correlated with electron density gradient. The ring shape of laser spot is produced by the refraction of laser beam in plasma column due to radial plasma density gradient. The value of electron density increases significantly by increasing main discharge current as a result of refraction ring shape laser spot is produced. Figures 3(a)-3(c) show that as we increased the current, a concave electron density profile developed, it is of course a good thing because it will provide guiding to laser beam. Our results show best agreement with the theoretical simulation result of electron density presented in Figs. 3(a)-3(c).
Knowledge of spatial distribution of laser beam intensity is of both practical and basic interest. Spatial intensity distribution results show that the beam divergence increases as the amplitude of main current pulse increases. From Fig. 9 it is clear that the divergence observed for 40kA is slightly larger than that observed for currents 30kA and 36kA. It can be seen that when main current is increased to 40kA the spatial distribution of laser spot is more uniform because increment in the main current pulse decreases the difficulty of pinching the plasma to axis. Thereby reducing the instability during the Z-pinch process and hence spot uniformity gets improved.
In conclusion, the simulation has been conducted for plasma parameters by MHD code for 4.8mm inner diameter capillary to study the impact of main current on the intensity of 46.9nm laser. Theoretical calculation results showed that increasing the main discharge current amplitude properly may be beneficial to obtain higher gain coefficient to produce high intensity 46.9nm laser. The characteristics of 46.9nm laser produced by 4.8mm alumina capillary were compared firstly for the main currents of 30, 36 and 40kA. When the main current amplitude was increased from 30 to 36 and 40kA, the intensity of capillary discharge 46.9nm soft X-ray laser at the optimum pressure increased to1.5 and 2 times respectively. When the main current was 30, 36 and 40kA, the optimum pressure of producing laser was 43, 53 and 61Pa respectively. The theoretical analysis established in the present work was used to analyze the effect of main current amplitude on the intensity of 46.9nm laser. The simulated results by MHD code were consistent with our experimental results. It is experimentally observed that laser intensity is strongly affected by predischarge current. Therefore, main discharge current must be initiated in a uniformly predischarge plasma. The laser intensity was observed maximum at the predischarge current of 140A as a result of sufficient preionization. Based on experimental and theoretical results, it is concluded that the enhancement of laser intensity in a larger inner diameter capillary for higher main currents perhaps due to the larger gain coefficient, high electron temperature, high electron density, high Ne-like Ar density, less ablation of capillary wall, less absorption of gas and higher consumption of main currents. Therefore, increasing the main current amplitude along with best predischarge current for larger alumina capillary of larger inner diameter is the effective way to enhance the intensity of capillary discharge 46.9nm laser. The estimated maximum value of laser pulse energy is approximately 1.5mJ for main discharge current of 45kA. Consequently, our estimated value of laser pulse energy is higher than all the reported results about pulse energies, in case of capillary discharge platform, this work may enhance the applications of 46.9nm laser. In future, 4.8mm inner diameter capillary, high main current pulse and best pre-discharge plasma will be used to study gain coefficient to produce high intensity 46.9 nm laser in order to expand its applications.
National Natural Science Foundation of China (61275139, 61875045).
This project was supported by the National Natural Science Foundation of China (Grant No. 61275139, 61875045).
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