Oleksandr Frolov

Soft X-ray emission of a fast-capillary-discharge device

The paper reports on some technological modifications of the capillary discharge device, especially in the spark gap region. Passively generated pre-pulse plasma was replaced by an external driver, which can independently control a pre-ionisation capillary current. The time development of the axial soft X-ray radiation of the modified capillary discharge device is reported as well.

Amplification of spontaneous emission of neon-like argon in a fast gas-filled capillary discharge

The evolution of the CAPEX facility and its basic diagnostics are described. The experiments carried out in the last modification of this facility accomplished with the demonstration of amplified spontaneous emission of neon-like argon (Ar8+) at the wavelength 46.88 nm. The first version of the facility, CAPEX1, operated with a plastic capillary and had a short high-power passive prepulse and an imperfect gas-filling system. In the second version, CAPEX2, a ceramic capillary was used, the prepulse amplitude was lowered, and the gas-filling system was improved. In the third, most successful version, CAPEX3, the capillary bending was reduced, a longer external prepulse was used, and the gas-filling system was further optimized. For each version, results of X-ray measurements are presented and interpreted.

A new method of determination of ablation threshold contour in the spot of focused XUV laser beam of nanosecond duration

It is well known that at interaction of femtosecond Extreme Ultraviolet Radiation (XUV) with a surface it is possible – according to local fluency - to distinguish two main regions: the desorption region (when efficiency η of removing particles is <10%), and the ablation region (when efficiency η ~ 100%). In this case, the ablation threshold determination is very simple and relatively accurate. It was e.g. shown that with the help of mapping of morphology of the ablationdug- craters it is possible to determine the fluency distribution in/near the beam focus. However, recently we found that (1) the desorption efficiency η for nanosecond pulses is much higher than that for femtosecond ones and spans from zero at the periphery imprint to ~90% at the ablation threshold; this complicates the ablation threshold determination; (2) the direct nano-structuring of solid surfaces is possible only in the desorption region (e.g. the diffraction pattern generated in windows of in-proximity-standing-grid [K.Kolacek et.al., Laser and Particle Beams 30, 57-63, (2012)] is visible only in these parts of laser-beam-spot, which correspond to the desorption region). This prompted us to use this nano-patterning for determination of ablation threshold contour. The best possibility seems to be covering the laser beam spot by interference pattern. For that, it was necessary to develop a new type of interferometer, which (a) provides as dense interference pattern as possible, (b) uses practically all the energy of laser beam, (c) works with focused beams. Such interferometer has been designed and is described in this contribution.

Recent progress in discharge-based soft X-ray lasers at IPP ASci CR

Activity of our laboratory in the field of pulsed high-current proximity-wall-stabilised discharges - media for XUV/soft X-ray generation and amplification (XUV/soft X-ray lasing), and our effort to demonstrate lasing of discharge-based sources at the wavelength <15 nm are described. While lasing on the wavelength of 46,9 nm (Ne-like Ar ions) is in capillary discharge routinely achieved (due to excitation pumping scheme), lasing on the wavelength below 15 nm either has been demonstrated in laser-created plasma in Ne-/Ni-like ions of metal vapours (also due to excitation pumping scheme) or has been predicted for H-like N ions (recombination pumping scheme). For the excitation pumping of metal vapours a wire exploding in water locally compressed by focused shock wave is being prepared. The recombination pumping of N is examined in the capillary discharge geometry known from the former experiments with Ar, but at higher discharge currents (>60 kA).

Strong Amplification of Ne-like AR line in the Source Based on Capillary Discharge

For the first time we have used a surveillance homemade flat field XUV spectrograph for the measurement of soft X-ray emission from the capillary discharge device CAPEX when using of an external capillary pre-pulse driver. The time-resolved XUV spectrum is presented. It was found that a very short intense spike that had been measured by a vacuum photodiode likely corresponds to a laser pulse. Modifications/assemblies of the spark gap switch the capillary, and the fast electromagnetic shutter are described as well.

Gas-filled laser-triggered spark gap

We report 200 ps jitter operation of high-voltage air/nitrogen pressurized spark gap triggered by 850 mJ/5 ns Nd:YAG laser. It is studied the delay and the jitter of this spark gap as a function of voltage, energy of laser pulse, gas pressure and angle of incidence of the laser beam. These experiments will resulted in recommendation for construction of low-jitter spark gap in our new pulse-capillary-discharge driver.

Generation and application of the soft X-ray laser beam based on capillary discharge

In this work we report on the generation and characterization of a focused soft X-ray laser beam with intensity and energy density that exceed the threshold for the ablation of PMMA. We demonstrate a feasibility of direct ablation of holes using a focused soft X-ray laser beam. Ablated craters in PMMA/gold-covered-PMMA samples were obtained by focusing the soft X-ray Ar8+ laser pulses generated by a 46.9 nm tabletop capillary-discharge-pumped driver with a spherical Si/Sc multilayer mirror. It was found that the focused beam is capable by one shot to ablate PMMA, even if the focus is significantly influenced by astigmatism. Analysis of the laser beam footprints by atomic force microscope shows that ablated holes have periodic surface structure (similarly as Laser-Induced Periodic Surface Structure) with period ~2,8 ?m and with peak-to-peak depth ~5-10 nm.

Repetitive XUV laser based on the fast capillary discharge

For testing and application purposes we have built a new small Marx generator capable to run in a repetitive regime. Its repeating frequency is currently up to 1 Hz. The generator is covered by metal sheets and feeds the CAPEX facility and ensures its full independence on the CAPEX-U machine (using another Marx generator). This paper reports on the first experimental results of a new experimental set-up of the CAPEX apparatus (repetitive lasing at 46.9 nm), mainly on set-up description, electrical parameters, and laser pulse stability in the repetitive regime.

Interaction of extreme ultraviolet laser radiation with solid surface: ablation, desorption, nanostructuring

The area, where interaction of focused XUV laser radiation with solid surface takes place, can be divided according to local fluency into desorption region (if fluency is larger than zero and smaller than ablation threshold) and ablation region (if fluency is equal or larger than this threshold). It turned out that a direct nanostructuring (e.g. imprinting diffraction pattern created on edges of windows of proximity standing grid) is possible in the desorption region only. While for femtosecond pulses the particle (atom/molecule) removal-efficiency η in the desorption region is very small (η < 10%), and hence, it can be easily distinguished from the ablation region with η ~ 100%, for nanosecond pulses in desorption region this η rises at easily ablated materials from 0% at the periphery up to ~90% at the ablation contour and, therefore, the boundary between these two regions can be found with the help of nanostructuring only. This rise of removal efficiency could be explained by gradually increased penetration depth (due to gradually removed material) during laser pulse. This is a warning against blind using crater shape for fluency mapping in the case of long laser pulses. On the other hand it is a motivation to study an ablation plum (or ablation jet) and to create a knowledge bank to be used at future numerical modeling of this process.

Experiment CAPEX‐U: Present and Future

After successful experiments with our device CAPEX with lasing on Ne‐like Ar (λ = 46.9 nm) we report on our new apparatus CAPEX‐U with larger transversal dimensions, with transparency along its axis, and with four‐channel laser‐triggered spark gap, which enables exact synchronization of the detectors and/or attached experiments. The modelling of this new apparatus consists of three steps: an electrostatic field mapping (to determine the minimum safe dimensions of insulators and the optimum roundness of the conductor edges), an equivalent circuit analysis (to predict capillary current amplitude and current rise time) and a testing single‐channel laser‐triggered spark gap (to find appropriate regime of operation of laser‐triggered spark gap). The description of the new apparatus CAPEX‐U, the results of the modelling and the first electrical tests are presented.