J. Straus

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.

Design of a laser-triggered driver for fast capillary discharge

We are developing a new upgraded capillary discharge device. The assembled set-up consists of a Marx generator, a pulse forming line, a gas-filled laser-triggered spark gap and a ceramic capillary. The special attention has been paid to the design of the spark gap. The laser triggering of this spark gap ensures a very low jitter in comparison with the present capillary discharge device CAPEX, where the main spark gap works in a self-breakdown regime. The description of the assembled apparatus, the main predicted parameters of the designed device (such as capillary current, capillary current rise-rte, and/or voltage at the end of pulse forming line) are presented.

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.

Wire Exploding in a Focus of Converging Cylindrical Shock Wave in Water - Introductory Remarks

It is expected that, just as the proximity of a wall stabilizes a Z-pinch in a capillary, the proximity of a wall (solid or liquid) can also stabilize an exploding wire placed in a capillary or in liquid (water), the density of which can be further increased by a suitably focused shock wave. A wire explosion will be used in our new apparatus CAPEX-U (current amplitude 100 kA, quarter period 70 ns) with free optical axial access to both (high voltage and ground) electrodes and with a four-channel laser- triggered main spark gap. As a driver for converging cylindrical shock waves in water a separate test apparatus was designed and assembled: a strong acoustical wave will be generated at the cylindrical chamber-surface (covered by porous ceramics) by a multi-streamer corona-like discharge that burns to a co-axial cylindrical mesh/foil electrode. Expected pressure amplitude at the axis approaches 1 GPa. Applications of wire explosion in the focus of a cylindrical shock wave in water to inertial confinement fusion as well as to soft X-ray lasers will be briefly mentioned.

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.

Application of EUV optics to surface modification of materials

We report results of experiments connected with surface modification of materials with an intense extreme ultraviolet (EUV) laser beam. Irradiated by the laser beam from a discharge-plasma EUV source (with wavelength of 46.9 nm) based on a high-current capillary discharge driver, the samples have been investigated by atomic-force microscope (AFM). The laser beam is focused with a spherical Si/Sc multilayer-coated mirror on polymethylmethacrylate (PMMA), gold-covered- PMMA and gallium arsenide (GaAs) samples. It turned out that desorption and ablation regimes, which are observed in all these cases strongly depends on substrate materials.