V. Prukner

{\rm N}_2 \left({A\,^3\Sigma_{\rm u}^+} \right) behaviour in a N2?NO surface dielectric barrier discharge in the modulated ac regime at atmospheric pressure

Optical, electrical and discharge product measurements were performed in order to reveal the behaviour of metastables in a surface dielectric barrier discharge (DBD) driven in N2 with small NO admixtures in a modulated ac regime. Metastable species were detected both in a thin surface plasma layer and in the space afterglow through plasma induced emission and optical?optical double resonance laser induced fluorescence, respectively. Basically, the observed species follow the discharge current oscillations in the plasma layer, while they evolve on a millisecond time scale due to diffusion through the space afterglow. Emission spectrometry provides a rough estimation of average metastable concentration in a thin surface plasma layer ~9 ? 1013?cm?3. Fluorescence measurements then allow an upper estimation of metastable concentration in the space afterglow ?1 ? 1012?cm?3.

Optical and electrical characteristics of a single surface DBD micro-discharge produced in atmospheric-pressure nitrogen and synthetic air

Basic opto-electrical characteristics of a single micro-discharge generated in a surface DBD reactor with a coplanar electrode arrangement were studied with nanosecond time resolution. The discharge electrode geometry based on machinable glass?ceramic was optimized in order to get a system free of the circulating dielectric liquids that are frequently used to insulate and cool metallic electrodes. The build-up and decay of UV?vis?NIR emission by a single micro-discharge unaffected by concurrent or preceding discharge events was inspected during the first 10??s of the micro-discharge evolution in nitrogen and in synthetic air. Obtained emission waveforms show a great similarity between the surface and volume streamers. The streamer volume-averaged N2(A) concentrations of ~8 ? 1014?cm?3 were estimated at t = 1??s decay time and concentrations of ~(2?4) ? 1015?cm?3 were estimated during the streamer propagation phase in nitrogen.

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.

ICCD microscopic imaging of a single micro-discharge in surface coplanar DBD geometry: determination of the luminous diameter of N2 and Ar streamers

Time-resolved images of a single-surface dielectric barrier micro-discharge were obtained by employing ICCD microscopy with high spatial and time resolution. The micro-discharge was repetitively produced using amplitude-modulated ac high voltage waveforms applied to the coplanar electrode geometry in a reactor, fed either with high-purity Ar or N2 at atmospheric pressure. The driving high-voltage amplitude was set to initiate just one single micro-discharge per one ac half-cycle. Images recorded within the two successive ac cycles provided evidence that the track left by the first micro-discharge influenced the propagation trajectory of consecutive streamers. Images of individual micro-discharges allowed evaluation of the luminous streamer channel diameter and its evolution along the streamers propagation trajectory. Minimum luminous streamer channel diameters of 30 ? 6??m and 50 ? 10??m were fixed in Ar and N2, respectively. Furthermore, direct comparison between a single micro-discharge image and an image integrating many micro-discharges allowed the estimation of potential errors associated with methods based on accumulating optical emission produced by many micro-discharge events.

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.

Formation of

Evolution of , v = 0−3) metastable species produced by triggered single filamentary streamer discharge was investigated utilizing emission spectrometry and laser-induced fluorescence techniques. Triggered streamers were produced repeatedly in point-to-plane DBD electrode geometry in pure nitrogen at 200 Torr and metastable species were monitored in the centre of the discharge gap. Populations of all four monitored vibronic levels increase during streamer channel decay phase, reaching local maximum in a microsecond timescale and decreasing afterwards. Maximum population of an individual , v) vibronic level occurs with a certain delay after streamer onset τd(v), which decreases with increasing vibrational number. The longest delay τd = 5 ± 0.5 µs was fixed for the lowest metastable level v = 0, whereas τd values of 4.4 ± 0.5, 1.75 ± 0.25 and 0.85 ± 0.25 µs were fixed for v = 1, 2 and 3, respectively. Coupled evolution of the two lowest , v = 0, 1) levels was monitored independently by applying an indirect method based on emission intensity of the N2 Herman infrared system. Very good agreement between emission and fluorescence based diagnostics was achieved for later post-discharge times t > τd. A simple 0-D model of , v) post-discharge kinetics based on vibrational relaxation and electronic quenching of individual vibronic levels reproduces experimental observations fairly well. According to the model, observed maxima likely occur due to the collisional cascade that transfers metastable species from higher even/odd vibrational levels towards v = 0/v = 1 terminal levels through the Δv = 2 vibrational relaxation mechanism.

{\rm N}_{2}(A\,^{3}\Sigma_{{\rm u}}^{+}

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).

, v = 0–3) metastable species in decaying nitrogen streamer

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.