Viktor Martišovitš

Transient spark: a dc-driven repetitively pulsed discharge and its control by electric circuit parameters

The paper presents an analysis of electrical characteristics of streamer-to-spark transition discharge in air at atmospheric pressure named transient spark (TS). The TS is applicable for flue gas cleaning or bio-decontamination and has potential in plasma shielding, combustion and flow control applications. Despite the dc applied voltage, TS has a pulsed character with short (~10–100 ns) high current (>1 A) pulses, with repetitive frequencies 1–20 kHz. Estimation of the temporal evolution of electron density shows that ne ≈ 1016 cm−3 at maximum and ~1011 cm−3 on average are reached using relatively low power delivered to the plasma (0.2–3 W). Thanks to the high repetition frequency, ne between two current pulses does not fall below a critical value and therefore plasma exists during the whole time. A detailed analysis of the TS control by electrical circuit parameters is presented. With appropriate circuit components, the current pulse tail (>1 mA) can be extended and the electron density can be held above ~1013 cm−3 for several tens of μs.

DC Discharges in Atmospheric Air and Their Transitions

We present images and basic characteristics and describe transition mechanisms between three dc discharges in atmospheric air point-to-plane or point-to-water gap. With increasing applied voltage, a streamer corona transits to a transient spark: a repetitive spark with very short (~100-ns) current pulses (~1 A) of very limited energy. With an appropriate ballast resistor, this transient regime evolves into a pulseless glow discharge. These three discharges generate nonequilibrium plasmas of high chemical activity interesting for environmental and biomedical applications.

The streamer-to-spark transition in a transient spark: a dc-driven nanosecond-pulsed discharge in atmospheric air

We present a study of the streamer-to-spark transition in a self-pulsing dc-driven discharge called a transient spark (TS). The TS is a streamer-to-spark transition discharge with short spark duration (?10?100?ns), based on charging and discharging of the internal capacity of the electric circuit with repetition frequency 1?10?kHz. The TS can be maintained under relatively low energy conditions (0.1?1?mJ?pulse?1). It generates a very reactive non-equilibrium air plasma applicable for flue gas cleaning or bio-decontamination.Thanks to the short spark current pulse duration, the steady-state gas temperature, measured at the beginning of the streamers initiating the TS, increases from an initial value of ?300?K only up to ?550?K at 10?kHz. The streamer-to-spark transition is governed by the subsequent increase in the gas temperature in the plasma channel up to ?1000?K. This breakdown temperature does not change with increasing repetition frequency f. The heating after the streamer accelerates with increasing f, leading to a decrease in the average streamer-to-spark transition time from a few ?s to less than 100?ns.

Staggered superconductivity in UPt3: A new phenomenological approach.

We present a new Ginzburg-Landau theory for superconductivity in UPt{sub 3}, based upon a multicomponent order parameter transforming under an irreducible space group representation; the phase is staggered in real space. Our model can explain the {ital H}-{ital T}-{ital P} phase diagram including the tetracritical point for all field directions. We motivate this unconventional superconducting state in terms of odd-in-frequency pairing that may arise in one- or two-channel Kondo models, and suggest experimental tests.

Fast imaging of intermittent electrospraying of water with positive corona discharge

The effect of the electrospraying of water in combination with a positive direct current (dc) streamer corona discharge generated in air was investigated in this paper. We employed high-speed camera visualizations and oscilloscopic discharge current measurements in combination with an intensified charge-coupled device camera for fast time-resolved imaging. The repetitive process of Taylor cone formation and droplet formation from the mass fragments of water during the electrospray was visualized. Depending on the applied voltage, the following intermittent modes of electrospraying typical for water were observed: dripping mode, spindle mode, and oscillating-spindle mode. The observed electrospraying modes were repetitive with a frequency of a few hundreds of Hz, as measured from the fast image sequences. This frequency agreed well with the frequency of the measured streamer current pulses. The presence of filamentary streamer discharges at relatively low voltages probably prevented the establishment of a continuous electrospray in the cone?jet mode. After each streamer, a positive glow corona discharge was established on the water filament tip, and it propagated from the stressed electrode along with the water filament elongation. The results show a reciprocal character of intermittent electrospraying of water, and the presence of corona discharge, where both the electrospray and the discharge affect each other. The generation of a corona discharge from the water cone depended on the repetitive process of the cone formation. Also, the propagation and curvature of the water filament were influenced by the discharge and its resultant space charge. Furthermore, these phenomena were partially influenced by the water conductivity.

Measurement of the electron density in Transient Spark discharge

This paper presents our measurements of the electron density in a streamer-to-spark transition discharge, which is named transient spark (TS), in atmospheric pressure air. Despite the dc applied voltage, TS has a pulsed character with short (~10–100 ns) high current (>1 A) pulses, with a repetition frequency on the order of kHz. The electron density ne ~ 1017 cm−3 at maximum is reached in TS with repetition frequencies below ~3 kHz, using relatively low power delivered to the plasma (0.2–3 W).The temporal evolution of ne was estimated from the resistance of the plasma discharge, which was obtained by a detailed analysis of the electric circuit representing the TS and the discharge diameter measurements using a fast intensified charge-coupled device (iCCD) camera. This estimate was compared with ne calculated from the measured Stark broadening of several atomic lines: Hα, N at 746 nm, and O triplet at 777 nm. Good agreement was obtained, although the method based on the plasma resistance is sensitive to an accurate determination of the discharge diameter. We have found that this method is also limited for strongly ionized plasmas. On the other hand, a lower ne detection limit can be obtained by this method than from the Stark broadening of atomic lines.

Vacuum UV and UV spectroscopy of a N2–Ar mixture discharge created by an RF helical coupling device

Optical emission spectroscopy in vacuum ultraviolet and UV spectral ranges is applied to study densities, and vibrational and rotational temperatures of the N2 molecule in a nitrogen–argon (0–95% Ar) plasma sustained at a pressure of 400 Pa by a helical cavity supplied with a power of 28 W and an excitation frequency of 27 MHz. The spatial investigation of all emission systems from UV to NIR shows a minimum situated in the middle of the helical structure and two maxima located at the positions where the RF power is transmitted to the gas and at the end of the helix. The minimum was deepest for emission of the first positive (1+) nitrogen system. This hollow shaped density profile due to the presence of a non-linear phenomenon in the discharge is constant whatever the gas composition. The emissions related to Lyman–Birge–Hopfield and the second positive (2+) systems of molecular nitrogen, and N(2P) atoms, are analyzed versus the Ar percentage. Additionally, the NO(A 2Σ+ → X 2Π) and OH(A 2Σ+ → X 2Π) emission systems coming from impurities are investigated. All the densities of the considered species increase with Ar amount. The rotational and vibrational temperatures of the emitter species are determined through the comparison between experimental and simulated spectra. In the case of a N2 discharge, all the rotational temperatures deduced through the nitrogen emission systems are equal and can be assimilated to the gas temperature. With the increase in the Ar amount, only the rotational temperature obtained from the 1+ system is close to the gas temperature. The rotational and vibrational temperatures related to the NO(A 2Σ+) species are constant whatever the gas composition. The vibrational distribution function of N2(a 1Πg) state presents a Boltzmann law with a vibrational temperature in the range 5600–8000 K (±1000 K) for the N2–x% Ar mixture with x < 75%. When the Ar percentage increases above this limit, we observe strong deviations from the Boltzmann law and no temperature can be deduced. Some kinetic considerations, where the nitrogen and argon metastables play an important role, are discussed to explain the strong dependence of the temperatures and density species toward the Ar amount in the gas mixture.

Radial structure of the collision-dominated plasma column

The ambipolar diffusion problem is solved when charged particles inertia and heating of positive ions together with the ion mobility variation in the radial electrostatic field are taken into consideration. For both constant mean free time and constant mean free path situations approximate formulae are found for the variation of drift velocity of ions and radial potential close to the wall when the collisionless sheath is assumed. It is shown that the plasma column can be subdivided into three regions in the radial direction: the region of ambipolar diffusion, the transition region where the ion inertia predominates and the collisionless sheath at the wall. The influence of this structure on the mass spectrometric sampling technique of a positive column is also noted.

Study of transient spark discharge focused at NOx generation for biomedical applications

The paper is focused at nitrogen oxides generation by transient spark (TS) in atmospheric pressure air. The TS is a DC-driven self-pulsing discharge with short duration (~10-100 ns) high current pulses (>1A), with the repetition frequency 1-10 kHz. Thanks to the short spark duration, highly reactive non-equilibrium plasma is generated, producing ~300 ppm of NOx per input energy density 100 J.l-1. Further optimization of NO/NO2 production to improve the biomedical/antimicrobial effects is possible by modifying the electric circuit generating the TS.

Vibrational population of the O2(b1Σ g + ) state in a low-Pressure oxygen pulsed discharge

The study of the vibrational state population of the O2(b1Σg+) metastable state in a pure oxygen DC pulsed discharge is presented. The vibrational temperature is evaluated from the relative populations of v = 0, 1 and 2 states. The populations are determined from the intensities of (0-0), (1-1), (1-0) and (2-1) transitions of the atmospheric system. The most intensive (0-0) and (1-1) bands are used in the time-resolved measurements in order to determine the time evolution of the vibrational temperature during the discharge pulse.