Influence of a Nitrogen Admixture on the Anomalous Memory Effect in the Breakdown of Low-Pressure Argon in a Long Discharge Tube

The memory effect (the dependence of the dynamic breakdown voltage U _b on the time interval τ between voltage pulses) in pulse-periodic discharges in pure argon and the Ar + 1%N₂ mixture was studied experimentally. The discharge was ignited in a 2.8-cm-diameter tube with an interelectrode distance of 75 cm. The measurements were performed at gas pressures of P = 1, 2, and 5 Torr and discharge currents in a steady stage of the discharge of I = 20 and 56 mA. Breakdown was produced by applying positive-polarity voltage pulses, the time interval between pulses being in the range of τ = 0.5–40 ms. In this range of τ values, a local maximum (the anomalous memory effect) was observed in the dependence U _b (τ). It is shown that addition of nitrogen to argon substantially narrows the range of τ values at which this effect takes place. To analyze the measurement results, the plasma parameters in a steady-state discharge (in both pure argon and the Ar + 1%N₂ mixture) and its afterglow were calculated for the given experimental conditions. Analysis of the experimental data shows that the influence of the nitrogen admixture on the shape of the dependence U _b (τ) is, to a large extent, caused by the change in the decay rate of the argon afterglow plasma in the presence of a nitrogen admixture.     

Effect of the direction of a preionization current on X-ray emission from a capillary discharge plasma

The effect of the direction of a preionization current on the generation of 469-Å X-ray emission from the plasma of a fast capillary discharge in argon was studied experimentally in the SIGNAL facility (the discharge current I = 25–40 kA and the current rise rate dI/dt ～ 10¹² A/s). The experiments were performed with 3.1-mm-diameter 157-mm-long ceramic capillaries filled with argon at a pressure of 0.2–1.0 Torr.     

Experimental investigation of X-ray lasing in a capillary argon discharge

Results are presented from experiments on the laser generation of X-ray radiation at the wavelength λ=469 ? (ε=26.4 eV) on the 3p(J=0)−3s(J=1) transition of Ne-like Ar ions. Experiments were carried out on the SIGNAL electrophysical facility with a 3.1-mm-diameter 157-mm-long Al₂O₃ ceramic capillary filled with argon at a pressure of 0.2–1.0 Torr. The discharge current amplitude was I ∼ 25–40 kA, the current rise rate being dI/dt ∼ 10¹² A/s. By a vacuum X-ray diode tuned to detect X-ray photons with energies in the range 10–40 eV, laser pulses with a duration of t ₁ ∼ 1 ns and maximum energy of E _1,max ∼ 1 μJ were recorded. The pulses were generated 35 ns after the discharge current was switched on. The line spectra in the wavelength range of 150–500 ? showed the bright λ=469 ? line. The angular divergence of the generated X-ray laser beam was estimated to be Δϑ ∼ 2 mrad. Original Russian Text ? O.N. Gilev, V.I. Afonin, V.I. Ostashev, V.Yu. Politov, A.M. Gafarov, A.L. Zapysov, A.V. Andriyash, é.P. Magda, L.N. Shamraev, A.A. Safronov, A.V. Komissarov, N.A. Khavronin, N.A. Pkhaĭko, L.V. Antonova, L.N. Shushlebin, 2006, published in Fizika Plazmy, 2006, Vol. 32, No. 2, pp. 160–165.     

Experimental investigation of the shock wave in a fast discharge with cylindrical geometry

The work is devoted to the registration and the study of the properties of cylindrical shock waves generated in the fast discharge (dI/dt ～ 10¹² A/s) inside the ceramic tube (Al₂O₃) filled by argon at pressures of 100 and 300 Pa. The shock wave appears at the inner wall of the insulator and moves to the discharge axis with a velocity of about (3?4) × 10⁶ cm/s with subsequent cumulation. The plasma behind the front is heated enough to produce radiation in the vacuum ultraviolet (VUV) region, which makes it possible to study its structure by means of a pinhole camera with a microchannel plate detector. The time resolution of the registration system was 10 ns. The analysis of VUV spectra of the plasma shows that the electron temperature behind the shock wave front is about several eV; after the moment of cumulation, its temperature increases to 20–30 eV.     

Characteristics of a Pulse-Periodic Corona Discharge in Atmospheric Air

Pulse-periodic corona discharge in atmospheric air excited by applying a voltage pulse with a subnanosecond or microsecond rise time to a point electrode is studied experimentally. It is shown that, at a voltage rise rate of dU/dt ~10¹⁴ V/s, positive and negative ball-shaped streamers with a front velocity of ≥2 mm/ns form near the point electrode. As dU/dt is reduced to 10¹⁰?10¹¹ V/s, the streamer shape changes and becomes close to cylindrical. The propagation velocity of cylindrical streamers is found to be ~0.1 mm/ns at dU/dt ~ 2 × 10¹⁰ V/s. It is shown that the propagation direction of a cylindrical streamer can be changed by tilting the point electrode, on the axis of which the electric field strength reaches its maximum value. It is established that, for the negative polarity of the point electrode and a microsecond rise time of the voltage pulse, a higher voltage is required to form a cylindrical streamer than for the positive polarity of the point electrode.     

Experimental study of the processes accompanying argon breakdown in a long discharge tube at a reduced pressure

Results are presented from experimental studies of the breakdown stage of a low-pressure discharge (1 and 5 Torr) in a glass tube the length of which (75 cm) is much larger than its diameter (2.8 cm). Breakdowns occurred under the action of positive voltage pulses with an amplitude of up to 9.4 kV and a characteristic rise time of 2–50 μs. The discharge current in the steady-state mode was 10–120 mA. The electrode voltage, discharge current, and radiation from the discharge gap were detected simultaneously. The dynamic breakdown voltage was measured, the prebreakdown ionization wave was recorded, and its velocity was determined. The dependence of the discharge parameters on the time interval between voltage pulses (the socalled “memory effect”) was analyzed. The memory effect manifests itself in a decrease or an increase in the breakdown voltage and a substantial decrease in its statistical scatter. The time interval between pulses in this case can reach 0.5 s. The effect of illumination of the discharge tube with a light source on the breakdown was studied. It is found that the irradiation of the anode region of the tube by radiation with wavelengths of ≤500 nm substantially reduces the dynamic breakdown voltage. Qualitative explanations of the obtained results are offered.     

Long-lived Ar-Hg plasma in the afterglow of a high-current pulsed discharge

High-density (n > 10¹² cm^?3) argon-mercury plasma produced by a short (t ～ 20 μs) high-power pulsed discharge in argon with an admixture of mercury vapor at a discharge current of ～50 A, an argon pressure of ～4 mm Hg, and a mercury vapor pressure of ～10^?3 mm Hg was studied using optical spectroscopy and radio physics methods. It is found that the lifetime of this plasma after the end of the discharge pulse is up to 10^?2 s. It is shown that such an abnormally long lifetime of such an afterglow plasma, as compared to the plasma of an argon discharge without an admixture of mercury vapor, is related to the long residence time of atoms and ions of both argon and mercury in highly excited states due to chemi-ionization processes involving long-lived metastable argon ions. It is suggested that dissociative recombination of highly excited molecular ions of argon play an important role in the transfer of excitation to argon atoms and ions that are close to autoionization states.     

Nanosecond electric explosion of a tungsten wire in different media

The effect of surrounding media of different densities and electric strengths on the heating dynamics of a micron wire during its nanosecond electric explosion is investigated. Tungsten wires with diameters of d = 25–50 μm were exploded in air and water at a current rise time of (dI/dt) ～ 10¹⁰ A/s. The diagnostic complex is described.     

Breakdown in helium in high-voltage open discharge with subnanosecond current front rise

Investigations of high-voltage open discharge in helium have shown a possibility of generation of current pulses with subnanosecond front rise, due to ultra-fast breakdown development. The open discharge is ignited between two planar cathodes with mesh anode in the middle between them. For gas pressure 6 Torr and 20 kV applied voltage, the rate of current rise reaches 500 A/(cm² ns) for current density 200 A/cm² and more. The time of breakdown development was measured for different helium pressures and a kinetic model of breakdown in open discharge is presented, based on elementary reactions for electrons, ions and fast atoms. The model also includes various cathode emission processes due to cathode bombardment by ions, fast atoms, electrons and photons of resonant radiation with Doppler shift of frequency. It is shown, that the dominating emission processes depend on the evolution of the discharge voltage during the breakdown. In the simulations, two cases of voltage behavior were considered: (i) the voltage is kept constant during the breakdown; (ii) the voltage is reduced with the growth of current. For the first case, the exponentially growing current is maintained due to photoemission by the resonant photons with Doppler-shifted frequency. For the second case, the dominating factor of current growth is the secondary electron emission. In both cases, the subnanosecond rise of discharge current was obtained. Also the effect of gas pressure on breakdown development was considered. It was found that for 20 Torr gas pressure the time of current rise decreases to 0.1 ns, which is in agreement with experimental data.     

High-frequency underwater plasma discharge application in antibacterial activity

Plasma discharge is a novel disinfection and effectual inactivation approach to treat microorganisms in aqueous systems. Inactivation of Gram-negative Escherichia coli (E. coli) by generating high-frequency, high-voltage, oxygen (O₂) injected and hydrogen peroxide (H₂O₂) added discharge in water was achieved. The effect of H₂O₂ dose and oxygen injection rate on electrical characteristics of discharge and E. coli disinfection has been reported. Microbial log reduction dependent on H₂O₂ addition with O₂ injection was observed. The time variation of the inactivation efficiency quantified by the log reduction of the initial E. coli population on the basis of optical density measurement was reported. The analysis of emission spectrum recorded after discharge occurrence illustrated the formation of oxidant species (OH^?, H, and O). Interestingly, the results demonstrated that O₂ injected and H₂O₂ added, underwater plasma discharge had fabulous impact on the E. coli sterilization. The oxygen injection notably reduced the voltage needed for generating breakdown in flowing water and escalated the power of discharge pulses. No impact of hydrogen peroxide addition on breakdown voltage was observed. A significant role of oxidant species in bacterial inactivation also has been identified. Furthermore the E. coli survivability in plasma treated water with oxygen injection and hydrogen peroxide addition drastically reduced to zero. The time course study also showed that the retardant effect on E. coli colony multiplication in plasma treated water was favorable, observed after long time. High-frequency underwater plasma discharge based biological applications is technically relevant and would act as baseline data for the development of novel antibacterial processing strategies.     

Hydrodynamic model of a dielectric-barrier discharge in pure chlorine

A one-dimensional hydrodynamic model of a dielectric-barrier discharge (DBD) in pure chlorine is developed, and the properties of the discharge are modeled. The discharge is excited in an 8-mm-long discharge gap between 2-mm-thick dielectric quartz layers covering metal electrodes. The DBD spatiotemporal characteristics at gas pressures of 15–100 Torr are modeled for the case in which a 100-kHz harmonic voltage with an amplitude of 8 kV is applied to the electrodes. The average power density deposited in the discharge over one voltage period is 2.5–5.8 W/cm³. It is shown that ions and electrons absorb about 95 and 5% of the discharge power, respectively. In this case, from 67 to 97% of the power absorbed by electrons is spent on the dissociation and ionization of Cl₂ molecules. Two phases can be distinguished in the discharge dynamics: the active (multispike) phase, which follows the breakdown of the discharge gap, and the passive phase. The active phase is characterized by the presence of multiple current spikes, a relatively high current, small surface charge density on the dielectrics, and large voltage drop across the discharge gap. The passive phase (with no current spikes) is characterized by a low current, large surface charge density on the dielectrics, and small voltage drop across the discharge gap. The peak current density in the spikes at all pressures is about 4 mA/cm². In the multispike phase, there are distinct space charge sheaths with thicknesses of 1.5–1.8 mm and a mean electron energy of 4.3–7 eV and the central region of quasineutral plasma with a weak electric field and a mean electron energy of 0.8–3 eV. The degree of ionization of chlorine molecules in the discharge is ~0.02% at a pressure of 15 Torr and ~0.01% at 100 Torr. The DBD plasma is electronegative due to the fast attachment of electrons to chlorine atoms: e + Cl₂ → Cl + Cl^–. The most abundant charged particles are Cl ₂ ⁺ and Cl^? ions, and the degree of ionization during current spikes in the active phase is (4.1–5.5) × 10^–7. The mechanism of discharge sustainment is analyzed. The appearance of a series of current spikes in the active phase of the discharge is explained.     

Experimental study of a low-pressure glow discharge in air in large-diameter discharge tubes: I. conditions for the normal regime of a glow discharge

The initiation and characteristics of a low-pressure glow discharge in air in large-diameter discharge tubes are studied. A deviation from the Paschen law is observed: the breakdown curves U _dc(pL) shift toward the higher values of U _dc and pL as the interelectrode distance L increases. It is shown that the normal regime of a glow discharge is accompanied by gas ionization in the anode sheath. This takes place only for pL values lying to the right of the inflection point in the breakdown curve. The cathode-sheath characteristics in the normal and abnormal regimes of an air discharge for a duralumin cathode are determined. The axial profiles of the ion density, electron temperature, and plasma potential, as well as the anode voltage drop, are measured at various air pressures.     

Shear stress and sediment resuspension in relation to submersed macrophyte biomass

We examined the impacts of macrophyte beds dominated by a canopy-forming (Myriophyllum sibiricum) and a meadow-forming (Chara canescens) species on bottom shear stress (τ) and resuspension in shallow Lake Christina, Minnesota (U.S.A.). Studies were conducted in late summer, 1998, when macrophyte biomass levels exceeded 200 g m^?2, and in early summer, 2000, when biomass was greatly reduced (<20 g m^?2) in both plant beds. The critical shear stress (τ_c) of sediments, measured experimentally in the laboratory, was low (1.4 dynes cm^?2) indicating potential for resuspension in the absence of macrophytes. During 1998, turbidity was low at the M. sibiricum and Chara station, rarely increasing when calculated bottom τ (calculated from wave theory assuming no biomass obstruction) exceeded τsub c sub, indicating that both beds reduced sediment resuspension at high biomass levels. In situτ (estimated τ), measured via gypsum sphere dissolution, did not exceed τ_c above the sediment interface in either bed during 1998. In contrast, sediment resuspension occurred in both beds during similar high winds in 2000. However, estimated τ was lower than calculated bottom τ, suggesting that at low biomass, macrophytes were having some impact on τ.     

Generation of uniform low-temperature plasma in a pulsed non-self-sustained glow discharge with a large-area hollow cathode

Generation of plasma in a pulsed non-self-sustained glow discharge with a hollow cathode with an area of ≥2 m² at gas pressures of 0.4–1 Pa was studied experimentally. At an auxiliary arc-discharge current of 100 A and a main discharge voltage of 240 V, a pulse-periodic glow discharge with a current amplitude of 370 A, pulse duration of 340 μs, and repetition rate of 1 kHz was obtained. The possibility of creating a uniform gas-discharge plasma with a density of up to 10¹² cm^?3 and an electron temperature of 1 eV in a volume of >0.2 m³ was demonstrated. Such plasma can be efficiently used to treat material surfaces and generate pulsed ion beams with a current density of up to 15 mA/cm².     

Dynamics of Breakdown in a Low-Pressure Argon–Mercury Mixture in a Long Discharge Tube

Breakdown dynamics in the course of glow discharge ignition in a long discharge tube (80 cm in length and 25 mm in diameter) filled with argon at a pressure of 3–4 Torr and mercury vapor at room temperature was studied experimentally. Rectangular voltage pulses with amplitudes from 1 to 2.5 kV were applied to the tube anode, the cathode being grounded. Complex electrical and optical measurements of the breakdown dynamics were carried out. Breakdown begins with a primary discharge between the anode and the tube wall. In this stage, a jump in the anode current and a sharp decrease in the anode voltage are observed and prebreakdown ionization wave arises near the anode. The cathode current appears only after the ionization wave reaches the cathode. The wave propagation velocity was measured at different points along the tube axis. The wave emission spectrum contains Hg, Ar, and Ar⁺ lines. The intensities of these lines measured at a fixed point exhibit very different time behaviors. The effect of the tube shielding on the breakdown characteristics was examined. It is found that, at a sufficiently narrow gap between the shield and the tube, this effect can be substantial.     

On the relation between the index of the interelectrode gap filling with spark channels and the current of a pulsed multichannel sliding discharge in Ne,Ar, and Xe

Experimental results and model concepts concerning the relation between the index K of the interelectrode gap filling with spark channels and the peak current I _peak of a single-pulse submicrosecond multichannel complete sliding discharge on an alumina ceramic surface are discussed. The spatial structure of an incomplete discharge at the threshold for the surface spark breakdown of gas is considered. The experiments were performed with three gases, Ne, Ar, and Xe, at pressures of 30 and 100 kPa and opposite polarities of the discharge voltage, with two discharge chambers differing in the geometry of the discharge gap and the thickness of the ceramic plate. It is shown that, although the structure of the incomplete discharge at the threshold for spark breakdown varies from diffuse homogeneous to pronounced filamentary, the dependence \(K\left( {\sqrt[6]{{I_{peak} }}} \right)\) for a complete discharge is close to linear and can be qualitatively explained by the earlier proposed semiempirical model of the time evolution of the structure of a multichannel discharge. In particular, the estimated steepness of the dependence \(K\left( {\sqrt[6]{{I_{peak} }}} \right)\) agrees best with the experimental results when the local density of free electrons at the threshold for spark breakdown is 10¹⁶ cm^?3 or higher.     

The properties of high-current high-pressure discharges

The properties of high-current high-pressure gas discharges (I = 60 kA, p = 1 atm, τ_1/2 = 1.6 ms, r ～ 10 cm) just before the end of the discharge are investigated experimentally. It is shown that the anomalously high rate of gas cooling after the current is switched off is related to the turbulent hydrodynamic processes induced by the Rayleigh-Taylor instability at the discharge boundary in the stage of volume radiative cooling of the discharge channel. The turbulent thermal conductivity is estimated using experimental data on the recovery of the electric strength of the discharge gap.     

Generation and interaction of intense counterpropagating plasma flows

Results are presented from experimental studies of the parameters of two counterpropagating (colliding) plasma flows generated by discharges in crossed electric and magnetic fields. It is shown that the conversion efficiency of the energy deposited in the discharges into the energy of directed plasma flows is 0.3–0.6. For discharge current pulses with a duration of ∼10 μs, the energy flux density in the plasma flow reaches ∼10 J/cm² and the total energy of the flow is on the order of 300 J. The density of deuterons in the flows is ∼10¹⁵ cm⁻³, and the flow velocity is ≤2×10⁷ cm/s. The total number of particles carried by the flows is about 10¹⁹. The possibility of using counterpropagating plasma flows to study reactions involving light nuclei (dd, pd, dt, and dHe reactions) in the range of ultralow collision energies is discussed. __________ Translated from Fizika Plazmy, Vol. 29, No. 8, 2003, pp. 714–721. Original Russian Text Copyright ? 2003 by Dudkin, Nechaev, Padalko, Bystritsky, Stolupin, Bystritskii, Voznyak.     

Light-saturated photosynthesis — Limitation by electron transport or carbon fixation?

The minimal turnover time, τ, for in vivo electron transport from water to CO₂, was calculated from oxygen flash yields and steady-state light-saturated photosynthetic rates in the marine chlorophyte, Dunaliella tertiolecta, cultured at different growth irradiance levels. As cells adapted to lower growth irradiance levels, τ increased from 3.5 to 14.5 ms, in parallel with increases in the contents of chlorophyll a, Photosystem II, PQ, cytochrome b₆f, Photosystem I and thylakoid surface density. Thus, at all growth irradiance levels examined, the relative proportion of these membrane-bound electron-transport components remained constant. However, the cellular pool size of ribulose-1,5-bisphosphate carboxylase/oxygenase, determined by radioimmunoassay, was independent of growth irradiance. Hence the ratio of the enzyme to electron-transport chain components varied between 4.8 and 1.2 as a function of growth irradiance levels. The change in this ratio was related quantitatively to the minimal turnover time of electron transport from water to carbon dioxide. Taking into account thylakoid surface density, cellular contents of electron-transport components and diffusion coefficient of plastoquinol, a diffusion time of 2.3 ms was calculated for transport of PQH₂ from Photosystem II to cytochrome b₆f. This rate is 1.5- to 13-times faster than τ. The data strongly suggest that under nutrient saturated conditions the absolute rate of light-saturated photosynthesis is limited by carbon fixation rather than electron transport. It is predicted, however, that in cells grown above 3000 μmol quanta per m² per s, electron transport rather than carbon fixation would become the rate-limiting step of light saturated photosynthesis.     

C-terminal truncation modulates both nucleation and extension phases of tau fibrillization

Proteolytic post-translational modification has been proposed as an early stage event in the aggregation of τ protein and formation of neurofibrillary lesions in Alzheimer’s disease. Caspases and other proteases cleave τ in vivo at discrete locations including Asp⁴²¹ and Glu³⁹¹. Both cleavage products are prone to aggregation relative to wild-type, full-length τ protein. To determine the mechanism underlying this effect, the fibrillization of τ truncated after Asp⁴²¹ and Glu³⁹¹ residues was characterized in a full-length four-repeat τ background using quantitative electron microscopy methods under homogeneous nucleation conditions. Both C-terminal truncations decreased critical concentration relative to full-length τ, resulting in more filament mass at reaction plateau. Moreover, truncation directly augmented the efficiency of the nucleation reaction. The results suggest the mechanism through which C-terminal proteolysis can modulate τ filament accumulation depending on whether it precedes or follows nucleation.