Prebreakdown stages of a barrier discharge in air

The initial stages of a barrier discharge in a short air gap at atmospheric pressure are investigated by means of numerical simulations. A highly nonuniform electric field caused by the residual surface charges on the dielectric surfaces was taken into account. The results of calculations of the two-dimensional dynamics of the discharge radiation are in good agreement with the experiment.     

Inertial electrostatic confinement and nuclear fusion in the interelectrode plasma of a nanosecond vacuum discharge. I: Experiment

Properties of an aerosol substance with a high power density in the interelectrode space of a nano- second vacuum discharge are studied. The possibilities of emission and/or trapping of fast ions and hard X-rays by ensembles of clusters and microparticles are analyzed. The possibility of simultaneous partial trapping (diffusion) of X-rays and complete trapping of fast ions by a cluster ensemble is demonstrated experimentally. Due to such trapping, the aerosol ensemble transforms into a “dusty” microreactor that can be used to investigate a certain class of nuclear processes, including collisional DD microfusion. Operating regimes of such a microreactor and their reproducibility were studied. On the whole, the generation efficiency of hard X-rays and neutrons in the proposed vacuum discharge with a hollow cathode can be higher by two orders of magnitude than that in a system “high-power laser pulse-cluster cloud.” Multiply repeated nuclear fusion accompanied by pulsating DD neutron emission was reproducibly detected in experiment. Ion acceleration mechanisms in the interelectrode space and the fundamental role of the virtual cathode in observed nuclear fusion processes are discussed.     

Inertial electrostatic confinement and nuclear fusion in the interelectrode plasma of a nanosecond vacuum discharge. II: Particle-in-cell simulations

Results of particle-in-sell simulations of ion acceleration by using the KARAT code in a cylindrical geometry in the problem formulation corresponding to an actual experiment with a low-energy vacuum discharge with a hollow cathode are presented. The fundamental role of the formed virtual cathode is analyzed. The space-time dynamics of potential wells related to the formation of the virtual cathode is discussed. Quasi-steady potential wells (with a depth of ∼80% of the applied voltage) cause acceleration of deuterium ions to energies about the electron beam energy (∼50 keV). In the well, a quasi-isotropic velocity distribution function of fast ions forms. The results obtained are compared with available data on inertial electrostatic confinement fusion (IECF). In particular, similar correlations between the structure of potential wells and the neutron yield, as well as the scaling of the fusion power density, which increases with decreasing virtual cathode radius and increasing potential well depth, are considered. The chosen electrode configuration and potential well parameters provide power densities of nuclear DD fusion in a nanosecond vacuum discharge noticeably higher than those achieved in other similar IECF systems.     

Study of the dark phase in the initial stage of the positive column formation in an argon glow discharge

The initial stage of the positive column formation in an argon glow discharge is investigated both experimentally and theoretically. A decrease in the plasma radiation intensity (the so-called “dark phase”) was observed experimentally over a time period of about 1 ms. A similar dip was also observed in the time dependence of the electric field strength. The time evolution of the population of the lowest metastable state of Ar was measured. A relevant theoretical model has been developed and used to perform calculations for the actual experimental conditions. A comparison between the numerical and experimental results shows that the model adequately describes the processes that occur during the formation of the positive column in an argon glow discharge. Experimental and theoretical study shows that the dark-phase effect is related to an excessive amount of metastable Ar atoms at the beginning of a discharge and, consequently, to high rates of stepwise ionization and chemionization.     

Partially constricted glow discharge in an argon-nitrogen mixture

The characteristics of a glow discharge in Ar:N₂ mixtures with a low content of nitrogen (0.02–1%) are studied experimentally. Most studies were performed at pressures higher than 10 Torr, at which the discharge constriction goes by a jump and the hysteresis effect is well pronounced. It is found that the time during which the discharge switches from the diffuse to the constricted mode (and back) can reach ～1 s. The transition between these modes begins with the development of a constriction at one end of the positive column. Then, the constricted part of the discharge extends toward the other end until it occupies the entire column. The reverse transition occurs in a similar way. By varying the parameters of the electric circuit during the transition, the constriction front can be stopped to form a steady-state partially constricted discharge. It is shown that this type of discharge corresponds to points lying inside the hysteresis loop of the conventional I-V characteristic measured without affecting the discharge during a transition. A comparative analysis of the discharge characteristics in Ar:N₂ mixtures and in pure argon is performed.     

Attainment of the Pease-Braginskii current in an ultra-high-pressure discharge

Results are presented from experimental studies of the contraction of the channels of discharges in hydrogen and helium at current amplitudes of 0.5–1.6 MA and initial gas pressures of 5–35 MPa. The observed decrease in the brightness temperature of the discharge channel with increasing deposited energy is caused by the heating of the ambient gas. The channel contraction observed near the maximum of the discharge current is due to the attainment of the Pease-Braginskii critical current. Previously, it was shown that megampere discharges operate in a fully metallic plasma of the eroded electrodes. The theoretical value of the Pease-Braginskii current for discharges in vacuum is ～100–200 kA. The observed increase in the critical current to ～1 MA is attributed to the absorption of channel radiation in the dense ambient gas.     

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.     

Low-pressure discharge induced by microwave radiation with a stochastically jumping phase

Results are presented from experimental studies on the unique beam-plasma generator of microwave radiation with a stochastically jumping phase (MWRSJP). To interpret the experimental results, a computer code was developed that allows one to simulate the process of gas ionization by electrons heated in the MWRSJP field and the behavior of plasma particles in such a field. The conditions for ignition and maintenance of a microwave discharge in air by MWRSJP are found both experimentally and theoretically, and the pressure range in which the power required for discharge ignition and maintenance is minimum are determined.     

Sedimentation of chlorophylls in an Arctic fjord under freshwater discharge

Sedimentation of chlorophylls was studied during summer 1997 in Adventfjorden (Spitsbergen, Arctic). During the period of study, the water column was found to be well stratified by a freshened surface layer (salinity <31 PSS). A high load of suspended particulate matter from riverine discharge reduced the euphotic zone to an interval of 0.4–1.1m. Total particulate matter sedimentation rates were about twice as high in June as in July. The following chlorophylls were distinguished in the sedimented particles: chl a and its degradation products (allomer chl a, phaeophytin a, phaeophorbide a, chlorophyllide a), chl b and chl c ₁+c ₂. The quantitatively most important derivative of chl a was phaeophorbide a (31--41% of porphyrin a). Generally, the sedimentation rate of chlorophylls increased with depth. Linear relationships between concentrations of chl a and phaeophorbide a (r ²=0.92), as well as between concentrations of chl a and phaeophytin a (r ²=0.90) indicated a strong connection between phytoplankton abundance and zooplankton grazing. The significant correlation between chl a and chlorophyllide a concentrations (r ²=0.82) showed that most of the sinking chl a belonged primarily to diatoms, and low chlorophyllide a:chl a ratio (0.03) indicated that cellular senescence was not an important reason for the sinking of chl a. Moreover, very low chl b:chl a ratios (about 0.05 calculated for samples where chl b was detectable) suggest that contributions of green algae and/or higher plant detritus were negligible in sinking particles. The ratio of chl c ₁+c ₂:chl a was 0.85 indicating that chl c-containing algae were dominating.     

Singlet oxygen in the low-temperature plasma of an electron-beam-sustained discharge

Results are presented from experimental and theoretical studies of the production of singlet delta oxygen in a pulsed electron-beam-sustained discharge ignited in a large (～18-1) volume at a total gas mixture pressure of up to 210 Torr. The measured yield of singlet oxygen reaches 10.5%. It is found that varying the reduced electric field from ～2 to ～11 kV/(cm atm) slightly affects singlet oxygen production. It is shown experimentally that an increase in the gas mixture pressure or the specific input energy reduces the duration of singlet oxygen luminescence. The calculated time evolution of the singlet oxygen concentration is compared with experimental results.     

Heat transfer in dust structures in an RF discharge plasma

Results are presented from experimental studies of heat transfer in liquid-like plasma-dust structures. The experiments were performed with aluminum oxide grains ～3–5 μm in size in an RF discharge plasma. The heat capacity of the dust grains in plasma is measured. The thermal conductivity and thermal diffusivity of liquid-like plasma-dust structures are deduced under the assumption that the observed temperature gradients and the propagation of a thermal perturbation in a dusty plasma are related to heat conduction within the dust component. The measured temperature dependences of the thermal conductivity and thermal diffusivity are in qualitative agreement with the results of numerical simulations performed in the model of a simple single-atom liquid. It is shown that quantitative discrepancy between the experimental and numerical results is related to the energy loss of dust grains in their collisions with the neutral particles of the ambient gas.     

Dark phase effect in the evolution of the positive column of a glow discharge in nitrogen

The characteristics of the initial stage of the formation of the positive column of a glow discharge in nitrogen at reduced pressures are studied experimentally and numerically. A dip in the plasma emission intensity in the initial stage of the discharge (the so-called “dark phase”) is observed experimentally at the positive polarity of the high-voltage electrode (the cathode is grounded). The dark phase is preceded by an ionization wave (IW). When the anode is grounded, neither an IW nor a dip in the discharge emission intensity are observed. A theoretical model capable of describing the discharge development under the actual experimental conditions is constructed. It is shown that the dark phase effect may be caused by the high electron density (above the steady-state one) produced in the gas during the passage of the IW across the discharge gap. This mechanism of the dark phase formation differs from the mechanism proposed earlier to explain a similar effect in noble gases. Additional experiments carried out with pure argon, helium, and helium with a nitrogen admixture have shown that, in the case of a grounded cathode, gas breakdown is also accompanied by the passage of an IW, whereas in the case of a grounded anode, no IW is observed; however, the dark phase is present in both cases. It is shown using computer simulations that, in nitrogen (in contrast to noble gases), the mechanism resulting in the dark phase effect does not operate in the absence of an IW.     

Fucoidan blocks macrophage activation in an inductive phase but promotes macrophage activation in an effector phase

It has been suggested that some polysaccharides play important roles in immune responses. Therefore, we used various types of polysaccharides for analysis of macrophage-mediated tumor cell killing. We report here that fucoidan blocked macrophage activation occurs in an inductive phase but enhanced macrophage activation appears in an effector phase.     

Spatial structure of emission from an electrode microwave discharge in hydrogen

The structure of electrode microwave (2.45 GHz) discharges in hydrogen with electrodes of various shapes and sizes at pressures of 1–8 torr and incident powers of 2–150 W is studied. It is found that the discharges exhibit a common feature that is independent of the antenna-electrode design: near the electrode surface, there is a thin bright sheath surrounded by a less bright, sharply bounded region, which is usually shaped like a sphere. It is suggested that the structure observed arises because the microwave field maintaining the discharge is strongly nonuniform. Near the electrode, there exists a thin dense plasma sheath with a high electron density gradient. A strong dependence of the electron-impact excitation coefficient on the electric field makes the effect even more pronounced. As the electron density decreases due to dissociative recombination, the microwave field gradient decreases and the discharge emission intensity tends to a nearly constant value. Presumably, in the boundary region of the discharge, there exists a surface wave, which increases the emission intensity at the periphery of the discharge.     

Numerical simulations of the development of an open discharge

Plasma Physics Reports - Results are presented from numerical simulations of the time evolution of open discharges in helium that are excited in the presence of an anode grid and generate electron...     

Generation of ion-acoustic and magnetoacoustic waves in an RF helicon discharge

A study is made of the generation of ion-acoustic and magnetoacoustic waves in a discharge excited in an external magnetic field by an electromagnetic wave in the whistler frequency range (ω_LH ? ω ? ω_He, where ω_LH = $\sqrt {\omega _{He} \omega _{Hi} } $ and ω_He and ω_Hi are the electron and ion gyrofrequencies, respectively). The excitation of acoustic waves is attributed to the decay of a high-frequency hybrid mode forming a plasma waveguide into low-frequency acoustic waves and new high-frequency waves that satisfy both the decay conditions and the waveguide dispersion relations. The excitation of acoustic waves is resonant in character because the conditions for the generation of waveguide modes and for the occurrence of the corresponding nonlinear wave processes should be satisfied simultaneously. An unexpected effect is the generation of magnetoacoustic waves by whistlers. A diagnostic technique is proposed that allows one to determine the thermal electron velocity by analyzing decay conditions and dispersion relations for waves in the discharge channel.     

Coagulation of dust grains in the plasma of an RF discharge in argon

Results are presented from experimental studies of coagulation of dust grains of different sizes injected into a low-temperature plasma of an RF discharge in argon. A theoretical model describing the formation of dust clusters in a low-temperature plasma is developed and applied to interpret the results of experiments on the coagulation of dust grains having large negative charges. The grain size at which coagulation under the given plasma conditions is possible is estimated using the developed theory. The theoretical results are compared with the experimental data.