Evolution of the radial structure of a negative corona during its transformation into a glow discharge and a spark

With the proper stabilization of a negative corona, it is possible to increase the threshold current at which the corona discharge in the point-plane gap in air transforms into a spark. Then, in the current range corresponding to the transition region between the corona discharge and the spark, a new type of discharge arises—an atmospheric-pressure diffuse glow discharge. The transformation of the negative corona into a glow discharge and then into a spark is accompanied by the rearrangement of the discharge structure. The experiments show that, as the corona current increases, the radial current profile at the anode shrinks and the glow diameter near the anode increases. The radial profiles of the current and the corona glow during the transition to a glow discharge are measured. The longitudinal structure of the corona is computed using a 1.5-dimensional model that, unlike the other available models, includes gas ionization in the drift region of the corona. The experimental data are used to determine the effective cross section of the current channel at the anode. The radial glow profile near the anode is calculated using the measured current profile and assuming that the field profile is parabolic.     

Phenomenology of a high-current negative point-to-plane corona in nitrogen

Results are presented from experimental studies of a steady-state negative point-to-plane corona in atmospheric-pressure nitrogen at anomalously high (milliampere) currents. The evolution of the corona glow with increasing discharge current is traced up to the corona conversion into a spark. It was found that the structure and current-voltage characteristic of the corona change significantly when the working gas is slowly blown trough the discharge cell. It is shown that, starting from a current of I≥0.5 mA, a negative corona in a nitrogen flow acts as a streamer corona, whereas there are no streamers in the absence of gas blowing. In contrast to the quadratic dependence of the current versus voltage in a low-current corona, the current-voltage characteristic of a high-current corona is linear. The time evolution of the radial profile of the current density at the anode is studied under different experimental conditions.     

Pulsed mode of a negative corona in nitrogen: II. Numerical calculations

A simplified model of a cathode sheath sustained by electron avalanches is presented. The model is used to calculate the pulsed mode of a negative corona in nitrogen in order to establish the physical picture of the processes occurring in a pulsed corona. The most important point is that, in the pulsed mode, both the averaged and dynamic current-voltage characteristics of a glow cathode sheath are found to have a negative slope. Lowering the degree to which the glow cathode sheath is subnormal (by sharply reducing the sheath area) or switching on additional ionization mechanisms (e.g., stepwise ionization) that force the cathode sheath to evolve into a prearc spot causes the negative slopes of the averaged and dynamic current-voltage characteristics of the sheath to become more gradual and even positive, thereby stabilizing the discharge current.     

Simulation of nonstationary phenomena in atmospheric-pressure glow discharge

Nonstationary processes in atmospheric-pressure glow discharge manifest themselves in spontaneous transitions from the normal glow discharge into a spark. In the experiments, both so-called completed transitions in which a highly conductive constricted channel arises and incomplete transitions accompanied by the formation of a diffuse channel are observed. A model of the positive column of a discharge in air is elaborated that allows one to interpret specific features of the discharge both in the stationary stage and during its transition into a spark and makes it possible to calculate the characteristic oscillatory current waveforms for completed transitions into a spark and aperiodic ones for incomplete transitions. The calculated parameters of the positive column in the glow discharge mode agree well with experiment. Data on the densities of the most abundant species generated in the discharge (such as atomic oxygen, metastable nitrogen molecules, ozone, nitrogen oxides, and negative oxygen ions) are presented.     

Spatiotemporal evolution of the current and the integral and spectral emission characteristics of a negative corona in nitrogen during its transformation into a spark

Results are presented from experimental studies of the conversion of a steady-state negative corona into a spark. It is found that a spark in a negative corona in nitrogen and air is formed in the absence of fast primary streamers. It is shown that, in atmospheric-pressure nitrogen, the conversion of a corona into a spark begins with the propagation of a plasma channel (secondary streamer) from the point electrode (cathode) to the plane electrode (anode). In contrast, the plasma channel in air originates near the plane electrode and then propagates towards the point electrode. The propagation velocity of the secondary streamer is very low, V=10³–10⁴ cm/s. Two possible scenarios of the formation of the spark channel in a negative corona in nitrogen are described on the basis of the concept of a contracted volume glow discharge. Results are presented from time-resolved spectral measurements of plasma emission from different regions of the corona during its transformation into a spark.     

Alternating nonsteady gas-discharge modes in an atmospheric-pressure air flow blown through a point-plane gap

The electric and spectral characteristics of a nonsteady discharge in an atmospheric air flow blown through a point-plane interelectrode gap were investigated experimentally. The discharge was produced by applying a constant positive voltage to the point electrode, the amplitude of the applied voltage being much higher than the corona ignition voltage. The nonsteady character of the discharge is due to the spontaneously repeating streamer-spark breakdown, followed by the formation of either a diffuse ultracorona or a filamentary glow discharge. In the latter case, the length of the plasma column increases progressively, being blown off by the gas flow from the discharge gap. The extinction of a filamentary discharge is unrelated to the break of the current channel: the discharge decays abruptly when the filament length reaches its critical value. The distribution of active particles (O, OH, and N*₂) carried out from the discharge gap is determined from the data of spectral measurements.     

Development of a linear corona torch discharge

An anomalous kind of the positive corona—a linear corona torch discharge—is investigated. The discharge is nearly steady-state and operates with various electrode configurations, e.g., wire-plane, needle-plane, wire-cylinder, and two or more wires placed in parallel. It is found that the discharge exists in the form of a pulsed corona or an ordinary positive corona, which alternatively change each other under the action of a spark-gap switch.     

Expansion of the cathode spot and generation of shock waves in the plasma of a volume discharge in atmospheric-pressure helium

The expansion of the cathode spot and the generation of shock waves during the formation and development of a pulsed volume discharge in atmospheric-pressure helium were studied by analyzing the emission spectra of the cathode plasma and the spatiotemporal behavior of the plasma glow. The transition of a diffuse volume discharge in a centimeter-long gap into a high-current diffuse mode when the gas pressure increased from 1 to 5 atm and the applied voltage rose from the statistical breakdown voltage to a 100% overvoltage was investigated. Analytical expressions for the radius of the cathode spot and its expansion velocity obtained in the framework of a spherically symmetric model agree satisfactorily with the experimental data.     

Glow Pause in a Helium Plasma at Room and Liquid Nitrogen Temperatures on Applying a Nanosecond Voltage Pulse to a Glow Discharge

The features of the relaxation of a quasi-steady glow discharge after extra excitation by a nanosecond high-voltage pulse are studied experimentally. It is shown that the plasma relaxation is characterized by the existence of a time interval with a low emission intensity—a glow pause. A kinetic model of the helium plasma relaxation is developed. It is shown that the nanosecond discharge that creates extra ionization and metastable atoms enables one to keep the electron temperature at a quasi-steady level within the range 0.05–0.5 eV for several hundred microseconds during the glow pause. The effect of the helium temperature on the glow pause features is investigated.     

Specific features of radiation from a negative air corona operating in the Trichel-pulse mode

Experimental studies of spatiotemporal characteristics of radiation from a negative corona operating in the Trichel-pulse mode in the point-to-sphere electrode geometry have revealed two emission zones. In addition to the well-known glow near the point electrode, there is also an anode glow, whose intensity depends substantially on the shape of the anode. It is found that the anode glow is delayed with respect to the beginning of the Trichel pulse by a time that depends on the gap length and gap voltage. The emission spectrum of the anode glow in the wavelength range 300–400 nm is identified as the spectrum of the second positive system of nitrogen (the C ³Π _u -B ³Π _g transition).     

Structure of the surface streamers of an AC barrier corona in argon

Results are presented from experimental studies of the structure of an ac surface discharge excited by a metal needle over a plane dielectric surface. A barrier corona discharge was ignited in atmospheric-pressure argon at frequencies of the applied sinusoidal voltage from 50 Hz to 30 kHz. In experiments, the area of a dielectric covered with the discharge plasma increased with applied voltage. The discharge structure in diffuse and streamer modes was recorded using a digital camera and a high-speed image tube operating in a frame mode. It is found that, in the positive and negative half-periods of the applied voltage, the structure of the surface discharge is substantially different. The statistical characteristics of the branching surface streamers in the positive and negative half-periods are determined as functions of the voltage frequency. The most intense lines in the emission spectrum of the barrier corona are determined for both half-periods. The correlation between the dynamics of the emission intensity and the dynamics of the discharge current and voltage is investigated.     

Drift model of the cathode region of a glow discharge

A one-dimensional drift model of the cathode region of a glow discharge with allowance for both electron-impact ionization and charged particle loss is proposed. An exact solution to the model equations is obtained for the case of similar power-law dependences of the ion and electron drift velocities on the electric field strength. It is shown that, even in the drift approximation, a relatively wide transition layer in which the ion-to-electron current ratio approaches a constant value typical of the positive column of a glow discharge should occur between the thin space-charge sheath and the quasineutral plasma, the voltage drop across the space-charge sheath being comparable to that across the transition layer. The calculated parameters of the normal and anomalous glow discharges are in good agreement with available experimental data.     

Transverse glow discharges in supersonic air and methane flows

Transverse glow discharges in supersonic air and methane flows are studied both experimentally and theoretically. The experiments show that a diffuse volume discharge filling the whole cross section of the flow can easily be initiated in air, whereas a diffuse discharge in a methane flow shows a tendency to transition into a constricted mode. The electron transport coefficients (mobility and drift velocity) and the kinetic coefficients (such as collisional excitation rates of the vibrational levels of a methane molecule, as well as dissociation and ionization rates) are calculated by numerically solving the Boltzmann equation for the electron energy distribution function. The calculated coefficients are used to estimate the parameters of the plasma and the electric field in the positive column of a discharge in methane.     

Experimental study of a multipoint cathode corona in an argon flow

Results are presented from experimental studies of a multipoint negative corona in an atmospheric-pressure argon flow. It is shown that a decrease in the interpoint distance, gas circulation through the discharge gap, and the adjustment of ballast resistances in the corona supply circuit allow one to stabilize the discharge and enlarge the operating range of discharge currents.     

Process of commutation of a vacuum electric-discharge gap by laser plasma

The temporal parameters of a process of vacuum gap commutation under exposure to a nanosecond pulse of laser radiation incident on the cathode has been studied depending on the radiation energy. Based on the experiment data, it is suggested that a glow discharge is initially ignited in electrode erosion products under exposure to the laser pulse, which due to development of the ionization-overheating instability undergoes the contraction of current channel and transits to an arc discharge. With the radiation energy exceeding a threshold value, the radiation (incident on the cathode) accelerates directly the instability development and the glow discharge transition to the arc discharge due to the radiation absorption in the discharge plasma.     

Shape of the current tube of a negative point-to-plane corona in air

Results are presented from the experimental studies of a negative point-to-plane air corona discharge with controlled geometry. It is shown that, in a certain range of parameters, the steady-state current-voltage characteristic of the corona can be fitted by a parabola. The results obtained are used to determine the time-averaged shape of the current tube of a negative corona.     

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.     

Boundary of the Transition to Hollow Dust Structures in a DC Discharge in Neon with Microparticles

Plasma Physics Reports - The boundary (line) of the transition from homogenous dust structures to hollow dust structures in the coordinates gas pressure–discharge current in a glow discharge...     

Charging of Macroparticles in a Corona Discharge in an Air Flow

The charging of Al₂O₃ macroparticles with dimensions ranging from 20 to 40 μm in a gas flow passing through a multielectrode corona discharge is investigated. The corona discharge is produced by a system of wire electrodes arranged across the gas flow. The particle charge and mass are measured using a linear electrodynamic trap. For a corona voltage of 18 kV, the average charge-to-mass ratio is found to be 1.69 × 10¹³e/g for particles charged in the positive corona and 1.35 × 10¹³e/g for particles charged in the negative corona.     

Accumulation of products of ferroelectric target sputtering in the plasma of an RF glow discharge

The zone of macroparticle accumulation was revealed based on laser radiation scattering at copper vapor in the plasma of an RF glow discharge upon sputtering of multicomponent nanosize ferroelectric films. This zone was detected at the interface between the negative glow and the dark cathode space of the glow discharge. It is supposed that these particles are charged products of ferroelectric target sputtering.