Discharge Formation Time Research Articles

Longitudinally excited short-pulse CO2 laser with large discharge tube without preionization

A longitudinally excited CO2 laser with a large discharge tube having an inner diameter of 16 mm and a length of 80 cm produced short laser pulses with an energy of 38.1 mJ at a repetition rate of 700 Hz. The CO2 laser did not have a preionization system or a fast gas flow system. The dependence of the laser output and discharge voltage on the repetition rate, the gas pressure and the structure of the discharge tube in 1:1:4 and 1:1:2 mixtures of CO2/N2/He gas was investigated. At a repetition rate of 300 Hz or more, the laser energy decreased as the repetition rate increased. The 1:1:4 mixture of CO2/N2/He gas produced a higher laser energy than the 1:1:2 mixture of CO2/N2/He gas. At a repetition rate of 300 Hz or less, the laser beam profile was doughnut-shaped. At a repetition rate of 400 Hz or more, the center of the laser profile had a high energy. At a high repetition rate, the fast discharge may have collected in the center of the discharge tube. Moreover, an increase in the repetition rate decreased the discharge formation time and the energy of a pulse tail part of the laser pulse waveform.

A dynamical model of microwave window breakdown at vacuum/dielectric interface

A new dynamical model is built to describe the process of microwave window breakdown at the vacuum/dielectric interface. In this model, the effects of gas desorption and gas diffusion on breakdown are first taken into account. The evolution of the density of electrons and the neutral gas is analyzed. Particle-in-cell simulations are employed to validate this model, and the agreements of comparisons are favorable. The results show that the density of neutral gas decreases exponentially with the distance away from the dielectric surface. It is concluded that the gas diffusion is the main factor in the density reduction of neutral gas. With the influence of gas ionization, the number of electrons striking the dielectric surface increases. As a result, the gas pressure above the dielectric surface increases rapidly. In addition, the discharge formation time tc of microwave window breakdown decreases exponentially with the gas desorption rate. Besides, tc decreases with the energy of the first crossover point of the secondary electron emission curve when the strength of microwave electric field Erf is small. This dynamical model provides a solution to designing the microwave window.

Nanosecond volume discharge in air initiated by a picosecond runaway electron beam

A voltage pulse with an amplitude of 250 kV and duration of 1ns was used to study discharge in atmospheric air. The discharge commences with the emergence of a field emission current from a cathode field enhancer. Next, a beam of runaway electrons with an amplitude of 0.5 – 1 A and duration of 10−11 s appears. Interaction between the beam and the voltage pulse was investigated using the reflectometry method. The discharge event was identified by the reversal of the reflected pulse polarity. If the emergence of the runaway electron beam is delayed by a time interval Δt1, polarity inversion is delayed by the time Δt2 = Δt1 = 200 ps. This is due to the small discharge formation time, 33 ps, as a result of the large number (about 108) of runaway electrons that initiate the discharge. The time interval between the beginning of the discharge and the voltage inversion is no longer than 100 ps. This process is theoretically estimated based on the concept of multielectron initiation of discharge. It is shown that what is observed in the experiment is the nanosecond multielectron-initiation discharge produced by runaway electrons of the discharge itself.

Study of the Formation Time of a Self-Sustained Subnanosecond Discharge at High and Ultrahigh Gas Pressures

The formation times of self-sustained subnanosecond discharges in nitrogen at pressures of 1‒40 atm and in hydrogen at pressures of 1–60 atm are analyzed in terms of the avalanche model. In experiments, a subnanosecond voltage pulse with an amplitude of 102 ± 2 kV was applied to a 0.5-mm-long discharge gap with a uniformly distributed electric field (the curvature radii of both the cathode and anode ends were 1 cm). The rise time of the voltage pulse from 0.1 to 0.9 of its amplitude value was about 250 ps. Breakdown occurred at the leading edge of the pulse. The discharge formation time was measured at different gas pressures with a step of 5–10 atm. Analysis of the experimental results shows that, in nitrogen at pressures of 10–40 atm and in hydrogen at pressures of 20–50 atm, breakdown occurs earlier than the electron avalanche reaches its critical length and that the critical avalanche length lies in the range of (2–8) × 10–2 mm, which is one order of magnitude shorter than the discharge gap length. This means that the avalanche–streamer model is inapplicable in this case. The fast formation of a conducting channel under these conditions can be explained by ionization of gas by runaway electrons. In this case, the conducting column develops as a result of simultaneous development of a large number of electron avalanches in the gas volume. An increase in the hydrogen pressure from 50 to 60 atm leads to an abrupt increase in the discharge formation time by about 50%. As a result, the growth time of the electron avalanche to its critical length becomes shorter than the discharge formation time. In this case, the electrons cease to pass into the runaway regime and the discharge is initiated from the cathode due to field emission from microinhomogeneities on its surface. Under these conditions, the discharge formation time is well described by the avalanche–streamer model.

Dependence of pre-breakdown time on ionization processes in a pseudospark discharge

The formation and development of pseudospark discharge, especially the onset of the breakdown, are of great technological interests in multiple applications due to their influences on the limits of current rising and fast switching performances of the devices. In this work, the development of pseudospark discharge in the pre-discharge and hollow cathode phases in a single-gap device are investigated by a time-dependent model to calculate the temporal development of total ionization cross section in varying times and regions. The simulations in our work are performed using the two-dimensional kinetic plasma simulation code XOOPIC. The time-dependent evolutions of the ionization cross section in pre-discharge and hollow cathode phases are presented under varying electric fields and hollow cathode configurations. Thus the electron multiplications and plasma generation processes by ionizing collisions in varying phases are examined and their dependences on a variety of external parameters are determined in different regions in the pseudospark device. A sequence of physical events and their influences in different regions are also identified via the quantitative analysis of time-dependent ionization cross section. The discharge formation time shows highest dependences on the cathode aperture diameters and anode voltages. Additionally, a linear dependence of the pseudospark breakdown time on the time-averaged ionization cross section is illustrated under varying external parameters. It indicates that the influences of the external parameters on the discharge performances can be determined and estimated via the total and average ionization cross sections under varying external conditions. In this work, both a qualitative understanding of the pseudospark onset mechanism and a quantitative approach to estimate the formation time in a pseudospark device with varying parameters are developed via this model.

The one-particle approximation in the reflecting discharge simulation

The method of some reflecting discharge (Penning discharge) characteristics computation, based on the one-particle approximation is proposed. This discharge is widely used in ion sources aimed at surface modification. However, only the steady state of this discharge is sufficiently described, whereas pulsed modes are preferable in many cases. In fact, the proposed method is similar to the approach used in the early times of first glow discharge investigations and crossed fields ion sources. It may be applied for the early discharge stages (the Townsend regime) description. It is somehow simpler than the diffusion-drift approximation used as a rule for the stationary state description, because plasma does not exist yet. On the other hand, one need not use most of usual diffusion-drift simplifications e.g. 1 or 2D models, uniform magnetic field etc. So the process of discharge formation may be described exactly for different kinds of Penning cells geometries and fields configurations. The discharge ignition condition for the Penning cell, analogous to the Townsend law is evaluated. It allows one to appreciate the discharge formation time as a function of cell geometric parameters, field configurations, anode voltage and Townsend’s coefficients α and γ. This time, or exactly the trajectory length during this time, plays the role of the Townsend parameter d – the distance between electrodes. The calculated values of such times show good agreement with experimental data.

Exoemission Properties of PDP Protective Layers

The priming properties of four different types of protective layers inside plasma display panels (PDPs) are compared, using undoped MgO, Si-doped MgO, Sc-doped MgO, and MgCaO (15%) plasma protective layers. In a two-electrode type of PDP, the statistical spread in discharge formation time is measured, as a function of the waiting time from 5 μs to 50 ms, after excitation by a different number of sustain cycles (1-1024) at 200 and 10 kHz. The results are interpreted in terms of the delayed emission of exoelectrons from these oxide cathodes. The kinetics of the emission process is compared with that of the existing recombination emission models. An exocurrent contribution is present in all of the materials that decays with the square root of the waiting time. It is proportional to the electron density itself, and can qualitatively be explained by a three-particle Auger electron loss mechanism.

Relation Between Discharge Length and Laser Pulse Characteristics in Longitudinally Excited CO2 Laser

A longitudinally excited CO2 laser pumped by a fast discharge emits a short laser pulse, similarly to TEA and Q-switched CO2 lasers. We investigated the relation between the discharge length and the laser pulse characteristics to develop a longitudinally excited CO2 laser producing a high spike laser pulse. We examined discharge lengths of 30, 45, and 60 cm, using the same mirrors and the same excitation circuit with the same input energy. A longer discharge length increased the discharge volume and improved the laser output energy. However, the longer discharge length caused a long discharge formation time (a slow fall time of the discharge voltage) due to the higher discharge impedance, which resulted in a long laser pulse tail. Therefore, the longitudinally excited CO2 laser had optimum conditions for obtaining a high spike laser pulse effectively.

New models and distributions of the electrical breakdown time delay in neon

The measurements of the electrical breakdown time delay td for a wide range of working voltages and at different preionization levels are presented. The statistical breakdown time delay ts and the discharge formative time tf are experimentally separated and theoretical models of their dependencies on the overvoltage and number densities of residual charges during relaxation are suggested. Several empirical and semiempirical models are used to describe the formative time delay dependence on working voltages tf (U). The empirical and theoretical models from the literature are also applied to the experimental data, without and with empirical corrections. Moreover, several new distributions are experimentally obtained: Gauss-exponential, Gaussian and double Gaussian ones for the statistical time delay, as well as Gaussian and double Gaussian distributions for the formative time. The measurements of the breakdown time delay at different preionization levels (afterglow periods) td (τ) obtained with a galvanic layer of gold and a sub-layer of nickel on the copper cathode are compared to the measurements with a vacuum deposited gold layer on the cathode surface. It was found that the surface charges retaining on a galvanic layer of gold influence the breakdown time delay which leads to double Gaussian distributions of the formative and statistical time delay.

Transition of window breakdown from vacuum multipactor discharge to rf plasma

In high-power microwave systems, the transition of window breakdown from single surface vacuum multipactor discharge to rf plasma with increasing gas pressure is investigated using particle-in-cell simulations. An intermediate pressure regime where multipactor discharge and rf plasma coexist was found. The pressure range where the multipactor can be maintained is summarized in the plot of the secondary electron emission yield as a function of the gas pressure. As the gas pressure increases, electron-neutral collisions prevail against secondary electron emissions and the electron energy probability function changes from the bi-Maxwellian at low pressures to Druyvesteyn at high pressures as a result of the change in electron heating and cooling processes. The discharge formation time in argon, neon, and xenon is shown for different gas pressures. Different scaling laws in the discharge formation time are presented at low and high pressures, respectively.

Very Fast Rise-Time Short-Pulse High-Voltage Generator

This paper relates to the development of an ultrafast compact system readily applicable to the field of ultrawideband microwave applications such as transient radar or laser drivers. The design, production, and experimental results of a short-pulse generation system are presented in this paper. The coaxial generator technology based on the principle of a line discharge by means of a high-pressure gas switch in a simple transmission line arrangement is described. In the first stage of the process, the coaxial generator was charged by a high-voltage direct current source. Interesting results were obtained, but due to the jitter of the discharge formative time, the reproducibility of the output pulses was not sufficient. To improve the performance, a pulsed source was developed in order to apply an overvoltage to the gap switch and thus reduce the jitter of the discharge formative time. The characteristics of the pulsed source are defined. A thyristor Marx generator and a pulse transformer using magnetic properties of ferrite cores were combined in order to produce this pulsed source. The main performances of this source are a pulse amplitude of 60 kV, a rise time of 250 ns, and a maximum frequency of 900 Hz from a 1-kV dc supply. Finally, the complete generation system is able to generate pulses with 25-kV amplitude and 70-ps rise time through a 50-Omega impedance from the single shot mode up to 900 Hz. The output pulses reproducibility obtained is better than plusmn5%

Statistical analysis of the electrical breakdown time delay distributions in krypton

The statistical analysis of the experimentally observed electrical breakdown time delay distributions in the krypton-filled diode tube at 2.6mbar is presented. The experimental distributions are obtained on the basis of 1000 successive and independent measurements. The theoretical electrical breakdown time delay distribution is evaluated as the convolution of the statistical time delay with exponential, and discharge formative time with Gaussian distribution. The distribution parameters are estimated by the stochastic modelling of the time delay distributions, and by comparing them with the experimental distributions for different relaxation times, voltages, and intensities of UV radiation. The transition of distribution shapes, from Gaussian-type to the exponential-like, is investigated by calculating the corresponding skewness and excess kurtosis parameters. It is shown that the mathematical model based on the convolution of two random variable distributions describes experimentally obtained time delay distributions and the separation of the total breakdown time delay to the statistical and formative time delay.

On the similarity law in picosecond gas discharges

It has been shown that the product of the gas pressure p and the discharge formation time τ is unambiguously related to the reduced electric-field strength E/p for air and argon in the picosecond range of τ. This dependence was previously found for the nanosecond range. The available experimental data are satisfactorily explained in the framework of the theory of the multielectron initiation of pulsed gas discharges that was proposed in my previous works.

The application of convolution-based statistical model on the electrical breakdown time delay distributions in neon

The convolution-based model of the electrical breakdown time delay distribution is applied for statistical analysis of experimental results obtained in neon-filled diode tube at 6.5 mbar. At first, the numerical breakdown time delay density distributions are obtained by stochastic modeling as the sum of two independent random variables, the electrical breakdown statistical time delay with exponential, and discharge formative time with Gaussian distribution. Then, the single characteristic breakdown time delay distribution is obtained as the convolution of these two random variables with previously determined parameters. These distributions show good correspondence with the experimental distributions, obtained on the basis of 1000 successive and independent measurements. The shape of distributions is investigated, and corresponding skewness and kurtosis are plotted, in order to follow the transition from Gaussian to exponential distribution.

Use of a glow discharge in a magnetic field for production of broad ion beams for technological applications

The results of investigations of the conditions of existence and ion-emission properties of a plasma of the high-current variety of a low-pressure glow discharge with a hollow cathode in a magnetic field are given. It has been shown that an applied weak magnetic field (∼10–3 T) not only reduces the minimum threshold gas pressure, but also makes the radial distribution of the plasma density more uniform (∼10%). A decrease in statistical straggling of the delay time to breakdown (∼102 times) and in discharge formative time (∼10 times) with increasing pulse repetition rate has been revealed to occur as a result of the transition to multielectron discharge initiation. The mass constitution of the discharge and the charge state of ions have been investigated and ways of lowering the content of the basic types of impurities have been determined. The influence of the parameters of the space charge cathode layer on the beam formation by electrostatic ion optics was investigated. The design of a source of gas ions possessing an increased lifetime and high reliability is described. The source is capable of producing an ion beam of cross section ∼100 cm2 with an energy of ions of up to 40 keV at an average current density of up to 1 mA/cm2.

Effect of cathode surface state on the characteristics of pulsed hollow-cathode glow discharges

A study is made of the effect of pulse repetition rate (0.1−103 s−1) and average discharge current (0–1 A) on the breakdown delay time and burning voltage of low-pressure glow discharges (p<0.1 Pa) in an electrode system of the reverse magnetron type with a large cathode surface area (≈103 cm2). It is shown that increasing the repetition rate leads to a many-fold reduction in the statistical spread in the delay time and in the discharge formation time, while the average discharge current has a significant effect on the burning voltage. The mechanism for the observed phenomena is interpreted qualitatively in terms of the presence of thin dielectric films on the cathode surface.

On the role played by UV resonant photons in the electrical breakdown of an argon discharge initiated with an electron source

A Monte Carlo model for the initial phase of an argon discharge breakdown (1 Torr, 2.5 cm distance between two plane electrodes) is presented. The growth of the first avalanche and the beginning of the second one are simulated to determine which physical process is responsible for the secondary electron emission of the cathode. Measurements of the discharge formation time are made under the same conditions as for the model. The discharge is initiated with an electron source. We show that, in the frame of our study, the ions are too slow to play a role in the initial phase of the breakdown and that the UV resonant photons create the succession of electronic avalanches.

Subnanosecond ultraviolet nitrogen TEA laser

A description is given of the construction and operation of an ultraviolet nitrogen TEA laser (λ = 337.1 nm) excited by a travelling discharge wave. The dependences of the stimulated emission parameters on the voltage and the interelectrode gap are reported. The optimal conditions for the emission of stimulated radiation at atmospheric pressure of nitrogen were obtained for E/p = 0.76–0.79 V cm-1 Pa-1 and an interelectrode gap of 2.9 nm when the discharge formation time was matched to the lifetime of the upper active level. Output pulses with a power in excess of 300 kW and of 1 ns duration were generated under the optimal conditions. When operating in atmospheric air the optimal conditions were an interelectrode gap of 2.1 mm and E/p = 0.90–0.94 V cm-1 Pa-1; in this case the output power of the pulses was 100 kW and their duration was 0.5 ns. A simple model of such a laser was used to account for the experimentally observed dependences and to calculate the optimal laser parameters.

Long formative time lags in nitrogen below the Paschen minimum

The formative time lag of an electric discharge for an overvoltage of 1-8%, below the minimum of the Paschen curve in nitrogen is investigated. The Laue distribution of the breakdown time delays is found to apply. The formative time rises with decreasing pd below pdmin from values of 10-3 s, up to the order of seconds. The suggested explanation is that avalanches in nitrogen are not independent. This poses the question of the definition of the discharge formative time.

THE DELAY EFFECT OF PULSE AVALANCHE DISCHARGE PROCESSES OF XeCl LASER AT HIGH GAS PRESSURE

We analysed the pulse avalanche discharge processes of XeCl laser at high gas pressure up to 10 atm. The influences of delay effect on the discharge formation time were discussed. By comparing the calaulated results with experimental data observed in the XeCl laser pulse avalanche discharge, the critical avalanche track length was obtained as ξ≌1 mm.