Picosecond runaway electron beams in air

Experimental data on the generation of picosecond runaway electron beams in an air gap with an inhomogeneous electric field at a cathode voltage of up to 500 kV are presented. The methods and equipment developed for these experiments made it possible to measure the beam characteristics with a time resolution of better than 10⁻¹¹ s, determine the voltage range and the beam formation time in the breakdown delay stage, and demonstrate the influence of the state of the cathode surface on the stability of runaway electron generation. It is demonstrated that the critical electron runaway field in air agrees with the classical concepts and that the accelerated beam can be compressed to ∼20 ps. It is unlikely that, under these conditions, the beam duration is limited due to the transition of field emission from the cathode to a microexplosion of inhomogeneities. The maximum energy acquired by runaway electrons in the course of acceleration does not exceed the value corresponding to the electrode voltage.     

A nanoparticle in plasma

Charge and energy fluxes onto a nanoparticle under conditions typical of laboratory plasmas are investigated theoretically. Here, by a nanoparticle is meant a grain the size of which is much smaller than both the electron Larmor radius and Debye length and the thermionic emission from which is not limited by the space charge. Under conditions at which thermionic emission plays an important role, the electric potential and temperature T _p of a nanoparticle are determined by solving a self-consistent set of equations describing the balance of energy and charge fluxes onto the nanoparticle. It is shown that, when the degree of plasma ionization exceeds a critical level, the potential of the nanoparticle and the energy flux onto it increase with increasing nanoparticle temperature, so that, starting from a certain temperature, the nanoparticle potential becomes positive. The critical degree of ionization starting from which the potential of a nanoparticle is always positive is determined as a function of the plasma density and electron temperature. The nanoparticle temperature T _p corresponding to the equilibrium state of a positively charged nanoparticle is found as a function of the electron density for different electron temperatures.     

Formation and development of a leader channel in air

A self-consistent model is constructed that makes it possible to investigate the formation of a leader channel in air and the evolution of the channel parameters in the developed stage, when the leader is as long as several meters or more. The initial stage of the formation of the channel is characterized by a rapid increase in the electron density and gas temperature, which is a consequence of the onset of thermal-ionizational instability. The radius of a fully developed plasma column at the current I=1 A in air at atmospheric pressure is R _h?10^?2 cm. Then, because of the gas-dynamic and thermal expansion, the plasma radius R _h increases considerably; as a result, the electric field and the reduced field E/N in the corresponding parts of the channel decrease. In the case under consideration, the field in the “oldest” parts of the leader drops to 200 V/cm and even lower and the reduced field becomes as weak as E/N≤10 Td. In this case, the densities of the main species of neutral and charged particles at the center of the channel remain close to their thermodynamically equilibrium values. The results of calculations are compared with the available experimental data.     

Plasma parameters in the channel of a long leader in air

The time evolution of the electric field in the leader channel and other characteristics of the leader plasma in long air gaps are simulated. Calculations are performed in the one-dimensional time-dependent model with allowance for the time-varying energy deposition in the channel, the channel expansion, and the nonequilibrium ionization kinetics in the leader plasma. The calculations show that, at a gas temperature of 4500–6000 K, associative ionization becomes a dominant ionization mechanism in the leader channel; as a result, the electric field decreases to 100–200 V/cm in 10^?4–10^?3 s under the conditions typical of the leader discharge. The calculated electric field agrees well with the data from the experimental modeling of long leaders by a spark discharge in short gaps.     

Theoretical investigation of the bistability effect in non-self-sustained discharges in Kr and Ar

The electron energy distribution function and the related plasma parameters in non-self-sustained discharges in Kr and Ar are studied theoretically. The investigations are carried out by numerically solving the corresponding Boltzmann equation for the electron energy distribution function with allowance for electron-electron collisions. The electron energy distribution and electron density are calculated self-consistently as functions of the intensity q of the source of secondary electrons and the magnitude of the reduced electric field E/N. The main goal of the investigations was to determine the conditions under which the plasma exhibits bistable parameters. Calculations show that, for discharges in Kr, there is a certain range of q and E/N values in which the Boltzmann equation has two different stable solutions. For an Ar plasma, such a bistability effect was not found: over the parameter range under consideration, the Boltzmann equation has a unique solution. Various plasma parameters (such as the effective electron temperature, electron drift velocity, and electron current density) are calculated for different discharge conditions, including those corresponding to the bistability effect.     

Two-component structure of the current pulse of a ranaway electron beam generated during electric breakdown of elevated-pressure nitrogen

Conditions are investigated at which two current pulses of ranaway electron beams are generated in elevated-pressure nitrogen during one voltage pulse. It is shown that the regime with two runaway electron beam current pulses takes place at decreased values of the electric field strength E in the gap (or decreased values of the parameter E/p, where p is the gas pressure). The regime with two runaway electron beam current pulses is observed both at high (1500?C3000 Torr) and low (below 100 Torr) pressures. It is shown that, for the second runaway electron beam current pulse to form, the voltage across the gap should be partially reduced during the first pulse. At low nitrogen pressures (~10 Torr), the regime in which two runaway electron beams are generated can be implemented by increasing the breakdown strength of the gap and/or increasing the value of E/p. In experiments carried out in atmospheric-pressure air with a picosecond time resolution, a rather complicated structure of the beam current pulse is observed at a voltage rise time of ~300 ps.     

Effect of nonlocal electron kinetics on the characteristics of a dielectric barrier discharge in xenon

The established dynamics of a dielectric barrier discharge in xenon at a pressure of 400 Torr is simulated in the framework of a one-dimensional fluid model in the local and nonlocal field approximations. It is shown that taking into account the nonlocal character of the electric field does not qualitatively change physical processes in a dielectric barrier discharge, but significantly affects its quantitative characteristics. In particular, the sheath thickness decreases, plasma ionization intensifies, the spatiotemporal distribution of the mean electron energy changes, and the discharge radiation efficiency increases. Electron kinetics in a dielectric barrier discharge in xenon is analyzed using the nonlocal field approximation.     

Comparison of the AC barrier corona with DC positive and negative coronas and barrier discharge

Results are presented from experimental studies of ac corona discharges between a point electrode and a dielectric-coated plate in nitrogen, argon, helium, and air in the voltage frequency range f=50 Hz–50 kHz. The characteristic features of this type of discharge are compared with the well-known features of dc positive and negative coronas and a barrier discharge between plane electrodes. It is shown that the presence of a dielectric barrier on the plane electrode significantly changes the electric characteristics and spatial structure of the corona, whereas the main phases of the discharge evolution remain unchanged as the voltage increases. With a point electrode, the breakdown voltage of the barrier corona decreases substantially as compared to the breakdown voltage of a barrier discharge with plane electrodes. This leads to softer conditions for the streamer formation in a barrier corona, which becomes more stable against spark generation.     

Analysis of ionization and recombination processes under the coronal equilibrium conditions by using a statistical atomic model

Ionization and recombination processes accompanying collisions of free electrons with plasma ions are considered using a statistical atomic model in which ionization and recombination are regarded as the processes of pair electron collisions in the electron gas of an atom. An expression for the ionization rate as a function of the ionization energy I and temperature T is derived. According to this expression, the ionization rate at I ? T is proportional to exp(?I/T). The statistical atomic model provides an estimate of the recombination rate for an ion with an arbitrary nuclear charge number Z, whereas more exact calculations of the recombination rate can be performed only for large Z. The model explains relatively low values of I/T (as compared to those given by the Saha equation) under the coronal equilibrium conditions and predicts a reduction in I/T with increasing Z. The values of I/T and the average ion charge number obtained from the balance equation for multielectron ions with the use of one fitting coefficient agree with the tabulated data calculated in the multilevel coronal model.     

Effect of space charge on the energy spectrum of a multispecies ion beam

The parameters of a multispecies metal ion beam extracted with the help of a set of grids from a plasma jet of a pulsed vacuum arc are studied experimentally. It is shown that the beam contains ions with energies that are both significantly lower and higher than the expected energy E _Z = \(\bar Z\) eU _acc, where \(\bar Z\) is the average ion charge number and U _acc is the extracting voltage. As a result, the mean ion energy is lower than E _Z and the ion energy spectrum is substantially wider than that in the plasma jet. It is found that this effect weakens with decreasing discharge current amplitude and that the shape of the spectrum depends on the accelerating voltage. Probe measurements show that, at accelerating voltages higher than 1 kV, a positive space charge forms in the drift gap, due to which the electric potential in the drift gap increases to a few hundred electronvolts. Analysis of experimental data indicates that the observed features of the ion spectrum can be attributed to the effect of the unsteady electric field of the space charge of the ion beam transported through the drift gap.     

Parameters of a supershort avalanche electron beam generated in atmospheric-pressure air

Conditions under which the number of runaway electrons in atmospheric-pressure air reaches ∼5 × 10¹⁰ are determined. Recommendations for creating runaway electron accelerators are given. Methods for measuring the parameters of a supershort avalanche electron beam and X-ray pulses from gas-filled diodes, as well as the discharge current and gap voltage, are described. A technique for determining the instant of runaway electron generation with respect to the voltage pulse is proposed. It is shown that the reduction in the gap voltage and the decrease in the beam current coincide in time. The mechanism of intense electron beam generation in gas-filled diodes is analyzed. It is confirmed experimentally that, in optimal regimes, the number of electrons generated in atmospheric-pressure air with energies T > eU _m , where U _m is the maximum gap voltage, is relatively small.     

Ecton processes in the generation of pulsed runaway electron beams in a gas discharge

As was shown earlier for pulsed discharges that occur in electric fields rising with extremely high rates (10¹⁸ V/(cm s)) during the pulse rise time, the electron current in a vacuum discharge is lower than the current of runaway electrons in an atmospheric air discharge in a 1-cm-long gap. In this paper, this is explained by that the field emission current from cathode microprotrusions in a gas discharge is enhanced due to gas ionization. This hastens the initiation of explosive electron emission, which occurs within 10^–11 s at a current density of up to 10¹⁰ A/cm². Thereafter, a first-type cathode spot starts forming. The temperature of the cathode spot decreases due to heat conduction, and the explosive emission current ceases. Thus, the runaway electron current pulse is similar in nature to the ecton phenomenon in a vacuum discharge.     

An Occam’s razor approach to chemical hardness: <Emphasis Type="Italic">lex parsimoniae</Emphasis>

The term “chemical hardness” refers to the resistance to deformation of the electronic density of a system; the greater this resistance, the “harder” the system. Polarizability, a physical property, is an inverse measure of resistance to deformation and thus should be inversely related to hardness. This is indeed generally accepted. Hardness has been postulated to be the second derivative of a system’s energy with respect to its number of electrons, despite the fact that this involves the differentiation of a noncontinuous function. This second derivative is typically approximated as the difference between the ionization energy I and the electron affinity A of the ground-state system, which results in ambiguity in that many molecules do not form stable negative ions. For atoms, the quantity I ? A does vary approximately inversely with polarizability, but this is only because the electron affinity is usually relatively low and ionization energy is known to be inversely related to polarizability for atoms. However, molecular polarizability depends primarily upon volume, and so does not show an acceptable inverse correlation with I ? A. Since both hardness and polarizability refer to the same property of a system—its resistance to deformation of the electronic density, we propose that the reciprocal of polarizability be taken to be a measure of hardness. We show that polarizabilities that are not known can be estimated quite accurately in terms of the average local ionization energies on the atomic or molecular surfaces and, for molecules, their volumes.     

Evolution of the channel of a long leader in air after a sharp decrease in the discharge current

The dynamics of the plasma parameters in a given cross section of a long-lived leader channel in air after a jumplike decrease in the discharge current is simulated numerically with the help of a one-dimensional non-steady-state model constructed with allowance for the dynamics of the energy input into the channel, the expansion of the channel, and the nonequilibrium ionization kinetics in the leader plasma. It is shown that, after a decrease in the current, the electric field in the channel, first, rapidly decreases and, then, increases gradually as the gas cools. The higher the energy input into the discharge before the decrease in the current, the longer the time scale on which the electric field increases. The results of simulations of the electric field in the channel agree with the data from the experimental modeling of the actual leader channel by a short spark.     

Electron cyclotron resonance acceleration of electrons to relativistic energies by a microwave field in a mirror trap

Results are presented from experiments on the acceleration of electrons by a 2.45-GHz microwave field in an adiabatic mirror trap under electron cyclotron resonance conditions, the electric and wave vectors of the wave being orthogonal to the trap axis. At a microwave electric field of ≥10 V/cm and air pressures of 10^?6–10^?4 Torr (the experiments were also performed with helium and argon), a self-sustained discharge was initiated in which a fraction of plasma electrons were accelerated to energies of 0.3–0.5 MeV. After the onset of instability, the acceleration terminated; the plasma decayed; and the accelerated electrons escaped toward the chamber wall, causing the generation of X-ray emission. Estimates show that electrons can be accelerated to the above energies only in the regime of self-phased interaction with the microwave field, provided that the electrons with a relativistically increased mass penetrate into the region with a higher magnetic field. It is shown that the negative-mass instability also can contribute to electron acceleration. The dynamic friction of the fast electrons by neutral particles in the drift space between the resonance zones does not suppress electron acceleration, so the electrons pass into a runaway regime. Since the air molecules excited by relativistic runaway electrons radiate primarily in the red spectral region, this experiment can be considered as a model of high-altitude atmospheric discharges, known as “red sprites.”     

Langmuir probe study of an inductively coupled magnetic-pole-enhanced helium plasma

This study reports the effects of RF power and filling gas pressure variation on the plasma parameters, including the electron number density n _e , electron temperature T _e , plasma potential V _p , skin depth δ, and electron energy probability functions (EEPFs) in a low-pressure inductively coupled helium plasma source with magnetic pole enhancement. An RF compensated Langmuir probe is used to measure these plasma parameters. It is observed that the electron number density increases with both the RF power and the filling gas pressure. Conversely, the electron temperature decreases with increasing RF power and gas pressure. It is also noted that, at low RF powers and gas pressures, the EEPFs are non-Maxwellian, while at RF powers of ≥50 W, they evolve into a Maxwellian distribution. The dependences of the skin depth and plasma potential on the RF power are also studied and show a decreasing trend.     

The 5′-untranslated region of the maize alcohol dehydrogenase gene provides efficient translation of mRNA in plants under stress conditions

The reduced level of expression of most cell proteins under stress conditions is determined by the low efficiency of cap-dependent translation of corresponding mRNAs. The maize gene encoding alcohol dehydrogenase, adh1, is a gene whose mRNA is efficiently translated in hypoxia. The reporter gene assay showed that the leader sequence of the adh1 mRNA provided for efficient translation of the reporter gfp gene in Nicotiana benthamiana cells in hypoxia or heat shock. The presence of this sequence in the 5′-UTR of mRNA did not change the level of expression under aerobic conditions, but the levels of gfp expression in hypoxia or heat shock were reduced five-to tenfold in the absence of this leader and remained unaffected when the adh leader sequence was present in the 5′-UTR. The adh1 leader sequence did not change the mRNA stability nor exhibited a promoter activity. Thus, the adh leader sequence acted as a translational enhancer, providing for efficient mRNA translation in plant cells under stress conditions. Introduction of this sequence into standard expression cassettes was proposed for the development of new systems to efficiently express the target proteins in plants under stress conditions.     

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.     

Study of Electromagnetic Electron Cyclotron Waves for Kappa Distribution with AC Field in the Magnetosphere of Saturn

Parallel propagating electromagnetic electron cyclotron (EMEC) waves in the extended plasma sheet (~12R_S) and in the outer magnetosphere (~18R_S) of Saturn have been studied. A dispersion relation for parallel propagating relativistic EMEC waves has been applied to the magnetosphere of Saturn, and comparisons have been made with the data of Voyager 1 at these radial distances. The detailed investigations for EMEC waves have been done in the presence of the perpendicular AC electric field, using the kappa distribution function. The relativistic temporal growth rate is calculated by the method of characteristic solution with the data provided by Voyager 1. The effect of the suprathermal electron density, temperature anisotropy, frequency of AC electric field, thermal energy of ions, and relativistic factor on the temporal growth rate of EMEC wave emission has been studied. The simulation results show that the growth of parallel propagating EMEC waves is significantly affected by variations in the temperature anisotropy, electron density, ion thermal energy, and relativistic factor in both the extended plasma sheet and the outer magnetosphere of Saturn. The temperature anisotropy (T⊥/T_║), ion thermal energy (K_BT_║i), and electron density (n₀) have been found to be a major source of free energy for parallel propagating EMEC waves in both regions.     

Characteristics of Electron Drift in an Ar–Hg Mixture

The characteristics of electron drift in a mixture of argon with mercury vapor at reduced electric fields of E/N = 1–100 Td are calculated and analyzed with allowance for inelastic collisions. It is shown that even a minor additive of mercury to argon at a level of a fraction of percent substantially affects the discharge parameters, in particular, the characteristics of inelastic processes. The influence of the concentration of mercury vapor in argon on the kinetic characteristics, such as the diffusion and mobility coefficients and ionization frequency, is investigated.