S. I. Yakovlenko

High-power subnanosecond beams of runaway electrons and volume discharge formation in gases at atmospheric pressure

A non-local criterion for runaway electrons generation is proposed, which is a universal two-valued dependence of the critical voltage U cr on pd for a certain gas (where p is the pressure and d is the interelectrode distance). This dependence subdivides the (U, pd) plane into an area of efficient electron multiplication and an area in which electrons leave the gas gap without multiplication. Electron beams of subnanosecond pulse duration and an amplitude of hundreds of amperes are generated at atmospheric pressure in various gases. A volume nanosecond discharge with a high specific excitation power was realized without the pre-ionization of the discharge gap by using an additional external source. The role of the electron avalanches propagating from the cathode to the anode in a dense gas was considered.

Runaway of electrons in dense gases and mechanism of generation of high-power subnanosecond beams

New understanding of mechanism of the runaway electrons beam generation in gases is presented. It is shown that the Townsend mechanism of the avalanche electron multiplication is valid even for the strong electric fields when the electron ionization friction on gas may be neglected. A non-local criterion for a runaway electron generation is proposed. This criterion results in the universal two-valued dependence of critical voltage Ucr on pd for a certain gas (p is a pressure, d is an interelectrode distance). This dependence subdivides a plane (Ucr, pd) onto the area of the efficient electron multiplication and the area where the electrons leave the gas gap without multiplication. On the basis of this dependence analogs of Paschen’s curves are constructed, which contain an additional new upper branch. This brunch demarcates the area of discharge and the area of e-beam.The mechanism of the formation of the recently created atomospheric pressure subnanosecond e-beams is discussed. It is shown that the beam of the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the runaway electrons is formed at an instant when the plasma of the discharge gap approaches to the anode. In this case a basic pulse of the electron beam is formed according to the non-local criterion of the runaway electrons generation.The role of the discharge gap preionization by the fast electrons, emitted from the plasma non-uniformities on the cathode, as well as a propagation of an electron multiplication wave from cathode to anode in a dense gas are considered.

X-ray radiation due to nanosecond volume discharges in air under atmospheric pressure

The formation of nanosecond discharges in atmospheric-pressure air versus the applied pulse polarity and discharge gap geometry is studied. It is shown that the polarity of high-voltage nanosecond pulses and the electrode configuration have a minor effect on the volume discharges under a variety of experimental conditions. When the spacing between needle-like electrodes is large, the discharge is asymmetric and its glow is weakly dependent on the sign of the potential applied to the electrode. Negative voltage pulses applied to the potential electrode generate X-ray radiation from both the surface and volume. For a subnanosecond rise time of the voltage pulse and diffusion character of the discharge, the X-ray radiation comes from the brightly glowing region of a corona discharge. The average values of the fast electron velocity and energy in nitrogen are calculated. At field strengths E/p < 170 kV/cm atm, the average velocity of a fast electron bunch is constant because of central collisions. At field strengths E/p > 170 kV/cm atm, fast electrons run away. Central collisions are the reason for X-ray radiation from the volume.

Escaping electrons and discharges based on the background-electron multiplication wave for the pumping of lasers and lamps

A review of the recent papers devoted to the phenomena realized upon nanosecond discharges in dense gases pumped with high-voltage pulses with subnanosecond edges is presented. The following problems are considered: (i) the dynamics of the fast (including escaping) electrons that provide for the background ionization of the dense gas, (ii) the propagation of the background-electron multiplication wave, and (iii) the discharges based on the background-electron multiplication wave. It is demonstrated that discharges based on the background-electron multiplication wave are promising for lasing at transitions that were shown to be involved in lasing in dense gas upon electron-beam pumping and in the afterglow of a pulsed discharge. In particular, they are promising for pumping excimer and exciplex lasers. In addition, such discharges can be used in excilamps that generate high-power spontaneous pulsed radiation.

Similarity law for a homogeneous discharge in the presence of a strong field and analogues of the Stoletov constant for the pulsed regime

It is demonstrated that the similarity relationships (breakdown curves), which establish a dependence of the field strength divided by the pressure on the product of the pressure and the delay time of the breakdown, are realized upon the uniform breakdown of the gas gap in the presence of both rectangular and triangular voltage pulses, which is interesting for the physics of gas and plasma discharges, and remain valid for strong fields. The breakdown criterion is described with a two-valued curve such that the effective multiplication of electrons in gas becomes possible in the presence of both weak and strong fields and at small products of the pressure and the pulse time. An analogue of the Stoletov effect, which corresponds to a maximum in the current with respect to pressure at a given voltage pulse, is demonstrated for the pulsed discharge. The analogues of the Stoletov constant are calculated for non-self-sustained pulsed discharges in various gases. The minimum delay time of the breakdown is also determined by these constants.

Production of powerful electron beams in dense gases

Subnanosecond electron beams with the record current amplitude (∼70 A in air and ∼200 A in helium) were produced at atmospheric pressure. The optimal generator open-circuit voltage was found for which the electron-beam current amplitude produced in a gas diode was maximal behind a foil. It was established that the electron beam was produced at the stage when the cathode plasma closely approaches the anode. It was shown that a high-current beam can be produced at high pressures because of the presence of the upper branches in the curves characterizing the electron-escape (runaway) criterion and the discharge-ignition criterion (Paschen curve).

Metastable State of Supercooled Plasma

The computer ab initio simulation and analytical theory, that revealed unexpected non-ergodic properties of a classical Coulomb plasma, is overviewed. The results of a many-charged-particles system simulation predict the possible existence of a real metastable plasma, supercooled with respect to its ionization degree. The three-body recombination at this state is suppressed. The existence of such a plasma state is a consequence of the entropy conservation in isolated Hamiltonian systems free from any stochastic action from the outside (external stochastic disturbance). The occurrence of a metastable supercooled plasma (rather similar to a supercooled vapor or superheated liquid) depends on two conditions: First, all the charged particles should behave exactly according to the laws of classical mechanics (hence, most negatively-charged particles should preferably be heavy ions). Second, the plasma ionization degree should be sufficiently high (> 10−3). It is shown from thermodynamic consideration that a mixture of supercooled plasma with a perfect (ideal) gas might form a plasmoid of the ball-lightning type.

On the Mechanism of the Runaway of Electrons in a Gas: The Upper Branch of the Self-Sustained Discharge Ignition Curve

Based on the results of simulation by the method of particles, it is shown that the Townsend mechanism of electron multiplication in a gas at a sufficiently large electrode spacing is valid at least up to such large values of E/p at which relativistic electrons are generated. On the other hand, the phenomenon of electron runaway in a gas is determined by the electrode spacing, which must be either comparable with or smaller than the characteristic electron multiplication length, rather than the local criteria accepted presently. It is shown that, for a particular gas, the critical voltage across the electrodes at which the runaway electrons comprise a significant fraction is a universal function of the product of the electrode spacing by the gas pressure. This function also determines the condition of self-sustained discharge ignition. It not only incorporates the known Paschen curve but also additionally contains the upper branch, which describes the absence of a self-sustained discharge at a high voltage sufficiently rapidly supplied across the electrodes.

Possibility of lasing on the Ne transitions in the afterglow of a background electron multiplication wave

The possibility of lasing with the Ne + H2 Penning mixture in the afterglow of a background electron multiplication wave that emerges in the presence of a nanosecond high-voltage pulse with the subnanosecond leading edge across the discharge gap filled with the atmospheric-pressure gas is considered. The needed nonuniformity of the field is created with small-curvature-radius electrodes. It is demonstrated that lasing is possible at a relatively small (several centimeters) length of the active medium.

On the mechanism of subnanosecond electron beam formation in gas-filled diodes

Subnanosecond electron beams formed in diodes filled in with a gas at atmospheric pressure and X-rays emitted from nanosecond-discharge plasmas are studied. Both phenomena hold promise for lasing technology. A three-group separation of fast electrons in a gas-filled diode is proposed. It is found that the duration of the beam current in a diode filled with air at atmospheric pressure does not exceed 0.1 ns. It is also shown that the amplitude of the beam current attains maximum with a certain delay after the application of voltage to the discharge gap. A current of ∼400 A is detected behind the foil of a diode filled with air at atmospheric pressure. At a subnanosecond duration of the voltage pulse and the diffuse discharge, X-ray radiation is observed from the brightly glowing area of corona discharge. The mean steady-state velocities and energies of fast electrons in nitrogen are calculated. Head-on collisions are shown to control the constancy of the mean velocity of fast electrons for the field strengths E/p < 170 kV/(cm atm). At E/p > 170 kV/(cm atm), the escape of fast electrons takes place. It is particularly the head-on collisions that are decided to be responsible for the emission of X-rays from the bulk.