Peculiarities of detecting pulses of runaway electrons and X-rays generated by high-voltage nanosecond discharges in open atmosphere

Peculiarities of detecting pulses of high-energy runaway electrons and the accompanying X-rays generated by high-voltage nanosecond discharges at high overvoltages in dense gases are discussed. Methods for overcoming difficulties encountered in such measurements are demonstrated. Different techniques for detecting runaway electrons and X-rays generated by discharges in open atmosphere are described, and typical errors that may be done when interpreting measurement results are considered. Experiments with the use of a small-size generator of nanosecond pulses with the idle-running voltage of 250–270 kV and stored energy of ～0.5 J have been carried out. It is shown that, in measuring runaway electron pulses by using an experimental configuration with a grid anode, a major portion of the recorded signal is attributed to electromagnetic disturbances. It is found that X-rays are mainly generated due to the deceleration of runaway electrons in the anode, rather than in gas. The number of runaway electrons with energies ～300 keV does not exceed 10⁹ per shot, and the X-ray dose is 60–200 μR/shot, depending on the anode material.     

Plasma Decay in the Afterglow of High-Voltage Nanosecond Discharges in Unsaturated and Oxygenated Hydrocarbons

Results of experimental and theoretical study of plasma decay in the afterglow of high-voltage nanosecond discharges in gaseous ethylene and dimethyl ether at room temperature and pressures from 2 to 20 Torr are presented. Using a microwave interferometer, the time behavior of the electron density in the range from 2 × 10¹⁰ to 3 × 10¹² cm^–3 during plasma decay is investigated. By processing the experimental data, the effective coefficients of electron–ion recombination as functions of the gas pressure are obtained. It is found that these coefficients substantially exceed the recombination coefficients of simple hydrocarbon ions. This distinction, as well as the increase in the effective recombination coefficient with pressure, is explained by the formation of cluster ions in three-body collisions, which recombine with electrons more efficiently than simple molecular ions. The coefficients of three-body conversion of simple molecular ions into cluster ions in the plasmas of ethylene and dimethyl ether, as well as the coefficients of recombination of electrons with cluster ions in these gases, are determined by analyzing the experimental data.     

Ion acceleration in a dipole vortex in a laser plasma corona

Particle-in-cell simulations show that the inhomogeneity scale of the plasma produced in the interaction of high-power laser radiation with gas targets is of fundamental importance for ion acceleration. In a plasma slab with sharp boundaries, the quasistatic magnetic field and the associated electron vortex structure produced by fast electron beams both expand along the slab boundary in a direction perpendicular to the plasma density gradient, forming an extended region with a quasistatic electric field, in which the ions are accelerated. In a plasma with a smooth density distribution, the dipole magnetic field can propagate toward the lower plasma density in the propagation direction of the laser pulse. In this case, the electron density in an electric current filament at the axis of the magnetic dipole decreases to values at which the charge quasineutrality condition fails to hold. In electric fields generated by this process, the ions are accelerated to energies substantially higher than those characteristic of plasma configurations with sharp boundaries.     

Plasma decay in high-voltage nanosecond discharges in oxygen-containing mixtures

Plasma decay in high-voltage nanosecond discharges in CO₂: O₂ and Ar: O₂ mixtures at room gas temperature and a pressure of 10 Torr is studied experimentally and theoretically. The time dependence of the electron density during plasma decay is measured using microwave interferometry. The time evolution of the charged particle density, ion composition, and electron temperature is simulated numerically. It is shown that, under the given conditions, the discharge plasma is dominated for the most time by O ₂ ⁺ ions and plasma decay is determined by dissociative and three-body electron?ion recombination. As in the previous studies performed for air and oxygen plasmas, agreement between measurements and calculations is achieved only under the assumption that the rate of three-body recombination of molecular ions is much greater than that for atomic ions. The values of the rate constant of three-body recombination of electrons with О₂ ⁺ ions in a wide range of electron temperatures (500–5500 K), as well as for thermal (300 K) electrons, are obtained by processing the experimental results.     

2.5-Dimensional numerical simulation of a high-current ion linear induction accelerator

Results are presented from numerical particle simulations of the transport and acceleration of a high-current tubular ion beam through one to five magnetically insulated accelerating gaps. The ion beam is neutralized by an accompanying electron beam. The possibility of transporting a high-current neutralized ion beam through five cusps is demonstrated. It is shown that the quality of the distribution function of a high-current ion beam at the exit from the accelerator can be substantially improved by optimizing the energy of the neutralizing electron beam. It is also shown that, by injecting additional high-current electron beams into the cusps, the accelerated ion beam can be made more monoenergetic and its divergence can be reduced.     

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.     

Means for efficient electron beam generation in wide-aperture open-discharge light sources

The interaction of a plasma in the accelerating gap of an open discharge with a strong external electric field and with the cathode surface has been investigated theoretically and experimentally. In a pulsed nanosecond discharge, the ion inertia and plasma screening of the electric field cause a fast growth of the electric field E in the cathode region and a decrease in the length of the latter. Along with a reduction of the electron multiplication factor at high electric fields, this leads to a substantial decrease in the ion flux toward the cathode, which allows one to develop highly efficient open-discharge light sources with a long lifetime and low cathode sputtering. In this respect, continuous and quasi-continuous discharges are less advantageous because of the smaller increase in the electric field in the cathode region. The Townsend coefficients of charge multiplication and electron emission at high electric fields typical of open discharges have been measured for the first time. Fast ions and atoms extracted from the plasma of the accelerating gap significantly affect the cathode emission properties. In particular, photoemission is enhanced by more than one order of magnitude and becomes the main mechanism for electron generation. This also increases the efficiency and lifetime of open-discharge light sources.     

Plasma decay in the afterglow of a high-voltage nanosecond discharge in air

The decay of air plasma produced by a high-voltage nanosecond discharge at room temperature and gas pressures in the range of 1–10 Torr was studied experimentally and theoretically. The time dependence of the electron density was measured with a microwave interferometer. The initial electron density was about 10¹² cm⁻³. The discharge homogeneity was monitored using optical methods. The dynamics of the charged particle densities in the discharge afterglow was simulated by numerically solving the balance equations for electron and ions and the equation for the electron temperature. It was shown that, under these experimental conditions, plasma electrons are mainly lost due to dissociative and three-body recombination with ions. Agreement between the measured and calculated electron densities was achieved only when the rate constant of the three-body electron-ion recombination was increased by one order of magnitude and the temperature dependence of this rate constant was modified. This indicates that the mechanism for three-body recombination of molecular ions differs from that of the well-studied mechanism of atomic ion recombination.     

Electrostatic plasma turbulence and spitzer longitudinal conductivity in tokamaks

An analysis has revealed that there may be three radically different steady states of a tokamak plasma: (i) discharges in which the electron and ion transport can be described by neoclassical theory; (ii) discharges with the Spitzer longitudinal conductivity, neoclassical ion transport, and “anomalous” electron transport; and (iii) discharges in which the electron transport and ion transport are both “anomalous.” The dimensionless parameters responsible for the occurrence of the three types of discharges are determined. In accordance with the criteria derived for the achievement of different steady states, discharges of the second type are most typical of modern tokamaks and discharges of the third type can occur only if the turbulence is sufficiently strong. Discharges of the first type cannot occur in the range of the working parameters of present-day tokamaks and future tokamak reactors, but they can be ignited in a large class of magnetic confinement systems. The physical reason for the onset of different types of discharges is associated with the fact that turbulent fluctuations play very different roles in the dynamics of the ion and electron components of a finite-size magnetized plasma. The question of the self-consistency between the profiles is considered. A law is derived according to which the thermal diffusivity increases toward the plasma edge.     

Distributions of the ion temperature,ion pressure,and electron density over the current sheet surface

The distributions of the ion temperature, ion pressure, and electron density over the width (the major transverse dimension) of the current sheet have been studied for the first time. The current sheets were formed in discharges in argon and helium in 2D and 3D magnetic configurations. It is found that the temperature of argon ions in both 2D and 3D magnetic configurations is almost uniform over the sheet width and that argon ions are accelerated by the Ampère force. In contrast, the distributions of the electron density and the temperature of helium ions are found to be substantially nonuniform. As a result, in the 2D magnetic configuration, the ion pressure gradient across the sheet width makes a significant contribution (comparable with the Ampère force) to the acceleration of helium ions, whereas in the 3D magnetic configuration, the Ampère force is counterbalanced by the pressure gradient.     

LET and ion-species dependence for cell killing and mutation induction in normal human fibroblasts.

We have been studying LET and ion species dependence of RBE values in cell killing and mutation induction. Normal human skin fibroblasts were irradiated with heavy-ion beams such as carbon (290 Mev/u and 135 Mev/u), neon (230 Mev/u and 400 Mev/u), silicon (490 Mev/u) and iron (500 Mev/u) ion beams, generated by Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences (NIRS). Cell killing effect was detected as reproductive cell death using a colony formation assay. Mutation induction in hprt locus was detected to measure 6-thioguanine resistant colonies. The RBE-LET curves of cell killing and mutation induction were different each ion beam. So, we plotted RBE for cell killing and mutation induction as function of Z*2/beta2 instead of LET. RBE-Z*2/beta2 curves of cell killing indicated that the discrepancy of RBE-LET curves was reconciled each ion species. But RBE-Z*2/beta2 curves of mutation induction didn't corresponded between carbon- and silicon-ion beams. These results suggested that different biological endpoints may be suitable for different physical parameter, which represent the track structure of energy deposition of ion beams.     

Generation of a Beam of Fast Electrons,Plasma Bremsstrahlung,and Characteristic Radiation in a High-Current Z-Pinch

Plasma Physics Reports - The generation of accelerated electron beams in a high-current Z-pinch formed by the implosion of wire cylindrical tungsten arrays on an Angara-5-1 facility is studied. The...     

Suprathermal electrons in a vacuum spark discharge

Results of experiments on the detection of suprathermal electron beams in the plasma of a highcurrent low-inductance vacuum spark by means of space-resolved spectral X-ray polarimetry are presented. It is shown that the observed polarization of bremsstrahlung may be caused by an ~100-keV electron beam propagating along the discharge axis from the pinching region toward the anode. The influence of the discharge initiation conditions on the parameters of the generated electron beams is analyzed.     

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.     

Molecular characterization of microbial mutations induced by ion beam irradiation

A positive selection system for gene disruption using a sucrose-sensitive transgenic rhizobium was established and used for the molecular characterization of mutations induced by ion beam irradiations. Single nucleotide substitutions, insertions, and deletions were found to occur in the sucrose sensitivity gene, sacB, when the reporter line was irradiated with highly accelerated carbon and iron ion beams. In all of the insertion lines, fragments of essentially the same sequence and of approximately 1188bp in size were identified in the sacB regions. In the deletion lines, iron ions showed a tendency to induce larger deletions than carbon ions, suggesting that higher LET beams cause larger deletions. We found also that ion beams, particularly "heavier" ion beams, can produce single gene disruptions and may present an effective alternative to transgenic approaches.     

Effect of the electron drift in a transverse electric field on the width of spark channels in a multichannel sliding discharge

The effect of electron drift on the transverse size of the spark channels in a multichannel sliding discharge on a dielectric surface is considered in a semianalytic approximation. The strength of an electric field transverse to the channel axis is estimated by the mirror image method. The estimate obtained is used to analyze the differential equation for the density of electrons with allowance for their drift from the channel into the surrounding layers. It is shown that the channel expands to a certain steady-state radius at which an increase in the local electron density due to the ionization of atoms is balanced by its decrease due to the electron drift from the surface channel layer into the surrounding layers. Numerical estimates are carried out for the conditions of earlier experiments with discharges in He, Ne, Ar, and Xe. The analysis applies to the initial nanosecond stage of the spark development, when the hydrodynamic expansion of the channels is still insignificant.     

Mechanism for efficient electron beam generation in a pixel of an open-discharge plasma display

A concept is proposed of a plasma pixel based on an open-discharge microstructure. The concept employs the capability of an open discharge to generate an electron beam at moderate (1–3 kV) discharge voltages with an efficiency close to 100%. To determine the possible application of this type of discharge, the parameters of the electron beams generated in open discharges operating in different working gases at various geometries of the discharge cell and various dimensions of the discharge channel were investigated. The electric potential distributions in the dielectric plate channel and in the cathode cavity were measured. The effect of additional illumination by radiation generated in the drift space on the current-voltage characteristic of the discharge is studied. Based on the results obtained, a noncontradictory model of a discharge capable of very efficiently generating an electron beam is proposed. According to this model, the main contribution to the electron beam comes from the photoelectron emission from the cathode under the action of radiation from the working-gas atoms excited by fast heavy particles in a highly nonuniform electric field in the cathode cavity. Such a field also scatters ions and fast atoms, thus reducing their fluxes toward the cathode. The results obtained indicate that highly efficient light sources and plasma panels can be created on the basis of open-discharge microstructures with a cathode cavity. Such microstructures allow very efficient conversion of electric energy into light.     

Pulsed high-voltage discharge in air with a pressure gradient

Results of experiments on high-voltage discharges in air with a pressure gradient are presented. The experiments were carried out at the setup developed at the Institute of Applied Physics, Russian Academy of Sciences. The goal of the experiments was laboratory modeling of high-altitude atmospheric discharges―sprites and jets. The setup and diagnostic techniques are described. The experimental results include the distribution of the gas pressure in the vacuum chamber formed by means of pulsed air puffing, photographs of discharges in air with a pressure gradient, and the dependences of the discharge current and optical emission intensity on the initial conditions.     

Interaction of an ion bunch with a plasma slab

Charge neutralization of a short ion bunch passing through a plasma slab is studied by means of numerical simulation. It is shown that a fraction of plasma electrons are trapped by the bunch under the action of the collective charge separation field. The accelerated electrons generated in this process excite beam?plasma instability, thereby violating the trapping conditions. The process of electron trapping is also strongly affected by the high-frequency electric field caused by plasma oscillations at the slab boundaries. It is examined how the degree of charge neutralization depends on the parameters of the bunch and plasma slab.     

Nonlinear theory of the low-frequency interaction of ion beams with the virtual cathode of a relativistic electron beam

A study was made of the nonlinear low-frequency interaction of a longitudinal ion beam with a virtual cathode of a relativistic high-current electron beam injected into a cylindrical drift chamber. Cases are considered in which the electron and ion beams have the same radii and in which the radius an ion beam is greaterthan that of an electron beam.