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.”     

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.     

Experimental studies of the behavior of the ion plasma component during disruptive instability in a tokamak

Results are presented from experimental studies of the behavior of the plasma ion component during disruptive instability in the TVD and DAMAVAND tokamaks. It is shown that the ion temperature increases during a major disruption by a factor of 1.5–2. The ions are accelerated predominantly across the magnetic field near the rational magnetic surfaces. Results on the ion acceleration along the magnetic field indicate that disruptions are accompanied by the generation of longitudinal electric fields that are aligned in opposite directions at the plasma periphery and near the plasma axis.     

Effect of the longitudinal momentum of electrons on their surfatron acceleration by an electromagnetic wave in space plasma

By numerically calculating the second-order nonlinear time-dependent equation for the wave phase on a particle trajectory, the effect of the longitudinal (with respect to the external magnetic field) momentum of electrons on the dynamics of their surfatron acceleration by an electromagnetic wave propagating across the external magnetic field in space plasma is analyzed. It is shown that, for strongly relativistic initial values of the longitudinal component of the electron momentum (the other parameters of the problem being fixed), the electrons are trapped into the ultrarelativistic regime of surfatron acceleration within a definite interval of the initial wave phase Ψ(0) on the particle trajectory. It was assumed in the calculations that Ψ(0) ≤ π. For the initial wave phases lying within the interval of 0 < Ψ(0) ≤ π, the electrons are immediately trapped by the wave, whereas at π ≤ Ψ(0) ≤ 0, no electron trapping is observed even at long computation times. This result substantially simplifies estimates of the wave damping caused by particle acceleration. The dynamics of the velocity components, momentum, and relativistic factor of electrons in the course of their ultrarelativistic acceleration are considered. The obtained results present interest for the development of modern concepts of the mechanisms for the generation of ultrarelativistic particles in space plasma, correct interpretation of experimental data on the flows of such particles, explanation of possible reasons for the deviation of the fast particle spectra observed in the heliosphere from the standard power-law scaling, and analysis of the relation between such deviations and the space weather.     

Investigation of ion acceleration in an expanding laser plasma by using a hybrid Boltzmann-Vlasov-Poisson model

The acceleration of ions of different species from a plasma slab under the action of a charge-separation electric field driven by hot and cold electrons is studied by using a hybrid Boltzmann-Vlasov-Poisson model. The obtained spatial and energy distributions of light and heavy ions in different charge states demonstrate that the model can be efficiently used to study the ion composition in a multispecies expanding laser plasma. The regular features of the acceleration of ions of different species are investigated. The formation of compression and rarefaction waves in the halo of light ion impurity, as well as their effect on the energy spectrum of the accelerated ions, is analyzed. An approach is proposed that makes it possible to describe the production of fast ions by laser pulses of a given shape. It is shown that the energy of fast ions can be increased markedly by appropriately shaping the pulse. The effect of heating of the bulk of the cold target electrons on the ion acceleration is discussed.     

Laser acceleration of light ions from a thin homogeneous foil of complex atomic composition

A model of the acceleration of light impurity particles from a plane ultrathin foil of complex ion composition by a high-power ultrashort high-contrast laser pulse is proposed. A study is made of both purely Coulomb ion acceleration, typical of extremely high electron energies, and ion acceleration under the conditions of space charge separation, determined by the finite typical electron temperature. Exact and approximate analytic approaches to describing impurity particle acceleration are developed. The spatial and spectral parameters of accelerated light particles are obtained, and their dynamics is investigated as a function of their relative charge density in a model of test impurity particles and in a model in which their own electrostatic field is taken into account. Optimum conditions for the generation of quasi-monoenergetic ions are discussed, depending on the laser radiation parameters and target composition.     

Plasma dynamics and generation of hard radiations in experiments with cylindrical Z-pinches

A survey of experimental studies on the generation of hard ionizing radiations from dynamic cylindrical Z-pinches is presented. Comprehensive experimental data do not confirm the hypothesis that charged particles responsible for the generation of hard radiations (neutrons and X-rays) are accelerated in short-scale Z-pinch necks (m = 0). Analysis of the experimental data indicates that, in discharges in pure hydrogen and deuterium, these particles are most probably accelerated in the axial direction along H _φ ≈ 0 lines by the induction electric field generated during the initiation of the secondary near-wall breakdown, which disconnects the pinch from the power supply. In discharges excited in heavy gases and at high initial current growth rates (İ ₀ ≥ 10¹² A/s) in experiments with hydrogen and deuterium contaminated with admixtures arriving from the chamber wall, there is an additional acceleration mechanism related to the growth of the resistance of a radiatively cooled Z-pinch.     

Acceleration of electron bunches injected into a wake wave

The process of trapping and acceleration of nonmonoenergetic electron bunches by a wake wave excited by a laser pulse in a plasma channel is investigated. The electrons are injected into the vicinity of the maximum of the wakefield potential with a velocity lower than the wave phase velocity. The study is aimed at utilizing specific features of a wakefield with substantially overlapped focusing and accelerating phases for achieving monoenergetic electron acceleration. Conditions are found under which electrons in a finite-length nonmonoenergetic bunch are accelerated to high energies, while the energy spread between them is minimal. The effect of energy grouping of electrons makes it possible to obtain compact high-energy electron bunches with a small energy spread during laser plasma acceleration.     

Wakefield acceleration of nonmonoenergetic electron bunches

The process of trapping and acceleration of a nonmonoenergetic electron bunch of finite length is investigated analytically in terms of a one-dimensional model, and relevant three-dimensional simulations are performed. The bunch is assumed to be injected into the region of maximum wake wave potential, the injection energy being such that the electron velocities are lower than the wave phase velocity. The study is aimed at clarifying how the spatial and energy parameters of the injected bunch in the trapping and acceleration stages depend on its initial energy spread. Formulas are obtained that describe the change in the bunch length and its energy spread in the course of acceleration. In some important limiting cases, the formulas are simple enough for them to be conveniently used for practical estimates. The injection conditions are discussed under which the electrons of a nonmonoenergetic bunch can be accelerated to high energies and the energy spread of the bunch electrons after acceleration is weakly sensitive to their initial energy spread. The analytical results agree well with the results of numerical simulations.     

Modeling of particle dynamics in a thruster

Numerical solutions to the equations describing the process of ion acceleration in a Hall current plasma accelerator (thruster) are studied. The system itself represents a three-component plasma: neutral atoms, free electrons, and singly-ionized atoms. The ions in the acceleration tract move without collisions, i.e., the length of the free path of ions is larger than that of the acceleration tract, while electrons move in a diffusion mode across the magnetic field. It is shown that in case the Poisson equation for an electric field is used the set of dynamic equations does not have an acoustic peculiarity that appears when solving a quasineutral set when the velocity of the ion flow and the ion-acoustic velocity coincide.     

On the longitudinal distribution of electric field in the acceleration zones of plasma accelerators and thrusters with closed electron drift

The long-term experience in controlling the electric field distribution in the discharge gaps of plasma accelerators and thrusters with closed electron drift and the key ideas determining the concepts of these devices and tendencies of their development are analyzed. It is shown that an electrostatic mechanism of ion acceleration in plasma by an uncompensated space charge of the cloud of magnetized electrons “kept” to the magnetic field takes place in the acceleration zones and that the electric field distribution can be controlled by varying the magnetic field in the discharge gap. The role played by the space charge makes the mechanism of ion acceleration in this type of thrusters is fundamentally different from the acceleration mechanism operating in purely electrostatic thrusters.     

Experimental study of electron vortex structures in an electrostatic plasma lens

Results are presented from experimental studies of electron vortex bunches in a cold ion-beam plasma consisting of strongly magnetized electrons and a beam of almost free positive ions. The existence of electron vortex bunches was detected from local minima of the electric potential on surfaces perpendicular to the magnetic field lines. It is found that the vortices have the form of magnetic-field-aligned filaments, in which electrons rotate with a velocity significantly exceeding both the velocity of the vortex as a whole and the electron velocity in the ambient plasma. It is shown that, in a sufficiently strong magnetic field, the accumulation of electrons in the vortices terminates when the condition for the longitudinal confinement of electrons by the electric field fails to hold.     

Adiabatic Heating of Electrons in the Magnetospheric Current Sheet

Electron dynamics and acceleration in an electromagnetic field configuration modeling the current sheet configuration of the Earth’s magnetotail region is investigated. A focus is made on the role of the dawn?dusk magnetic field component B_y in the convection electron heating by an electric field E_y. For numerical integration of a large number of test particle trajectories over long time intervals, the equations of motion written in the guiding center approximation are used. It is shown that the presence of a B_y ≠ 0 magnetic field significantly changes the electron heating and allows electrons with small pitch angles to gain energy much more efficiently than the equatorial electrons. As a result, the convection heating in the current sheet with B_y ≠ 0 leads to the formation of an accelerated anisotropic population of particles with energies higher than a few hundred electronvolts. The obtained results and spacecraft observations in the Earth’s magnetotail are compared, and possible limitations in the proposed model approaches are discussed.     

Effect of the magnetic field on the resonant particle acceleration

The acceleration of charged particles trapped by a potential wave in a magnetic field is investigated as applied to the problem of the generation of fast particles in a laser plasma. The conditions for unlimited particle acceleration are determined, and the spectra of fast particles are found.     

Dynamics of beam instability in a finite plasma volume: Numerical experiment

Results are presented from numerical simulations of the dynamics of beam instability in a finite plasma volume (plasma-filled cavity) in a weak magnetic field. It is shown that, in such a system, the low group velocity of the plasma waves excited by an electron beam can result in the generation and amplification of an electric field; strong electron heating in the axial region; and, as a consequence, the generation of a high potential at the axis. The quasistatic radial electric field so produced accelerates ions toward the periphery of the plasma column, forming a directed ion beam with an energy much higher than the thermal energy of the bulk plasma electrons.     

Phosphorylation of 1,N6-etheno-ADP in flash groups and the concomitant decay kinetics of the absorption change at 515 nm

The accelerated decay of the transmembrane electric field under phosphorylating conditions as well as the concomitant generation of [³²P]-ATP after excitation with flash groups has been measured. An acceleration of the decay and a corresponding synthesis of ATP is found on addition of ADP and 1,N6-etheno-ADP (?-ADP). Both effects are smaller in the presence of optimal concentrations of ?-ADP than in the presence of an optimal ADP concentration. A quantitative evaluation of $\text{H}{}^{\mathrm{+}}\text{ATP}$ ratio under comparable conditions gives values of 3.3 for ADP and 3.5 for ?-ADP. The results are consistent with the interpretation that the accelerated decay of the transmembrane field is caused directly by a proton flux via the ATPase.     

Physical nature of the electric current produced by the asymmetry of particle motion in toroidal magnetic confinement systems

A new type of longitudinal electric current is revealed by analyzing the drift trajectories of charged particles in a tokamak—the current that may be referred to as the asymmetry current because it is associated with the asymmetry of the boundary between trapped and transit particles in phase space. The generation of this current is explained by the fact that the motions of the particles that cross the magnetic surface at a given point in opposite directions are qualitatively different. The asymmetry current results from the toroidal variations of the magnetic field and is maintained by the radial momentum flux of transit particles. The contribution of the particles of different species to the asymmetry current density is proportional to their pressure, is independent of the gradients of the plasma parameters, is maximum at the magnetic axis, and decreases toward the plasma periphery. In contrast to standard neoclassical theory, the asymmetry current can be found only from exact particle trajectories. The asymmetry current is calculated for tokamaks with differently shaped magnetic surfaces and for a model stellarator. By exploiting the newly revealed asymmetry current, together with the bootstrap current, it may be possible to substantially simplify the problem of creating a tokamak reactor.     

Analysis of the dependence of surfatron acceleration of electrons by an electromagnetic wave in space plasma on the particle momentum along the wave front

Based on the numerical solution of the nonlinear nonstationary second-order equation for the wave phase on the particle trajectory, the dynamics of surfatron acceleration of electrons by an electromagnetic wave propagating across the external magnetic field in space plasma is analyzed as a function of the electron momentum along the wave front. Numerical calculations show that, for strongly relativistic initial values of the electron momentum component along the wave front g _y (0) (the other parameters of the problem being the same), electrons are trapped into the regime of ultrarelativistic surfatron acceleration within a certain interval of the initial wave phase Ψ(0) on the particle trajectory. It is assumed in the calculations that |Ψ(0)| ≤ π. For strongly relativistic values of g _y (0), electrons are immediately trapped by the wave for 19% of the initial values of the phase Ψ(0) (favorable phases). For the rest of the values of Ψ(0), trapping does not occur even at long times. This circumstance substantially simplifies estimations of the wave damping due to particle acceleration in subsequent calculations. The dynamics of the relativistic factor and the components of the electron velocity and momentum under surfatron acceleration is also analyzed. The obtained results are of interest for the development of modern concepts of possible mechanisms of generation of ultrarelativistic particle fluxes in relatively calm space plasma, as well as for correct interpretation of observational data on the fluxes of such particles and explanation of possible reasons for the deviation of ultrarelativistic particle spectra detected in the heliosphere from the standard power-law scalings and the relation of these variations to space weather and large-scale atmospheric processes similar to tropical cyclones.     

Dynamics of an Electron Bunch Accelerated by a Wakefield

The energy characteristics of an electron bunch accelerated by a wakefield are largely determined by the initial bunch dimensions. Present-day injectors are still incapable of ensuring the initial spatial parameters of the bunches required for their acceleration without increasing the energy spread of the bunch electrons. In connection with this, the possibility is studied of improving the energy characteristics of an accelerated bunch by precompressing it in the longitudinal direction in the stage of trapping by a wakefield. Analytic formulas are derived that describe the one-dimensional dynamics of the spatial and energy characteristics of a short (much shorter than the wakefield wavelength) electron bunch in both the trapping and acceleration stages. The analytical results obtained are shown to agree fairly well with the results from one-dimensional and three-dimensional simulations, provided that the electrons are injected into the region that is optimum for acceleration. The possibility is discussed of forming compressed bunches so as to ensure the high quality of the bunch in the course of its acceleration to high energies.