Trapping of electrons and acceleration of the electron bunch in a wake wave

The process of electron trapping by a wake wave excited by a laser pulse in a plasma channel in the case where the electron bunches are injected into the vicinity of the maximum of the wakefield potential at a velocity lower than the wave phase velocity is considered. The mechanism for the formation of a compact electron bunch in the trapping region when only the electrons of the injected bunch that are trapped in the focusing phase mainly undergo the subsequent acceleration in the wakefield is analyzed. The influence of the spatial dimensions of the injected bunch and its energy spread on the length of the trapped electron bunch and the fraction of trapped electrons is studied analytically and numerically. For electron bunches with different ratios of their spatial dimensions to the characteristic dimensions of the wake wave, the influence of the injection energy on the parameters of the high-energy electron bunch trapped and accelerated in the wake-field is studied.     

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.     

Acceleration of an electron bunch injected in front of a laser pulse generating an accelerating wake wave

A study is made of a promising method for injecting an electron bunch into an accelerating laser-plasma system. A bunch is injected ahead of the front of a laser pulse generating a wake wave that propagates in a direction collinear with the pulse and has a velocity lower than the pulse group velocity. The influence of the initial nonmonoenergetic character of the bunch on its trapping and acceleration is investigated. By appropriately choosing the laser pulse parameters and the bunch injection energy, it is possible to create such conditions for the trapping of an initially nonmonoenergetic bunch by the wake wave that, over a certain acceleration distance, there will be no energy spread of the bunch due to its initial nonmonoenergetic character, a circumstance that allows compact electron bunches to be accelerated to high energies, with a minimum energy spread.     

Design of an experiment on wakefield acceleration on the VEPP-5 injection complex

Relativistic beams produced by the VEPP-5 injection complex (Budker Institute of Nuclear Physics, Siberian Division, Russian Academy of Sciences) can be used to generate plasma waves with a longitudinal electric field of 1 GV/m. A part of the electron (or positron) driver bunch is accelerated by this field over a distance of up to 1 m. The main advantage of the proposed design over the previous wakefield acceleration experiments is the beam preparation system capable of compressing bunches to a length of σ_z = 0.1 mm in the longitudinal direction and producing an optimal longitudinal profile of the beam density. The main parameters of the planned device are as follows: the electron energy at the entrance to the plasma is 510 MeV, the number of particles in the bunch is 2 × 10¹⁰, the plasma density is up to 10¹⁶ cm^?3, the number of accelerated particles is up to 3 × 10⁹, and their energy spread is less than 10%. The physical project of the experiment is presented, and the results of computer simulations of the beam-plasma interaction are described.     

Acceleration of dense electron bunches at the front of a high-power electromagnetic wave

The acceleration of dense electron bunches (e.g., those produced by the ionization of thin films) at the front of a high-power electromagnetic wave in vacuum is considered. It is shown that the reaction force of the intrinsic radiation of a bunch can play a significant role in the acceleration process because it gives rise to an additional accelerating force acting on the bunch and to forces that compress the bunch in the longitudinal direction. As a result, all of the bunch electrons can be synchronously accelerated during the first several half-periods of the external electromagnetic field.     

Electron acceleration by Langmuir waves excited by a laser pulse in a semi-infinite plasma

Results are presented from a theoretical investigation of the acceleration of test electrons by a Langmuir wave excited by a short laser pulse at half the electron plasma frequency. Such a pulse penetrates into the plasma over a distance equal to the skin depth and efficiently excites Langmuir waves in the resonant interaction at the second harmonic of the laser frequency. It is shown that the beam of electrons accelerated by these waves is modulated into a train of electron bunches, but because of the initial thermal spread of the accelerated electrons, the bunches widen and begin to overlap, with the result that, at large distances, the electron beam becomes unmodulated.     

Wakefield excitation by a relativistic electron bunch in a magnetized plasma

The excitation of a wake wave by a relativistic electron beam in an unbounded magnetized plasma and a plasma waveguide is studied theoretically. It is shown that, in a waveguide partially filled with a plasma, the energy that the electrons of the accelerated beam can gain is 37 times higher than the energy of the electrons of the beam generating wakefield.     

Nonlinear properties of two-dimensional wake waves excited by relativistic electron bunches in plasma

The properties of a nonlinear plasma wake wave excited by an axially symmetric relativistic electron bunch are studied. It is shown that the nonlinear dependence of the wake wavelength on the transverse coordinate leads to distortion of the phase front of the wake wave and to steepening and oscillations of the transverse profile of the wakefield. The magnetic field of the wake wave is nonzero and oscillates at a frequency higher than the plasma electron frequency. Because of nonlinearity, the amplitude of the excited wake wave changes with distance from the bunch. The increase in nonlinearity leads to the development of turbulence and chaotization of the wakefield and results in the switching-on of the thermal effects and plasma heating.     

Dependence of the ion energy on the parameters of the laser pulse and target in the radiation-pressure-dominated regime of acceleration

When the dominant mechanism for ion acceleration is the laser radiation pressure, the conversion efficiency of the laser energy into the energy of relativistic ions may be very high. Stability analysis of a thin plasma layer accelerated by the radiation pressure shows that Raleigh-Taylor instability may enhance plasma inhomogeneity. In the linear stage of instability, the plasma layer decays into separate bunches, which are accelerated by the radiation pressure similarly to clusters accelerated under the action of an electromagnetic wave. The energy and luminosity of an ion beam accelerated in the radiation-pressure-dominated regime are calculated.     

Study of the development of relativistic plasma bunches in a long mirror trap by optical and X-ray imaging and numerical simulations

The work presents experimental results demonstrating the feasibility of autoresonance acceleration of electrons in a long mirror trap with a reverse magnetic field. It is shown that gyromagnetic autoresonance results in the formation of a plasma bunch with average electron energy of several hundred keV, which is confined for a long time in the trap. The results of computer simulations of the regime of reverse gyromagnetic autoresonance agree well with the experimental data.     

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.     

Quasilinear relaxation of a low-density electron beam in a plasma

The quasilinear relaxation of a low-density electron beam under the action of plasma turbulence, which is generated during the development of a beam instability, when the beam is formed due to rapid local electron heating (acceleration) is analyzed in the one-dimensional approximation. It is shown that quasilinear diffusion results in the formation of a local plateau at the top of the electron distribution function without causing any significant spread in velocities of the beam electrons and that the relaxation process proceeds primarily through the spatial expansion of electrons with different velocities.     

Charged prticle aceleration by an itense ultrashort electromagnetic pulse excited in a plasma by laser radiation or by relativistic electron bunches

A review is given of theoretical and experimental investigations and numerical simulations of the generation of intense electromagnetic fields in accelerators based on collective methods of charged particle acceleration at rates two or three orders of magnitude higher than those in classical resonance accelerators. The conditions are studied under which the excitation of accelerating fields by relativistic electron bunches or intense laser radiation in a plasma is most efficient. Such factors as parametric and modulational processes, the generation of a quasistatic magnetic field, and the acceleration of plasma electrons and ions are investigated in order to determine the optimum conditions for the most efficient acceleration of the driven charged-particle bunches.     

An efficient method for extracting plasma ions in laser isotope separation systems

The possibility of using a Hall-current accelerator to extract ions from a partially ionized plasma produced by selective laser isotope photoionization of atomic vapor is examined. A mechanism for ion acceleration is investigated using one-dimensional time-dependent equations of two-fluid magnetohydrodynamics. The current cutoff due to the ion space charge is prevented by electron emission. It is shown that, at an accelerating voltage of 25–50 V and emission current density of several mA/cm², the ion component is accelerated throughout the entire plasma volume up to a velocity of ～10⁵ cm/s in a few microseconds. The influence of resonant charge exchange and secondary ionization by electrons on both the acceleration dynamics and selectivity degradation is taken into account. It is shown that the Hall-current extractor allows one to avoid selectivity degradation even when the plasma size exceeds the charge-exchange mean free path by one order of magnitude.     

Electron bunch acceleration in the wake wave breaking regime

Results are presented from theoretical analysis and 2D PIC simulations of electron acceleration in a breaking wake plasma wave generated by a short intense laser pulse during its interaction with a finite-length underdense plasma layer. The high energy electron energy spectrum and transverse emittance are obtained. It is shown that, for laser pulse lengths above the plasma wake wavelength, the wakefield-accelerated electrons are further accelerated by the electromagnetic wave. Published in Russian in Fizika Plazmy, 2006, Vol. 32, No. 4, pp. 291–310. The text was submitted by the authors in English.     

Energy of electrons emitted as a result of separation of surfaces

Experiments indicating acceleration of charged particles as a result of separation of solid surfaces are analyzed. As a possible mechanism of such acceleration, generation of surface charge on the separated surfaces of a cleaved ionic crystal is considered. The maximum electric field generated due to the charging of the separated surfaces and the energy of electrons accelerated in such a field are estimated. It is shown that, for the maximum attainable electric field, conditions are created for the generation of runaway electrons that, even at atmospheric pressure, electrons are accelerated to high energies, not experiencing collisions with gas particles.     

Effect of a strong monochromatic electromagnetic wave with circular polarization on one-dimensional wake waves in plasma

A study is made of the excitation of wake waves by a one-dimensional bunch of charged particles in an electron plasma in the presence of an intense monochromatic pump wave with circular polarization. In the main state (in the absence of a bunch), the interaction between a pump wave and a plasma is described by the Maxwell equations and the nonlinear relativistic hydrodynamic equations for a cold plasma. The excitation of linear waves by a one-dimensional bunch is investigated against a cold plasma background. It is shown that, in a certain range of the parameter values of the bunch, pump wave, and plasma, the amplitude of the excited transverse waves grows as the energy of the bunch particles increases until the relativistic factor of the bunch reaches a certain threshold value above which the transverse wave amplitude becomes essentially independent of the bunch particle energy and grows as the intensity and frequency of the pump wave increase. The amplitude and wavelength of the longitudinal field, which is shown to depend weakly on the energy of the bunch particles, grows with increasing the pump wave intensity.     

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.