Process of compensation of the space charge of a negative ion beam in a gas

The process of compensation of the space charge of a negative ion beam propagating through a neutral gas is investigated numerically. A comparison of the results obtained with experimental data unambiguously proves that, at high gas pressures, when the beam space charge is overcompensated, the electric field within the beam is determined by Coulomb collisions of the beam ions with plasma electrons. At low pressures, when the space charge is undercompensated, the field within the beam is determined by the dynamic processes related to oscillations of the beam current.     

Dynamics of a relativistic electron beam in the vicinity of a spherical injecting body in space plasma

The dynamics of a relativistic electron beam in the vicinity of an injector in the form of a spherical conducting body in a space plasma is considered. An equation describing the radial evolution of a steady electron beam with a self-similar density profile in the electric field of the injector is formulated. A method for calculating the radial evolution of a relativistic electron beam in the vicinity of an injector is developed. The method is based on the numerical integration of a set of ordinary differential equations for the beam radius and field potential in the space charge region under the relevant boundary conditions at the injector surface. Results are presented from numerical simulations of the radial dynamics of an electron beam in the vicinity of a spherical screen system for neutralizing the electric charge carried away by the beam. The numerical results show that the electric field of the injector hastens the beam expansion.     

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.     

Coherent interaction of a relativistic electron beam with a plasma

Results are presented from experimental studies of the interaction of a modulated relativistic electron beam with a plasma. The electron energy spectra at the exit from the interaction chamber are measured for electron beams with energies of about 50 and 20 MeV. The coherent interaction of an electron beam with a microwave-driven plasma is studied. It is shown that, in strong electric fields that can be generated in the coherent interaction, the beam current is very sensitive to the phase of the microwave field.     

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.     

On the nonlinear theory of collective Cherenkov interaction of a high-density relativistic beam with a plasma

The nonlinear dynamics of the instability of a straight high-density relativistic electron beam under the conditions of the stimulated Cherenkov effect in a plasma waveguide is studied both analytically and numerically. It is shown that, for a beam of sufficiently high density such that the stabilizing factors are nonlinear frequency shifts and for a plasma described in a linear approximation, the basic equations have soliton-like solutions and the electron beam after saturation of the instability relaxes to its initial, weakly perturbed state, provided that only one harmonic of the plasma and the beam density is taken into account. The analytical solutions obtained here for this case correlate well with the numerical ones. A more general model that accounts for the generation of higher harmonics of the plasma and the beam density does not yield soliton-like solutions for the time evolution of the amplitudes of the plasma and beam waves. In such a model, the instability will be collective again: it can be described analytically (at least, up to the time at which it saturates) by using equations with cubic nonlinearities and the method of expansion of the electron trajectories and momenta.     

Nonlinear theory of the collective Cherenkov interaction of a dense relativistic electron beam with a dense plasma in a waveguide

A nonlinear theory of the instability of a straight relativistic dense electron beam in a plasma waveguide is derived for conditions of the stimulated collective Cherenkov effect. A study is made of a waveguide with a dense plasma such that the plasma wave excited by the beam during the instability can be escribed, with a good degree of accuracy, as a potential wave. General relativistic nonlinear equations are btained that describe the temporal dynamics of beam-plasma instabilities with allowance for plasma nonlinearity and the generation of harmonics of the initial perturbation. Under the assumption that the resonant interaction between the beam waves and the plasma waves is weak, the general equations are reduced to relativistic equations with cubic nonlinearities by using the method of expansion in small perturbations of the trajectories and momenta of the beam and plasma electrons. The reduced equations are solved analytically, the time scales on which the instability saturates are determined, and the nonlinear saturation amplitudes are obtained. A comparison between analytical solutions to the reduced equations and numerical solutions to the general nonlinear equations shows them to be in good agreement. Nonlinear processes caused by the relativistic nature of the beam are found to prevent stochastization of the system in the nonlinear stage of the well-developed instability. In contrast, a nonrelativistic electron beam is found to be subject to significant anomalous nonlinear stochastization.     

Beamline characterization of a dielectric-filled reentrant cavity resonator as beam current monitor for a medical cyclotron facility

At PSI (Paul Scherrer Institute), Switzerland, a superconducting cyclotron called “COMET” delivers proton beam of 250 MeV pulsed at 72.85 MHz for proton radiation therapy. Measuring proton beam currents (0.1–10nA) is of crucial importance for the treatment safety and is usually performed with invasive monitors such as ionisation chambers (ICs) which degrade the beam quality. A new non-invasive beam current monitor working on the principle of electromagnetic resonance is built to replace ICs in order to preserve the beam quality delivered. The fundamental resonance frequency of the resonator is tuned to 145.7 MHz, which is the second harmonic of the pulse rate, so it provides signals proportional to beam current. The cavity resonator installed in the beamline of the COMET is designed to measure beam currents for the energy range 238–70 MeV. Good agreement is reached between expected and measured resonator response over the energy range of interest. The resonator can deliver beam current information down to 0.15 nA for a measurement integration time of 1 s. The cavity resonator might be applied serving as a safety monitor to trigger interlocks within the existing domain of proton radiation therapy. Low beam currents limit the abilities to detect sufficiently, however, with the potential implementation of FLASH proton therapy, the application of cavity resonator as an online beam-monitoring device is feasible.     

Nonlinear oscillations of a semiconductor plasma with a nonrelativistic electron beam

Nonlinear oscillations of a semiconductor plasma with a low-density electron beam in the absence of an external magnetic field are studied in the hydrodynamic approximation. The beam is assumed to be nonrelativistic and monoenergetic. Cases are studied in which the Langmuir frequency of the electron oscillations in a semiconductor is much higher or much lower than the electron momentum relaxation rate. The self-similar solution obtained for the first case describes the damping of the nonlinear oscillations of the wave potential. Numerical analysis of the second case shows that the electric field distribution in the beam may correspond to that in a shock wave.     

Energy characteristics of beam-plasma interaction in a closed volume

Energy exchange between an electron beam and plasma during a beam-plasma discharge in a closed cavity excited by the electron beam is analyzed using computer simulations by the KARAT code. A method allowing one to analyze the beam-plasma interaction in the quasi-steady stage of the discharge is proposed. Qualitative characteristics of energy exchange (such as beam energy losses and the energy distributions of beam electrons and plasma particles leaving the discharge) both during spontaneous discharge excitation and in the presence of initial beam modulation by regular or noiselike signals are determined. The results obtained enable one to estimate the energy characteristics of a plasma processing reactor based on a beam-plasma discharge.     

Stimulated ionization scattering of a wave beam forming a discharge channel in a magnetic mirror trap

The stimulated scattering of a whistler wave beam forming an extended discharge channel in a magnetic mirror trap is discovered and investigated experimentally. It is shown that the beam is scattered by relaxaction oscillations of the lattice of plasma inhomogeneities excited by the beam field. The spectrum of the pump field in the RF discharge plasma is found to broaden considerably and to contain individual modulation peaks corresponding to lattice oscillations. The peaks are observed at working gas pressures at which the electron mean free path is close to the wavelength of the standing wave forming the discharge channel. A physical model describing the phenomena observed is developed.     

Surface waves on the boundary of a conducting medium and their excitation by a relativistic electron beam

Excitation of surface waves by a relativistic electron beam propagating over a conducting cylindrical medium (metal or highly ionized plasma) is investigated theoretically. Dispersion relations describing the linear interaction of surface electromagnetic waves with a monoenergetic electron beam are derived, and the growth rates and spatial amplification factors of excited waves are determined. Condition for the nonlinear trapping of the beam electrons by a surface wave is used to determine the maximum amplitude of the excited wave and the optimal radiator length. The electric field of a surface wave excited by an electron beam is estimated for a particular case.     

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.     

Simulation of electron beam formation and transport in a gas-filled electron-optical system with a plasma emitter

The results of computer simulations of the electron-optical system of an electron gun with a plasma emitter are presented. The simulations are performed using the KOBRA3-INP, XOOPIC, and ANSYS codes. The results describe the electron beam formation and transport. The electron trajectories are analyzed. The mechanisms of gas influence on the energy inhomogeneity of the beam and its current in the regions of beam primary formation, acceleration, and transport are described. Recommendations for optimizing the electron-optical system with a plasma emitter are presented.     

Noninvasive diagnosis of a single cell with a probe beam

It is important to distinguish a living/dead cell in cell culture, especially in the regenerate medicine field including cell therapy, since those cells are usually in short supply and consequently the ex vivo culture process should be operated strictly. Conventional methods for distinguishing a living from a dead cell usually require labeling with a dye, which spoils the culture of the cell. Here we show a simple noninvasive method for diagnosing a dead or alive cell with a probe beam. If a cell is alive, the active transport of materials across the cell membrane causes a change of concentration gradients, and this change further induces a change of deflection of a probe beam passing through a vicinity of the cell membrane. If a cell is dead, no or little change in deflection of the probe beam is induced because no or little active materials movement across the cell membrane exists. The deflection of the probe beam is monitored, and judgment on whether a cell is dead or alive from the deflection signal agreed with the conventional decision.     

Relativistic diamagnetic equilibrium of a thin-walled annular electron beam in an external magnetic field

A self-consistent equilibrium state of a thin-walled annular electron beam in an external magnetic field is investigated with allowance for diamagnetic effect and relativistic effects in the beam rotational motion. An equation for the relativistic angular velocities of the beam rotation is derived in the hydrodynamic approximation. The main parameters of the beam equilibrium state are obtained analytically and are calculated numerically. The parameters of a longitudinally homogeneous, relativistic diamagnetic high-density electron beam are determined.     

Role of the magnetic field in electron transportation in a beam-plasma generator

The passage of a concentrated beam through the extraction system of a beam plasma generator is investigated on the basis of analysis of variations in the beam diameter and emittance. Evolution of these parameters is analyzed in the final section of the extraction device, which is characterized by the high density gradient of the supplied gas. Much attention is given to the influence of the magnetic field on the features of electron beam propagation. It is demonstrated that focusing is favorable to decreasing not only the beam transverse size, but also the growth rate of the beam emittance.     

Interaction of a high-current electron beam with hybrid waveguide and plasma modes in a dielectric Cherenkov maser with a plasma layer

The characteristics of a high-current electron beam-driven microwave amplifier—a dielectric Cherenkov maser—are investigated in the framework of linear theory for the case of a plasma layer present at the surface of the maser slow-wave structure. The dispersion relation for axisymmetric perturbations is obtained for the conventional configuration (a circular dielectric-lined waveguide and a thin annular beam propagating within the vacuum region inside the annular plasma) in the model of a fully magnetized plasma and beam. The results of numerically solving the dispersion relation for different beam and plasma parameters are presented, and an analysis based on these results is given with regard to the features of the beam interaction with the hybrid waves of the system (both hybrid waveguide and hybrid plasma modes). For the hybrid waveguide mode, the dependences of the spatial growth rate on the frequency demonstrate an improvement in the gain at moderate plasma densities, along with narrowing the amplification band and shifting it toward higher frequencies. For the hybrid plasma mode, the interaction with a mildly relativistic (200–250 keV) beam, when the wave phase velocity is close to the speed of light in the dielectric medium, is most interesting and, therefore, has been studied in detail. It is shown that, depending on the beam and plasma parameters, different regimes of the hybrid plasma mode coupling to the hybrid waveguide mode or a usual, higher order plasma mode take place; in particular, a flat gain vs. frequency dependence is possible over a very broad band. The parameters at which the ?3-dB bandwidth calculated for the 30-dB peak gain exceeds an octave are found.     

Unsteady processes during stimulated emission from a relativistic electron beam in a quasi-longitudinal electrostatic pump field

The problem of stimulated emission from a relativistic electron beam in an external electrostatic pump field is studied. A set of nonlinear time-dependent equations for the spatiotemporal dynamics of the undulator radiation amplitude and the amplitude of the beam space charge field is derived. The beam electrons are described by a modified version of the macroparticle method. The regimes of the single-particle and collective Cherenkov effects during convective and absolute instabilities are considered. The nonlinear dynamics of radiation pulses emitted during the instabilities of the beam in its interaction with the forward and backward electromagnetic waves is investigated.