Electron Beam Dynamics Research Articles

Particle-in-Cell Simulation and Optimization for a 220-GHz Folded-Waveguide Traveling-Wave Tube

The folded-waveguide slow-wave structure (SWS) is a robust beam-wave interaction circuit for millimeter-wave traveling-wave tubes. We applied a particle-in-cell solver (Computer Simulation Technology Particle Studio) to analyze and optimize a 220-GHz folded-waveguide SWS. The beam-wave interaction and the electron beam dynamics were studied based on simulation results. The influence of the beam tunnel, with respect to its transverse shape and size, on the circuit performance was investigated in detail. A beam tunnel with a square cross section exhibits lower gain and efficiency and similar bandwidth compared with a beam tunnel with a circular cross section. With a proper phase-velocity taper employed in the nonlinear region of the circuit, the small-signal gain, peak saturated output power, and efficiency were significantly improved, increasing from 21.5 to 27.8 dB, 41 to 70.5 W, and 4.5% to 8%, respectively, for a 36-mm-long loss-free circuit. A simulated 3-dB bandwidth of ~ 15 GHz is demonstrated.

Back bombardment for dispenser and lanthanum hexaboride cathodes

The back bombardment (BB) effect limits wide usage of thermionic rf guns. The BB effect induces not only ramping-up of a cathode's temperature and beam current, but also degradation of cavity voltage and beam energy during a macropulse. This paper presents a comparison of the BB effect for the case of dispenser tungsten-base (DC) and lanthanum hexaboride (${\mathrm{LaB}}_{6}$) thermionic rf gun cathodes. For each, particle simulation codes are used to simulate the BB effect and electron beam dynamics in a thermionic rf gun cathode. A semiempirical equation is also used to investigate the stopping range and deposited heat power of BB electrons in the cathode material. A numerical simulation method is used to calculate the change of the cathode temperature and current density during a single macropulse. This is done by solving two differential equations for the rf gun cavity equivalent circuit and one-dimensional thermal diffusion equation. High electron emission and small beam size are required for generation of a high-brightness electron beam, and so in this work the emission properties of the cathode are taken into account. Simulations of the BB effect show that, for a pulse of $6\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ duration, the DC cathode experiences a large change in the temperature compared with ${\mathrm{LaB}}_{6}$, and a change in current density 6 times higher. Validation of the simulation results is performed using experimental data for beam current beyond the gun exit. The experimental data is well reproduced using the simulation method.

Quantum fluctuations of electrons in free-electron laser undulator working at nanowavelength

A new (nonrelativistic) model for electron-beam dynamics in a free-electron laser undulator was formulated using the method of linear invariants of quantum-mechanical charge particle. The magnetic field varies periodically along the undulator. We obtained an exact solution for this quantum-mechanical problem described by quadratic Hamiltonian. The time-evolutions of the tree independent quantum fluctuations [cov(q,p), var(q), and var(p)] of the electrons were determined. The quantum behavior shows covariance states which are different from the well-known coherent and squeezed states. On the basis of the quantum approach applied here, further exploration of Madey formula for Δλ of emitted photons was done and the low limit of the electron energy was determined, below which the quantum uncertainty prevails the energy spread of the beam.

The VUV/IR/THz free electron laser program at Jefferson Lab

Jefferson Lab operates a pair of oscillator-based continuous-wave free electron lasers (FELs) as a linac-based next generation light source with pulse repetition rates up to 75 MHz. The facility uses an energy recovered linac design for efficiency of operation. Recent advances in superconducting technology have been implemented to produce higher acceleration gradients in the linac to produce higher electron beam energies that result in higher photon energies. Thus, while the system originally operated only in the IR, it now covers the photon energy range from the UV to THz, with harmonics upwards of 10 eV with an average spectral flux that is calculated to be 5×1017 photons/s/0.1% BW. Pulse lengths are in the sub-picosecond regime, and the fully coherent nature of the source, both transversely and longitudinally, results in peak and average brightness values that are several orders of magnitude higher than storage rings. The system provides an R&D test-bed for studies of electron beam dynamics in a regime appropriate for next generation light sources operating at MHz repetition rates.

Experimental study and numerical simulations of a plasma relativistic microwave amplifier

The dependences of the radiation parameters of a plasma relativistic microwave amplifier on the external factors have been studied both experimentally and numerically. The calculated dependences are found to agree qualitatively with the measured ones. In contrast to experimental studies, numerical simulations make it possible to examine physical processes occurring inside the plasma waveguide. Good agreement between the measured and calculated dependences of the radiation parameters on the external factors shows that information provided by numerical simulations of the processes occurring inside the plasma waveguide can be considered quite reliable. The electromagnetic field structure and electron beam dynamics inside the plasma waveguide have been investigated.

Wall-Plug Efficiency and Beam Dynamics in Free-Electron Lasers Using Energy Recovery Linacs

In a high average power free-electron laser (FEL) the wall-plug efficiency is of critical importance in determining the size, complexity, and cost of the overall system. The wall-plug efficiency for the FEL oscillator and amplifier (uniform and tapered wiggler) is strongly dependent on the energy recovery process. A theoretical model for electron beam dynamics in the energy recovery linac is derived and applied to the acceleration and deceleration of nano-Coulomb electron bunches for a tapered FEL amplifier. For the tapered amplifier, the spent electron beam exiting the wiggler consists of trapped and untrapped electrons. Decelerating these two populations using different phases of the radio-frequency wave in the recovery process enhances wall-plug efficiency. For the parameters considered here, the wall-plug efficiency for the tapered amplifier can be ~10% using this approach.

Nonlinear dynamics of electron beam with virtual cathode in external inhomogeneous magnetic field

Nonlinear dynamics of virtual cathode (VC) formed in electron beam in a strongly inhomogeneous external magnetic field generated by a ring magnet has been numerically simulated using a two-dimensional model. It is established that the field characteristics strongly influence the VC dynamics. Physical processes responsible for this strong dependence are considered. The output power of generation in a model system with VC is determined as a function of the field parameters.

Modal description of longitudinal space-charge fields in pulse-driven free-electron devices

In pulsed-beam free-electron devices, longitudinal space-charge fields result in collective effects leading to an expansion of short electron bunches along their trajectory. This effect restricts an application of intense ultrashort electron pulses in free-electron radiation sources. A careful theoretical treatment is required in order to achieve an accurate description of the self-fields and the resulted electron beam dynamics. In this paper, longitudinal space-charge fields are considered in the framework of a three-dimensional, space-frequency approach. The model is based on the expansion of the total electromagnetic field (including self-fields) in terms of transverse eigenmodes of the (cold) cavity, in which the field is excited and propagates. The electromagnetic field, originally obtained in the model as a solution of the wave equation, is shown to satisfy also Gauss's law. We applied the theory to derive an analytical expression for the longitudinal electric field of a pointlike charge, moving along a waveguide at a constant velocity. This enables consideration and study of the role played by different terms of the resulted expressions, such as components arising from forward and backward waves, propagating waves, and under cutoff frequencies, and so on. Possible simplifications in evaluation of longitudinal space-charge fields are discussed.

Experiments on the physics of charged particle beams at the VEPP-4M electron-positron collider

The VEPP-4M accelerator facility with a universal KEDR detector is designed to conduct experiments with colliding electron-positron beams. High-energy physics, nuclear physics, and studies using synchrotron radiation are the main directions of research with this facility. In addition, experiments on poorly explored issues in the physics of beams in the electron-positron storage ring and methodological studies to prepare an experiment aimed at testing corollaries of the CPT theorem for an electron and a positron are regularly conducted at the VEPP-4 facility. A number of works performed in recent years are described: studies to increase the accuracy of comparing the electron and positron spin precession frequencies by the resonant depolarization method; measurements of the count rate of Touschek electrons as a function of the beam energy in a wide range; comparison of the methods for measuring the beam energy spread; a study of the electron beam dynamics when a nonlinear betatron resonance is crossed.

Self-shielded electron linear accelerators designed for radiation technologies

This paper describes self-shielded high-intensity electron linear accelerators designed for radiation technologies. The specific property of the accelerators is that they do not apply an external magnetic field; acceleration and focusing of electron beams are performed by radio-frequency fields in the accelerating structures. The main characteristics of the accelerators are high current and beam power, but also reliable operation and a long service life. To obtain these characteristics, a number of problems have been solved, including a particular optimization of the accelerator components and the application of a variety of specific means. The paper describes features of the electron beam dynamics, accelerating structure, and radio-frequency power supply. Several compact self-shielded accelerators for radiation sterilization and x-ray cargo inspection have been created. The introduced methods made it possible to obtain a high intensity of the electron beam and good performance of the accelerators.

Transverse dynamics of high-density relativistic electron beams propagating in plasma in the direction of the external magnetic field

Based on analysis of the effective potential, a possibility is found to realize a single type of radial evolution of high-density relativistic electron beams. This operating mode is characterized by periodic oscillations of the beam radius within a certain range. Conditions are found for radial beam stabilization under this beam transport regime.

Transverse dynamics of a low-density relativistic electron beam propagating in the plasma along an external magnetic field

It is shown that a low-density relativistic electron beam can be transported under conditions of (i) periodic oscillation of the beam radius, (ii) unlimited beam expansion, and (ii) pinching. Analytical expressions for the radial evolution of the beam under these transport condition are obtained.

100-femtosecond MeV electron source for ultrafast electron diffraction

A near-relativistic 100-fs MeV electron beam is developed by using a photocathode rf gun for revealing the hidden ultrafast dynamics of intricate molecular and atomic processes in materials through experimentation of ultrafast time-resolved electron diffraction (UED). The transverse and longitudinal dynamics of femtosecond electron beam in the rf gun were studied theoretically by particle simulation. The growths of the emittance, bunch length and energy spread due to the rf and space charge effects were investigated by changing the laser parameters, field gradient and electron charge. The theoretical studies indicate that a 100-fs MeV electron beam with the transverse emittance of 0.1 mm mrad and the relative energy spread of 10 −3–10 −4 at bunch charge of 0.1–2 pC (10 6–10 7 electrons per pulse) is achievable for UED, in which the intensity is three orders of magnitude higher than that produced by the conventional dc or pulsed guns.

Single-bunch emittance preservation in the presence of trajectory jitter for FERMI@elettra-seeded FEL

The electron beam dynamics in the presence of the linac structural wake field is studied. Trajectory manipulation is used to gain control of the transverse wake field-induced instability and this technique is also validated in the presence of shot-to-shot trajectory jitter. A specific script working with Courant–Snyder variables has been written to evaluate the residual banana shape after instability suppression in the presence of shot-to-shot trajectory jitter.

Terahertz laser modulation of electron beams

The study of modulated electron beams is important because they can be used to produce coherent radiation, but the modulations can cause unwanted instabilities in some devices. Specifically, in a free electron laser, proper prebunching at the desired emission frequency can enhance performance, while bunching resulting from instabilities and bunch compression schemes can degrade performance. In a photoinjector accelerator, tailoring the shape of the drive laser pulse could be used as a technique to either enhance or mitigate the effect of these modulations. This work explores the possibility of creating deeply modulated electron beams at the photocathode by using a modified drive laser designed to produce multiple subpicosecond pulses repeated at terahertz frequencies. Longitudinal space charge forces can strongly influence the evolution of modulations by converting density modulations to energy modulations. Experiments at the Source Development Laboratory electron accelerator at Brookhaven National Laboratory and PARMELA simulations are employed to explore the dynamics of electron beams with varying charge and with varying initial modulation. Finally, terahertz light generated by a transition radiator is used to confirm the structure of the electron beam.

End-to-end absolute energy calibration of atmospheric fluorescence telescopes by an electron linear accelerator

The Telescope Array (TA) is a hybrid detector composed of atmospheric fluorescence telescopes and a ground array of particle detectors for the precise energy measurement of ultra high-energy cosmic rays. We propose an end-to-end absolute energy calibration of fluorescence telescopes by using air showers induced by electron beams from a linear accelerator, which will be installed at the site of the TA experiment and will have a well-measured beam energy. We have simulated electron beam dynamics and interaction of electrons in air to evaluate the accuracy of the beam energy determination, beam currents, air showers, and detector response using a beam simulation code (PARMELA) and GEANT4. We found that the systematic error of energy measurement estimated for the TA fluorescence telescopes can be reduced from 23% to 17% by this end-to-end energy calibration. We are developing and constructing a compact linear accelerator with a maximum electron energy of 40 MeV and an intensity of 6.4 mJ/pulse. In this article, we describe the method of absolute energy calibration and the design of the accelerator.

Spectrum of velocities and dynamics of electron beams in divergent magnetic fields

The dynamics of an electron beam in divergent magnetic fields has been studied. The changes in the longitudinal and transverse electron velocities in the beam and the effect of the magnetic field parameters on the dispersion of longitudinal electron velocities have been analyzed. A possibility of high-efficiency conversion modes in divergent magnetic fields has been established by methods of numerical simulation.

Spectrum of velocities and dynamics of electron beams in divergent magnetic fields

Spectrum of velocities and dynamics of electron beams in divergent magnetic fields

General Expressions for the Magnetic Flux Density Produced by Axially Magnetized Toroidal Permanent Magnets

This paper presents analytical-integral expressions which evaluate the magnetic flux density radial and axial components' on- and off-axis of axially magnetized toroidal permanent magnets. The pieces, when used in magnetic electron beam focusing structures called periodic-permanent magnets (PPMs), can be employed in microwave vacuum electronics devices. These expressions can be adapted to computational algorithms, especially those developed based on mathematical commercial codes, aiding the investigation of the effect of magnetic fields on electron beam dynamics. Comparisons between analytical-integral expressions and theoretical and experimental data are also presented and discussed.

Simulated Dynamic Spectra: A Group of Coronal Type III Bursts

First simulations are presented for the dynamic spectra of a group of coronal type III bursts that are observed remotely at Earth. A three-dimensional model is developed, including the structure of the source region, dynamics of electron beams, Langmuir, ion-sound, and electromagnetic waves within the source, and propagation of radiation to Earth. For realistic coronal and beam acceleration parameters, the simulation results are consistent with the general characteristics of type III bursts. We demonstrate that multiple radio bursts at high frequencies in the group merge at low frequencies, due mainly to a previously proposed mechanism which leads to merging of beams in the presence of beams and Langmuir waves only. The radio flux after the merging, however, depends jointly on beam-wave and wave-wave interactions and propagation effects.