Beam Self-fields Research Articles

Cherenkov radiation of electromagnetic waves by electron beams in the absence of an external magnetic field

In conventional sources of coherent Cherenkov electromagnetic radiation, the electrons move linearly, guided by external magnetic fields. In the absence of such fields, the electrons can move radially, being affected by the beam self-fields as well as by the radial component of the electric field of the wave. This radial motion can, first, improve the coupling of electrons to the field of a slow wave localized near the wall of a slow-wave structure, and second, cause an energy exchange between the electrons and the wave due to an additional transverse interaction. This interaction, in particular, can lead to an experimentally observed excitation of nonsymmetric transverse electric waves in Cherenkov devices. In plasma-filled sources, the beam self-fields can be compensated for by ions, leading to a known ion focusing of the beams. In such regimes, the beam can be surrounded by an ion layer creating a potential well for electrons which can be displaced from stationary trajectories by transverse fields of the wave. The operation of such sources when the presence of ions and the radial electric field of the wave play competing focusing and defocusing roles, and electron interception by the walls restricts the output power level, is analyzed in stationary and nonstationary regimes.

On the design of electron guns for cyclotron resonance masers utilizing trochoidal electron beams (trochotrons)

The properties of crossed-field electron guns which are capable of providing the required trochoidal beams for efficient cyclotron resonance interaction in the trochotron configuration are analyzed. A discussion of the beam parameters that are used to characterize the device performance is included in a general description of the operation of these guns. Two concrete designs are presented and are used to demonstrate the performance capabilities of these devices with respect to the beam self-fields, the externally applied electric and magnetic fields, and the electrode geometry. It is shown that these devices, which typically generate beams with large ratios of perpendicular to parallel (drift) velocity, are capable of producing extremely small drift velocity spreads over a wide range of parameters.

Generation of low-emittance electron beams in electrostatic accelerators for FEL applications

This paper reports results of transverse emittance studies and beam propagation in electrostatic accelerators for free electron laser applications. In particular, we discuss emittance growth analysis of a low current electron beam system consisting of a miniature thermoionic electron gun and a National Electrostatics Accelerator (NEC) tube. The emittance growth phenomenon is discussed in terms of thermal effects in the electron gun cathode and aberrations produced by field gradient changes occurring inside the electron gun and throughout the accelerator tube. A method of reducing aberrations using a magnetic solenoidal field is described. Analysis of electron beam emittance was done with the EGUN code. Beam propagation along the accelerator tube was studied using a cylindrically symmetric beam envelope equation that included beam self-fields and the external accelerator fields which were derived from POISSON simulations.

Three-dimensional particle simulation of heavy-ion fusion beams*

The beams in a heavy-ion-beam-driven inertial fusion (HIF) accelerator are collisionless, nonneutral plasmas, confined by applied magnetic and electric fields. These space-charge-dominated beams must be focused onto small (few mm) spots at the fusion target, and so preservation of a small emittance is crucial. The nonlinear beam self-fields can lead to emittance growth, and so a self-consistent field description is needed. To this end, a multidimensional particle simulation code, warp [Friedman et al., Part. Accel. 37-38, 131 (1992)], has been developed and is being used to study the transport of HIF beams. The code’s three-dimensional (3-D) package combines features of an accelerator code and a particle-in-cell plasma simulation. Novel techniques allow it to follow beams through many accelerator elements over long distances and around bends. This paper first outlines the algorithms employed in warp. A number of applications and corresponding results are then presented. These applications include studies of: beam drift-compression in a misaligned lattice of quadrupole focusing magnets; beam equilibria, and the approach to equilibrium; and the MBE-4 experiment [AIP Conference Proceedings 152 (AIP, New York, 1986), p. 145] recently concluded at Lawrence Berkeley Laboratory (LBL). Finally, 3-D simulations of bent-beam dynamics relevant to the planned Induction Linac Systems Experiments (ILSE) [Fessenden, Nucl. Instrum. Methods Plasma Res. A 278, 13 (1989)] at LBL are described. Axially cold beams are observed to exhibit little or no root-mean-square emittance growth at midpulse in transiting a (sharp) bend. Axially hot beams, in contrast, do exhibit some emittance growth.

Effects of electron dynamics on spontaneous emission spectra in free electron lasers

Abstract Spontaneous emission spectra caused by electrons in realizable undulators (helical, linear) are investigated. The trajectories of an ensemble of electrons are calculated numerically, and the spectra of individual electrons are superimposed. The code accounts for a homogeneous guide field and the mean beam self-fields. The ponderomottive forces are not considered. Computational results are presented and supported by published measurements of FEL emission spectra.

Chaotic behavior of electron orbits in a free electron laser near magnetoresonance

A single-particle computer model is used to calculate the electron trajectories in combined helical wiggler and guide fields. Off-axis terms of the wiggler field as well as the beam self-fields are included. Special attention is focussed on the stability of the electron orbits near magnetoresonance. Upon calculating Poincaré surface-of-section maps and Liapunov exponents, it is shown that the electron dynamics is chaotic. Omission of the off-axis terms of the wiggler field and the beam self-fields yields an integrable problem with always regular orbits. In this case, the dynamical state near magnetoresonance can be described in terms of a bistable pseudopotential: Orbits with high longitudinal (low transversal) velocities are separated by a potential barrier from orbits with low longitudinal (high transversal) velocities. Due to the nonintegrable perturbation of the Hamiltonian, for certain initial conditions, an unpredictable (chaotic) transition occurs between these two modes.

Design of low velocity-spread cusp guns for axis encircling beams

The design of a novel electron gun suitable for intense beam cyclotron resonance devices is introduced here. An annular Pierce-type gun is used in conjunction with an unbalanced nonadiabatic field reversal and an adiabatic compression region to produce an axis-encircling beam. This beam is ideally suited for interaction with electromagnetic waves that have strong on-axis electric fields (e.g., the TE011 mode). Low velocity spreads are achieved by utilizing the beam self-fields in the compression region and by focusing in the Pierce gun.

Resonance Excitation by Distortion of the B-Function Coupled with a Single Beam Space Charge Force

It is shown how a single beam space charge force can combine with a lattice design having large ß variations, such as occurs in machines with low ß insertions, to induce resonance behavior. Although the single beam space charge force may be highly nonlinear, it will, by itself, excite no resonances, since its azimuthal Fourier decomposition is essentially composed of 0th harmonic. However, low periodicity, large ß variations are harmonically rich and can provide the necessary azimuthal harmonics for resonance excitation. The resonance characteristics of this type of system are developed. A strength parameter involving the linear tune shift and the ßmax value is introduced. In particular, two cases are discussed: (1) where the force arises from the beam self-field and (2) where the force is induced by images in the surrounding boundaries. A comparison is made with the beam-beam force in terms of both the strength parameters (related to the linear tune shifts), and the nonlinear resonance behavior.