Electron Beams In Magnetic Fields Research Articles

Towards MR-guided electron therapy: Measurement and simulation of clinical electron beams in magnetic fields

In the current era of MRI-linac radiotherapy, dose optimization with arbitrary dose distributions is a reality. For the first time, we present new and targeted experiments and modeling to aid in evaluating the potential dose improvements offered with an electron beam mode during MRI-linac radiotherapy. Small collimated (1 cm diameter and 1.5 × 1.5 cm2 square) electron beams (6, 12 and 20MeV) from a clinical linear accelerator (Varian Clinac 2100C) are incident perpendicular and parallel to the strong and localized magnetic fields (0-0.7T) generated by a permanent magnet device. Gafchromic EBT3 film is placed inside a slab phantom to measure two-dimensional dose distributions. A benchmarked and comprehensive Monte Carlo model (Geant4) is established to directly compare with experiments. With perpendicular fields a 5% narrowing of the beam FWHM and a 10mm reduction in the 15% isodose penetration is seen for the 20MeV beam. In the inline setup the penumbral width is reduced by up to 20%, and a local central dose enhancement of 100% is observed. Monte Carlo simulations are in agreement with the measured dose distributions (2% or 2mm). A new range of experiments have been performed to offer insight into how an electron beam mode could offer additional choices in MRI-linac radiotherapy. The work extends on historic studies to bring a successful unified experimental and Monte Carlo modeling approach for studying small field electron beam dosimetry inside magnetic fields. The results suggest further work, particularly on the inline magnetic field scenario.

Linear theory of instabilities generated by an electron beam in a metamaterial-loaded waveguide

We present the linear theory of the starting current of Cherenkov-cyclotron and Cherenkov instabilities generated by an electron beam passing through a metamaterial-loaded waveguide. Effective medium theory is used to represent the metamaterial structure properties. The theory predicts that the instabilities compete with the Cherenkov-cyclotron mode dominating at a lower magnetic field and the Cherenkov instability at a higher magnetic field. The theoretical results are compared to results from recent experiments at MIT using a 490 kV, 84 A electron beam in magnetic fields of 300 G to 1500 G. For an effective medium model fitted to the MIT experimental parameters, theory predicts that the Cherenkov-cyclotron mode will dominate below 780 G and the Cherenkov mode above 780 G, in good agreement with experimental observations of switching between these modes at 750 G. The analytical theory allows a better understanding of the mode competition and the dependence of the instabilities on key parameters such as voltage, current, and magnetic fields.

Dynamics of an orbital polarization of twisted electron beams in electric and magnetic fields

Relativistic classical and quantum dynamics of twisted (vortex) Dirac particles in arbitrary electric and magnetic fields is constructed. The relativistic Hamiltonian and equations of motion in the Foldy-Wouthuysen representation are derived. Methods for the extraction of an electron vortex beam with a given orbital polarization and for the manipulation of such a beam are developed. The new effect of a radiative orbital polarization of a twisted electron beam in a magnetic field resulting in a nonzero average projection of the intrinsic orbital angular momentum on the field direction is predicted.

Relativistic Quantum Dynamics of Twisted Electron Beams in Arbitrary Electric and Magnetic Fields.

Relativistic quantum dynamics of twisted (vortex) Dirac particles in arbitrary electric and magnetic fields are constructed for the first time. This allows us to change the controversial contemporary situation when the nonrelativistic approximation is used for relativistic twisted electrons. The relativistic Hamiltonian and equations of motion in the Foldy-Wouthuysen representation are derived. A critical experiment for a verification of the results obtained is proposed. The new important effect of a radiative orbital polarization of a twisted electron beam in a magnetic field resulting in a nonzero average projection of the intrinsic orbital angular momentum on the field direction is predicted.

Possible mechanism for radial ion acceleration in a beam-plasma discharge

A mechanism is proposed that can lead to radial ion acceleration in a plasma discharge excited by an electron beam in a relatively weak longitudinal magnetic field. The mechanism operates as follows. The beam generates an azimuthally asymmetric slow potential wave, which traps electrons. Trapped magnetized electrons drift radially with a fairly high velocity under the combined action of the azimuthal wave field (which is constant for them) and a relatively weak external longitudinal magnetic field. The radial electron flux generates a radial charge-separation electric field, which accelerates unmagnetized plasma ions in the radial direction. The ion flux densities and energies achievable in experiments with kiloelectronvolt electron beams in magnetic fields of up to 100 G are estimated.

Quantum‐mechanical analysis of scattering of an electron beam in magnetic fields by a finite height potential by means of BEM

AbstractThis paper analyzes quantum‐mechanically the scattering of electron beams due to a finite and constant scattering potential in a magnetic field by using the boundary element method. The system is divided into parts with and without the scattering potential. In each region, the boundary integral equation is established. Then, the wave function and its normal derivative at the potential boundary common to both regions are included. Although both are unknown, they can be obtained as direct solutions by solving the two integral equations simultaneously. The present method is applied to a magnetic electron focusing structure with a finitely high square potential between the emitter and collector and the probability distribution of the electrons is computed. The results are interpreted physically and the effectiveness and problems of the present method are discussed. © 2002 Scripta Technica, Electron Comm Jpn Pt 2, 85(3): 1–8, 2002

Optimal management of the parameters of a focusing field for obtaining a minimum-emittance beam

We formulate and solve the problem of optimal management for the equations of the dynamics of a Gaussian high-current relativistic electron beam in magnetic fields with a quadrupole and octupole symmetry and present the results of numerical simulation.

On envelope equations for electron beams in magnetic fields

A self-similarity approach is used to obtain envelope equations for an annular beam propagating along a magnetic field, and also an electron beam injected at an angle to a magnetic field. An exact solution is constructed for the self-consistent problem of transverse oscillations of a cold annular beam in a magnetic field, and a comparison is made with approximate results from the method of envelope equations.

Transport of a high-current relativistic electron beam by magnetic multipoles

With the aid of moments of the distribution function a system of differential equations is obtained to describe the dynamics of a Gaussian high-current electron beam in magnetic fields with quadrupole and octupole symmetries. Results of its numerical solution are reported.

Quantum-Mechanical Collimation and Diffraction of Electron Beams in Magnetic Fields

Behavior of an electron beam injected from a waveguide into an open space in magnetic fields is studied by the method analogous to that Kirchhoff applied to the diffraction of light. It is found that magnetic field quantum-mechanically collimates electron beams. An electron beam injected from an orifice travels longer distance than that classical theory predicts before reflection on the boundary. Substructure of electron beams around the reflection point is interpreted by introducing the effect of magnetic fields to the diffraction theory in the absence of a magnetic field.

Submillimeter-wave harmonic gyrotron experiment

A theoretical and experimental investigation of the operation of a harmonic gyrotron at submillimeter wavelengths is reported. Using a waveguide cavity with an iris at the output end of the straight section, 14 different second-harmonic modes were observed with frequencies of 301-503 GHz, output powers of 1-22 kW, and a 12-MHz emission frequency bandwidth. The highest output power was 22 kW, with a total efficiency of 3.5% at 467 GHz, and an output power of 15 kW with a 6% efficiency was obtained at 417 GHz. Research was conducted using a 65-75 kV up to 10-A electron gun with a 1/1.5- mu s pulse length and a 4-Hz repetition rate, which produced a helical electron beam in magnetic fields of up to 14 T. These results represent the first operation of a high-power harmonic gyrotron in the submillimeter region. >

Behaviour of Electron Beams in Increasing Magnetic FieldsZα(α><1)†

ABSTRACT The behaviour of electron beams in magnetic fields varying as Z α ( α><l) is described, both for compression and scalloping, for electrons emitted from completely or partially shielded cathodes. It is shown that the beams emitted from completely shielded cathodes experience a stronger compression with their cross section reducing as the square of the increase in the magnetic field, while this convergence for the electrons emitted from partially shielded cathodes is considerably less, being only proportional to the increase in the magnetic field. From the point of view of scalloping the latter beams are superior, in that their fractional ripple remains constant regardless of the amount of compression, whereas in the former class of beams this parameter increases as the square root of the ratio of the magnetic field to the entrance value. These results are independent of the manner of the increase of magnetic field, making it, therefore, possible to use increasing fields that go over smoothly to th...

Note on the Focusing of Electron Beams in certain Magnetic Fields

Equations are set out which determine the focusing properties of electron beams in magnetic fields whose scalar potential has a plane of antisymmetry. From these is derived the condition that a proposed ray-axis and associated focusing requirements should be physically realizable. It is also shown that the fringe-effect of fields with sharply defined boundaries may be characterized by a pair of focal lengths for which formulae are given.