Magnetic Lens Research Articles

Shape design of magnetic circuit of the magnetic lens based on medial axis transform

This paper proposes a method of designing a magnetic lens circuit based on axial transform theory. A unified mathematical description of the magnetic circuit profile is given within a medial axial transform in differential geometry. The objective constraint and geometric constraint equations of the magnetic circuit form a closed set of differential equations. Lastly, the optimized magnetic circuit is obtained using the solutions of the equations. The optical performance of the magnetic lens is significantly improved.

Third- and Fifth-Order Geometric Aberrations in Magnetic Exponential Lens Model for Object Magnetic Immersion

We numerically employ the differential algebraic technique to calculate the third and fifth-order geometric aberrations coefficients, which are derived by using the map method, of the magnetic exponential lens model. These coefficients are calculated for object magnetic immersion (OMI). The magnetic exponential model is used as an example for the magnetic round lens to calculate the coefficients. The numerical electron optical results are perfectly in match with the analytically results with a very small relative error (10-10- 10-12). The DA advantage technique is a very compendious, effective, proper, and accurate for analysis of electron optical aberration. The COSYINFINITY 10 code is used in our calculations.

Pressure field and its gradient in electron microscopes

In this work the analytical and numerical calculations for the modeling carried out for obtaining the pressure field and its gradient along the structure of an electronic microscope are presented. Electron microscopes in their various types generally have a tubular structure where the electron trajectory occurs. For the proper functioning of the electron microscope, the electron gun is operated at a pressure of the order of 10−7 mbar (10−5 Pa) and on the tubular structure at a pressure of the order of 10−6 mbar (10−4 Pa). The modeling of the vacuum system is based on the assumption that the transport of gases, in the molecular regime, can be considered a diffusion phenomenon. The specific degassing rate and the specific conductance of the tubular structure are defined, as well as the effective pumping speed. Both the steady and transient pressure fields are obtained. In the steady case only the gas source due to the degassing of the tube wall is considered. In the transient case, in addition to the natural degassing of the wall, a pulsed gas source in space and time is considered. This gas source may be originated from the electron beam hitting the wall of the electron microscope tube, due to a malfunction of the magnetic lenses. Through this analysis the time required for the vacuum system to restore the steady pressure field, for the electron microscope to return to normal operation can be determined.

Design of Symmetric Magnetic Lenses with Optimum Operational Conditions

In the present paper, the geometrical and optical object properties of symmetrical magnetic lens are designed and analyzed using the electron optical design (EOD) program. The effect of the axial aperture diameter (D), the air gap between the poles (s), the thickness of the poles (t), the excitation parameter NI of the lens are all studied for the best optical properties like the object focal length fo, the spherical and chromatic aberrations coefficients (Cs and Cc respectively). It was found that the optical properties of the object significantly improved with a decrease in the axial aperture diameter and width of the air gap of this lens. Upon our study, it was found that the best properties are achieved when the air gap width (S2 mm), and the axial hole diameter (D=6 mm). The effect of the electrode face thickness on the magnetic properties of the proposed magnetic lens is studied, it was found that the best magnetic properties and the highest value of magnetic flux and the lowest value of the axial magnetic field strength bandwidth were at electrode face thickness equal to (3 and 4 mm).Therefore, the best magnetic lens in our design with the best optical properties was S=2mm, D=6mm and t=3mm.

Vortex particles in axially symmetric fields and applications of the quantum Busch theorem

The possibilities to accelerate vortex electrons with orbital angular momentum (OAM) to relativistic energies and to produce vortex ions, protons, and other charged particles crucially depend on whether the OAM is conserved during the acceleration and on how phase space of the wave packet evolves. We show that both the OAM and a mean emittance of the packet, the latter obeying the Schrödinger uncertainty relation, are conserved in axially symmetric fields of electric and magnetic lenses, typical for accelerators and electron microscopes, as well as in Penning traps. Moreover, a linear approximation of weakly inhomogeneous fields works much better for single packets than for classical beams. We analyze quantum dynamics of the packet’s rms radius ⟨ρ 2⟩, relate this dynamics to a generalized form of the van Cittert–Zernike theorem, applicable at arbitrary distances from a source and for non-Gaussian packets, and adapt the Courant–Snyder formalism to describe the evolution of the packet’s phase space. The vortex beams can therefore be accelerated, focused, steered, trapped, and even stored in azimuthally symmetric fields and traps, somewhat analogously to the classical angular-momentum-dominated beams. We also give a quantum version of the Busch theorem, which states how one can produce vortex electrons with a magnetized cathode during either field- or photoemission, as well as vortex ions and protons by using a magnetized stripping foil employed to change a charge state of ions. Spatial coherence of the packets plays a crucial role in these applications and we provide the necessary estimates for particles of different masses.

New type of hybrid magnetic lens for practical applications of HTS bulk superconductors

This is a viewpoint on the letter by S. Namba et al 2019 (Supercond. Sci. Technol. 32 12LT03).

Quantum mechanics of round magnetic electron lenses with Glaser and power law models of [formula omitted

Scalar theory of quantum electron beam optics, at the single-particle level, derived from the Dirac equation using a Foldy–Wouthuysen-like transformation technique is considered. Round magnetic electron lenses with Glaser and power law models for the axial magnetic field B(z) are studied. Paraxial quantum propagator for the Glaser model lens is obtained in terms of the well known fundamental solutions of its paraxial equation of motion. In the case of lenses with the power law model for B(z) the well known fundamental solutions of the paraxial equations, obtained by solving the differential equation, are constructed using the Peano–Baker series also. Quantum mechanics of aberrations is discussed briefly. Role of quantum uncertainties in aberrations, and in the nonlinear part of the equations of motion for a nonparaxial beam, is pointed out. The main purpose of this article is to understand the quantum mechanics of electron beam optics though the influence of quantum effects on the optics of present-day electron beam devices might be negligible.

Simple program for computing objective optical properties of magnetic lenses

The paper describes the basic features of a new program denoted Magnetic Electron Lens Optical Properties (MELOP), primarily intended for providing free and simple way to calculate the objective focal properties of rotationally-symmetric electron magnetic lenses with the presence of its axial magnetic field distribution. The calculation is done by solving the paraxial ray equation using the fourth-order Runge-Kutta formula. For specific beam voltage, the program computes the excitation parameter, the object or image plane, the objective principal plane, the objective focal length, the objective magnification, the spherical aberration coefficient, the chromatic aberration coefficient and the magnetic flux density at the object or image plane. These parameters are solved for zero, low, high or infinite magnification condition. The program can handle instantaneously plotting the variations of the calculated parameters relatively. In addition to that, the data can be transferred to xlsx or txt file format.

Role of wavefront and velocity distribution with magnetic lens on matter-wave diffraction in near-field regime

Near-field diffraction has been widely studied and applied in many research fields including optics, quantum mechanics and sensor technology. High accuracy sensors, for instance, have been developed based on the Talbot effect, i.e., the self-imaging effect, due to the high sensitivity of the near-field diffraction pattern. Since the visibility of the diffraction pattern depends intensively on velocity distribution and wavefront of the incoming wave, the Talbot-Lau effect is generally used to improve the spatial coherence of the incident wave and hence a better fringe contrast. This research aims to investigate the influences of the wavefront spreading out radially involving the Gaussian velocity distribution on the diffraction pattern of a matter-wave which are key points in design optimization and development of tools for improving fringe quality. The probability density to find the particles in the Talbot-Lau interference pattern can be obtained by employing Feynman path integrals. The theoretical simulation results show that the alteration of wavefront leads to changes in fringe contrast and the width of fringe peaks, whereas the velocity distribution affects only the fringe visibility.

Measurement of Energy Distribution of Output Electrons From a Microchannel Plate Based on Vacuum Photodiode

The energy distribution of output electrons (EDOE) from a microchannel plate (MCP) is measured using electrons emitted by a vacuum photodiode excited by a 266 nm Ti-sapphire femtosecond laser. The photoelectrons emitted from the photocathode are focused on a specific location of the MCP by a magnetic lens. While exist an enough voltage difference between the two ends of MCP, the energy distribution curve of output electrons form MCP can be obtained by analyzing the data as function of the bias voltage applied to the charge collector. The result shows that the energy distribution curve obtained by this means consists of a sharp and a long tail when using pulse light source. The most probable electron energy (MPE) is 12.64 eV and the full width at half maximum (FWHM) of the energy distribution curve is about 29.23 eV while the applied voltage difference is 700 V in the experimental setup. The MPE is 10.63 eV and the FWHM of the energy distribution is 22.436 eV in simulation. The measured curve is consistent with the expectation from simulation.

Injection of a short electron bunch into THz radiation section with an undulator and strong guiding magnetic fields

To implement an efficient source of coherent radiation with negative mass longitudinal stabilization, the methods of the formation and injection of a dense electron bunch onto a stationary helical trajectory in a combined helical undulator and a strong uniform magnetic field are studied. Using a magnetic or electric lens permits sending particles almost along the converging lines of the magnetic field (magnetic following) and obtaining a nearly rectilinearly moving compressed bunch inside a solenoid. After that, the bunch can be injected into an adiabatically increasing field of the helical undulator. In this way, it is possible to excite operating undulator oscillations of particles, significantly mitigating the effects of destructive bunch expansion, the excitation of parasitic cyclotron oscillations, and velocity spread, thereby providing stabilization and terahertz radiation of a dense bunch. Due to a significant mode selection for an axis-encircling bunch, as well as due to the long-term interaction of the particles with a dominant mode, which is closest to the group synchronism conditions, radiation with a relatively narrow spectrum and high efficiency can be obtained even in a strongly oversized waveguide. An additional efficiency enhancement can be obtained due to the reduction of the velocity spread caused by the mutual Coulomb repulsion of electrons during injection due to the initial energy chirp of the bunch.

Simple program for computing objective optical properties of magnetic lenses

Simple program for computing objective optical properties of magnetic lenses

AC/DC magnetic device for safe medical use of potentially harmful magnetic nanocarriers

Continuing our previous research work on a drug delivery system based on combined AC/DC magnetic fields, we have developed a prototype AC/DC magnetic syringe device for stimulation of drug release from drug carriers, with the options of injecting/removing drug carriers. The porous Fe3O4 carrier, in a dose-dependent manner, causes acute oxidative damage and reduces the viability of differentiated SH-SY5Y human neuroblastoma cells, indicating the necessity for its removal once it reaches the therapeutic concentration at the target tissue. The working mechanism of the device consists of three simple steps. First, direct injection of the drug adsorbed on the surface of a carrier via a needle inserted into the targeted area. The second step is stimulation of drug release using a combination of AC magnetic field (a coil magnetised needle with AC current) and permanent magnets (DC magnetic lens outside of the body), and the third step is removal of the drug carriers from the injected area after the completion of drug release by magnetising the tip of the needle with DC current. Removing the drug carriers allows us to avoid possible acute and long term side effects of the drug carriers in the patient's body, as well as any potential response of the body to the drug carriers.

Optimization of proton imaging system including fringe field of magnetic lens

The proton imaging system is composed of four quadrupole magnetic lenses and a collimator. The quadrupole magnetic lenses can realize point-to-point imaging, and the collimator can improve image quality by controlling proton flux and realize material diagnosis. The magnetic field gradient of an ideal quadrupole lens becomes zero at the edge. Inside the lens, the magnetic field gradient is constant along the axis, while the magnetic field boundary of the actual lens extends outward. In the proton imaging system, the fringing field will affect the proton transport state and the performance of the imaging system as well. In this paper, a method to optimize the system is presented when the fringe field is considered. A proton imaging system of 1.6 GeV is established with the Geant 4 program, in which the magnetic field gradient distribution of the actual lens is approximated by the Bell function. In an ideal imaging system, the external drift length is 1.2 m, the internal drift length is 0.5 m, the length of the magnet is 0.8 m, and the magnetic field gradient is 8.09 T/m. The parameters of the practical imaging system can be obtained by using the optimization method: when the integral difference in magnetic field gradient distribution between the actual lens and the ideal lens is equal to zero, the outer drift length of the imaging system is 1.203 m and the inner drift length is 0.506 m; when the integral difference in the magnetic field gradient distribution between the actual lens and the ideal lens is equal to 1%, the outer drift length is 1.208 m and the inner drift length is 0.516 m. In the numerical simulation, a 1mm-thick copper plate and a concentric ball are chosen as the objects, and the influence of the fringing field on the collimator aperture and that on the proton flux error are studied. The results show that the optimized imaging system can reduce the flux error of protons passing through the object, and the difference in the aperture of collimator is on the order of 10<sup>–2</sup> when the integral difference is on the order of 10<sup>–2</sup> in magnitude.

Effect of Pole Face Thickness on Magnetization of the Single-pole Magnetic Lens

Effect of Pole Face Thickness on Magnetization of the Single-pole Magnetic Lens

Relativistic magnetic lensing of electron beams using superconducting spheres

In this paper, the theoretical design of magnetic electron lens made of macroscopic superconducting spheres in rotation is proposed. The relativistic quantum behavior of an electron in the magnetic field induced by rotating bulk superconductors is described based on the Dirac equation, and electron beam transport is manifested in specific cases. Based on this approach, the focal length of magnetic electron lens made of macroscopic superconductors is theoretically calculated. Subsequently, the characterization of relativistic electron beam manipulating capability of newly designed magnetic electron lens is carried out by variation of the system parameters. Owing to the ability to focus the relativistic electron beam in versatile and precise manner, this approach offers a new way to study a macroscopic system of superconductors based on special relativity, and it sets a milestone in the construction of a new generation of magnetic electron lenses with superconducting spheres. The model is inherited and developed based on previous calculations of the field induced by spherical and cylindrical superconductors (L.B. Holdeman, NASA TR R-443 (1975); R.G. Rystephanick, Can. J. Phys. 51 (1973) 789–794) and the quantum theory of electron propagation through an electron lens (R. Jagannathan, Phys. Rev. A 42 (1990) 6674).

FANM: A software for focus and aberrations of nuclear microprobe

Focus and Aberrations of Nuclear Microprobe (FANM) is a new beam optics package to achieve fast and accurate design of a nuclear microprobe. FANM achieves a balance between speed of focusing and accuracy of high order aberrations. A combined method proposed in FANM is to achieve focusing conditions using a matrix method and to calculate aberration coefficients using a numerical ray tracing method. FANM has two optional optimization algorithms (derivative-free method and particle swarm optimization), offering a powerful choice for multi-variable optimization. Numeric variables in FANM are stored as 64-bit (8-byte) double-precision floating-point values to reduce calculation errors. Results obtained with FANM are compared with six existing computational tools (PRAM, TRANSPORT-PSI, TRANSPORT-PBO, WinTRAX, GEANT4-nanobeam and Zgoubi). The reasons for discrepancies between the different packages are discussed. Calculated currents of magnetic quadrupole lenses obtained with FANM are compared with experimental values of a nuclear microprobe in the National University of Singapore.

Topologically optimized magnetic lens for magnetic resonance applications.

Improvements to the signal-to-noise ratio of magnetic resonance detection lead to a strong reduction in measurement time, yet as a sole optimization goal for resonator design, it would be an oversimplification of the problem at hand. Multiple constraints, for example for field homogeneity and sample shape, suggest the use of numerical optimization to obtain resonator designs that deliver the intended improvement. Here we consider the 2D Lenz lens to be a sufficiently broadband flux transforming interposer between the sample and a radiofrequency (RF) circuit and to be a flexible and easily manufacturable device family with which to mediate different design requirements. We report on a method to apply topology optimization to determine the optimal layout of a Lenz lens and demonstrate realizations for both low- (45 MHz) and high-frequency (500 MHz) nuclear magnetic resonance.

Deuterium ion beam focusing for the point neutron source development

This work is a continuation of the studies on the point neutron source development. In the neutron source scheme proposed, neutron emission occurs in a relatively small region (less than 1 mm) as a result of the D–D fusion reaction initiated when the focused deuterium ion beam hits the deuterium-loaded target. The main goal of the project is to develop a simple and compact device that can be used in the high-quality fast neutron radiography. In the scheme proposed, the ion beam is extracted from the dense ECR discharge plasma that is confined in the simple magnetic mirror trap. After upgrading the extraction system and the magnetic lens, the deuterium ion beam with a total current in the focal plane of up to 150 mA was obtained, which considerably exceeds the current obtained previously. The numerical simulations of the beam magnetic focusing were performed using the IBSimu code and the results were compared to the experimental data.

Brightness as a function of lens excitation

The Langmuir limit, defined by B. D. Langmuir, was derived from Lambert’s law. For convex cathode or curved trajectories, beams with brightness higher than the Langmuir limit are obtainable. Measured brightness varies dramatically when we vary the excitation in the first lens of a system consisting of an electron gun and two magnetic lenses. The ratios of maximum to minimum brightness were measured as 717, 403, and 310 for beam energies of 10, 20, and 30 keV, respectively. These results indicate that the brightness was not conserved when lens excitation was varied. The maximum brightness for beam energies of 10, 20, and 30 keV were observed to be, respectively, 143, 208, and 92.1 times higher than the Langmuir limit.