Dmitry Greenfield

Computer modeling of a subfemtosecond photoelectron gun with time-dependent electric field for TRED experiments

In the paper, theoretical and numerical studies on temporal focusing of photoelectron bunch in time-dependent fields are continued. Presented are the results of computer modeling on electron-optical system with combined time-dependent electric and static magnetic fields to ensure both spatial focusing and temporal compressing of photoelectron bunch down to sub-femtosecond level. The peculiarity of space charge effect contribution to the bunch broadening in the case of time-dependent electric field is discussed.

Perturbation techniques in the problems of computational charged particle optics

Basing on perturbation technique, discussed are some urgent problems of computational charged particle optics including numerical evaluation of fringe fields and field perturbations caused by small deviation of the shape of electrodes from rotational or planar symmetry as well as aberrational analysis of charged particle beams in the most general tensor form. It is shown that perturbation technique, being combined with other numerical approaches such as the tau- variations method and generalized method of initial parameters variation, proves to be most promising for evaluation of mechanical tolerances and simulation of Coulomb repulsion effects and charged particles scattering upon a fine-structure grid.

New Computer Modeling and Experimental Results on Photoelectron Gun with Time-Dependent Electric Field

Presented are the recent, numerically-supported experimental results on temporal compressing of electron bunch in timedependent electric field, derived with the use of a unique photoelectron gun that has been simulated, designed, manufactured, and tested at the Photoelectronics Department of A.M. Prokhorov General Physics Institute, RAS. An original photoelectron bunch generated from the photocathode by a 7 ps laser pulse was compressed down to ~fs. The future prospects of temporal compressing of electron bunch in time-dependent fields are discussed.

Sub-100 fs streak tube: computer-aided design, manufacturing, and testing

In the present communication we describe the design of the sub-100 fs streak-tube that may be used for commercial streak cameras manufacturing. Careful attention is paid to preparing of a very smooth input photocathode substrate on which a low surface resistance (1-5 Ohm/) photocathode of S-1 type is deposited. Our estimations have shown that the photocathode surface roughness of about tens of nanometers may restrict the ultimate time resolution at the level of 100 fs. This is the reason why the photocathode substrate surface has to be smooth within the units of nanometers. The curvature of the photocathode surface is also very important to compensate the difference in the time-of-flight of electrons emitted from the central and peripheral photocathode areas. Further modernization was conducted with a photocathode-accelerating mesh assembly. The assembly may operate with 2 - 3 ns (FWHM) electrical pulses of 12 - 15 kV amplitude. In order to improve the S/N ratio in the streaked images, a shuttering system was incorporated inside the tube. As the result, a completely new femtosecond streak tube of PV-FS-M type was designed, manufactured, and tested.

New computer modeling and experimental results on a photoelectron gun with time-dependent electric field

The paper elucidates some new computer modeling and experimental results on the design of a photoelectron gun with time-dependent electric field. The main essence of the new approach is based on the fact that the properly chosen electric field ramp ensures first-order temporal focusing of photoelectron bunch, which is principally impossible in static field previously used. This new technology allows a real breakthrough in time resolution of photoelectron guns and diffractometers intended for time-resolved electron diffraction experiments (TRED).

Chapter 8 Static and Time‐Analyzing Image Tubes With Axial Symmetry

Publisher Summary This chapter focuses on the static and time-analyzing image tubes with axial symmetry. This chapter also presents spatial aberrations of the electron image formed by electrostatic systems. This chapter also discusses temporal aberrations in streak image tubes. This chapter discusses the boundary-layer effect in cathode lenses and electron mirrors. The formal procedure of the tau-variation technique allows correct and computationally effective construction of aberration expansions of charged particle trajectories in the vicinity of any arbitrarily chosen principal trajectory. The essence of the tau-variation approach lies in the possibility of performing two numerical procedures synchronously: solving the differential equations for the isochronous variations and transforming the tau-variations into the aberration coefficients according to the contact transformation formulas.

Chapter 9 Spatial and Temporal Focusing of Photoelectron Bunches in Time‐Dependent Electric Fields

Publisher Summary This chapter discusses the peculiarities of spatial and temporal focusing of electron bunches in time-dependent fields as applied to the problem of generating the ultrashort electron probes for time-resolved electron diffraction (TRED) experiments. This chapter also consider the principal difference in requirements that electron bunch should obey in conventional streak image technique and TRED, and thus reach the necessity of the time-dependent electric fields for temporal focusing. This chapter also presents some theoretical grounds of first-order temporal focusing on the basis of the aberration approach and the tau-variation technique. This chapter discusses some aspects of numerical simulation of a photoelectron gun with time dependent electric field and presents some experimental results on temporal focusing, which have been recently obtained with the use of the photoelectron gun in question.

Chapter 7 General Properties of Emission‐Imaging Systems

Publisher Summary This chapter discusses the most general characteristics of the images formed by bunches of charged particles moving in electromagnetic fields. This chapter also derive the integral correlations between the initial (perhaps, nonstationary) probability distributions of charged particles on the emitter surface (the input image) and the probability distribution of charged particles at a given time moment or on a given smooth surface. This chapter also presents the concept of the output electron or ion image and its principal characteristic, the spatiotemporal spread function. The general isoplanatism condition is introduced in this chapter as a condition for the spread function to be invariant with respect to the shifts in a sufficiently small spatiotemporal region of the emitter. This chapter discusses the Fourier representation of the spread function and introduces the concept of modulation and phase transfer functions, which leads to the definition of the spatiotemporal resolution for static and dynamic emission-imaging systems.

Chapter 1 Integral Equations Method in Electrostatics

Publisher Summary This chapter discusses the integral equations approach or the boundary element method (BEM), which is the most suitable for imaging charged particle optics applications. This chapter presents the mathematical statement of the general three dimensional (3D) electrostatic problem as the Dirichlet problem for the Laplace-Poisson equation, introduces the basic definitions, and outlines the principles of the BEM. The details of the electrode surfaces representation and the charge density approximation are presented in this chapter. Extension of the BEM to the case of dielectric materials is considered in this chapter. This chapter concludes with a special type of electrostatic problem—namely, simulation of the electric field in the vicinity of periodic structures, such as electron-optical grid.

Chapter 4 Some Aspects of Magnetic Field Simulation

Publisher Summary This chapter discusses some methods for numerical simulation of magnetic fields. This problem is more complicated compared with electrostatic field simulation because the vector potential function is generally needed to describe magnetic field. This chapter also presents the Biot-Savart law is used to calculate vector magnetic potential of the current-conducting circuits. General representation of magnetic field as a sum of the scalar and vector potential components makes a basis for analysis of the 3D magnetic problems discussed in this chapter. The nonlinear problems for systems containing saturable magnetic materials can be effectively approached with the variational principle are devoted to magnetic fields simulation in planar and axisymmetric systems using the finite element method (FEM). This chapter also considers the magnetic super-elements method, which allows essential gain in the efficiency of finite element simulation in the nonlinear magnetic problems.