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.
To analyze in detail the effect of Coulomb repulsion on ultrashort electron bunch formation, a special numerical procedure based on the combination of the improved Barnes-Hut method and the aberrational technique is proposed. The results of numerical experiments on the bunch formation in dynamic photoelectron tubes are presented and discussed.
N. V. Ageeva, S. V. Andreev, V. S. Belolipetski, G. I. Bryukhnevich, D. E. Greenfield, S. R. Ivanova, A. M. Kaverin, A. N. Khohlova, E. A. Kuz’menko, G. P. Levina, V. A. Makushina, M. A. Monastyrskiy, M. Ya. Schelev, Z. M. Semichastnova, Yu. Serdyuchenko, T. A. Skaballanovich, V. E. Sokolov
In contrast to the conventional image intensifier with large work area, a streak image tube should possess additional
important feature - the comparatively small temporal distortion at the entire work area of the photocathode. With this
additional engineering restriction taken into account, a novel small-size meshless streak image tube has been developed
by means of numerical optimization. The tube with 25-mm wide work area contains a pair of deflection plates to sweep
the electron image along the 25 mm output phosphor screen that is separated by 100 mm from the photocathode. The
electron image can be shuttered with a 300 V blanking electric pulse. Electron-optical magnification of the tube is unit;
spatial resolution reaches 30 lp/mm over the entire photocathode work area; temporal resolution lies in the 20 - 50 ps
range, depending on the accelerating voltage (6 - 15 kV).
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.
High-energy physics experiments sometimes require exceptionally large areas of photo detection. In many of these cases,
low noise and fast response make a photoelectric tube such as a photomultiplier the detector of choice. Hexagonal
photomultipliers have been made for a number of years to meet these requirements. This paper describes the design and
development of hexagonal image tubes to fulfill the need for improved spatial resolution combined with the ability to
closely pack detectors over a large working area.
Mathematical simulation of electron-optical systems often requires high accuracy of electric and magnetic field calculation and smoothness of its results in the entire computational volume, not just near the symmetry axis. This requirement is necessary to construct aberrational series with respect to an arbitrary principle trajectory. To solve this problem, we propose an effective method for modeling non-linear magnetostatic systems. The method is based on decomposition of the computational domain into three parts, in which we employ different calculation approaches: the finite-element method and the boundary element method. Solution joining at the subdomains' boundaries is guaranteed by the use of common iteration procedure of minimizing the energy functional. The developed method allows calculating magnetic field with higher accuracy and less calculation difficulties.
The first part of the work is devoted to the problem of computer simulation of static image intensifiers and streak
image tubes. The most peculiar and sensible points of the simulation, such as image curvature surface, aberration
coefficients, spatial and temporal resolution along the work area, are illustrated on typical examples. The problem of
image curvature correction is discussed as well.
The second part of the work outlines the theoretical grounds and results of computer-aided design of a principally
new photoelectron-optical system with time-depending fields capable of essential compressing of electron bunches for
the needs of time-resolved electron diffraction (TRED) experiments. Some recent experimental results on the bunch
compressing obtained with the newly designed and manufactured photoelectron gun are presented.
Some theoretical milestones, in definite sense summarizing our studies on temporal compressing of photoelectron
bunch with time-depending electric fields, are elucidated. The recent experimental results on dynamic compression of
photoelectron bunches of picosecond duration, gained with the use of a newly designed photoelectron gun employing the
electric field ramp of about 1.5 kV/ns, are presented and compared with the results of computer simulation.
Space-charge interaction effect in ultrashort electron bunches is considered. It is shown that, along with the
already known possibility of effective (first-order) temporal focusing, the time-depending electric fields offer the
advantage of partial compensating for space charge effects in the bunch, which, in turn, seems most promising from the
viewpoint of dynamic range increase in photoelectron tubes and diffractometers.
The peculiarity of the numeric simulation of the thermoemission electron guns consists in the principal necessity of
taking into account the contribution of the electrons' charge into the potential distribution in the beam formation region.
Ths fact makes the mathematical model essentially nonlinear especially in the high-perveance operation mode.
Moreover, the charge density is extremely high in the vicinity of emitting surfaces, rising infinitely in the limit of zero
initial velocities. A special semi-analytical approach has been applied to deal with the charge singularity. Being
combined with traditional finite-element numerical techniques, this approach provides an efficient way to simulate
thermoemission electron guns.
KEYWORDS: Control systems, Thermoelectric materials, Automatic control, Ions, Power supplies, Vacuum equipment, Process control, Analog electronics, Cryogenics, Ion beams
Discussed are the principles of building automatic control systems for electron- and ion-beam technological equipment.
It is shown that such systems must provide unbreakable control of the technological processes, failure-safe evacuation
and quality control.
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.
The principle limitation of the temporal resolution in the streak tubes is the first-order temporal aberration rising from the initial energy spread of the photoelectrons. The conventional way to reduce this aberration lays in increasing the near-cathode electric field. However the danger of electric breakdown leads to practical limitation of the field strength and, in its turn, the temporal resolution.
Another approach to the problem consists in using a fine-mesh structured photocathode instead of a solid one. The paper presents calculations, both analytical and numerical, which have allowed estimation of required mesh parameters. As the needed mesh period is rather small, the approach under consideration could be hardly brought into practice until the nowadays advance in microelectronics and micromechanics.
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.
In the paper new theoretical aspects of temporal focusing of photoelectron bunch in time-dependent fields are elucidated. The results of computer modeling on electron-optical system with combined time-dependent eletric and static magnetic fields which ensure both spatial focusing of the bunch and its temporal compressing up to the sub-femtosecond level are presented.
The paper is devoted to the problem of temporal resolution in electron-optical streak tubes intended for ultrafast processes investigation. The temporal threshold of about 150-200 femtoseconds can be hardly overcome without a principally new approach to the design of electron-optical scheme of such tubes. One of the possibilities, namely the use of a photocathode having rather fine grid structure, is numerically analyzed on the basis of 3D field calculations.
The problem of ideal first-order temporal focusing of photoelectron bunches with quasi-stationary electromagnetic fields is discussed on the basis of theoretical electron optics and computer modelling.
The algorithm of three-dimensional numerical computation of electrostatic fields based on the first-kind Fredholm integral equations with finite element approximation of electrodes' surface geometry and charge distribution on a triangular mesh is proposed. Using the Fichera's theorems for special treatment of surface charge singularities in the vicinity of electrodes' vertexes and ribs provides high calculation accuracy with acceptable CPU time consumption. It makes the software based on the proposed algorithm effective for solving the problems of precise charge particle optics.
The experimental study of charge relaxation while electron beam processing of dielectrics has shown that under irradiation with an intensive and concentrated electron beam some dielectrics reveal strange behavior. The electron emission from their surface much exceeds the value expected frmo the conventional theory of the secondary electron emission. Moreover, the time dependence of the emission is quite different. The measured current changes rapidly with time and, under some circumstances, oscillates while the secondary emission is always proportional to the electron beam's current. The target of the present paper is to construct the physical model of this effect employing the theory of hot electrons.
Under e-beam irradiation of insulators the introduced electron charge is accumulated in the material. As a result an electric field appears and increases with time causing parasitic deflection of the beam and decrease of the energy of incident electrons. This general problem creates seemingly insuperable obstacles to the e-beam processing and electron microscopy of insulators. However the presented extended study of the physical processes accompanying electron irradiation of insulators has allowed to find the ways around this problem, especially of the case of e-beam engraving studied with full details.
Electron-beam processing of insulating materials faces some difficulties connected with electric charge accumulation during electron irradiation. In this paper investigation of charging dynamics of glasses subjected to electron beam, is presented. It was found that secondary emission from the surface is not the only way of excess charge vanishing in some insulating materials. Under certain circumstances, strong electric field of introduced charge may result in intensive hot electron emission from the surface. This emission, in turn, leads to this charge rapid disappearing. This mechanism prevents such insulators from charging under the electron beam irradiation and facilitates processing them significantly. Main features of emission of this kind are discussed.
The paper deals with the problems that appear in the electron- beam processing of insulators and consist in distortion of the surface pattern created by an electron beam. There are presented the results of theoretical and experimental studies on the phenomena in insulators, which are powered by the electron-beam processing of their surfaces, and the effect of these phenomena on the precision of the processing.
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