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.
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.
S. V. Andreev, V. S. Belolipetski, S. R. Ivanova, T. P. Kulechenkova, G. P. Levina, V. A. Makushina, M. A. Monastyrskiy, N. D. Polykarkina, M. Ya. Schelev, Z. M. Semichastnova, V. E. Sokolov
The paper reports on the development of a new streak image tube with accelerating mesh and large (18 mm)
photocathode work area. The tube's temporal resolution is close to one picosecond. To govern photoelectronic images
the tube possesses shutter and deflector plates. Its geometric design allows uniform spatial resolution (more than 25
lp/mm) along the entire photocathode work area at 1.3 electron-optical magnification and negligibly small distortion.
Being a continuation of the well-known PV and PIF - type streak image tubes developed in due time in GPI, the tube
represents a promising tool for taking pictures of ultrafast processes in wide radiation spectrum range.
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).
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.
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.
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.
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.
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.
A new approach to the theory of temporal aberration for the dynamic electron optical imaging systems is given in the present paper. A new definition of temporal aberrations is given in which a certain initial energy of electron emission along the axial direction εz1(0≤εz1≤ε0max) is considered. A new method to calculate the temporal aberration coefficients of dynamic electron optical imaging system, which is named "Direct Integral Method", is also presented. All of the formulae of the temporal aberration coefficients deduced from "Direct Integral Method" and "-Variation Method" have been verified by an electrostatic concentric spherical system model, and contrasted with the analytical solutions. Results show that these two methods have got identical solution and the solutions of temporal aberration coefficients of first and second-order are the same with the analytical solutions. Thus it can be concluded these two methods given by us are equivalent and correct, but the "Direct Integral Method" is related to solve integral expressions, which is more convenient for computation and could be suggested to use in the practical design.
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.
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.
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 Photochron 5 Streak Tube was designed at St Andrews University by Professor W Sibbett & et al. In its original form, the tube had four times magnification and was calculated to have a temporal response of circa 250 fs. Prototypes made by Photek were tested and showed temporal resolution of 400 fs. The electron optical design of the tube has been recalculated, which enables the tube to be operated at magnification as low as 2, to take advantage of advances in CCD design and photon counting/centroiding technology. We also studied the theoretical possibility of operating the tube with a small slot and no mesh. This would enable a gain in quantum efficiency inversely proportional to mesh transmission -- i.e. a factor about 1.5 times improvement. The focus is intensely anisotropic, with magnification x 2.6 in the spatial axis and 1.3 in the temporal axis. The working area of the tube is considerably reduced, to less than 100 microns width. A second iteration of the anisotropic design has been built and tested. Magnification in the spatial direction is x 2 and x 1 in the temporal direction. Working area is approximately 1.5 mm x 12 mm. The low magnification in the temporal direction can be used to increase the working area of the tube, so increasing light throughput and dynamic range, without effecting time resolution. Alternatively, time resolution can possibly be improved, providing that the readout spatial resolution can take advantage of this factor.
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.
It is shown that the simulation problems originated in femtosecond photoelectronic imaging in the beginning of 80s resulted in the development of a new aberration approach called 'the tau-variation technique.' The approach proves to be most fruitful for computer electron optics of non- stationary electromagnetic fields, and at the same time allows unifying the aberration theories of narrow and wide beams. The results illustrating new features of 'ELIM/DYNAMICS' Package specially oriented to the problems of femtosecond imaging are considered, and some aspects of the unsolved computer problems closely connected with femtosecond photoelectronic imaging are thereupon outlined.
Two different approaches to image tube computation, numerical optimization and design being developed in GPI (Russia) and BIT (China) are presented and comparatively discussed as applied to a Russian-made streak image tube.
A new approach to solve the self-consistent problem for electron gun with arbitrary-shaped cathode is suggested. The main feature of the approach is that the most effective numerical techniques of charged particle optics, namely finite-difference and integral equation methods for field calculation on the one hand, and direct ray-tracing and aberration analysis for trajectory calculation on the other, are integrated within a versatile iterative procedure. Some testing problems are considered and analysed in details.
The paper summarizes the results on developing and testing numerical algorithms and software for computer modeling of emission electron beam devices. As examples, some numerical experiments carried out with the newly developed Applied Program Package `CHARGE' are presented and discussed.
The paper is devoted to software development for simulation, optimization, and computer-aided design of photo/thermo- emission electron optical systems and units. The first part of the paper presents the applied program package (APP) 'ELIM\DYNAMICS\ intended for computer-aided design of dynamic photo-emission image tubes with electro/magnetostatic focusing and deflection (streak tubes). The developed software allows highly precise computation of basic image quality characteristics both in static and streak modes. One of the main advantages of the new program version presented is that 'through' electron beam computation from the photocathode to image receiver is available with regard to dynamic aberrations caused by scattering fields located nearby the edges of deflecting plates. In the second part, the possibility is shown to generalize some numerical techniques being effectively applied in photo-emission imaging electron optics (namely, the (tau) -variation -- and the first kind integral equations techniques) to simulation of the thermo-emission electron beam technology units. Functions of the new APP 'CHARGE' are presented, and some numerical aspects of the self-coordinated problem are discussed.
Streak tubes design in General Physics Institute of Russian Academy of Sciences is traditionally supported by computer modeling. Numerical examples which illustrate the main phases of electron image formation inside such tubes are presented and discussed in this paper. Some factors resulting in spatial and temporal resolution decrease are examined in details. These factors include various geometric and chromatic aberrations of electron-optical focusing system, dynamic aberrations of deflection system, temporal instrumental function broadening due to Coulomb interaction effects inside the electron beam. Contribution of photoelectrons spread in energy and angles to spatial and temporal resolution is also studied for various photocathode types and streak speeds.
Current GPI status on femtosecond cameras and diffractometers research and development is overviewed. Discussed are the key components of the femtosecond diffractometer which is now under design. The first experimental prototype of 20 - 40 kV, femtosecond photoelectron diffractometer is computer designed, manufactured and tested.
Applied program package ELIM\DYNAMICS\ is designed for simulation, optimization and computer-aided design of dynamic image tubes with electrostatic focusing and deflection. The package allows highly precise computation of basic electrical and electron-optical characteristics including spatial/temporal line spread functions, spatial and temporal resolution along the photocathode working area in static and streak modes, spatial and temporal scale distortion, as well as assembling tolerances associated with image errors due to weak disturbances of axial symmetry. One of the main advantages of the new version presented is that `through' computation of electron beam from photocathode to screen is available with regard to dynamic aberrations caused by scattering fields located nearby the edges of deflecting plates. Some examples of image tube computations made with ELIM\DYNAMICS\package are given.
A new theoretical line of attack to solve the self- coordinated problem in charge particle optics is developed basing on the aberration theory and (tau) -variation technique. The self-coordinated problem is treated as initial-value problem for the evolution integro-differential equation which directly follows from the Lorentz equation with taking into account the space charge effects. The Coulomb potential of a beam is presented in the form of integral taken over the initial parameters domain, and the induced potential (the `mirror image' potential) is presented in the convolution form with a Green function satisfying the adjacent Fredholm integral equation of the first kind. Some considerations to numerical solution of the evolution equation obtained are applied.
The results of many-years activity in elaboration of numerical techniques and software for static and dynamic image tube design are presented. The developed software is based on the first kind integral equations technique, aberration theory, and special algorithms which allow high-precise image quality computation. The software was carefully tested and used in numerous calculations for spatial and temporal parameters of image tubes operating in static and dynamic modes. Some examples of these calculations are graphically presented and discussed in details.
A complete procedure for computer modeling of static and dynamic performances of numerous electron optical systems is described. Basic equations for estimation of electromagnetic field distribution inside electron optical lens are presented. Use is made of aberration analysis for estimation of electrons trajectories and their transit times. On the basis of the developed (tau) -variations technique, integral parameters of various electron optical systems may be precisely evaluated. A short description of the developed software package is given together with some examples of its application for different image tubes computation.
Coulomb interaction in electron beam is one of the main factor limiting maximum time resolution of subpicosecond image converter tubes. Among the investigations devoted to theoretical analysis of this problem one can distinguish two different approaches. In one of them1 the simplified model concept for the beam motion equations is treated in order to estimate the typical system parameters within the accuracy of one order of magnitude .Suchapproach can not give the detailed characterization of the beam structure required for formation of space—time modulation transfer function (MTF). Another approach2 is related to the analysis of closed system of equations describing the initial boundary nonstationary selfcoordinated problem for electron beam. It is well known that in general case the correct numerical solution of a such kind problem may be obtained only by application of rather complicated and massive numerical procedure which realization is difficult even for modern powerful computers. The important condition describing the behaviour of Coulomb interaction in dynamic emissionimaging system incorporates the necessity of space charge calculation at the level of "optical" accuracy when initial photoelectron angular and energy distribution have to be taken into account. It is also highly important that in the mentioned above systems the inherent Coulomb field of the beam proves to be as a rule several orders of magnitude integrally weaker than the applied external field and therefore Coulomb field may be considered as a perturbation3' Two indicated circumstances, namely, the necessity of high accuracy attainment and the possibility of application the perturbation theory are put into consideration in this work.
The results of two different approaches in simulation of image converter tube
electron optics are given. The first approach is based on aberrations technique, and
the second one is connected with trajectories analysis. To compare the efficiency of
both techniques we have chosen an electron-optical system of the very well- known
time analysing image-converter tube of PVOO1 type. Our digital results reveal much
higher calculation stability for the aberration technique when the latter was used
for calculation of the image tube parameters near the tube axis. For the
photocathode points shifted of the tube axis, both approaches exhibit the similar
behaviour. Furthermore, for the photocathode peripheral areas the software based
on trajectories analysis seems to be more preferable. It is evident that further
development of both techniques will provide the same accuracy and calculation
stability over the whole photocathode area for various type of electron lenses.
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