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.
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.
Presented are the experimental results on femtosecond streak tubes measurements in dynamic mode. Several streak tube prototypes have been manufactured, with either distributed coaxial-strip line or capacitor-type photocathode-accelerating mesh assembly. Electrical field transition time in the photocathode-accelerating mesh gap was investigated. Tubes have been tested in a variety of regimes, in order to define the most efficient ones. Dynamic parameters of the developed femtosecond streak tubes were measured inside the streak camera prototype. The following dynamic parameters were evaluated: ultimate time resolution, dynamic range, and signal/noise ratio, spectral range, input sensitivity, streak speed and its nonlinearities, etc. The developed and optimized femtosecond streak tubes represent a reliable basis for design of streak cameras being required for photographic recording of ultrafast events in laser and plasma physics, time-resolved spectroscopy, laser interaction with matter, laser fusion, etc.
The investigations of residual gas influence on EBI in streak tubes were continued. Model of the microdischarges in streak tubes, grounded on the theory of reciprocal ion emission is considered and it was shown, it is agree with experimental data on EBI fluctuations. The secondary electron emission from a material of an anode diaphragm was detected and investigated.
New generation of streak tubes intended for single-shot and synchroscan operations with femtosecond time resolution was computer modelled, designed, manufactured, tested and adopted for further application in laser research. The developed PV-FS type tubes provide close to 100 fs-time resolution in single-shot streak mode. It is important to note that the PV-FS tubes may be equipped with Peltier cooled S1-photocathodes and their spectral sensitivity may cover the range of 115 - 1550 nm. The developed photocathodes have very low surface resistance (tens of Ohm per square unit). New tubes offer a high (more than 50 line pairs/mm) spatial resolution when recording ultrafast optical images with femtosecond time resolution. Due to keeping the PV-FS external geometry similar to the well-known PV-type tubes it becomes possible to install new devices into available streak cameras (AGAT, Imacon 500, etc.).
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.
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.
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.
A variety of streak/framing/synchroscan image tubes are now under design and manufacturing in the Department of Photoelectronics, General Physics Institute. Among them are: a series of the well-known PV001 image tubes introduced into wide practice since 1978, a set of more advanced PIF001 tubes originally designed in 1979, specially developed femtosecond streak tubes of BSV-type, which were initially proposed in 1987, and finally a number of PF-type tubes placed in service last year. The whole set of these image tubes may cover the spectral range from 115 nm up to 1.55 micrometers , providing maximum sensitivity of 0.5 (mu) A/W at 1.55 micrometers (S1/IR) and up to 3 mA/W at 900 nm (S25/ER). Various input photocathode windows may be used: fiber-optics or borosilicate substrates which blue transparency starts at 350 nm. UV-glass windows (> 200 nm), MgF2 input window (> 115 nm). All tubes with photocathode- accelerating mesh geometry have photocathode area of 6 mm in diameter, while the tubes in non-mesh configuration (PV and PF0 have a rectangular photocathode area of not less than 4 mm by 18 mm. The described tubes may be supplied with any type of phosphor screen (red, orange, blue, green) deposited onto fiber-optics faceplate.
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.
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.
The current demand for inexpensive streak camera manufacturing leads to the necessity in development of a variety of relatively simple and low cost image-converter tubes. One such tube, known as PIF-C, designed and manufactured in the Photoelectronics Department of the General Physics Institute (GPI), is now commercially available. Its experimentally measured time resolution in streak mode has approached one picosecond, and 3 ps in synchroscan mode at 82 MHz operation frequency. In single frame mode at 100 ns exposure time, the spatial resolution over 6 mm input area is within 15 lp/mm. Electron optical magnification of the tube is 1.5 x. PIF-C tubes may be supplied with one of the S1/S20/S25 photocathodes, fabricated either on borosilicate glass, UV-glass, or MgF2 substrate. Its P11 phosphor screen is deposited onto the fiber optic window. EBI of the PIF-C/S1 tube is in the range of 5 (DOT) 10-10 A/cm2.
A. Babushkin, Gennadii Bryukhnevich, Valentina Degtyareva, S. Kaidalov, B. Moskalev, Valdis Postovalov, Alexander Prokhorov, Evgenij Titkov, V. Fedotov, Mikhail Schelev
An experimental prototype of a femtosecond streak image converter camera was built around a specially designed femtosecond streak image tube having a cylindrical type electron focusing lens. Experimentally measured temporal resolution of the camera is better than 500 fs while its spatial resolution is not worse than 40 lp/mm across the slit direction.
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 revised version of the PIFO1 image tube was computer-simulated,
designed and manufactured. For dynamic measurements of this tube, two laser
systems were employed: YA1O3:Nd solid state laser with fibre optics compressor
generating a 200 ns train of single pulses of 0.3 Ps pulse duration at
1.08 ,im wavelength, as well as two-modes Nd:glass laser generating sinusoidallymodulated
radiation with variable picosecond modulation period at 1.06 im
wavelength. Time-dispersed pictures from the PIFO1 phosphor screen were recorded
by SIT-vidicon readout system through crossover 1ype microchannel intensifier.
Temporal resolution of 1.3 Ps was measured at 10 cm/s streak speed and dynamic
range of better than 30. The PIFO1 tube spatial resolution was 25 line pairs/mm
over 6 mm input photocathode area, and its electron-optical magnification
was 1.5x. Comparative technical parameters of PIFO1 and PVOO1 will be presented, as
well as parameters of an experimental streak camera prototype built around PIFO1
image tube.
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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