NEW INSTRUMENTS AND METHODS OF MEASUREMENT

APPLICATION OF AN ELECTRON-OPTICAL CONVERTER FOR THE STUDY OF RAPIDLY OCCURRING PHENOMENA

M. P. Vanyukov

INTRODUCTION

In the investigation of various phenomena associated with the rapid motion of parts in machines and mechanisms, the process of material destruction under impact loads, combustion, detonation, electric discharge in gases, the flight of bullets, projectiles, etc., one has to deal with the need for photographic recording of rapidly occurring processes. This is accomplished either by direct photography of the entire phenomenon under study with a sufficiently short exposure, or by photographing a space-time sweep of some part of it.

In the case of photographing the entire phenomenon, short exposures can be obtained by two methods. One of them is based on the use, in photography, of pulsed illumination; the other is carried out with the aid of various kinds of high-speed shutters.

By using short light flashes for illumination, obtained during an electric discharge in a gas, it is possible to bring the exposure in photography down to \(10^{-7}\) sec. A further reduction of the exposure in this case is apparently impossible because of the inertia of the processes responsible for the emission of light during an electric discharge in gases[^1]. Exposures shorter than \(10^{-7}\) sec can be obtained with the aid of high-speed shutters. Until recently, the shortest exposures were achieved with the aid of the Kerr electro-optical shutter and the Faraday magneto-optical shutter. However, these shutters have a number of substantial shortcomings. Thus, for example, although the Kerr shutter makes it possible to obtain exposures down to \(10^{-8}\) sec, nevertheless, owing to the large loss of light in the polaroids, impossi-

...of using light beams with a considerable aperture; owing to the strong absorbing power of nitrobenzene in the wavelength region shorter than \(\lambda = 3800\text{--}4000\ \text{\AA}\), with this shutter it is possible to photograph only brightly luminous objects.

In recent years, in photographing rapidly occurring phenomena, electro-optical converters have found application; these make it possible to realize both a fast-acting shutter and a high-speed space-time sweep.

Electro-optical converters, as is known, make it possible to transform an optical image, obtained on a photocathode, into an electronic one and then, by means of an electron-optical system and a fluorescent screen, again into a visible image possessing a different spectral composition of the radiation. At the same time there is also the possibility of converting an image in invisible, infrared rays into a visible image, and also of increasing to a considerable degree the brightness of the image, which, as is known, cannot be accomplished by means of ordinary light optics. These properties of electro-optical converters were the main reason that stimulated their development \(^{3,4,5}\).

The use of electro-optical converters for recording rapidly occurring phenomena is based on the exploitation of the possibility of practically inertia-free control of the electronic image by means of electric and magnetic fields.

Control of the electronic image can be carried out in two ways. One of them consists in the fact that, by means of a voltage pulse applied to the electrodes of the converter, an electronic image is produced which exists during a specified short interval of time. In this case the electro-optical converter serves as a high-speed inertia-free shutter. Such a shutter has a number of substantial advantages in comparison, for example, with the Kerr shutter, since in this case there are no light losses in the polarizers used in magnetic and electro-optical shutters; there is the possibility of using high-aperture optics when projecting the phenomenon under investigation onto the photocathode of the converter; amplification of the brightness of the image is possible; and radiation is recorded in a broader spectral range. At the present time electro-optical shutters have been constructed which make it possible to obtain exposures of the order of \(10^{-9}\) sec \(^{6}\). There is reason to expect that with this type of shutter the exposure can be brought down to \(10^{-10}\) seconds \(^{6}\).

The other method of controlling the electronic image consists in the fact that, by means of a magnetic or electric field varying with time, the electron beam in the converter is deflected, as a result of which a space-time sweep of the rapidly occurring phenomenon under investigation is obtained on its screen. In this case, from the entire phenomenon, by means of a narrow

of the target usually only a certain part of it is singled out, which is then projected onto the photocathode of the converter.

The speed of electro-optical scanning may far exceed the speed of optical scanning. Thus, for example, if motion-picture cameras with a moving film make it possible to obtain scanning at a speed of up to 200 m/sec^7, cameras with a rotating mirror—up to 4000–5000 m/sec^7,8, then an electro-optical converter with magnetic deflection provides a scanning speed of up to 300,000 m/sec^7, and with high-frequency electric deflection—approximately up to 20,000,000 m/sec^9.

In the present review, works published over the last 5–6 years on the application of the electro-optical converter as a high-speed shutter and for obtaining electron-optical scanning are considered; and, as an illustration, the results of certain investigations performed using these new and highly effective methods of high-speed photography are presented.

1. THE ELECTRO-OPTICAL SHUTTER AND ITS APPLICATIONS

§ 1. Methods of implementing an electro-optical shutter

The first experiments on implementing a high-speed electro-optical shutter were carried out with standard electro-optical converters with electrostatic focusing.

In the Soviet Union, work on the application of the electro-optical converter in a dynamic regime was begun at the suggestion of Academician A. A. Lebedev as early as 1949 and, as M. P. Vanyukov and E. V. Nilov^10 indicate, already in 1950 positive results were obtained in applying three-electrode converters with electrostatic focusing of the AEG type for implementing a fast-acting shutter.

Fig. 1. Circuit for pulsed switching-on of an AEG-type electro-optical converter.

In these works, the electro-optical converter was switched on by voltage pulses of duration 1 µsec with an amplitude of 20 kV, obtained by means of a radar-type generator. The basic switching circuit is shown in Fig. 1. When a positive rectangular pulse was applied to the grid of tube L (GMI-83)

of small amplitude, formed in the submodulator, a partial discharge of the capacitor \(C\) takes place through the lamp \(\Lambda\), and a rectangular high-voltage pulse arises across the load resistance \(r_1 + r_2\). In the experiment described, the value of the load resistance did not exceed \(1000\ \Omega\). With the aid of a potentiometer on the electrodes of the converter, the potential distribution necessary for good focusing was achieved. The values of the resistances \(r_1\) and \(r_2\) must not be too large, so as not to increase appreciably the time constant of the circuits formed by these resistances and the corresponding interelectrode capacitances of the converter, which is essential for obtaining a good trailing edge of the pulse applied to the converter electrodes. Indeed, at the moment the lamp \(\Lambda\) is cut off, the interelectrode capacitances of the converter, having previously been charged, will begin to discharge through the resistances \(r_1\) and \(r_2\), causing an exponential decrease of the voltage on the converter electrodes. To obtain good image quality on the converter electrodes, it is necessary to apply voltage pulses of rectangular form, so that during the entire exposure the field strength at the photocathode remains the same; this is necessary to ensure the minimum circle of confusion of the electron-optical image, which depends on the field strength at the photocathode, as follows from Recknagel’s formula \(^{11}\):

\[ A = 4 \frac{\bar{\varepsilon}}{E}, \]

where \(A\) is the diameter of the circle of confusion in \(\mu\mathrm{m}\), \(\bar{\varepsilon}\) is the initial energy of the photoelectrons in volts, and \(E\) is the field strength at the photocathode in \(\mathrm{V}/\mu\mathrm{m}\).

Fig. 2. Voltage divider for pulsed switching of a type 1P25 converter.

In 1951, a paper by Hogan \(^{12,13}\) was published, in which pulsed switching of a standard five-electrode electron-optical converter of type 1P25 was carried out. To switch on this converter, a voltage of about \(5\ \mathrm{kV}\) is required, distributed in the corresponding manner among the electrodes. This distribution was produced with the aid of a chain (Fig. 2) made up of resistances \(R_1 \ldots R_4\) and capacitors \(C_{E_1} \ldots C_{E_3}\) \(^{12}\). The capacitances of the capacitors were selected in such a way that, in combination with the interelectrode capacitances of the converter, they made it possible, when applying pulsed voltage to the divider, to obtain the necessary potential distribution on the converter electrodes independently of the pulse duration.

The shutters described above were implemented on the basis of the use of standard converters with electrostatic focusing, developed for use in steady-state operation. A pulsed voltage was applied to the electrodes of these converters, equal in magnitude to the voltage required to form the image in steady-state operation. Shorter exposures and better field resolution were obtained with the converter described by Tarnokov4, as well as with the Jenkins and Chippendale[^15–^19,^33] converter with combined electrostatic and magnetic focusing, specially intended for operation in pulsed mode. To switch on these tubes, a comparatively low pulsed voltage, of the order of 3 kV, is required. A circuit for pulsed switching of such converters was also proposed[^15].

Figure 3 presents a schematic representation of the ME-1201[^18] electro-optical converter, specially developed for photographing with short exposures. The photocathode \(F\) and the fluorescent screen \(P\) are applied to the inner sides of the flat end walls of the tube. A special electrode \(G\) is introduced into the converter, playing the role of a grid in an electron tube and making it possible to control the flow of photoelectrons. The screen diameter of the ME-1201 converter is 115 mm, and the diameter of the working part of the photocathode is 30 mm; in the converter it is possible to obtain an image magnification of up to fourfold.

Fig. 3. Three-electrode electro-optical converter of type ME-1201 with combined electrostatic and magnetic focusing.

Usually the electro-optical converter operates at the following electrode voltages: \(V_{\text{кат}} = 0\), \(V_{\text{сетка}} = -+3\ \text{kV}\), \(V_{\text{экран}} = -+6\ \text{kV}\). A focusing coil is arranged near the cathode part of the tube. By changing the magnitude of the current in the coil, one can obtain different degrees of image magnification in the converter. The best image quality is obtained at a magnification of from two to four times. The current in the focusing coil must be stabilized within 1% in order to obtain a resolving power of 20 lines/mm on the screen. The electron-optical system, consisting of a combination of electric and magnetic fields, has the advantage that it makes it possible to obtain an image having practically uniform resolution over the entire screen with a flat photocathode and a flat screen. Such tubes make it possible to use ordinary

photographic lenses for projecting the optical image onto the photocathode of the converter.

The most significant distortions in electron-optical converters with combined electrostatic and magnetic focusing are pincushion and anisotropic distortion^18. The great influence of the field gradient at the photocathode on image quality is evident from a series of photographs (Fig. 4) obtained with an ME-1201 type converter at the same anode voltage, but at different values of the voltage on the grid electrode. The distortions become small if the potential difference between the photocathode and the grid exceeds 2.5 kV. When changing the potential of the grid electrode \(G\), in order to obtain a high-quality image on the converter screen it is necessary each time to select the current strength in the focusing coil. To lock the stream of photoelectrons in the converter onto the intermediate electrode, it is necessary to apply to it a potential negative with respect to the photocathode, with a magnitude of 60 V.

For high-speed photography it is advisable to manufacture electron-optical converters with antimony-cesium photocathodes, since the spectral distribution of their sensitivity, having a maximum near 4000 Å, agrees well with the spectral distribution of the radiation of flash lamps, which are usually used to create a high level of instantaneous illumination.

The fluorescent screens of the ME-1201 converter are made of willemite or zinc sulfide, giving green and blue luminescence respectively. The spectral emission curve of a screen with zinc sulfide agrees well with the spectral sensitivity of high-sensitivity panchromatic photographic films. In photographic action, ZnS is 5 times more effective than willemite. The screens are coated with a layer of aluminum, which protects them from poisoning by cesium and does not transmit light passing through the semitransparent photocathode. Both types of screen can be made with a resolving power of not less than 50 lines/mm.

The circuit for switching the ME-1201 electron-optical converter, used in photographing successive stages of the development of luminescence in a flash lamp, is shown in Fig. 5^18. A constant voltage of \(+6\) kV is applied to the converter screen. The intermediate electrode (grid) and the photocathode are connected to the anodes of two hydrogen thyratrons \(S_2\) and \(S_1\), respectively. The potential on the photocathode is selected to be approximately 100 volts more positive than on the grid. Under these conditions the converter is locked, and no image is present on the screen. The converter is switched on at the moment when a positive pulse is applied to the grid of the hydrogen thyratron \(S_1\), as a result of which the thyratron opens, and the voltage on the photocathode falls from 3 kV to \(+60\) V over a time interval of the order of \(10^{-8}\) sec (the opening time of the hydrogen thyratron). The converter is again closed at the moment when the positive

Fig. 4. Images obtained on the screen of the ME-1201 converter in the static mode at an anode voltage of 6 kV and at various values of the voltage \(V_z\) on the intermediate electrode.

the pulse is applied to the grid of thyratron \(S_2\), opens it, and the intermediate electrode of the converter is brought to ground potential. The time interval between the moments of opening of the thyratrons \(S_1\) and \(S_2\), which in this case play the role of inertialess switches, determines the exposure produced by the electro-optical shutter. This time interval, as well as the delay in opening the shutter relative to the moment of switching on the flash lamp \(E\), is produced by the remaining part of the circuit, which operates as follows. When switch \(S_0\) is closed, thyratron \(Ta\) opens, and through it and the primary winding of the step-up transformer \(Tp\) a capacitor discharges, producing in the secondary winding of transformer \(Tp\) the high-voltage pulse necessary for switching on the flash lamp. The signal from the cathode resistor of thyratron \(Tb\) is integrated and can produce a delay in the opening of thyratron \(Tc\) of up to \(100\ \mu\text{sec}\). The signal taken from the cathode resistor of thyratron \(Tc\) is divided into two channels. Thyratron \(T_1\) opens after a fixed time delay, determined by an integrating circuit with constant parameters. \(T_2\) opens with an adjustable time delay and may lag behind the opening of \(T_1\) by a time interval up to \(2\ \mu\text{sec}\). Simultaneously with thyratrons \(T_1\) and \(T_2\), thyratrons \(S_1\) and \(S_2\) open. From the circuit it is seen that the variable resistance \(R_1\) controls the moment of exposure, while \(R_2\) determines the exposure of the electro-optical shutter. The circuit described makes it possible to obtain exposures down to \(3 \div 4 \cdot 10^{-8}\ \text{sec}\). With further reduction of the time interval between the moments of opening of thyratrons \(S_1\) and \(S_2\), a tendency is observed for the shape of the voltage pulse applied to the electrodes of the converter to change; specifically, the rectangular pulse gradually becomes triangular, which leads to deterioration of the image quality. Synchronization in this circuit can be maintained with an accuracy down to fractions of a microsecond. A drawback of the thyratron circuit is the impossibility of operating it at a high repetition frequency; therefore it cannot be used, for example, for stroboscopic purposes.

Exposures of duration down to \(10^{-7}\ \text{sec}\) were obtained with an ME-1201 type converter when it was switched on by a rectangular voltage pulse produced by discharging, through a load resistance and a hydrogen thyratron, a section of coaxial cable \((RG\text{-}8/u)\) \(66\) feet long[^20]. Such a shutter was used for photographing explosions.

Shorter exposures, not exceeding \(4 \cdot 10^{-9}\ \text{sec}\), were obtained with two-electrode electro-optical converters whose design ensures minimum lead inductance[^6]. The rectangular voltage pulse required for switching on the converter is obtained in this case by discharging, into the load resistance, a section of coaxial cable,

as the dielectric, in which polyethylene is used. The wave impedance of the cable is 13 ohms. A schematic drawing of the converter and its connection to the coaxial line is shown in Fig. 9. The converter has a low-ohmic antimony–cesium photocathode 25 mm in diameter and a fluorescent screen 112 mm in diameter. The glass tube \(D\) introduced into the converter provides insulation between the photocathode and the anode, which is a conducting layer of aluminum deposited on the inner wall of the converter, starting from the place where tube \(D\) is sealed in and up to

Fig. 5. Circuit for obtaining single exposures with an exposure time from \(3\cdot10^{-8}\) to \(2\cdot10^{-6}\) sec with the ME-1201 converter.

the screen \(F\), which has an aluminum backing. The anode leads are made by means of four tungsten seals connected by a silver ring deposited on the glass. Springs provide contact between the electrode leads of the converter and the coaxial cable. The load resistances \(r\) are arranged around the converter. The tube withstands 8 kV at constant anode voltage and 15 kV under pulsed voltage, if the duration of the pulses does not exceed \(10^{-8}\) sec.

Still shorter exposures, down to \(10^{-10}\) sec, can probably be obtained with an electron-optical converter of the Holst cup type, switched by a rectangular voltage pulse formed during the discharge of a coaxial cable with a small (of the order of 1.5 ohms) wave impedance.

§ 2. Obtaining Single-Exposure Photographs

In studying readily reproducible phenomena it is advisable to use the method of single-exposure photography. By repeating the phenomenon and photographing it at various stages, one can obtain a series of photographs that show the development of the phenomenon in time sufficiently well. The choice of exposure is determined primarily by the brightness of the object being photographed. Thus, for example, photographs of a rotating disk illuminated by a flash lamp were obtained with the 1P25 converter at an exposure of \(2\ \mu\mathrm{sec}\) \(^{12}\). Various stages of the glow of the flash lamp itself were photographed with an exposure of \(0.1\ \mu\mathrm{sec}\) \(^{18}\). Good photographs of explosive phenomena were obtained with an exposure of \(0.03\ \mu\mathrm{sec}\) \(^{13}\), and the development of the streamer of a spark discharge with an exposure of about \(0.001\ \mu\mathrm{sec}\) when electro-optical converters with increased anode voltage were used \(^{6}\).

As an example, let us present several photographs obtained with the aid of an electro-optical converter used as a high-speed shutter. Figure 6 shows a series of photographs showing various stages in the development of the glow in an LSD2-type flash lamp \(^{18}\).

Fig. 6. Various stages of the glow of a discharge in an LSD2 flash lamp, photographed through an ME-1201 electro-optical converter with an exposure of \(0.1\ \mu\mathrm{sec}\). \(a\)—arrangement of the electrodes in the lamp; \(b, c, d, e, f\)—glow of the lamp at different instants of time from the beginning of the discharge.

Successive stages in the development of the channel of a spark discharge in argon are shown in Fig. 7, \(a, b, c\) \(^{10}\). In the afterglow stage (Fig. 7, \(d\)) a separation of the luminous cloud from the electrodes of the spark gap is observed, which apparently is caused by considerable cooling of the gas in contact with the metallic electrodes.

Interesting phenomena occur during the expansion of the luminous channel of a spark discharge in a hydrogen atmosphere, as shown by

Fig. 7. Photographs of the glow of a spark discharge in an argon atmosphere. a, b, c — glow of the discharge at a pressure of 15 atm, taken with an exposure of 0.4 μsec at various moments of time from the beginning of the discharge; d — glow of the discharge at a pressure of 8 atm, taken with an exposure of 2 μsec, 25 μsec after the beginning of the discharge (afterglow stage).

photographs taken through the ME-1201 electro-optical converter with an exposure of 0.5 μsec[^21]. Figure 8 shows the distribution of the radiation intensity across the channel at various moments of time. Approximately 1 μsec after the beginning of the discharge, the radiation intensity at the edges of the channel becomes greater than in its central part, and after 3 μsec an additional bright glow appears in the center of the channel. The authors of the work suppose that the latter phenomenon is due to metallic vapors arriving from the electrodes. In the afterglow stage (5 μsec after the beginning of the discharge), when the current in the discharge circuit ceases, a weakening of the glow brightness is observed at the edges of the channel.

The electro-optical converter with pulsed switching was used by E. K. Zavoisky, G. E. Smolkin, A. G. Plakhov, and M. M. Butslov[^22] to photograph the tracks of ionizing particles in luminescent substances. The track of an ionizing particle

projected onto the photocathode of the converter, which was switched on by a voltage pulse at the moment the luminous track appeared. Synchronization was carried out by means of a photoelectron multiplier.

In the work, photographs are presented of stars formed under bombardment by fast neutrons of a thallium-activated cesium iodide crystal (\(\pi-\mu\) decay), and others.

Fig. 8. Distribution of the radiation intensity in the channel of a spark discharge in hydrogen. Pressure \(p = 760\) mm; \(I = 1500\) a, exposure \(t = 0.5\) μsec.

The development of the streamer of a spark discharge was photographed by means of the electron-optical shutter by Saxe and Chippendale\(^6\), who used the two-electrode tube described in the preceding paragraph. The layout of their apparatus is shown in Fig. 9. The electric spark and the voltage pulse that switches on the electron-optical converter are produced by the discharge of a coaxial line, the central part of which consists of two sections \(A\) and \(B\). Through the resistance \(R\), section \(A\) is charged to a voltage of 30 kV. The distance between sections \(A\) and \(B\) is chosen in such a way that

Fig. 9. Diagram of the apparatus for studying a streamer discharge.

when the full voltage is reached, spark breakdown occurs. After a time interval \(t\), equal to the time of propagation of the electric wave along section \(B\) of the coaxial line, on the load resistance connected in parallel with the electrodes of the converter,

a rectangular voltage pulse with an amplitude of about 15 kV will appear, and the converter will be switched on. The light from the spark gap traverses the optical path \(STUVE\) and, with the aid of lenses \(L\) and mirrors \(M\), is focused on the photocathode \(E\) of the converter. By moving mirror \(M\), it is possible to vary the length of the optical path traversed by the beam of light and thereby introduce the required delay in the moment at which the light reaches the photocathode. By this method photographs (Fig. 10) were obtained of various stages in the development of the streamer, the total duration of which is about \(4 \cdot 10^{-9}\) sec.

Fig. 10. Growth of a streamer of a spark discharge. The electro-optical converter is opened for a time interval of the order of \(4.5 \cdot 10^{-9}\) sec. Relative time delay: (a)—0; (b)—\(1.3 \cdot 10^{-9}\) sec; (c)—\(3.4 \cdot 10^{-9}\) sec.

§ 3. Obtaining a series of successive photographs

In a number of cases it is necessary to obtain a series of successive photographs of one and the same phenomenon (in contrast to the series of photographs presented in the preceding paragraph, obtained by photographing repeatedly reproducible phenomena at different

stages of their development). Such a series can be obtained with the aid of an electro-optical converter in two ways. The converter can be switched on at definite time intervals and the screen photographed on moving film. The difficulty in carrying out such photography at a high repetition frequency lies in the fact that the afterglow of the fluorescent screen of the converter has considerable inertia. Thus, for example, a zinc sulfide screen has an afterglow whose duration, determined by the decrease in brightness to 75% of the maximum value, is \(\sim 10\ \mu\mathrm{sec}^{18}\). The most inertia-free phosphors presently available for making screens of electro-optical converters are characterized by an afterglow of \(4\ \mu\mathrm{sec}^{18}\) (determined by the decrease in brightness from the maximum value to half its magnitude). The possibilities of the method of photography under consideration can be considerably broadened if inertia-free phosphors with sufficient light output are developed.

Another method for obtaining a series of photographs consists in deflecting the image to different parts of the screen, while opening the converter at different instants of time\(^{23}\). This method is not limited by the duration of the screen afterglow, but it does not make it possible to obtain a large number of photographs. To move the image over the converter screen, a magnetic field produced by two pairs of deflecting coils may be used. The electron image is thereby displaced in two mutually perpendicular directions. With the aid of such a device, a propeller rotating at a frequency of 3600 rpm was photographed\(^{23}\). In this case the current in the coils was switched mechanically. In addition, systems with purely electronic commutation were implemented. One of them made it possible to obtain 8 photographs with an exposure of \(0.5\ \mu\mathrm{sec}^{21}\), another—9 photographs with an exposure of \(0.1\ \mu\mathrm{sec}\) and a time interval between photographs of \(0.5\ \mu\mathrm{sec}^{24}\). In the latter device, tubes of the ME-1201 AA type were used.

§ 4. High-Frequency Stroboscope

With the aid of an electro-optical converter, a stroboscope can be implemented that makes it possible to observe periodically recurring phenomena at a significantly higher frequency than can be achieved with ordinary stroboscopes. In this case the object under study is illuminated by a continuous light source and is viewed through an electro-optical converter switched on by short voltage pulses.

Electrical circuits intended for switching on electro-optical converters by voltage pulses should—

differing, with a high repetition rate, substantially from circuits used for one-time pulsed switching of converters. In high-frequency switching of converters, it is necessary to take into account the power consumed by the circuit, and the possibility of using thyratrons in the circuit is also excluded.

Figure 11 shows circuits for pulsed switching, at high frequency, of a converter of type 1P25A[^13]. A current of magnitude 100 ma normally flows through the 6BG6G tube. When a negative pulse is applied to its grid, the anode current is interrupted, and the energy stored in the magnetic field of the current flowing through the coil \(L\) is released in the resistance \(R_L\), whose value is 50,000 ohms.

The inductance of the coil \(L\) is chosen sufficiently large so that, through the resistance, a current flows approximately equal to 100 ma throughout the entire time during which the negative pulse is applied to the grid of the tube.

Fig. 11. Circuit for pulsed switching of the 1P25A converter in a stroboscope.

This current produces, across the resistance \(R_L\), a voltage pulse with an amplitude of about 5000 V, which is quite sufficient for good operation of the 1P25A converter. In this circuit, however, it is difficult to obtain voltage pulses of strictly rectangular form because of the presence of considerable distributed capacitance in the coil inductance.

Figure 12 gives a circuit for switching an electro-optical converter ME-1201 as a stroboscope[^17].

Electro-optical stroboscopes find application in studying the propagation of ultrasonic waves, flames, elastic waves in transparent media, etc. One such stroboscope was

implemented for operation with frequencies up to 20,000 cps at exposures of 1 μsec^15, another with frequencies from 50 to 300,000 cps^13. Further

Fig. 12. Circuit diagram for connecting the ME 1201 converter in a stroboscope.

increase of the frequency of the electron-optical stroboscope is possible, up to 1 Mcps^12.

§ 5. The possibility of implementing
color television

With the aid of pulsed switching of the electron-optical converter, a purely electronic color-television circuit can be implemented^13. The principle of operation of such a device is as follows. If images corresponding to images in the three primary colors arise alternately on the screen of an ordinary television tube, then, by splitting them optically into three images by means of a system of plane mirrors and viewing them through three electron-optical converters, which can be switched on alternately by voltage pulses, one can obtain a three-color image. The screen of the television tube must have a white luminescence. The screens of the electron-optical converters must give luminescences corresponding to the primary transmitted colors, which can be accomplished by installing the corresponding color filters in front of them. The images

The screens of all three converters are then combined optically on a projection screen. The converters are switched on by voltage pulses alternately for intervals of time during which an image corresponding to one of the single-color images appears on the television tube. This switching is accomplished by means of frame-synchronization pulses, which, with the aid of a special separator, are distributed over three channels. With a sufficiently rapid repetition of the superposition of the single-color images, the viewer will see on the projection screen a multicolored image of the transmitted object.

§ 6. Image errors arising during pulsed switching of the electron-optical converter

When the converter is switched on by short voltage pulses, an image appears on its screen with distortions not encountered when tubes operate in the steady-state mode. Such distortions in converters with combined electrostatic and magnetic focusing were described in Tarnok’s paper4. The corresponding distortions in converters with electrostatic focusing were observed by M. P. Vanyukov and E. V. Nilov[^10].

Figure 13 presents a series of photographs showing distortions in image transmission on an ME-1201 converter,

Fig. 13. Image distortions arising on the screen of the ME-1201 converter under strong local illumination of the photocathode. (a) — image of the reticle projected onto the photocathode; (b), (c), (d) — images of the reticle on the converter screen under pulsed switching and at different levels of illumination of the photocathode. The illumination in (c) and (d) is, respectively, 2 and 4 times greater than in (b).

which has combined electrostatic and magnetic focusing, when it is switched on by voltage pulses of duration \(0.1\ \mu\text{s}\) and at high illumination of the photocathode[^18].

To explain the observed phenomenon, it is assumed that during the exposure there occurs strong emission from the most

illuminated areas of the photocathode, as a result of which these areas are charged to a potential approximately equal to the potential of the middle electrode. Such local short-lived charges can create a tangential field on the surface of the photocathode, which has a strong effect on electrons emitted with small initial velocities. It is possible to establish the limiting illumination of the photocathode permissible for undistorted image transfer. In the case of a single switching-on of the converter with a specified exposure, this illumination ultimately determines the blackening density of the negative obtained when photographing an image from the converter screen. Thus, for example, when using standard ME-1201 tubes and an exposure of 0.1 μsec, it is practically impossible to obtain negatives with blackening densities greater than 0.5 with undistorted image transfer.

An examination of standard electro-optical converters with electrostatic focusing of the AEG type, when they were switched on by short voltage pulses of duration 0.4–10 μsec, showed¹⁰ that some of them nevertheless gave an image quality the same as with constant supply. At the same time, in other specimens the resolving power fell almost by a factor of two when they were supplied with pulses of duration on the order of 8–10 μsec.

In the case of further shortening of the duration of the switching pulses, the resolving power of these tubes decreased still more, and in some specimens the image was completely absent for pulse durations on the order of 1 μsec.

The phenomena described above are apparently due to the different rates of the transient processes in converters having different values of the resistance of the photocathode layer per unit of its surface.

Indeed, because the photocathode has resistance, when a rectangular voltage pulse is applied to the electrodes of the converter, the potential on the photocathode is not equalized instantaneously, but over a certain time interval depending on the magnitude of the layer resistance and the corresponding interelectrode capacitance. As a result, a noticeable distortion of the field in the converter occurs. The equipotential lines of the field at the instant the pulse is switched on, when between the center of the photocathode and its peripheral part there arises a potential difference equal to the voltage of the applied pulse \(+E\) kV, are shown in Fig. 14, a. At the photocathode an additional field \((E_1 - E_5)\) is formed, hindering the emission of photoelectrons. With time this field decreases, and the normal field of the electron-optical lens is established in the converter. The time interval required for the disappearance of the additional field depends on the magnitude of the surface resistance of the photocathode. This explains the fact that in some converters an absence of an image is observed

with short pulses (of the order of fractions of a μsec). Fig. 14, b shows the field at the photocathode formed at the moment the pulse is switched off. In this case the image will also be absent, since almost all the electrons will strike the walls of the bulb and only a small part of them will reach the screen. The shortcomings described above in the operation

Fig. 14. Equipotential field lines in an electron-optical converter at the moment of switching on (a) and at the moment of switching off (b) the pulse.

Fig. 15. Dependence of the diameter of the image of the spark-discharge channel on the converter screen on the illumination level at the photocathode.

of electron-optical converters can be eliminated to a considerable extent by using photocathodes with a small resistance per square of their surface.

It was established that, when a high illumination level is created at the photocathode, the quality of the image arising on the converter screen deteriorates significantly. High illumination (of the order of \(10^6\) lux) was produced by projecting onto the photocathode of the converter the channel of a spark discharge. The illumination level was varied by means of different degrees of diaphragm stopping of the projected-

of the projecting objective. The converter was switched on by voltage pulses of duration \(0.4\ \mu\text{sec}\).

It was found that, depending on the magnitude of the relative aperture of the objective projecting the channel onto the photocathode, the dimensions of the channel image on the converter screen change. Figure 15 presents a graph expressing the dependence of the diameter of the channel image \(d'\) on the screen on the relative value of the illumination on the photocathode, expressed through the quantity

\[ \left(\frac{d}{f}\right)^2, \]

where \(d\) is the effective diameter of the objective and \(f\) is its focal length. With a gradual increase in illumination, beginning from a certain value marked on the graph by the letter \(k\), the diameter of the transverse section of the channel at first gradually decreases and then, after reaching a minimum, increases again. The longitudinal size of the channel undergoes analogous changes. When the photocathode is overloaded, a redistribution of brightness in the image appearing on the converter screen is also observed. The edges of the image appear brighter than the middle.

Figure 16 shows a microphotogram of a photograph of the channel obtained from the converter screen under considerable overload of the photocathode. The photometry was carried out in a direction perpendicular to the axis of the channel. Along the abscissa axis of the graph the distances \(d'\) are plotted.

Fig. 16. Microphotogram of a photograph of the channel of a spark discharge obtained under strong overload of the photocathode.

With local overload of the photocathode, a disruption of the operation of the entire electron-optical system is observed; this can be illustrated by the following experiment. Immediately in front of the photocathode of the electron-optical converter a test pattern is placed, having the form of a rectangular grid with a circular aperture at the center. Into this aperture is projected the image of the discharge in a capillary tube. The body of the glow in such a spark gap has the shape of a circle whose diameter is equal to the diameter of the internal aperture of the tube1. The remaining part of the test pattern was illuminated by scattered light. If a high illumination is produced at the center of the test pattern, then a sharply pronounced pillow-shaped distortion is observed over the entire screen. By smoothly varying the illumination on the photocathode, one can readily establish that level of illumination which corresponds to re-

...of an undistorted image. For a converter operating well in the pulsed mode, an undistorted image on the screen was obtained at photocurrent densities not exceeding \(80\ \mu\text{A}/\text{cm}^{2}\).

The image distortions described above, which arise at large instantaneous illuminations, can be explained from the standpoint of the presence of a space charge near the photocathode surface and of a potential relief on it. The space charge arising near the photocathode surface acts on the emitted electrons in the following way. First, it hinders the emission of electrons, this effect being stronger in the central part of the image than at the edges. Accordingly, a redistribution of the image brightness on the screen occurs (Fig. 16). Similar phenomena were also observed when the photocathode of an ME-1201-type converter was overloaded \(^{21}\). Secondly, the formation of a space charge leads to a change in the trajectories of the electrons. Electrons passing through the central part of the space charge deviate little from their usual direction of motion, since, owing to symmetry, the forces acting on the electrons from this charge are small. The trajectory of an electron passing through the boundary regions of the space charge will be considerably distorted as a result of the repelling action of the entire charge. The presence of a surface resistance of appreciable magnitude in the photocathode leads to the fact that the potential of the illuminated part of the cathode cannot become equal to the potential of its unilluminated part. The illuminated region acquires a positive charge, and a “potential relief” is created at the photocathode surface, which distorts the trajectories of the photoelectrons. The electron lens that is formed, shown in Fig. 14, a, acts on the electrons as a converging lens.

The nonmonotonic course of the curve expressing the dependence of the image size on the illumination level (Fig. 15) may be ascribed to the combined action of the potential relief and the space charge on the electron image in the converter. It is natural to suppose that the potential relief first begins to have an effect, since the space charge is strongly dispersed by the field, which has a large gradient near the photocathode. When a certain illumination level is reached, marked in Fig. 15 by the point \(K\), the converging lens formed by the potential relief on the photocathode begins to act, and a decrease in the image size occurs. A further increase in the illumination leads to the accumulation of a space charge near the photocathode, which, as already indicated, leads to an increase in the image size.

In implementing high-speed shutters with electron-optical converters, attention is directed first of all to making photocathodes with a low value of resistance

layer. Thus, for example, in the development of the ME-1201 tubes the resistance of the layer was reduced from millions of ohms per square to hundreds of ohms per square[^15]. At the same time, the sensitivity of the cesium-antimony photocathodes used in the tubes fell from 60 μA/lm to 20 μA/lm. In addition, when obtaining single-shot photographs with short exposures, it is necessary to limit the level of illumination on the photocathode so as to avoid the formation of a considerable space charge.

§ 7. Possibility of increasing the resolving power of electron lenses under pulsed operation

In the preceding section, the image errors arising under pulsed operation of electron-optical converters and ways of eliminating them were considered. We shall now consider the fundamental possibilities of increasing the resolving power of electron lenses under pulsed operation.

As has already been pointed out[^11], the resolving power of electron-optical immersion objectives is determined by the initial energy of the photoelectrons and by the field gradient at the photocathode. Consequently, in order to increase resolving power it is necessary to increase the field strength at the photocathode. Increasing this parameter is usually hindered by breakdown occurring between the cathode and the other electrodes.

Experiments carried out by G. V. Spivak and E. M. Dubinina[^26][^27] with pulsed operation of an immersion electron-optical objective showed that, when a rectangular voltage pulse of duration on the order of 1–10 μsec is applied to the electrodes of the objective, the breakdown potential can be raised by a factor of 5–7 in comparison with the case of steady-state powering of the same system. Such an increase in the breakdown potential is due to the fact that the pulse duration is of the same order of magnitude as the time required for the development of breakdown.

In the experiments of G. V. Spivak and E. M. Dubinina, a three-electrode objective was used to obtain an image of the cathode. When this objective was powered by voltage pulses with an amplitude of 2 kV, in order to create the necessary field gradients at the cathode, the electrodes of the objective were brought together to a distance of about 0.08 mm.

The objective could withstand, without breakdown, 2000 V of pulsed voltage for a duration of the applied pulses of 2 μsec, whereas a continuously applied voltage of 300 V led to breakdown. The transition to pulsed voltage makes it possible to increase the field gradient at the cathode from 4.5 kV/cm to 30 kV/cm. In this case a considerable increase in the resolving power of the objective is observed, which was established by photographing the structure of an oxide cathode.

If the shape of the pulse applied to the electron-optical converter is represented as a trapezoid, then it can be shown that the resolved distance \(\delta\) will be determined by formula 26:

\[ \delta=\frac{8\varepsilon\left[(a-E_0)S+l\right]}{(a^2-E_0^2)S+2al}, \]

where \(\varepsilon\) is the mean initial energy of emission of the electrons, equal in the case of thermionic emission to \(kT\), \(E_0\) is the initial electric field at the cathode, which ensures minimum contrast on the screen, \(a\) is the amplitude of the pulse, \(l\) is the duration of the maximum voltage in the pulse

\[ S=\frac{1}{\operatorname{tg}\alpha}+\frac{1}{\operatorname{tg}\beta}, \]

where \(\alpha\) and \(\beta\) are the angles formed by the lateral sides of the trapezoid with the base.

The relation given above shows that the resolution improves with increasing pulse amplitude and as its shape approaches a rectangular one.

I. N. Prilezhaeva, V. V. Livshits, and G. V. Spivak \(^{28}\) carried out an electron-optical study of the nonstationary emission of an oxide cathode in vacuum and in gas with pulsed energizing of the immersion objective.

The experiments described above show that, when a fast-acting shutter is implemented by means of electron optics switched on by a voltage pulse, it is possible to obtain an image of better quality than the same optics gives when supplied with a constant voltage.

II. ELECTRON-OPTICAL SCANNING AND ITS APPLICATIONS

§ 1. Methods of carrying out scanning

The electron-optical converter may also be used to carry out high-speed space-time scanning of light phenomena. In contrast to a shutter, which makes it possible to obtain an image of the entire rapidly occurring phenomenon at certain discrete instants of time, scanning makes it possible continuously to follow the space-time changes that occur, in general, only in some part of the phenomenon under investigation. Usually optical scanning is used to study phenomena associated with strong self-luminescence. A narrow slit is placed in front of the luminescence under investigation; it is projected either onto a rotating photographic film, or, by means of a rotating mirror, onto a stationary film. The slit is positioned parallel to the axis of rotation of the drum carrying the photographic film, or, in the case of mirror scanning, parallel to the axis of rotation of the mirror. Owing to the non-simultaneous illumination of the scan-

of individual sections of the slit, a certain curved line is observed on the sweep photographs; from the slope of the tangent to this line one can determine the velocity of development of the light phenomena at different instants of time in the direction of the slit, knowing the scale of the photography and the sweep rate. The advantage of the electron-optical sweep over the ordinary optical sweep is that it is carried out without the use of moving devices and makes it possible to attain considerably higher speeds. Such a sweep can be obtained by deflecting the electron stream involved in forming the electron image by means of a linearly varying magnetic or electric field.

Fig. 17 shows the layout of the Courtney–Pratt apparatus²⁶, designed for sweeping, in an electron-optical converter, the luminescence arising during the detonation of an explosive film.

Fig. 17. Diagram of the arrangement of apparatus for obtaining a space-time sweep of the luminescence arising during detonation of an explosive.

The explosive film is placed between glass and metal blocks and its detonation is initiated by means of an electric spark, starting in the middle part of the slit. The slit, illuminated by the explosion, is projected onto the photocathode of the converter in the form of a luminous vertical line \(AB\). In the absence of deflecting fields, the same straight vertical line will be observed on the fluorescent screen. If, however, the electron stream of the electron-optical converter is acted upon by a vertical magnetic field varying linearly with time, then on its screen a temporal sweep of the luminescence selected by the slit will be observed.

In the first experiments, scanning was carried out on three-electrode converters of the AEG type2. However, when the image is deflected in these converters, considerable distortion is observed. Better results can be obtained with two-electrode converters of the ME-1201 type, specially developed for carrying out electron-optical scanning3 (Fig. 18). In this converter the photocathode \(A\) and the fluorescent screen coated with a thin layer of aluminum are deposited on the flat end walls of the bulb. The metallization applied to the inner surface of tube \(G\) is connected to the fluorescent screen, which is usually at a potential of \(+6\ \text{kV}\) relative to the photocathode. In the ME-1200 converter, combined electric and magnetic focusing is used. By changing the current in the focusing coil, a distinct image with various magnifications, up to fourfold, can be obtained on the tube screen.

Fig. 18. ME-1200 electron-optical converter, intended for obtaining image scanning by means of magnetic deflection.

When coil \(D\) is supplied with current stabilized within 1%, a resolution of 20 lines/mm can be achieved over the entire screen, which has a diameter of 115 mm.

The electrical circuit for obtaining a current pulse in the deflecting coils is shown in Fig. 194. Capacitor \(C\) is charged through resistance \(R\) from a constant-voltage source and is discharged through the deflecting coils \(L\) when thyratron \(V_1\) is opened. Although the current in the coil has a sinusoidal form, within the first \(30^\circ\) of the oscillatory cycle it may be considered to vary approximately according to a linear law. Diode \(V_2\) serves to limit parasitic oscillations in the deflecting coils \(L\).

Fig. 19. Electrical circuit for magnetic deflection of the image on an electron-optical converter.

\[ R = 470\ \text{k}\Omega,\quad C = 0.5\ \mu\text{F}, \]
\[ L = 1.1\ \text{mH},\quad V_1 = 2D21,\quad V_2 = 6 \times 5. \]

With a thyratron of the 2D21 type, having an argon filling, a scanning speed of \(1.1\ \text{km/sec}\) was obtained. With a hydrogen thyratron VT83, an image-deflection speed on the converter screen of \(22\ \text{km/sec}\) was achieved. To obtain scanning at a speed of

up to 300 km/sec a generator of undamped sinusoidal oscillations[^7] is used, into whose oscillatory circuit, as a component part, the inductance of the deflecting coils is included.

§ 2. Applications of electro-optical sweep

Electro-optical sweep was used to investigate the luminescence arising during the detonation of lead azide (Fig. 20)3. The velocity of propagation of the detonation front, determined from the photograph, is 2000 m/sec. The luminescence of individual grains of lead azide continues for \(7 \cdot 10^{-8}\) sec. The flaring-up of the luminescence of the grains occurs in \(10^{-8}\) sec. At a sweep speed exceeding 100 km/sec and a resolving power of the converter of 20 lines/mm on the fluorescent screen, it is possible to separate events with a time interval between them of \(10^{-9}\) sec. Using the high time resolution of electro-optical sweep, Courtney-Pratt experimentally observed the delay in the passage of a light signal along a path of only 2.6 m[^29].

Fig. 20. Space-time sweep of the luminescence arising during the explosion of a film of lead azide.

M. P. Vanyukov, V. I. Isaenko, and L. D. Khazov[^30] used an electro-optical converter with an oxygen-silver-cesium photocathode to study the space-time development of the luminescence of a spark-discharge channel in the visible and infrared regions of the spectrum and of the shock wave formed during the discharge. The optical arrangement of the setup for photographing the shock wave is presented in Fig. 21. The spark gap under investigation, \(S_2\), is projected by objective \(O_2\), with a magnification of \(3 \div 4\), onto the slit in diaphragm \(D_2\), placed in front of the photocathode of the converter. The illuminating flash was obtained in this work in the target spark-discharge gap \(S_1\). The luminescence of this gap is projected by objective \(O_1\) onto the slit in diaphragm \(D_1\), whose size corresponds to the image of the target source \(S_1\). Slit \(D_2\) is placed perpendicular to the axes of the spark gaps \(S_1\) and \(S_2\) and to slit \(D_1\). If in the spark gap \(S_2\)

if a shock wave arises, then it causes a deflection of the rays in the beam of light emitted by the gap \(S_1\), and in this case the rays do not fall on the slit \(D_1\), as a result of which a shadow image of the shock wave is formed in the plane of the photocathode. The image on the screen of the converter is photographed by the camera \(F\).

In the same setup, an investigation was made of the space-time development of the luminescence arising during a spark discharge. In this case the luminescence was photographed through a light filter \(C\Phi\), which in one case transmitted blue-violet radiation and in the other—

Fig. 21. Diagram of the optical setup for photographing from the screen of an electron-optical image converter the sweep of a shock wave and of the luminescence arising during a spark discharge.

infrared. Fig. 22 shows a photograph of the sweep of the blue-violet (a) and infrared (b) luminescence of a spark discharge in air, carried out with the following parameters of the discharge circuit: \(C = 0.05\ \text{mkf}\), \(V = 13\ \text{kv}\), \(L = 0.7\ \text{mkgn}\), distance between the electrodes \(l = 3\ \text{mm}\).

As is seen from Figs. 22, a, b, the boundaries of the blue-violet and infrared luminescence propagate with the same velocity only during the first \(0.4\)—\(0.5\ \text{mksec}\) after the beginning of the discharge, until the channel reaches a diameter equal to approximately \(2.5\ \text{mm}\). Subsequently the short-wavelength luminescence practically does not propagate farther; its front becomes strongly blurred, and its intensity rapidly decreases with time. The infrared radiation behaves quite differently. Its front moves with appreciable velocity up to the instant \(t = 2\)—\(3\ \text{mksec}\), and the diameter of the body of infrared luminescence reaches approximately twice the value compared with the diameter of the body of blue-violet luminescence. The front of the infrared radiation remains sharply outlined the whole time.

In addition, the infrared luminescence exhibits approximately the same brightness throughout the entire channel. A photograph of the sweep of the shock wave obtained with the aid of the electron-optical image converter is presented in Fig. 22, c.

A comparison of the sweep of the shock wave and of the infrared luminescence shows that the boundary of this luminescence approximately coincides with the boundary of the rear front of the shock wave. The blue-violet luminescence is attributed to the radiation of the discharge plasma, and the infrared—to the radiation of the gas located in the layer between the plasma and the shock wave.

Using this same method, data were obtained on the rate of expansion of the channel of a spark discharge in inert gases.[^30]

E. K. Zavoiskii and S. D. Fanchenko[^9] carried out a very rapid sweep on an electron-optical converter, using for deflection high-frequency fields from an ultrashort-wave generator with wavelengths of 95–150 cm. Special and sufficiently sensitive converters were employed. With the aid of such ultrahigh-speed sweeping, they studied short light flashes arising during the discharge in air of small capacitors with capacitances from 1 to 6 picofarads, with a spark gap length from 0.2 to 1 mm, as well as during the discharge in nitrogen (pressure from 10 to 30 atmospheres) of miniature Lebedev vibrators 8–12 mm long with a spark gap from 0.02 to 0.1 mm. In Fig. 23 a photograph is given of the elliptical sweep of the discharge glow

Fig. 22. Space-time sweep: (a) of the blue-violet glow of the channel of a spark discharge in air, (b) of the infrared glow of the same discharge, (c) of a shock wave.

of the Lebedev vibrator with a total duration of \(\sim 4.5\cdot 10^{-10}\) sec. The spark gap was set approximately perpendicular to the direction of the sweep. The direction of the sweep is indicated by an arrow. At sufficiently high magnification in the photographs it can be seen,

Fig. 23. Elliptical sweep of the glow arising during the discharge of a Lebedev vibrator, 12 mm long, in nitrogen at a pressure of 30 atm. One scale division corresponds to \(1\cdot 10^{-10}\) sec. The total duration of the spark glow is \(4.5\cdot 10^{-10}\) sec.

Fig. 24. Sweep of the glow oscillating between the electrodes during the discharge of a capacitor of capacitance 3 pF. The arrow indicates the direction of the sweep. One scale division corresponds to \(1\cdot 10^{-10}\) sec.

that against the background of the overall glow of the spark channel there stands out a bright glow which oscillates between the electrodes of the spark gap. The sweep of such a glow, arising during the discharge in air of a capacitor of capacitance 3 pF, is shown in Fig. 24, a. Fig. 24, b presents the sweep of the initial stage of the same

of the discharge. The photographs obtained show that the luminescence time of the radiation sources of the oscillating zone is \(1\cdot 10^{-11}\) sec. Such a short luminescence time is attributed by the authors of the work to the interaction of excited atoms, ions, or molecules with unexcited ones.

The time resolution achieved in the present work is \(3\cdot 10^{-12}\) sec. The limit on the resolution is imposed by the imperfection of the electron optics of the converter and by the finite time of passage of photoelectrons through the thickness of the photocathode. The latter is estimated as a quantity of the order of \(10^{-14}\) sec*).

Electro-optical scanning can also be applied to the direct photoelectric recording of spectra varying with time \(^{31,32}\).

The principle of operation of a spectrograph with an electro-optical converter \(^{32}\) can be followed from Fig. 25. Light from the source under study

Fig. 25. Diagram of a spectrograph with an electro-optical converter.

is resolved by the spectrograph into a spectrum, which is projected onto the photocathode of the electro-optical converter. The image of this spectrum arising in the converter is deflected by a magnetic field in the direction of the wavelength axis, which leads to the corresponding displacement of the spectrum image on the fluorescent screen. If, now, the image of the converter screen is projected onto slit \(II\), placed in front of the entrance window

*) Note added in proof. A justification of the limiting time resolution of electro-optical scanning of \(10^{-14}\) sec is given in the work of E. K. Zavoisky and S. D. Fanchenko (DAN SSSR 108, No. 2, 218, 1956), published after the present review was written. In this work a relation is derived between the limiting time resolution of scanning on an electro-optical converter, electronic chromatic aberration, and the thickness of the photocathode layer.

if a photomultiplier is used, radiation of different wavelengths will successively fall on its photocathode, and the current arising in the photomultiplier can be recorded on an oscilloscope in the form of a spectrum.

The triggering of the magnetic sweep is synchronized with the beginning of the sweep on the oscilloscope. By varying the moment at which the sweep is triggered relative to the beginning of the phenomenon under study, one can investigate the development of the spectrum at different moments in time.

A drawback of the proposed method is that, when standard electron-optical converters are used, for example three-electrode tubes of the AEG type, the persistence of the screen limits the possible sweep speed. At sweep speeds greater than 10 m/sec, considerable blurring is observed in the recorded spectrum. To increase the recording speed it is necessary to use screens with less persistent phosphors. It is also possible to construct a device for electron-optical sweeping of the spectrum without using a fluorescent screen[^31]. In this case, instead of the usual screen, a slit is installed in the converter; directly behind it, inside the tube, an electron multiplier is placed. By deflecting the electron image with a magnetic field, it is possible to achieve a practically inertia-free recording of the spectrum.

With the aid of a spectrograph with an AEG-type converter, the combustion spectrum of powders was investigated.

Spectrographs with electron-optical converters also make it possible to record radiation in the infrared region of the spectrum, up to wavelengths \(\lambda = 1.2\,\mu\).

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