Tube arrangement for recording ultra-high speed variations in high intensity light and method of operating the same

[57] ABSTRACT

An electron tube arrangement for recording ultrarapid variations in the intensity of light comprising means such operated that they impress upon the photoelectrons, as soon as emitted by the photocathode, an accelerating energy so high that the energy dispersion inherent to photoelectronic emission can be disregarded.

Claims, 11 Drawing Figures

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PATENTED MAR 12 1974

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3,796,901

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PATENTED**» 12 1974

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SHEET 2 Of 4

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PATENTED MAR 121974

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SHEET 3 OF 4

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PATENTED MAR 12 1974

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3,796, 1

TUBE ARRANGEMENT FOR RECORDING ULTRA-HIGH SPEED VARIATIONS IN HIGH INTENSITY LIGHT AND METHOD OF OPERATING THE SAME

This application is a continuation-in-part of our 5 pending application Ser. No. 800,056, filed Feb. 18, 1969 and now abandoned.

The present invention relates to devices for analysing rapid variations in light. The term rapid variations is intended to convey variations of duration equal to or less ¹⁰than one nanosecond, of the kind occurring in a laser for example.

At the present time, for purposes of this kind of analysis it is current practice to use:

either an arrangement of two tubes: a first tube, hav- ¹⁵ing a photocathode exposed to the light being analysed, produces an electrical signal which is supplied to the tube of an oscilloscope in order to produce the requisite image on the screen of the latter;

or a single incident tube: the beam of the photoelec- ²⁰trons emitted by the photocathode under the effect of the incident light being analysed, is employed to form the image upon the screen of the tube.

In either case, the resolution is in the order of 1 nano- ₂₅second: the image obtained does not make it possible to distinguish between two separate luminous phenomena unless they are at least 1 nanosecond apart.

It is an object of the invention to reduce this time interval. ₃₀

According to the invention there is provided an electron tube arrangement for recording ultra-rapid variations in light intensity comprising: a tube having a photoelectron source for receiving said light and producing an output of photoelectrons in the form of a narrow ₃₃strip, a luminescent screen for receiving said photoelectrons, meanson the path of said photoelectrons between said source and said screen for so accelerating said photoelectrons immediately after their emission from said source that the dispersion of the electrons si- 40 multaneously emitted by said source, in terms of instants at which they impinge upon said screen, due to the energy dispersion inherent thereto, may be disregarded compared to the acceleration energy applied thereto by said accelerating means, said accelerating 45 means including a first electrode immediately adjacent said photoelectron source, means for focusing the beam of electrons including said first electrode and two further electrodes, forming together a three consecutive electrode system, means for periodically deflecting ⁵θ said photoelectrons after emerging from said focusing means normally to their direction of propagation and to said narrow strip, and before they impinge upon said screen, said arrangement further comprising means for maintaining said first electrode at a sufficiently positive ⁵⁵potential with regard to said photoelectron source to cause in the immediate vicinity of said source an electric field better than 10⁴ volts/cm, so that the interval between the instants at which the electrons simultaneously emitted by said source reach said first electrode is less than three picoseconds, and means for applying to the centrally located electrode of said system with respect to said photoelectron source a lower positive potential than the potential of said first electrode, but not less than half thereof, while the other of said two further electrodes is maintained at the same potential as said first electrode.

,901

For a better understanding of the invention and to show how the same may be carried into effect reference will be made to the drawing accompanying the ensuing description and in which:

FIG. 1 is a diagram of an image tube portion limited to the region in the neighbourhood of the photoelectron source;

FIG. 2 is a diagram showing the essential elements of an image tube;

FIG. 3 is a schematic sectional view of a known tube for the analysis of high speed variations in high intensity light;

FIGS. 4 , 5, 6 and 7 are schematic perspective views of the tube arrangement according to the invention;

FIG. .8 is a schematic perspective view of an assembly comprising an accelerator electrode and a deflector electrode, in accordance with the invention;

FIG. 9 is an enlarged sectional view showing the structure of a photocathode for tube arrangements in accordance with the invention;

FIG. 10 is an enlarged schematic sectional view of an electrooptical system arranged between the photocathode and the accelerator electrode, in tube arrangements according to the invention; and

FIG. 11 is a full-size view of an image produced by a tube arrangement in accordance with the invention.

FIG. 1 schematically illustrates an image tube limited to the region in the neighbourhood of the photoelectron source. A photocathode 1, an accelerator electrode 2, spaced therefrom, by a distance d, and a deflecting electrode 3 can be seen. The electrode 2 is limited by a plane A at the photocathode side and by a plane B at the other side. The deflector electrode 3 is limited by a plane C opposite B.

A beam of incident light 4 impinges on the photocathode 1 which emits an electron beam represented by a central electron e_c, moving at an initial velocity v„.

The accelerator electrode 2 is at the voltage Ui relative to the photocathode 1.

This voltage produces between the electrodes 1 and 2 an electric field E, constant in time, which is directed along the abscissae axis Ox.

When the light beam 4 impringes on the photocathode 1, the latter emits photoelectrons of velocity v₀which varies within certain limits from one electron to the next in magnitude and direction. It will be first assumed that this velocity is directed along the axis Ox, as shown in FIG. 1.

Under these conditions, the electron e_c follows in the space comprised between the photocathode 1 and the electrode 2, a uniformly accelerated rectilinear movement along Ox, the equation of which is:

x=v_ot+ll2at², (i) where x designates the abscissa value of the electron on the axis Ox and t the time, and where a — e/m . U,ld, e and m being respectively the charge and the mass of the electron and U-Jd being the approximate expression for the field E.

This equation defines the time taken by the electron to travel the distance d separating the photocathode 1 from the entry plane A of the electrode 2. This time depends upon the initial velocity v₀ of the electron, and is the shorter the higher v_o is.

3,796,901

The invention is based upon the realization that, with the velocity dispersion generally encountered in electrons emitted by a photocathode, the dispersion in the time, taken by the electrons emitted by said photocathode at a given instant to reach the plane A, is only a 5 very small fraction of a nanosecond.

To understand this, it is merely necessary to compare the solutions of equation (1) for x = d, corresponding to extreme values of v_o, namely 0 and v_o.

These solutions differ by a quantity At the expression 10 for which is:

_______y « is in which ee represents the kinetic energy of the electron corresponding to the initial velocity of v_o. Since e is very small compared to U_t, this expression can with close approximation be written as: ²⁰ /-e;

C/fl/ X ________________(2) 25 introducing the field E = U-Jd, one gets

-Λ/Ι· __ J m________(2')

This latter formula results in the following numerical values for a value of v₀ corresponding to an energy ee of 1 eV, this being the value generally accepted for the energy dispersion in photoelectronic emission.

ee — 1 eV placement of a photoelectron from the cathode up to the screen.

In this figure, in addition to the elements shown in FIG. 1, one may see an electrode 2', connected to the electrode 2, and the screen 5 which is at the highest potential in relation to the photocathode 1 (connection not shown). The electrode 3 is limited by a plane F at the side facing the screen 5. The distance between the plane C and the plane F is L and the distance between the screen 5 and the central plane m of the deflector plates is L_P The electrode 3 is built up by two flat parallel components 3α and 3b separated by an interval 2D, and between which there is established a potential difi ference U_d varying in time. This potential difference is produced symmetrically in the form of + U_d/2 and — U_d/2 and at the axis a fixed accelerating voltage U] prevails.

This voltage develops between the electrodes 3α and 3b a substantially uniform field E_d varying in time and directed perpendicularly to the axis Ox.

The electrons emitted by the photocathode 1, after having passed the electrode 2, arrive at the plane C where they penetrate into the electrode 3.

Under the effect of the electric field E_rf prevailling between the components 3α and 3b which build up this electrode, they undergo a deflection, the amplitude of which depends upon the value of the voltage U_d, which varies in time, at the instant at which said electrons reach said plane C.

The electrons emitted by the photocathode 1 during the duration of a flash will thus produce upon the screen 5, after passing the electrode 3, an impact the distance of which from the axis Ox will vary as a function of the instant at which they were emitted by the photocathode. In this way, the screen 5 will display a spread image of the light flash received by the photocathode.

E = 3 10* 10 3 10³ 10³ 3 IO² voks/cm /=1 3 10 30 100 picoseconds

From these values, it will be appreciated that for an electric field E between photocathode 1 and accelerator electrode 2, of between 3 X 10³ and 10⁴ volt/cm, these being values currently used in the present state of the art, the interval between the instants at which two electrons, simultaneously emitted by the photocathode, reach the plane A, is at the most some few picoseconds.

It is therefore possible, in analysing the beam of incident light, to distinguish two instants separated by an interval of 1/10 of a nanosecond without the said interval having any disturbing effect, this because said interval is negligible compared with this value.

What is true at the entry end A of the accelerator electrode 2 also applies at the exit end B therefrom, the difference between the transit times in the space AB of two electrons having different initial velocities and emitted at the same instant by the photocathode being negligible compared to the time interval between the instants at which said two electrons arrive at A.

These conclusions are still valid for electrons whose initial velocity has a component perpendicular to Ox, said component not affecting the foregoing calculations.

FIG. 2 schematically illustrates the essential elements of an image tube and enables one to follow the disThe deflection suffered by a photoelectron on passing the electrode 3, can be derived from the equations ⁴⁰ of its motion in the field Ed prevailling between the electrode components 3α and 3b. Assuming the field Ed to be of the sawtooth form varying linearly as a function of time t in accordance with the expression Ed = Eo t/τ, where Eo is a constant and r is the time (for all ⁴⁵ practical purposes unaffected by fluctuations of the initial velocity v0) taken by an electron leaving the accelerator electrode 2 to traverse the electrode 3, one can write these equations in relation to the axes Oj x and ₅₀ O, y, as x = vt and d² y/dt² = e/m E₀/r t, v being the velocity of the electron at the time at which it reaches the plane C of the deflector electrode 3.

Considering an electron leaving the plane C at time t„ with a velocity v which is actually parallel to OX, and ₅₅ splitting the field E_d into two terms, one being E„ 1₀/τ which is independent of t, and the other E₀/t (/ — r_o), these equations give: x == v (/—/„) and y = o/2m EJt (t₀(t-r₀)² + (r-/₀)³/3).

From these equations one can derive by calculating ₆θ dy/dx, the deflection suffered by the electron in the electrode 3 between the entry plane C and the exit plane F, these planes being spaced apart by the distance L. The tangent of the deflection angle a is given by the value of dy/dx in the plane F, or, (dy/dx)_F.

₆₅ Assuming that, whatever the instant /„ at which the electron under consideration leaves the plane C, the voltage U_d continues to be applied between the components 3α and 3b during the whole of the time taken by

3,796,901 this electron to traverse the electrode 3, as is actually the case, one may write:

(dy/dx) _F = tg = e/2m E_o L/v² 2 (jJt) + 1 (3)

The deflection undergone by the beam in the electrode 3 is thus made up of two terms:

— e/2m E_o L/v², which term is independent of the time t„ at which the electron leaves the plane C, and — e/m E_o L/v² ί_ο/τ (4) which term is dependent upon this time.

The deflection corresponding to the first term, which is constant through the luminous phenomenon, is of no interest for the present invention, since the latter is concerned with the difference between the amplitudes of the deflections experienced, in the course of their trajectory towards the screen, by two electrons emitted at two different instants during the luminous flash. It can be eliminated by suitably shifting the frame.

As to the deflection corresponding to the second term, the equation (4) shows that for a field E_&lt;; varying linearly with time, it varies linearly with the instant t₀at which the electron in question passed the entry plane C of the deflection system. The formula (4) makes the calculation of the deflection corresponding to the various values of t„.

This leads, after the time definition, to the consideration of the spatial definition (or resolution) of the device. Limiting the number of lines to 5 par mm of screen, to achieve good contrast, if it is desired to analyse the luminous phenomenon into 200 phases, that is to say if a spatial definition of 200 lines is desired, the image will have to be spread over a height of 40 mm.

This value is readily obtained, with tubes of this kind of conventional dimensions, and the voltages at which they operate.

A numerical example will now be given by way of example.

The time interval Δζ between the instants at which two photoelectrons, simultaneously emitted by the photocathode, reach the plane A (FIGS. 1 and 2), is given by the formula (2) and, accordingly, for e — 1 volt, L\ — 5000 volts and between the photocathode and the electrode which is adjacent thereto a distance d = 1 cm, which gives a field of 5000 volts/cm, one has Δζ = 6.8 · 10~¹² seconds.

This time interval is, within a few percent, that within which the two photoelectrons reach the entry plane C of the deflection plates 3α and 3Z&gt; (distance A B being some few cm and electrodes at a potential in the order of 5 KV).

Assuming that in the formula (3) a is small, and this is always the case in the tubes considered here, one can say that the deflection β due to the first term of the equation (3), is given by:

tg β — E„ t₀/r L/2 U_t and the difference Δζ# β of the deflections experienced by the two photoelectrons, is given by:

&amp;tg β = E„ Δζ₀/τ L/2 i/„ (5) where Δζ_ο is equal to the interval between the instants at which the two photoelectrons arrive at the entry plane C of the deflector plates.

For L = 2 cm, τ = 0.47 nanoseconds; accordingly the equation (5), for a value of Δζ₀ equal to the foregoing interval Δζ, gives

Δί₈β = Ε₀ V/cm 2.85/10®, and a spread in height δ of the image, between the impact of said two photoelectrons on a screen 5 located at a distance L_x of 20 cm, of δ cm = 57/10® E_o V/cm or in other words, for the spatial definition of 5 lines per mm referred to hereinbefore, that is to say for δ = 0.2 mm, E_o = 350 volts, a value to which corresponds, during the time Δζ, a variation in the amplitude E_d of the deflecting field, of:

350 x Δζ/τ = 5 V/cm approximately.

The full 40 mm high image will be obtained for a variation E_(i max 200 times greater, in other words areound 1000 V/cm, this image corresponding to a flash lasting around 1.36 nanoseconds. The corresponding voltage Ud max between the two plates 3α and 3b (FIG. 2), said voltage being equal to E_d max x 2D, is around 600 V for 2D = 0.6 cm.

The limitation on time resolution is due, as stated hereinbefore, to the initial kinetic energy dispersion of the photoelectrons, which is in the order of one electron volt and which results in a difference between the times taken by two electrons simultaneously emitted to travel the distance between the photocathode and the deflecting electrode.

The above example shows that, for a given energy dispersion of the emitted photoelectrons, i.e., 1 eV, by providing a sufficient electric field between the photocathode and the accelerator electrode which follows, this limitation can be reduced to 6.8 X 10^-12 second. This reduction is the greater, the higher the electric field considered. It is therefore important to accelerate the electrons to their maximal velocity at the moment they leave the photocathode.

However, in conventional tubes such as that shown schematically in section in FIG. 3, this is not possible.

FIG. 3 shows a tube in the form of a solid of revolution, with a narrow slot 10 in a diaphragm 10' located on the trajectory of the light beam 4, concentrated by a lens 11 which provides a beam 12 at the centre 10” of the photocathode 1, which emits a beam 13 of photoelectrons. A camera c photographs the image displayed upon the screen 5 so that observation need not be limited by the latter’s persistence time. In addition there is provided between the photocathode 1 and the accelerating electrode or anode 2, a focusing electrode 9 and, if need be, a shutter electrode 8 at a lower potential.

In view of the space required to accommodate these two electrodes it would be necessary to apply voltages of prohibitive level to the accelerator electrode in order to create the high strength electric field needed in the neighbourhood of the photocathode.

In the tube arrangements in accordance with the invention, on the contrary, the first electrode located opposite the photocathode on the trajectory of the photoelectrons, is connected to means providing the highest

3,796,901 potential compatible with its distance from the photocathode. Electrodes performing the function of electrode 9, and electrode 8 if necessary, are provided elsewhere in the tube.

The above numerical example shows that, by means 5 of this arrangement, which is characteristic of the invention, due to the high potential difference between the photocathode and the accelerating electrode, and the small distance d therebetween, which should be reduced to the minimum compatible with the existing po- 10 tential difference, resulting in a high value of the electric field U_!/&lt;(it is possible to record in a 40 mm high image flashes whose duration does not exceed 1.36 nanoseconds.

Such a recording will make it possible to distinguish 15 two luminous phenomena, separated by 0.07 nanoseconds, this value being 10 times greater than that due to photoelectron emission dispersion.

The same apparatus makes it possible to analyse, with images of the same size, phenomena of longer du- 20 ration, for example 10 nanoseconds, using the same deflecting voltage U_rf max but with a poorer time definition of 50 picoseconds instead of 6.8 picoseconds. The only other distinction between these two cases will be a variation in the frame.

A number of embodiments of the invention have

The first electrode 15 is connected to the positive terminal 61 of a voltage source 60, the negative terminal 62 of which is connected to the photoelectron source 14 while the electrode 17 is connected to an intermediate point 63 of said source; the electrode 15' is also connected to the terminal 61.

The voltage source 60 maintains said first electrode 15 at a potential in the order of 5 to 10 kV with regard to the photocathode 14; the photoelectrons emerging from the photocathode 14 are accelerated by the electric field prevailing in the space between said photocathode and the first electrode 15.

The electrode 17 has applied through the connection 63 a potential at least equal to half the potential of the electrode 15.

The assembly made of electrodes 15,17 and 15' consitutes the focusing means. The use of an assembly of this kind is rendered necessary by the fact that, however narrow the slot 10 and the image 10 which it produces on the photocathode, the beam of photoelectrons emitted by the photocathode has a certain thickness which necessitates focussing perpendicularly to the axis of propagation of the beam. On its trajectory towards the screen 16 along the axis of the tube, the ²⁵ beam passes between the two plates 18 which act as the deflector means.

been schematically illustrated in FIGS. 4, 5, 6 and 7. In all these figures, a photoelectron source and a luminescent screen respectively occupy the two extremities of a sealed enclosure 50 in the form of a solid of revolu- ³θ tion. A light beam 4 propagates through a narrow slot 10 pierced in a diaphragm 10' and an optical system 11 which concentrates the beam 4 into a beam 12 which forms an image 10 at the centre of the photocathode. The screen 16 in FIG. 4 and 22 in FIGS. 5, 6 and 7, the ³⁵connection to which has not been illustrated, is at the highest potential, substantially equal to that of the first electrode opposite the photoelectron source, namely electrode 15 in FIG. 4, 20 in FIGS. 5, 6, and 7. A camera, which again has not been shown, enables a film to ⁴⁰be made of the image produced upon the screen. The darkening of the film is measured by means of a microdensitometer.

In these figures, the photoelectron source consists in a photocathode 14 or 19. Its output, emitting photo- ⁴^ electrons upon the effect of the image 10 produced as in FIG. 3 by a diaphragm 10' having an aperture 10 and a lens 11, is in the form of a thin strip as the image 10 itself.

In all the embodiments further described and cov- ⁵⁰ered by the invention, the electronic tube comprising said photoelectron source and said luminescent screen, furthermore comprises accelerating means such operated that they cause in the immediate vicinity of said source an electric field better than 10⁴ volts/cm, so that ⁵the discrepancy between said photoelectrons velocities is reduced, means for focusing the beam of electrons such accelerated and means for deflecting said beam as a function of time. ₆θ

In the embodiment shown in FIG. 4, said accelerating means comprises a first electrode 15 made of a cylinder of revolution about the axis of the enclosure 50.

Two further electrodes 17 and 15', also in the shape of cylinders of revolution having the same radius and ₆₅the same axes as the cylinder 15, are successively located behind the first electrode 15 on the path of the beam.

FIG. 5 illustrates a second embodiment of the invention, different from that of FIG. 4 in terms of the shape of the three electrodes 15, 17 and 15' shown in this figure. In FIG. 5, as well as in FIGS. 6 and 7, the voltage source 60 is missing.

The solids of revolution 15, 15' and 17, are replaced in FIG. 5 by pairs of plates 20, 20' and 21, respectively. The references 19, 22 and 23 respectively designate the photocathode, the screen and the pair of plates which constitute the deflection electrode. This structure is employed in the context of flat ribbon-shaped beams, within the core of which the mutual repulsive force exerted by the electrons is lowest and, accordingly, beams of this kind are more easy to focus than cylindrical beams.

FIG. 6 illustrates another modification of the invention which differs from the foregoing in that the FIG. 5 plates 20' of the accelerator electrode, in the immediate neighbourhood of the deflector electrode 23, are replaced in FIG. 6 by the plates 24 which are no longer parallel to the axis of the tube but perpendicular thereto. The object of this arrangement is, other things being equal, to reduce the distance between photocathode and deflector electrode and, consequently, albeit in a small measure (as the foregoing calculations demonstrate), to reduce the discrepancies between the transit times, between these two electrodes, of two electrons of different initial velocities.

In another embodiment of the invention shown in FIG. 7, deflector plates are employed as elements of the electronic lenses. In this case, the plates 25, located between the pairs of plates 20 and 20', fulfil the functions both of the deflector electrode (electrode 18 in FIGS. 4 and 23 in FIGS. 5 and 6) and the focusing electrode (electrode 17 in FIG. 4 and electrode 21 in FIGS. 5 and 6); there is therefore applied to them on the one hand a direct voltage U_c, across the resistors 26, and a sawtooth voltage U_d, across the capacitors 27.

The arrangement shown in FIG. 7 can be produced in a particularly compact manner, whose mechanical stability makes it possible to achieve high mechanical

3,796,901 precision. This embodiment is illustrated in FIG. 8. The plates 20 and 20' and the plates 25 connected to the plates 20 and 20' by staples 28 and insulator beads 28' build up a sort of flared box. On those edges of the box structure which face the photocathode, small cylinders 40 are soldered to the plates 20 in order to improve their breakdown resistance and reduce the corona discharge effect.

In order to improve the contrast of the image on the screen, in another embodiment of the invention, shown in FIG. 9, the optical system between the light beam and the photocathode is given the following form:

On a transparent substrate 29, forming part of the tube 50 and exposed to the light beam 4, a transparent conductive layer 30 (for example Sb-doped SnO₂) is deposited, on which there is applied by vaporization a metal layer 31 leaving at the centre a window 32 some tens of microns in height, and then, on the layer 31, the photoemissive layer 33 is applied opposite the accelerator electrode 20. The window 32 is thus illuminated directly by the light being analysed, without interposition of the slot 10 and the lens 11 of FIGS. 4, 5, 6 and 7. In this way, a sharper delimitation of the light image on the photocathode, is obtained.

In order to reinforce the effect of the invention arrangement, in another embodiment according to the invention, the photoelectron source comprises, inside the envelope 50 as shown in the enlarged view of FIG. 10, a photocathode which may be built up as in example of FIG. 9 (same references) and a pair of electron spectrographs, β spectrographs, 35α and 35b, located between said photocathode and the accelerator electrode 20, the latter being followed by the electrodes 25 and 20'.

The photoelectrons issuing from the slot 32 are focused by the electrode 38 and 39 on the slot 36 so that those photoelectrons whose initial velocity has the highest probability, form an image on the slot 36, of the spectrograph 35a, the others being picked up by the diaphragm 36α. The second spectrograph 35b makes it possible to standardise still further energies of the electrons leaving the slot 37 formed in the diaphragm 37α, and constituting the output of the photoelectron source, before entering the system of the three electrodes 20, 25 and 20'. Beyond the photoelectron source the tube system is the same as that hereinbefore described. In addition to the embodiments described and illustrated, the invention is capable of being embodied in other ways which are obvious to those skilled in the art and all of which fall within the scope of the appended claims.

FIG. 11 shows the overall appearance, at full-size of an image produced using the tubes considered (20 X 40 mm rectangle).

Of course the invention is not limited to the embodiments described and shown which are given solely by way of example.