EP0155890B1 - Slit-scanning image converter tube - Google Patents

Description

The present invention relates to a slit-scanning image converter tube.

The recording of images with an extremely short exposure time makes it possible to raise the profile of the evolution over time of very brief light phenomena. Thus, ultra-fast electronic cinematography applies to a wide field of research and very diverse disciplines: ballistics, detonics, study of living cells, experiment conducted around lasers, etc.

The slit camera makes it possible to record photographically the variation over time of the light level of an image, in one dimension, of the phenomena to be studied. The image of the phenomenon to be studied is made on a photocathode of a conventional image converter tube comprising, in addition to the photocathode, a control electrode, acceleration electrodes, a focusing electrode, a pair of deflectors and an electroluminescent screen, possibly associated with an electron multiplier device.

The number of electrons emitted at each point of the photosensitive layer of the photocathode is proportional to the level of light applied locally. The electrons are accelerated and focused at the location of the phosphor screen for example, where a visible image is produced. When the tube is at rest, the electrons are blocked at the photocathode by a negative potential applied to the control electrode or are deflected by a shutter electrode and then intercepted by another electrode.

In the slit camera, the image produced on the photocathode is delimited by a narrow slit of which the image is made on the screen, the scrolling of the image of the slit being obtained by applying to the deflectors of the deflection optics a scanning signal. On the exit window of the tube, the evolution of the brightness is observed along the scanning axis, as a function of the time of the light phenomenon under examination. According to the area perpendicular to the scanning axis, that is to say along the largest dimension of the slot, the spatial evolution of the same phenomenon is noted.

The applicant disclosed in patent FR-A-2 284 978 filed on September 13, 1974 a slit-scanning image converter tube in which the focusing of the electron beam is independent in the deflection plane, or time plane, and in the spatial plane. This makes it easier to focus on the image of the slot on the screen. Figures 1a and 1b schematically respectively represent the shape of the electron beam in the spatial plane and in the deflection plane in a slit-scanning image converter tube according to this patent.

In FIG. 1a, the shape of the electron beam is shown in the spatial plane between the photocathode 2 and the screen 4. In this plane, the image of the slit is obtained by means of a converging lens produced by electrodes 6 and 8, said image being made on the screen 4 or on a wafer of microchannels 10 in an output system comprising an electron multiplier. Preferably, the converging lens used is a quadrupole lens which has the advantage of not introducing large distortions in the spatial plane, since it is devoid of first order aberrations. The trace of the electron beam in the spatial plane bears the reference 12.

In Figure 1b, there is shown the deflection means of the tube and the shape of the electron beam in the deflection plane. Between the photocathode 2 and the screen 4 are successively arranged an accelerating electrode 14, the electrodes 16 and 18 of the quadrupolar lens forming a diverging lens in the deflection plane, a deflection and focusing lens 20 and possibly a microphone wafer channels 10.

The deflection and focusing optics consist of three pairs of plates 22, 24 and 26. This structure made it possible, in the tubes according to the prior art whose length was limited for technological reasons, to physically separate the lens from screen deflection, which helps reduce beam distortion. However, the corollary of this structure is the impossibility of independently optimizing the focusing and deflection of the beam.

An objective of the invention is to decouple the focusing and the deflection of the beam in the deflection plane. This makes it possible in particular to improve, compared with the prior art, the sensitivity of deflection of the beam.

On the other hand, it can be seen that in FIG. 1b, it is necessary to provide a diaphragm 28 at the input of the focusing and deflection optics 20, this so that the electron beam 30 which diverges at the output of the plates 16 and 18 of the quadrupole lens do not strike said focusing and deflection optics. The proportion of intercepted current is important. It is imposed by the dimension of the diaphragm 28 and by the divergence created by the quadrupole lens adjusted to make, in the spatial plane, the image of the slit on the screen.

The invention proposes to add a converging lens in the deflection plane, upstream of the quadrupole lens in order to limit the width of the beam at the output of said quadrupole lens and thus limit the importance of the intercepted current.

The presence of two converging lenses in the deflection plane before and after the quadrupole lens also gives an infinity of combinations of adjustment of the two lenses which makes it possible to have the image of the slit on the screen. These various settings make it possible to fix either the collected current or the thickness of the beam trace on the screen, that is to say the resolution, which depends on the width of the beam at the input of the deflector.

It is therefore thus possible in certain applications to optimize the current collected at the expense of the thickness of the trace. Conversely, it is possible to optimize the number of points along the spatial axis to the detriment of the collected current.

The invention is therefore an improvement to known slit-scanning image converter tubes. It allows, while retaining the resolved temporal tion of these tubes, to improve spatial resolution.

Specifically, the invention relates to a slit-scanning image converter tube intended to observe rapidly evolving light phenomena by scanning on a screen the image of a slit, said slit collecting on a photocathode the light sent by the light phenomenon to be studied, and emitting an electron beam, said tube comprising said photocathode, a control electrode, an accelerating electrode and an optic for deflecting and focusing the electron beam located between the electrode accelerator and the screen, said deflection and focusing optics comprising a first electronic means for making the image of the largest dimension of said slit on the screen and a second electronic means independent of the previous one for focusing and deflecting the beam, in the plane of the screen, in a direction perpendicular to the previous direction, said second electronic means comprising, between the accelerating electrode and the screen , a focusing optic followed by a deflection electrode, the focusing optic comprising a quadrupole lens and a planar converging lens, characterized in that the focusing optics further comprises a second planar converging lens upstream of the quadrupole lens and in that the deflection electrode is located downstream of the converging planar lens so that the focusing optics make the image of the smallest dimension of the slit on the screen and limit the width of the beam at the input of the deflection electrode.

Thus, the focusing and deflection of the electron beam in the deflection plane are independent. It is thus possible to optimize each of these two functions.

According to a preferred embodiment of the invention, the focusing and deflection optics comprises, between the accelerating electrode and the screen, a planar convergent lens, a quadrupole lens, another planar convergent lens and a deflection electrode .

According to another preferred embodiment, the deflection electrode constitutes a wave propagation line.

This improves the deflection sensitivity of the beam.

• - les figures 1 a et 1 b, déjà décrites, représentent l'allure du faisceau d'électrons dans le plan spatial et dans le plan de déflexion d'un tube convertisseur d'image à balayage de fente selon l'art connu,
• - la figure 2 est un schéma, en perspective d'un mode de réalisation du tube convertisseur selon l'invention,
• - les figures 3a et 3b représentent la forme du faisceau d'électrons dans le plan spatial et dans le plan de déflexion du tube convertisseur de la figure 2,
• - les figures 4a et 4b illustrent la modification du faisceau électronique due à l'électrode accélératrice dans le plan spatial et dans le plan de déflexion,
• - les figures 5a et 5b représentent deux vues de la lentille quadrupolaire, respectivement en coupe sur la figure 5a et de côté sur la figure 5b,
• - la figure 6 représente un mode de réalisation des lentilles planes convergentes dans le plan de déflexion,
• - la figure 7 représente un mode de réalisation de l'électrode de déflexion, et
• - la figure 8 illustre un mode de réalisation d'un moyen d'obturation du tube.

Other characteristics and advantages of the invention will appear better after the following description of exemplary embodiments given by way of explanation and in no way limiting, with reference to the appended drawings in which:

• FIGS. 1 a and 1 b, already described, show the shape of the electron beam in the spatial plane and in the deflection plane of a slit-scanning image converter tube according to the prior art,
• FIG. 2 is a diagram, in perspective of an embodiment of the converter tube according to the invention,
• FIGS. 3a and 3b represent the shape of the electron beam in the spatial plane and in the deflection plane of the converter tube of FIG. 2,
• FIGS. 4a and 4b illustrate the modification of the electron beam due to the accelerating electrode in the spatial plane and in the deflection plane,
• FIGS. 5a and 5b represent two views of the quadrupole lens, respectively in section in FIG. 5a and from the side in FIG. 5b,
• FIG. 6 represents an embodiment of the planar lenses converging in the deflection plane,
• FIG. 7 represents an embodiment of the deflection electrode, and
• - Figure 8 illustrates an embodiment of a means for closing the tube.

In Figure 2, there is shown an embodiment of a converter tube according to the invention. The light of the beam 32 is concentrated by an optical system (not shown) on the photocathode 2 in a rectangle 34 constituting the electron-emitting slit. The converter tube also comprises an accelerating electrode 14, a planar convergent lens 36, a quadrupole lens 38, another planar convergent lens 40, a deflection electrode 42 and a screen 4. In this embodiment, we have not shown the means for closing the tube. These will be described with reference to Figure 8.

Conventionally, the image on the screen is taken up by an image intensifier which can be either inside the tube (microchannel pancake) or outside the tube. In some cases, the presence of an image intensifier in the tube introduces significant background noise. It is therefore preferable to use an external image intensifier such as a matrix of cells with charge transfer devices (in English CCD).

The accelerator electrode 14 is connected to a positive voltage source 44; the three pairs of plates of the converging lens 36 are connected to a voltage source 46; the two electrodes 6 and 8 of the quadrupole lens 38 are connected to the same positive voltage supply 48, while the other two facing electrodes 16 and 18 are connected to the same negative voltage supply 50; the three pairs of plates of the convergence lens 40 are connected to a voltage source 52 and the deflection electrode 42 to a voltage source 54.

In the spatial plane, namely the xOz plane, the image of the slit 34 is obtained by virtue of the converging lens produced by the electrodes 6 and 8 of the quadrupolar lens, said image being made on the screen 4. In the plane of deflection, ie the plane yOz, the electrode 42 deflects in the direction Oy (time axis) the image of the photocathode on the screen 4.

In FIG. 3a, the shape of the electron beam is shown in the spatial plane between the photocathode 2 and the screen 4. The potential applied by the power supply 48 of FIG. 2 to the electrodes 6 and 8 is such that, in the xOz spatial plane, the image of the photocathode slot is produced substantially on screen 4. The electron beam is shown at 56.

In FIG. 3b, the shape of the electron beam is shown in the deflection plane yOz. The electron beam 58 is accelerated by the electrode accelerator 10, then it is prefocused by the converging planar lens 36 before being made divergent by the electrodes 16 and 18 of the quadrupole lens. It then enters the converging planar lens 40, after having been optionally diaphragmed by a diaphragm 60, to be focused on the screen 4. The current stopped by the diaphragm 60 can be adjusted by means of the prefocusing lens 36.

Downstream of the converging lens 40, the beam 58 passes through the deflection electrode 42 which performs the function of scanning the beam on the screen. Preferably, this deflection electrode constitutes a wave propagation line. The deflection voltage signal then propagates on the deflection plate (s) at the same speed as the electron beam.

In FIG. 4a, the photocathode 2 and the accelerating electrode 10 are shown, as well as the diagram of the beams coming from the photocathode such as the beams 62 and 64. The point of first convergence is located at 66 and the image of the photocathode given by the accelerating electrode 10 is shown in dotted lines at 68. The position of the point of first convergence 66 of the image of photocathode 68 and the height of the point of first convergence vary as a function of the ratio e / d, where e is the half width of the slot of the accelerator electrode 10 and d the distance between the photocathode and the accelerator electrode.

In FIG. 4b, the photocathode emitting electron beams such as 70, 72 and 74 is shown in the spatial plane, the image 76 of the photocathode given by the accelerating electrode 10 in this plane being located downstream.

In FIGS. 5a and 5b, the quadrupolar lens 38 has been shown. This lens is formed according to this embodiment of four equilateral hyperbolic arcs, the arcs facing 16 and 18 being brought to potential + V and the arcs 6 and 8 at potential - V. In FIG. 4b, the same quadrupole lens of length I has been represented, side view in section along the plane yOz.

The potential inside such a lens is of the form V = A (y ^2- x ² ); the lens is convergent in the xOz plane, 8V / ôx = -2Ax, and divergent in the yOz plane, avlay = + 2Ay.

The property of convergence of the quadrupole lens 38 is used to make the image of the slot 34 of the photocathode 2 on the screen. In the spatial plane, the beam coming from photocathode 2 has dimensions which are not negligible compared to the inter-electrode distance 2a. It is therefore in the spatial plane that the aberrations of the quadrupole lens will alter the quality of the image.

In the deflection plane, the height of the slit being for example 1 mm, the height of the beam will be small in front of 2a, of the order of a centimeter, and the aberrations negligible. The advantage of the quadrupole lens over the simple converging lens is that it does not introduce large distortions in the spatial plane since it is devoid of first order aberrations.

The shape of the electrodes allowing to realize the quadrupole field is, as we have seen an equilateral hyperbola branch. This form being difficult to machine, it is replaced, in a variant of the invention, by an arc of an osculating circle.

In Figure 6, there is shown an embodiment of a lens converging in the deflection plane such as the lenses 36 and 40. This lens consists of three pairs of plates 78, 80 and 82. The plates 78 and 82 are grounded, and plate 80 has negative potential. This potential is adjustable and can be adjusted independently for each of the lenses 36 and 40 located on either side of the quadrupole lens 38. This allows, while focusing the beam on the screen, to modify its thickness at the input deflection optics, which conditions the thickness of the trace on the screen. The time resolution can thus be adjusted according to each application. The minimum thickness of the trace on the screen is very significantly smaller than in tubes according to the prior art.

FIG. 7 shows an embodiment of the deflection electrode 42.

One of the major problems that arises when deflecting an electron beam is defocus deflection. When we deflect an electron beam by means of plates, brought to positive and negative potentials relative to an average potential, we see the trace thicken on either side of the median position. This thickening is due to the converging lens effect created by the application on the plates of the deflection voltages: the electrons close to the positive plates are accelerated and being faster are less deflected than the axial electrons.

On the contrary, the electrons closer to the negative plates are slowed down, therefore more deviated, so that the crossing of the trajectories is carried out closer than desired to the exit of the deflection plates. We show that the thickness of the trace is proportional to _W / (1 ₂ .L) where w is the width of the beam at the input of the deflection optic, 1 ₂ the length of the deflection plates and L the distance between the entry of the deflection plates and the screen.

The thickness of the beam cannot be reduced if it is desired to keep most of the current transported. In addition, the length of the plates 1 ₂ cannot be increased without reducing the bandwidth of the deflection system. It is therefore advantageous to increase the length L within the limits compatible with the length of the tube.

The bandwidth of the deflection system is limited by the transit time of the beam electrons between the plates. To achieve high scanning speeds, a divided wave propagation deflector system is used, that is to say a system in which the deflection signal accompanies the beam electrons. This makes it possible to obtain a simple deflector, with high sensitivity, therefore with a low deflection voltage, and with very high bandwidth.

The deflection optic 42 shown in FIG. 7 comprises a plate 84 brought to constant potential and a plate 86 forming a zigzag line such that the voltage ramp propagates, in the Oz direction, at the speed of the beam electrons . To allow good propagation of the si general sweep, the set of incoming wires, connectors and zigzag line must be adapted to the impedance and closed on the characteristic impedance. This is achieved by a resistor 87 disposed between the plate 86 and the mass. The adaptation is finally adjusted by means of a counter plate 88 brought to ground potential.

When the tube is at rest, the electrons can be blocked in known manner at the photocathode by a negative potential applied to a control electrode disposed between the photocathode and the accelerating electrode. An electrical signal of positive rectangular shape is then superimposed on this negative bias potential to obtain the opening of the tube. This embodiment is not always the most suitable, in particular when the distance between the photocathode and the accelerating electrode is of the order of only a few millimeters and the potential of the accelerating electrode is high, for example greater than 10 kV.

FIG. 8 shows another embodiment of a system for closing the electron beam. In this figure, representing in section the various elements of the tube, a first shutter lens 90 has been added between the converging lens 36 and the quadrupole lens 38, and a second shutter lens 92 between the converging lens 40 and the deflection electrode 42. The obturation is achieved by deflection of the electron beam by polarizing one of the electrodes of the lens 90.

The impact of the electrons on the lens 92 generates secondary electrons which it would be harmful if they could propagate in the tube. To prevent this, it suffices to confine the secondary electrons in the space delimited by the lens 92 by applying to said lens a potential greater than the potential of the converging lens 40. A voltage of a few hundred volts is sufficient to ensure obturation .

• - distance photocathode-écran: 500 mm,
• - dimensions de la fente de la photocathode: 1 x 12 mm,
• - grandissement dans le plan spatial: 2,
• - dimensions de la fente accélératrice: 2x12 mm,
• - lentille quadrupolaire, li = ₉6,5 mm, a= 14,4 mm,
• - électrode de déflexion: 1₂=69 mm, L= 223 mm,
• - w = 2,8 mm, sensibilité 0,08 mm/V,
• - plage utile de l'écran 24 x 32 mm,
• - potentiel d'accélération: 15000V,
• - potentiel de la lentille quadrupolaire: ±219 V,
• - potentiel de blocage de l'électrode d'obturation: - 500 V,
• - potentiel des lentilles planes convergentes: 500 V,
• - sensibilité de déflexion: 0,08 mm/V,
• - défocalisation de déflexion: s=25 m,
• - résolution spatiale dans le sens de la fente: 25 pl/mm,
• - distorsion inférieure à 2%,
• - épaisseur de la trace dans le plan de déflexion: 40 pm,
• - résolution temporelle le long de la fente: 1 pi- coseconde.

In conclusion, we will indicate the geometric and electrical characteristics of an embodiment of the invention:

• - photocathode-screen distance: 500 mm,
• - dimensions of the photocathode slot: 1 x 12 mm,
• - enlargement in the spatial plane: 2,
• - dimensions of the accelerating slot: 2x12 mm,
• - quadrupole lens, li = ₉ 6.5 mm, a = 14.4 mm,
• - deflection electrode: 1 ₂ = 69 mm, L = 223 mm,
• - w = 2.8 mm, sensitivity 0.08 mm / V,
• - useful range of the screen 24 x 32 mm,
• - acceleration potential: 15000V,
• - potential of the quadrupole lens: ± 219 V,
• - blocking potential of the shutter electrode: - 500 V,
• - potential of flat converging lenses: 500 V,
• - deflection sensitivity: 0.08 mm / V,
• - deflection of deflection: s = 25 m,
• - spatial resolution in the direction of the slit: 25 pl / mm,
• - distortion less than 2%,
• - thickness of the trace in the deflection plane: 40 μm,
• - temporal resolution along the slit: 1 picosecond.

Claims (7)

1. Image converter tube with slit scanning for observing rapidly evolving light phenomena by scanning the image of a slit (34) on a screen (4), said slit collecting on a photodiode (2) the light supplied by a light phenomenon to be studied, and emitting an electron beam (12, 30, 58), said tube having the aforementioned photocathode (2), a closing means (90, 92), an accelerating electrode (14) and a deflection and focusing optics for the electron beam located between the accelerating electrode and the screen, said deflection and focusing optics comprising a first electronic means for producing the image of the largest dimension of the slit on the screen and a second electronic means, independent of the first, for focusing and deflecting the beam, in the plane of the screen, in a direction perpendicular to the preceding direction, wherein said second electronic means having, between the accelerating electrode (14) and the screen (4), a focusing optics followed by a deflection electrode (42), said focusing optics incorporating quadrupole lens (38) and a convergent planar lens (40), characterized in that the focusing optics also comprises a second convergent planar lens (36) upstream of the quadrupole lens (38) and in that the deflection electrode (42) is located downstream of the convergent planar lens (40), so that the focusing optics forms the image of the smallest dimension of the slit on the screen and limits the width of the beam on entering the deflection electrode (42).

2. Image converter tube according to claim 1, characterized in that the deflection electrode (42) forms a wave propagation line.

3. Image converter tube according to either of the claims 1 and 2, characterized in that the slit of photocathode (2) is rectangular, having perpendicular axes Ox and Oy, the Ox axis being parallel to the large side of the rectangle of centre O, in that the accelerating electrode (14) is provided with a slit parallel to the Ox axis, the electron beam produced by light impact on the photocathode being accelerated along the Oz axis perpendicular to the Ox and Oy by a positive potential applied to the accelerating electrode (14), in that the convergent quadrupole lens (38) in the xOz plane, called the spatial plane, is divergent in the yOz plane, called the deflection plane, in that each plane lens (36, 40) is convergent in the yOz deflection plane, and in that an electric power supply applies a variable voltage between the deflection electrodes (42) in order to deflect the electron beam in direction Oy as a function of time.

4. Image converter tube according to any one of the claims 1 to 3, characterized in that the photocathode (32) is planar.

5. Image converter tube according to any one of the claims 1 to 4, characterized in that the quadrupole lens (38) is constituted by four cylindrical electrodes (6, 8, 16, 18) having generatrixes parallel to the Oz axis and whose cross-sections in a plane parallel to the xOy plane are substantially equilateral hyperbola portions, two facing electrodes being raised to a positive potential and the other two facing electrodes to a negative potential.

6. Image converter tube according to any one of the claims 1 to 4, characterized in that the quadrupole lens (38) is constituted by four cylindrical electrodes and with a generatrix parallel to the Oz axis and whereof the cross-sections in a plane parallel to the xOy plane are circular arcs, two facing electrodes being raised to a positive potential and the other two facing electrodes a negative potential.

7. Image converter tube according to any one of the claims 1 to 6, characterized in that the convergent plane lenses (36, 40) have three pairs of plates, the end plates being earthed.

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