GB2116359A - Streak tubes - Google Patents

Abstract

In a streak tube having a radiation input window 18, a photocathode 20, an accelerating mesh electrode 22, focus electrodes 34, 36, electron deflection plates 26, 28 and a sensitive receiving element, e.g. phosphor screen 38, the mesh electrode 22 (Fig. 3) has a regular array of apertures or slots 30 which are substantially longer in the direction of separation of the deflection plates 26, 28 than they are wide. Preferably there is a simple row of apertures of slots each having a length/width ratio of from 2 to 1 to 50 to 1. The arrangement provides improved focusing and this can also be assisted by making the photocathode concave towards the sensitive receiving element of the tube; and additionally or alternatively by making the transparent substrate of the photocathode, and/or the input window to the tube, positive lens shaped. The shaping of cathode or input window tends to equalise the time of transit of electrons from all parts of the photo cathode to the deflection plates. Advantageously the deflection plates may be arranged as part of a transmission line. Also the sensitive receiving element may be a structurally combined camera tube. <IMAGE>

Description

SPECIFICATION Improvements in or relating to streak tubes This invention relates to streak tubes. A streak tube is a vaccum electronic device whereby events of very short time duration may be resolved and displayed in analogous form. A streak tube is applicable more especially to pulses of radiation produced by a laser device; and in the context of the invention the phrase "of very short duration" may mean separated by a time interval in the order of 0.1 to 10 picosecond.

Streak tubes have been described by way of example, in United States Patent Specification No.

3761614 of D. J. Bradley. Conventional forms of streak tube have had some disadvantages arising from differences in total flight times of electrons emitted from different parts of the photocathode of the tube; and also from defocusing effects produced by accelerator electrodes. The present invention provides modifications to the conventional form of streak tube which go far in reducing these disadvantages.

According to the invention, in a streak tube having an envelope, a radiation input window, a photocathode to receive input radiation, a mesh electrode close to the photo cathode, a system of focus electrode to focus, in use, electrons emitted from said photo cathode, electron deflection plates, a sensitive receiving element eg a phosphor screen or camera tube target array to receive, in use, electrons from said photo cathode, and electrical connections through the envelope to said cathode, mesh, focus electrodes and deflection plates, the said mesh electrode has for the transmission there-through of electrons a regular array of apertures or slots which are substantially longer in the direction of spearation of the deflection plates than they are wide.

Desirably said regular array is a single row of apertures or slots, said row being parallel with the deflection plates.

The apertures or slots may be rectangular, and the ratio between longer and shorter dimensions thereof may be in the range from about two to one to about fifty to one. Conveniently the width of aperture or slot in the direction of the row is in the range from about one to about ten times the distance between adjacent apertures or slots in said direction.

The photo cathode may additionally be arranged so as to be concave towards the mesh and focus electrodes and deflection plates.

Additionally or alternatively the radiation input window or photo cathode substrate may be positive lens shaped.

The deflection plates may with advantage be arranged as part of a transmission line.

A camera tube, eg of the kind used in television, may conveniently be arranged in a common envelope with the streak tube, so as to coact with an electron beam therefrom.

The invention will be further described, by way of example only, with reference to the accompanying drawings in which Fig. 1 is a longitudinal sectional view of a streak tube.

Fig. 2 shows diagrammatically a different radiation input window; and electrical connections to the deflection plates.

Fig. 3 is an axial view of the mesh electrode.

Fig. 4 shows a streak tube combined with a camera tube.

Fig. 5 shows a modified photo cathode substrate.

Referring to Fig. 1, a streak tube has an envelope, indicated with an intermediate section 1 2 of metal. Reference 14 indicates a side tube through which the streak tube is evacuated prior to use. Radiation, in the general direction of the arrows 16, enters the streak tube through the radiation input window 1 8. The radiation is in general in the visible, infra red or ultra violet parts of the spectrum, and would usually be in brief pulses, generated directly or indirectly by a laser device. The pulses may be isolated or be generated at regular intervals.

Having entered the streak tube the radiation encounters the photo cathode at 20, which is supported by supports 21. Photo cathode of various compositions may be employed, but in this particular embodiment the cathode is supported on a glass plate, about 1 5 millimetre in diameter, and consists of oxidised silver coated with caesium (Ag-O-Cs). Such a cathode is suitable for use with radiation having a wavelength of the order of one micrometre. The Ag-O-Cs coating is on the right hand face of 20, as viewed in Fig. 1, and under the influence of radiation, electrons are emitted in the right hand direction. In the conventional streak tube the cathode has been flat.According to the present invention the cathode is made concave towards the right, as viewed in Fig. 1; and in this embodiment the cathode surface has a radius of 40 millimetre, so that there is a natural tendency for electrons emitted normal to the cathode surface, and at the same point in time, to reach the deflection plates at the same instant. In this embodiment it is found that the maximum difference in transit time for electrons from cathode to deflection plates is about 5 pico second in an average transit time of 1.6 nano second; that is a maximum difference of about 0.3 per cent, irrespective of whether an electron is emitted from the centre or the edge of the cathode.

Arranged about 2 millimetre from the cathode is the mesh electrode 22, supported by pillars 24.

In conventional streak tube the mesh electrode has usually comprised a foil, eg of stainless steel, typically having about 60 apertures per millimetre, each about 13 micrometre square. Alternatively, circular apertures of about the same area have been used. It has been found that such an electrode construction can give rise to serious electron defocusing effects.

In the present invention, such defocusing has been to a great extent avoided by constructing the mesh electrode so that the apertures are substantially longer in the direction of separation of the deflection plates than they are wide. The apertures are preferably made rectangular slot shaped, and may conveniently be in a single row, as indicated diagrammatically in Fig. 3; the dotted lines indicating the projection on the mesh electrode 22 of the deflection electrode 26, 28.

The mesh electrode is, in practice, made with more slots than have been shown at 30. In this embodiment of the invention the dimension L is 2 millimetre and the dimension W is 12 millimetre.

The ratio between the length, L, of a slot and its width may be in the range from about two to one to about fifty to one (although ratios outside that range are not excluded). The width of a slot may be in the range from about one to about ten times the width of a bar 32 between adjacent slots 30. If made too thin, the bars may not be strong enough to withstand assembly of the streak tube. A mesh electrode of the kind described may be constructed by methods analogous to those employed in the manufacture of integrated circuits and the like microelectronic devices. A voltage is applied, in use, between the cathode 20 and mesh electrode 22, in such a sense as to accelerate emitted electrons away from the cathode. A typical voltage difference is 1 Kilovolt.

Emitted electrons pass from the mesh electrode 22 through a system of focus electrodes 34, 36 which, in use, have applied to them voltages such as to focus electrons onto the phosphor screen 38.

The focus electrode system is in itself substantially of conventional nature. An electron beam leaving the focus electrode system passes between the deflection plates 26, 28, and finally arrives at the phosphor screen 38 where a visible image is formed. The inner surface of the envelope 10, between electrode 36 and the screen 38 is provided, except around the connections through to the deflection plates, with an electrically conducting coating 40, eg of aluminium, which, in use, is maintained at the same potential as electrode 36, to produce a field-free region inside the tube. In this embodiment the deflection plates are limited in length (axially along the tube) to 30 millimetre, the limit being imposed by transit time difterences between electrons traversing different paths between the plates.

In use, a ramp voltage is applied between the deflection plates 26, 28, in order to deflect an electron beam at a steady rate across the phosphor scrsen 38. For the detection of very brief pulses of radiation the ramp voltage must itself be in the nature of a pulse, or sequence of pulses, and with conventional arrangements of deflection plates, matching difficulties between voltage source and deflection plates have been encountered. Matching may be improved by making the plates part of a transmission line. For this purpose, each plate is provided with four connectors 42, 42A (Figs. 1 and 2) through the envelope of the tube, and the deflection plates are made as wide as possible consistent with an adequate clearance from the inside of the envelope.The ramp voltage is provided by a conventional generator 44 feeding through connectors 42 into one side of the deflection plates; the other side being led, through connectors 42A, to a suitable non-reflecting line termination 46. In this example the ramp generator 44 is shown, diagrammatically, as being triggered by the control circuit 48 of a laser device, the radiation from which is input to the streak tube.

It has been described above how transit times of electrons emitted from different parts of the photo cathode 20 may be equalised by making the cathode emitting surface concave in the direction of emission. Alternatively, or additionally, equalisation may be achieved by making the radiation input window in the shape of a lens, as indicated at 1 8A in Fig. 2. This has the effect of delaying radiation reaching the centre of the cathode in relation to radiation reaching the outer parts. Hence for a given radiation wave front, electrons are emitted later from the centre of the cathode than from the periphery. This compensates for the longer distance to focus which electrons from the cathode periphery must travel.In a modified construction, additionally, or alternatively, the transparent substrate 50 (Fig. 5) of the photo cathode 20 may be made in the shape of a positive lens.

One of the principal uses of a streak tube is to resolve the variation with time of a pulse of radiation produced by a laser device. Such a pulse may be of very brief duration, eg a few picosecond, but may nevertheless vary in intensity during that time period, eg having two intensity peaks. A pulse of radiation, or a proportion of a pulse, is input at 1 6 to the streak tube so as to be incident on the photo cathode 20. This causes emission of electrons from the cathode, the electrons being, in this invention, initially restricted substantially to a narrow rectangular section beam by the mesh electrode 22. The beam is focused by the electrodes 34, 36 through the deflection plates 26, 28 onto the phosphor screen 38, te produce a correspondiny narrow rectangular luminescent image.The ramp voltage applied to the plates 26, 28, to synchronise with the pulse of input radiation 16, drives the said luminescent image across the phosphor screen 38. The spatial variation of intensity of the luminescent image is then a measure of the time variation of intensity of the input radiation 1 6. The luminescent image may be recorded photographically, and then subjected to photo densitometer measurements, or by means of a video camera, from which the structure of the input radiation pulse may be determined. If a video camera is used, the phosphor screen 38 may be replaced by the sensitive receiving screen of the camera, and the luminescent image can then be made to appear direct on a video receiver screen, or the information for the image may be stored digitally.

The use of a separate video camera may be avoided by combining a camera tube 52 (Fig. 4) with a streak tube in the common envelope 10.

This provides a single compact device avoiding the necessity for separate setting up and adjustment of a camera tube.

Claims (13)

1. A streak tube having an envelope, a radiation input window, a photo cathode to receive input radiation, a mesh electrode close to the photo cathode, a system of focus electrodes to focus, in use, electrons emitted from said photo cathode, electron deflection plates, a sensitive receiving element to receive, in use, electrons from said photo cathode, and electrical connections through the envelope to said cathode, mesh, focus electrodes and deflection plates; in which the said mesh electrode has for transmission therethrough of electrons a regular array of apertures or slots which are substantially longer in the direction of separation of the deflection plates than they are wide.

2. A streak tube according to claim 1 in which the said regular array is a single row of apertures or slots, said row being parallel with the deflection plates.

3. A streak tube according to claim 1 or claim 2 in which the ratio between longer and shorter dimensions of the apertures or slots is in the range from about two to one to about fifty to one.

4. A streak tube according to claim 2 or claim 3 in which the width of aperture or slot in the direction of the row is in the range from about one to about ten times the distance between adjacent apertures or slots in said direction.

5. A streak tube according to any one of the preceding claims in which the apertures or slots are rectangular.

6. A streak tube according to any one of the preceding claims in which the photo cathode is arranged to be concave towards the mesh and focus electrodes and deflection plates.

7. A streak tube according to any one of claims 1 to 5 in which the radiation input window is positive lens shaped.

8. A streak tube according to any one of claims 1 to 5 in which the photo cathode substrate is positive lens shaped.

9. A streak tube according to any one of the preceding claims in which the deflection plates are arranged as part of a transmission line.

10. A streak tube according to any one of the preceding claims in which the sensitive receiving element is a phosphor screen.

11. A streak tube according to any one of claims 1 to 9 in which the sensitive receiving element is a camera tube target array.

12. A streak tube according to claim 11 having a camera tube arranged in a common envelope with the streak tube so as to coact with an electron beam therefrom.

13. A streak tube substantially as hereinbefore described with reference to any one of the figures of the accompanying drawing.