GB2103417A - Electron-optical image tubes and methods of operating them - Google Patents

Abstract

An electron-optical image tube, for example for high-speed cameras, achieves high temporal resolution, good spatial resolution, and small geometric aberrations in respect of off-axis object images on the photocathode (10) of a height up to 5mm by the provision, beyond the photocathode (10) and extraction electrode mesh (12), of a focussing region which includes three (or more) axially spaced focussing electrodes (14a, 14b and 14c) and an anode electrode (16), followed by an electron deflection region of length d. In an optimum configuration the mesh voltage Vm = 10kV; V2 = 10 to 18kV; V1 = 6kV; Va = 10kV; the mesh-focussing electrode spacing (g) = 10mm; the photocathode-mesh spacing (s) = 2mm; and the length of the deflection region (d) = 150mm. The three focussing electrodes (14a, 14b and 14c) which are preferably identically dimensioned and equally spaced are used to allow two dimensions of optimisation of the voltage configuration for a fixed image plane. <IMAGE>

Description

SPECIFICATION Electron-optical image tubes and methods of operating them This invention relates generally to electron-optical image tubes, and espectiallyto image tubes such as are used in electron-optical streak cameras, framing cameras and like systems. The invention also includes methods of operating such image tubes.

Over recent years efforts have been made to improve the electron-optical tubes which are used inter alia to produce streak images of high speed luminous events. It is the conventional practice to incorporate in such tubes a photocathode on to which the light images impinge and which in consequence produces electrons at its output side, an extraction electrode, preferably in the form of a mesh, on the output side of the photo-cathode and close to the photocathode, and various focussing and/or deflection electrodes spaced along the length of the image tube beyond the extraction electrode and which serve to produce an image on a phosphor screen or on some equivalent receiving surface at the end of the tube remote from the photocathode.

Almost all of the image tubes currently used in ultrafast cameras, in both streak and frame operation, use electron-optical imaging lens system adapted from image converter and image intensifier tubes. These have generally been designed to have high spatial resolution and a high dynamic range, as is required in such instruments. In the design of an ultrafast camera tube however the temporal response of the tube should be of at least equal importance. Indeed, if one isto achieve temporal resolution in the femtosecond region, the temporal dispersion of the imaging system is of paramount importance in its design.

In all such streak image tubes it is desirable to be able to achieve high spatial resolution coupled with low temporal dispersion. Particular problems arise if one is seeking to obtain a streak image from an object which is off the central axis of the tube.

It is an object of the present invention to provide an image tube in which one can achieve high spatial resolution and low temporal dispersion in streak and frame images of extended object sources.

It is another object of the invention to provide an image tube designed with high temporal resolution as its primary purpose. The tube is intended for use in either streak or frame operation or in a synchroscansystem. A temporal dispersion through the electron-optics, excluding the deflection system, of less than 200 femtoseconds can be achieved, and the spatial (geometric) aberrations are significantly lower than in other current image tubes for object heights on the photocathode of up to Smm. In addition the magnification of the tube may be 'zoomed' over a small range without significant loss of performance.

It is another object of the present invention to provide an image tube in which (a) temporal dispersion is as low as possible; (b) spatial resolution of the electron-optics is as high as possible; (c) magnification is as low as possible and vari able over a small range without significant loss of performance; and (d) the electrode structure is simple to manufac ture and align and requires voltages of not more than 18kV.

In accordance with the present invention there is provided an electron-optical image tube comprising a photocathode, an electron extraction region immediately adjacent to the output side of the photocathode, an electron focussing region beyond the extraction region having at least three axially spaced focussing electrodes and an anode electrode, and an electron deflection region beyond the focussing region.

The number and disposition of electrodes in the focussing region and the voltages applied to these electrodes can be arranged such that the image tube has a temporal dispersion of less than 250 fs over an extended image, and a spatial resolution of better than 50 line pairs /mm apart from the limits set by the phosphor grain size when one provides a phosphor electron-receiving surface beyond the deflection region.

In a preferred embodiment of the tube, the focussing region comprises three focussing electrodes, preferably cylindrical, plus an anode spaced along the length of the focussing region. The first focussing electrode operates at a predetermined potential which is lower than the extraction electrode potential, and the second focussing electrode at a potential equal to or higher than the extraction electrode potential. The anode, which in fact constitutes a fourth focussing electrode, operates at a high potential which is greater than or equal to the extraction potential and which determines the potential of the electron deflection region. The third focussing electrode is adjustably set at the potential required to achieve a focussed image on the electron-receiving surface.

Although the use of three focussing electrodes plus an anode is preferred, four focussing electrodes plus an anode could be used. Afurther relatively small increase in time-resolution is thereby achieved.

The length of the deflection region and the potential ofthe anode are less critical parameters. However, the deflection sensitivity of the image tube is determined by the ratio of the anode voltage to the length ofthe deflection region and in one preferred arrangement this ratio has a value of 1:15 where the voltage is expressed in kilovolts and the length is expressed in millimetres.

In a preferred embodiment of the tube, the three focussing electrodes are of equal length and are spaced at equal intervals, not necessarily equal to the electrode lengths, within the focussing region. In this embodiment, with a potential on the extraction electrode and the anode of 1 OkV for example, the potential on the first focussing electrode is preferably about 6kV and the potential on the second focussing electrode is preferably between 10kV and 18kV. The extraction electrode is positioned between 1 mm and 5mm, preferably 2mm, from the photocathode. The length of the deflection region is pref erably of the order of 150m m.

In order that the invention may be fully understood, a more detailed description will now be given of one presently preferred embodiment of image tube, together with various results based on the operation of an image tube according to this design.

In the accompanying drawings Figure 1 is a schematic representation of the image tube, and Figures 2 to 16 are various graphical representations illustrating the effect of varying various parameters of the image tube.

Referring first to Fig. 1, this shows a schematic representation of an electron-optical image tube comprising a photocathode 10; an extraction electrode 12, which is preferably a plane mesh, generating an extraction field; a series ofthree cylindrical focussing electrodes 14a, 14b, 14c which are all of the same length a and internal diameter a and which are spaced atthe same distance apartg; a cylindrical anode 16 which is also of length a and internal diameter a and which is spaced at a distanceg from the third focussing electrode 14c; and a phosphor screen 18 which has a transverse dimensions a equal to the length of the individual electrodes.The distance between the photocathode 10 and the extraction electrode 12 is denoted bys; the first focussing electrode 14a is positioned at a distanceg from the extraction electrode 12; and the distance between the anode 16 and the phosphor screen 18 is denoted byd. The anode 16 includes a diaphragm 17 containing an aperture to minimise interference between the focussing and deflection fields.

In Fig. 1 and in the following description the photocathode 10 is at OV., the voltage applied to the extraction electrode 12 is denoted by Vrn, the voltage applied to the three focussing electrodes 14a, 14b, 14c are denoted by V1, V2 and V3 respectively, and the voltage applied to the anode 16 is denoted by Va.

The deflection region is assumed to be field free and at the anode potential Va. Three focussing electrodes 14a, 14b and 14c are required to allow two dimensions of optimisation of the voltage configuration for a fixed image plane. This is necessary to achieve zoom without loss of performance.

It is well known that the temporal dispersion in the photocathode/mesh region is minimised by maximising the field strength in this region. Studies of the build up of temporal dispersion in the focussing regions of other image tubes have shown that the temporal dispersion increases most rapidly in regions where the electrons are travelling slowly, i.e.

regions with a low axial potential. Temporal dispersion in the focussing region is therefore minimised in the tube of the present invention by maximising the mean axial potential in the tube. The mesh voltage Vm is therefore set at a high value, e.g. Vm = 10kV. Assuming a uniform field between the photocathode 10 and the mesh 12 the temporal dispersion At is approximately given by At - Aus 17vim where 11 is the charge/mass ratio of the electron, s is the photocathode/mesh spacing, and Au is the spread of electron emission velocities. A value s=2MM is preferred which gives At= 1 10fas for an emission energy distribution of mean and half width 0.1eV.

The use of a high mesh voltage Vm and relatively large spacing to achieve the high extraction field has a secondary advantage. The mesh 12 acts to reduce the angular aperture of the cone of electron trajectories entering the focussing region, acting effectively as an aperture stop in the entrance pupil of the lens. The angular aperture of the cone of trajectories entering the lens is determined by the mesh voltage, not the extraction field strength, so that by using a high value for vim the aperture angle, and hence the spatial aberrations of the focussing electron-optice, are reduced.

The deflection sensitivity of the image tube is largely determined by the electron velocities in the deflection region, i.e. by Va. For maximum sensitivity, Va should therefore be minimised. However, to achieve gain at the phosphor screen 18 the electron energies must be at least 10keV and a value of Va=10kV is preferred. Avalue of d=150mm is preferred with the aforesaid voltage.

The figures for resolution in thex, y and t directions (Ax, Ay and At) are derived from the standard deviations of the distribution of the electrons in a given given image plane near the paraxial focal plane, although this is not necessarily the plane which gives the best overall resolution. The magnification figures are derived from the ratio of image height to object height.

The image tube as shown in Fig. 1 can be considered to comprise three sepate regions, namely an extraction region from the photocathode 10 to the extraction electrode 12, a focussing region from the extraction electrode 12 up to and including the anode 16, and a deflection region from the end of the anode 16 (denoted by the diaphragm 17) to the screen 18. Changes in the parameters in these three regions affect the operation of the image tube in different ways, as will be described hereinafter.

The only voltage configuration which gives a focus in the image plane with electrode voltages below 18kV and a high mean axial potential is as follows, V2 > 1 OkV > V1,V3 Voltage V3 is used to achieve the focus condition for pairs of values V1, V2. In other words, the voltage V3 applied to the third focussing electrode 14c is adjusted to a value such as to give the required paraxial focal position.

The off-axis spatial resolution of the electron optics is largely determined by the astigmatism and field curvature present. Therefore, the defocus of the image of a point object 4mm off-axis, Af4, is used hereinafter as a merit figure with which to compare the spatial performances of various voltage config urations. Similarly, the temporal dispersions at the image plane for a point object 4mm off axis, At4, is used as a merit figure of the temporal performance.

In a first series of tests on the image tube, in order to investigate the focussing region, the following parameters were set to constant values for this series of tests: Vm= 10kV,Va= 10kV,s= 2mm,d= 150mm,a= 52mm,g= 10mm,f= 1 wheref is a scaling factor applied to dimensionsa andg.

Figure 2 shows At4 with V2=18kVandV,=5.6to 6.4kV. at V,=6kV, At4 is a minimum and is in fact approximately equal to the temporal dispersion for an axial object point, 169fas. In this configuration the temporal dispersion is therefore independent of object height, at least up to 4mm on the photocathode. For this range of V1, the spatial resolution is well above 501p/mm over the whole extended image. Figure 3 shows Af4 for the same set of config urations. At V1 -6kV the spatial aberrations are very near a minimum. The configuration for which Qt4 is a minimum will be referred to as the optimum config uration and At4 and Af4 are defined as the temporal and spatial merit figures respectively in the configuration.

Optimum configurations are also found for lower values ofV2 (i.e. to 18 kV) and V,~6kV. The value of At4 rises as V2 is decreased as shown in Figure 4, whilst the value of Af4 actually decreases with V2 as shown in Figure 5. Figure 6 shows the magnification M in the optimum configuration as a function of V2.

Comparison of these three curves shows that the magnification can be varied over a range from 1.52 to 1.88 with no significant loss of performance. The linear variation indicates that the image tube is capable of "zoom" over a limited magnification range without loss of performance, as the spatial resolution is well in excess of 50 Ip/mm. Consequently, in use of the image tube by applying an increasing voltage to the focussing electrode 14b, either smoothly or in steps, one can achieve increasingly magnified image, the change in magnification being linear.

This is an important aspect of one method of using this tube.

Figure 7 shows the total temporal dispersion in the optimum configuration with V2=18kV for an axial object point as a function of the photocathode/mesh spacing overthe ranges = 1 mum to 5mm. For large values of s the dispersion in the photocathode/mesh region dominates the total dispersion which therefore rises linearly with s. For small values of s the dispersion in the mesh/phosphor region is dominant and no improvement in the total temporal dispersion can be gained by further reducing s. The foot of the linear section of the curve is the optimum design value, in this case s=2mm.

Figure 8 shows the variation of At4, ssf4 and M with changes in the anode/phosphor distance d. Although a small improvement in the temporal dispersion can be gained by increasing d, with only small deterioration of the spatial performance, the magnification increases markedly with an increase in d. conversely, by reducing dthe magnification can be reduced with little effect on the temporal and spatial performance. A reduction in d however worsens the deflection sensitivity. Figure 9 shows the effect of changing d and Va together so that the deflection sensitivity determined by Va/d is constant. The curvesforAt4 and Af4 are simiiarto Figure8 butthe rate of increase of M with d is far less.Hence the values of d and Va may be varied over a wide range with a trade-off of deflection sensitivity against magnification without significantly altering the temporal and spatial performance of the tube. Va= 1 OkV and d4150mm are preferred in the optimum configuration as values which give a practical compromise.

Various geometries of the focussing region with different length electrodes were also investigated.

No significant improvement in any performance parameter could be gained by altering any or all of the electrode lengths. In most cases the performance, in particular the spatial aberrations and magnification, deteriorated markedly with electrodes of different length. The geometry of Figure 1 is considered to be the optimum configuration.

The effect of scaling the dimensions of the electrodes in the mesh/anode region by a constant factor k has also been considered. Figure 10 shows At4, Af4 and M for values of k=0.5 to 1. As would be expected At4 falls as the focussing region is shortened. The value of Af4 rises rapidly however since the object height on the cathode, i.e. 4mm, is a greater distance from the axis when normalised to the electrode diameter, i.e. for k=0.5 an absolute object height of 4mm on the photocathode is equivalent to a height of 8mum for k=1. Although the image of the slit on the photocathode can be reduced in proportion to k by the relay optics the absolute size of the image on the phosphor would also be reduced.Since the spatial resolution of the phosphor will be the dominant factor in the overall spatial resolution of the tube independent of the value of k, a smaller image would cover fewer resolution elements on the phosphor and the information content of the image would therefore be reduced. The magnification ofthetube also increases ask decreases since the anode/phosphor distance d is constant (see Figure 8). The magnification can be reduced in a trade-off against deflection sensitivity by reducing d as referred to earlier. A more practical trade-off is achieved however by zooming the magnification with the voltage V2. Figure 11 shows At4, & 4 and the magnification M as a function of V2 for k=0.5. Although still high at 2.45, the magnification for V2=lOkV is not unusable.

In particular, such a configuration can be achieved in a tube with an overall length of about 276mm.; this should be compared with conventional tubes which have an overall length of about 320mm. A marked improvement in the spatial performance of the tube is seen with a decrease in V2. The reduction of the temporal dispersion in this configuration to At4 1 50fs indicates that even higher temporal resolution could be achieved, if a high magnification and a reduced object size could be tolerated, by further reducing k.

Figures 12 to 16 show the effect of varying some of the parameters of the basic configuration described above. Four tubes A, B, C and D are considered, having the following values: A:V2=18kV a=52mm g=10mm B: V2=i0kV a=52mm g=10mm C:V2=18kV a=26mm g= 5mm D: V2=10kV a=26mm g= 5mm In all these tubes Vm=Va=10kV s=2mm, d=150mm and V, and V3 are adjusted to give the optimum configuration, i.e. minimum temporal dispersion.

Figure 12 shows the field curvature and astigmatism of the four tubes A, B, C and D for object heights of O to 5mm. Figure 13 shows the image spot diame ter at the position of best focus (on the curved focal surface not the phosphor screen 18) for object height of O to 5mm. The diameter of the image spot on the plane surface defined by the phosphor screen 18 will depend on these factors and on the angular aperture of the ensemble of electron trajectories forming the image. The aperature stop action of the mesh electrode mentioned above and the relatively long image distance of these tubes ensures that the angu lar aperture of the electron beam in these tubes is small.This fact, considered with Figures 12 and 13, indicates that the image spot diameters on the phosphor screen of all these tubes is at least comparable with and in general significantly smaller than in existing image tubes.

In fact, the dynamic spatial resolution of a practical image tube will be limited principally by the dynamic spatial aberrations ofthe deflection system and by the spatial resolution of the phosphor screen and image intensifier system used to a much lower value than that implied by the image spot diameter for all four tubes.

Figure 14 shows the spatial (geometric) distortion of images formed by the four tubes. Distortion of 1.1% or less for object heights of up to 5mm is neg ligible in this type of tube.

Figure 15 shows the principal advantage of the image tube of the present invention, namely the temporal dispersion as a function of object height.

The flat-field temporal response of the tubes A, B, C, D of 200fs dispersion or less is in marked contrast to that of a well-known conventional tube, known as the "Photocron II", which falls from 560fas on axis to 1.95ps at 5mm off-axis, as indicated by the line Pull.

The temporal resolution of an image tube is determined by this temporal dispersion and by the dynamic temporal aberrations of the deflection system used. The temporal distortion ofthefourtubes A, B, C and D is shown in Figure 16.

With the image tube of the present invention, temporal resolution can be maintained at off-axis points of the slit, and with the improved spatial resolution more information can be recorded along the slit length, e.g. simultaneous temporal and spectral information could be obtained by dispersing a spectrum along the slit length, and the time behaviour of the spectral components would be time resolved by streaking the slit image. This advantage of higher spatial resolution for off-axis image points is, of course, available at slower streaking speeds when the ultimate in time-resolution is not required.

Thus, although the new tube design has consider ably improved time-resolution, it is also to be prefer red for use at slower streaking speeds because of the improved spatial resolution off-axis, giving more resolution points for a given length of streaking slit.

Again, because of the better off-axis spatial resolution, this tube could be used for a framing camera. In one mode of operation the streaked image would be gated on and offfora short period sothat during this time the streaked image was not blurred, e.g. using a fast laser-triggered switch the extraction mesh voltage could be switched on and off in less than 100 picoseconds. The simultaneous streaking voltage would be arranged so that in 100 picoseconds or less the image was deflected through less than one spatial resolution element.

In an extension of the use of the image tube, the object under investigation could be dissected into separate elements by the use of a mesh through which the object is imaged on to the photocathode 10, or by employing a dissected photocathode constructed as an array of point photocathodes. In this mannerthe complete image could be streaked at an angle to allow later processing of the streaked image to provide framing information. The present tube would permit the application of this concept for time-resolution of - 50 ps or less in framing mode, because ofthe high spatial resolution available offaxis.

Claims (30)

1. An electron-optical image tube comprising a photocathode, an electron extraction region immediately adjacent to the output side of the photocathode, an electron focussing region beyond the extraction region having at least three axially spaced focussing electrodes and an anode electrode, and an electron deflection region beyond the focussing region.

2. A tube as claimed in claim 1, in which the focussing electrodes are each cylindrical.

3. A tube as claimed in claim 1 or 2, in which the anode electrode is cylindrical.

4. Atube as claimed in any preceding cliam, in which the focussing electrodes are each of the same length.

5. A tube as claimed in any preceding claim, in which the focussing electrodes are axially equispaced from each other.

6. A tube as claimed in claim 5, in which the first focussing electrode beyond the extraction region is axially spaced from an extraction electrode in the extraction region by a distance equal to the spacing between the focussing electrodes.

7. A tube as claimed in claim 4, or cliam 5 or 6 when dependent on claim 4, in which the length of the anode electrode is equal to the length of each focussing electrode.

8. Atube as claimed in claim 7, in which the axial spacing between the final focussing electrode and the anode electrode is equal to the axial spacing between the individual focussing electrodes.

9. Atube as claimed in any preceding claim, in which there are three focussing electrodes each having an axial length of the order of 52mm with an axial spacing between said focussing electrodes of the order of 10mm.

10. Atube as claimed in any of claims 1 to 8, in which there are three focussing electrodes each having an axial length of the order of 26mm with an axial spacing between said focussing electrodes of the order of 5mm.

11. Atube as claimed in any preceding claim, in which the focussing electrodes are cylindrical and the axial length of each focussing electrode is equal to its diameter.

12. Atube as claimed in any preceding claim, in which the anode electrode is cylindrical and its axial length is equal to its diameter.

13. Atube as claimed in any preceding claim, in which the electron extraction region includes an extraction mesh spaced from and parallel to the photocathode.

14. Atube as claimed in claim 13, in which the photocathode-mesh spacing is from 1 mm to 5mm.

15. Atube as claimed in claim 14, in which the photocathode-mesh spacing is 2mm.

16. Atube as claimed in any preceding claim, in which the deflection region is separated from the focussing region by an apertured diaphragm extending transversely across the tube at the end of the anode electrode remote from the photocathode.

17. Atube as claimed in any preceding claim, in which the axial length ofthe deflection region is of the order of 150mm.

18. Atube as claimed in any preceding claim, in which the axial length ofthe deflection region in millimetres is of the order of 15 times the voltage in kilovolts at which the anode electrode is arranged to operate.

19. A method of operating an electron-optical image tube as claimed in any preceding claim, having a mesh as an extraction electrode, which comprises connecting the mesh to a voltage Vm, connecting the three focussing electrodes to voltages V1, V2 and V3 respectively in order away from the mesh, and connecting the anode electrode to a voltage Va, and arranging the voltages such that V2 > 1 OkV > V, and V3

20. A method as claimed in claim 19, in which Vm is of the order of 1 OkV.

21. A method as claimed in claim 19 or 20, in which Va is of the order of 10kV.

22. A method as claimed in any of claims 19 to 21, in which V2 is ofthe order of 18kV.

23. A method as claimed in any of claims 19 to 22, in which V, is of the order of 6kV.

24. A method as claimed in any of claims 19 to 23, in which the voltage applied to the focussing electrode most remote from the mesh is adjustable to give minimum temporal dispersion.

25. A method as claimed in claim 24, in which the voltage applied to the focussing electrode nearest to the mesh is also adjustable to give minimum temporal dispersion.

26. A method as claimed in any of claims 19 to 25, in which the voltage applied to at least one of the focussing electrodes is increased, smoothly or in steps, to achieve a linearly increasingly magnified image.

27. An electron-optical image tube substantially as hereinbefore described with reference to the accompanying drawings.

28. A method of operating an electron-optical image tube substantially as hereinbefore described with reference to the accompanying drawings.

29. A high-speed camera incorporating an image tube in accordance with any of claims 1 to 18 or 27.

30. A high-speed camera as claimed in claim 29 wherein the image tube is operated in accordance with the method as claimed in any of claims 19 to 26 or 28.