JPH0418418B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image pickup tube electron gun, and more particularly to an electrode configuration of an electron gun that suppresses expansion of the velocity distribution of a group of electrons forming an electron beam.

In a vidicon type image pickup tube, a charge pattern corresponding to the illuminance of the subject is generated on a photoconductive film, and the charge pattern is sequentially discharged by scanning the photoconductive film with an electron beam generated by an electron gun. The charging current corresponding to the current is taken out as a signal. The charge generated once by the object is usually not fully discharged in one beam scan, so even if the object disappears, a false signal corresponding to the residual charge will be generated in subsequent scans. This occurs as an afterimage and degrades the image quality of moving subjects.

In particular, in image pickup tubes using a blocking photoconductive film, the main problem is an afterimage with a time constant determined by the product of the capacitance of the photoconductive film and the beam resistance of the scanning electron beam. This is called capacitive afterimage. Beam resistance is equivalent to the velocity distribution of the electron group forming the electron beam, and for an electron beam to achieve low afterimages, it is necessary that the velocity distribution of the electron group be narrow.

The electron group emitted from the cathode has a Maxwellian velocity distribution, but in the process of forming a narrow beam, the current density of the beam increases,
It is known that the velocity distribution is expanded by the energy relaxation phenomenon given to the Coulomb force between electrons. This phenomenon is called the Beige effect, and the rate of expansion of the velocity distribution is determined by the axial current density J(z) and the axial potential V.
(z) is known to be approximately proportional to J(z) ^1/3 /V(z) ^1/2 .

In an electron gun that aims for such low image retention, it is necessary to suppress the increase in beam current density as much as possible, and a two-pole electron gun has been proposed in which the first grating facing the cathode is operated at a positive potential with respect to the cathode. There is.

An ideal low-afterimage electron gun would have a so-called laminar beam, with electrons emitted from the cathode parallel to the axis and without the formation of high current density crossovers. However, conventional bipolar electron guns have a cross-over type configuration due to the need to widen the dynamic range of the beam current so as not to cause insufficient beam even for high-illuminance objects. Since the over point was formed at a position near the first grid where the axial potential was low, the expansion of the velocity distribution width was not sufficiently suppressed.

The present invention aims to improve the conventional two-pole electron gun and provide an electron gun that can generate a larger beam current with less afterimage and a lower cathode load (cathode emission current density). do.

In the bipolar electron gun according to the present invention, a diverging electron lens is formed in the vicinity of the minute aperture of the first grating, which is placed opposite to the cathode and to which a positive voltage is applied to the cathode. By making the electron beam that has passed through the first lattice aperture diverge once and forming a crossover point at a position with a high axial potential away from the first lattice aperture, the expansion of the velocity distribution width of the electron group can be suppressed to a smaller extent. The basic principle is to simultaneously increase the beam current passing through the minute aperture provided in the second grating.

Hereinafter, the present invention will be explained in detail with reference to Examples.

Figure 1 shows the schematic configuration of a vidicon type image pickup tube.
1 is the cathode, 2 is the heater, 3 is the first grid, 4 is the second
A grating, 5 is a third grating, 6 is a fourth grating having a mesh-like electrode, 7 is a photoconductive film target, 8 is a focusing coil, and 9 is a deflection coil. It is composed of a first lattice 3 and a second lattice 4. The target 7 is formed into a thin shape by an electron gun, and a magnetic field lens generated by a focusing coil 8
An image is formed on the image and scanned by the magnetic field generated by the deflection coil 9. Although an electromagnetic focusing/electromagnetic deflection type image pickup tube is shown here as an example, the present invention relates to the electron gun section from the cathode 1 to the second grating 4, and is not concerned with the subsequent beam focusing and deflection methods. It can be applied to any type of image pickup tube.

FIG. 2 is an enlarged sectional view showing the main parts of a conventional two-pole electron gun, in which 1 is a cathode surface, 3 is a first grating, 13
1 is a minute opening provided in the first lattice, 4 is a second lattice, 14 is a minute opening provided in the second lattice 4,
0 indicates the electron trajectory emitted from the center of the cathode surface 1.

The electron beam passing through the first grating aperture 13 is focused immediately after passing through the aperture, and forms a crossover 15 at a position close to the first grating 3.

FIGS. 3 and 4 are enlarged sectional views showing essential parts of the bipolar electron gun according to the present invention, respectively. In the embodiment shown in FIG.
The thickness of the recess formed at the center of the flat electrode 31 is increased, and the depth _{l 1 of} the recess is set to d ₁ /
Make it 2 or more. By deepening the depression, the second
The effect of shielding the electric field generated by the grating 4 is generated, and a diverging lens is formed near the first grating aperture 13, so that the electron beam 12 passing through the aperture 13 diverges once and crosses at a point away from the first grating aperture 13. Form over 15.

In the embodiment shown in FIG.
An intermediate grid 20 having an opening is provided between the grids 4,
A voltage equal to or lower than that of the first grid 3 is applied to the intermediate grid 20 so that the first grid opening 1
A diverging lens is formed in the vicinity of 3. In particular, it is preferable to set the potential of the intermediate grid 20 to be the same as that of the cathode 1 without increasing the number of stem leads.

Figures 5 and 6 show the axial potential V(z) and axial current density J(z) with respect to the axial distance z from the cathode for the conventional example shown in Fig. 2 and the implementation example shown in Fig. 4. Examples are shown below. Compared to the characteristics of the conventional example shown in FIG. 5, the characteristics of the embodiment shown in FIG. J(z)
The peak value (corresponding to the crossover point) (shown by the dotted line) is formed at a position farther from the cathode where the axial potential is higher, and the expansion of the velocity distribution width of the electron group is more strongly suppressed.

FIG. 7 shows the beam current i _B passing through the second grating aperture 14 and the current velocity (cathode load) ρ _C at the cathode center point with respect to the voltage E _C1 of the first grating 3. The conventional example shown in FIG. 4 and the embodiment shown in FIG. 4 are shown respectively.

Here, two specific examples will be shown regarding the electrode dimensions of the embodiment shown in FIG. In Specific Example 1, the diameter of the second grating opening 14 is 30 μm, and in Specific Example 2, it is 20 μm, and the other dimensions are exactly the same. The plate thickness of the first grating 3, the diameter of the first grating opening 13, and the gap between the cathode 1 and the first grating 3 are the same as in the conventional example shown in FIG. 2, and are each 0.18 mm.
200μm, 0.15mm. The plate thickness of the intermediate grating 20, the plate thickness of the second grating 4, the gap between the first grating 3 and the intermediate grating 20, and the gap between the intermediate grating 20 and the second grating 4 are each 0.25
mm, 1.0mm, 0.2mm, 0.2mm. The diameter of the opening of the intermediate grating 20 and the inner diameter of the recesses of the first and second gratings 3 and 4 are all 0.65 mm.

The voltage applied to each electrode is 0V for the cathode 1, 300V for the second grid 4, 0V for the intermediate grid 20, which is the same as the cathode,
The first grating 3 has a voltage of several volts to several tens of volts, as shown in FIG.

In FIG. 7, curves 71, 7 shown as solid lines
2 and 73 indicate the beam currents i _B of specific example 1 and specific example 2 according to the present invention and the conventional example, respectively, and curves 74 and 75 shown by dotted lines indicate the electron guns of the present invention (concrete example 1 and specific example 2), respectively. and the cathode load ρ _C of the conventional example. Regarding FIG. 7, when comparing the characteristics of the electron gun according to the present invention with the conventional example, curves 71 and 7 are shown.
2 and curve 73, the electron gun of the present invention has a beam current i _B value larger than that of the conventional example for the same E _C1 , and moreover, the rise of i _B with respect to E _C1 is steeper than that of the conventional example. . This shows that the electron gun of the present invention having a diverging lens system has sharper beam focusing. On the other hand, when comparing the cathode load ρ _C (see curves 74 and 75), the electron gun of the present invention has a lower value for the same E _C1 ,
This almost matches the theoretical value given by the Child-Langmuir equation, and the relationship is ρ _C ∝E _C1 ^3/2 .
This is because, in the electron gun of the present invention, potential changes are gradual in the vicinity of the first grid opening, and the effect of the electric field formed by the potential of the second grid is shielded. On the other hand, in the conventional example, the potential change near the first lattice opening is large, and a stronger electric field acts on the cathode center due to the effect of the electric field formed by the second lattice potential, increasing the load. This effect is particularly noticeable when E _C1 is small, resulting in a large deviation from the theoretical value.

In a bipolar electron gun that applies a positive potential to the first grid, the life span and reliability of the cathode are the most important points. A beam current can be generated, which is extremely advantageous in terms of the lifetime reliability mentioned above.

Furthermore, as is clear from the above specific examples 1 and 2, in the electron gun of the present invention, even if the diameter of the second grating aperture 14 is made smaller, the decrease in beam current _iB is small. This makes it possible to use this method, and it is also advantageous for improving the resolution of the image pickup tube.

As explained above, according to the present invention, the cross-over point can be formed at a position with a high axial potential to further suppress the expansion of the velocity distribution of electron groups, and a large beam current can be generated with a lower cathode load. This makes it possible to realize an extremely advantageous electron gun in terms of lifetime reliability, improved image pickup tube resolution, and reduced afterimages.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a schematic configuration of an image pickup tube, FIG. 2 is an enlarged sectional view showing the main parts of a conventional dipole electron gun, and FIGS. 3 and 4 are a diagram showing a dipole electron gun according to the present invention. Enlarged sectional views showing the main parts of the gun, Figures 5 and 6,
and FIG. 7 are diagrams showing a comparison of beam characteristics of the present invention and a conventional example. DESCRIPTION OF SYMBOLS 1... Cathode, 2... Heater, 3... First grating, 4... Second grating, 5... Third grating, 6... Fourth grating, 7... Photoconductive film, 8... Focusing coil, 9... Deflection coil, 10,
DESCRIPTION OF SYMBOLS 11, 12... Electron beam, 13... First grating micro-aperture, 14... Second grating micro-aperture, 15... Crossover, 20... Intermediate grating.

Claims (1)

[Scope of Claims] 1. At least a first grating having a cathode that emits electrons, a first micro-aperture disposed in its rear stage and to which a positive voltage is applied to the cathode, and a a second grating having a second minute aperture to which a positive voltage is applied to one grating, and a diverging electron lens in the vicinity of the minute aperture of the first grating between the first grating and the second grating. is formed, and the first grating and the second grating are formed by the diverging electron lens.
An image pickup tube electron gun characterized in that it is configured to generate a crossover point between the grating and in the vicinity of the second grating. 2. The first grating includes a flat plate electrode having a recess centered on the first minute opening, the depth of the recess is 1/2 or more of its inner diameter, and the recess allows the second grating to be connected to the second grating. 2. The image pickup tube electron gun according to claim 1, wherein the diverging electron lens is formed by shielding an electric field generated by the electron beam. 3. The image pickup tube electron gun includes an intermediate grating disposed between the first grating and the second grating, the intermediate grating having an aperture to which a voltage equal to or lower than the voltage applied to the first grating is applied; 2. The image pickup tube electron gun according to claim 1, wherein said diverging electron lens is formed by an intermediate grating. 4. The image pickup tube electron gun according to claim 3, wherein the voltage applied to the intermediate grid is equal to the voltage applied to the cathode.