Channel Plate Electron Multiplier Tube Having Reduced Astigmatism

The invention relates generally to electron discharge tubes and particularly to tubes of the type wherein an electron image is intensified by passage through a channel plate electron multiplier.

Electron image intensifying tubes such as image tubes may include a channel plate electron multiplier. Such tubes are described in detail in, for instance, U.S. Pat. Nos. 3,260,876 and 3,487,258 both to B. W. Manley et al and U.S. Pat. No. 3,497,759 to B. W. Manley. A proximity focussed image tube generally includes a flat photocathode and a flat phosphor screen facing one another in an evacuated flat glass envelope. Spaced between the photocathode and the phosphor screen is a thin, flat channel plate whose faces are parallel to, and closely spaced from, the photocathode and phosphor screen. The channel plate has a large number of small, round, parallel channels extending from one face to the other. The channels are slanted at a bias angle of about 5.degree. with respect to the input and output faces of the plate. Both input and output faces are provided with a conducting electrode such as a coating of chromium. The inside surfaces of the channels are activated with hydrogen to increase secondary emission.

In operation of the tube, appropriate accelerating voltages are applied to the photocathode, the input electrode of the plate, the output electrode of the plate, and the phosphor screen, so that electrons from the photocathode strike the inside walls of the channels, are multiplied, while travelling through the channels exit at the output face of the plate, and strike the phosphor screen to produce a visible output image.

Increased spacing of the output screen from the output electrode of the plate permits a higher accelerating voltage to be applied between the output electrode and screen, and thus further increases the intensity of the output image without increasing risks of charging of the tube walls or causing high field breakdown. However, such increased spacing results in decreased resolution since the beam of electrons from individual channels rapidly spreads as it leaves the output face of the plate. Beyond a short distance, on the order of mils, beams from adjacent channels overlap. Such decreased resolution has, nevertheless, been avoided by endspoiling of secondary emission near the channel end at the output face to effectively decrease the output aperture of the channels. In previous devices, endspoiling is provided by extending the output electrode metallizing a uniform short distance into the end channel so that no multiplication can occur near the output face. The endspoiling in effect collimates each of the output beams from the channels.

The detection efficiency of the input image can also be increased by increasing the bias angle of the channels. Increase in bias angle also results in increased multiplication since there are then a larger number of electron impacts near the input of the channel. However, slant of the channels with respect to the output phosphor screen, results in astigmatism. That is, the output beam from each channel strikes an elongated spot on the output screen rather than a round spot, much as a round light beam impinging on a surface at an angle illuminates an elongated spot. The astigmatism decreases resolution with increasing bias angles of the channels and with increased spacing of the output screen from the channel plate.

The novel device comprises a channel plate electron multiplier with slanted endspoiling on the output face of the channel plate. In the novel device, slanted endspoiling is provided by extending the output electrode a non-uniform distance, shorter on one side and longer on the other, into the end of each channel. The end of each annular endspoiling segment lies substantially in one of a set of parallel planes which are slanted with respect to the output face in the same direction as, and at least to the same degree of magnitude as the channels are slanted with respect to the output face.

The novel device has substantially reduced astigmatism due to channel bias. Therefore, a relatively large bias angle plate may be used together with a relatively large spacing between the output electrode and the output screen. As a result, intensification is increased without substantial loss of resolution due to astigmatism.

FIG. 1 is a sectional view of an image tube according to the preferred embodiment of the invention.

FIG. 2 is an exaggerated sectional view of a fragment of a channel plate multiplier of the tube of FIG. 1.

The preferred embodiment of the invention is a proximity focussed image tube 10 shown in FIG. 1 of the drawings. The tube 10 includes an evacuated glass envelope 12 with two closely spaced parallel faceplates 14, 16. One of the faceplates, the input faceplate 14, has a photocathode 18 deposited on the inside surface. The other faceplate, the output faceplate 16, has an output screen 20 deposited on the inside surface. The output screen 20 is a layer of phosphor 22 covered with a thin aluminum layer 24, which prevents back emission of light and ion damage to the phosphor layer 22. Spaced between the photocathode 18 and the output screen 20 is a channel plate electron multiplier 26 having an input face 28 toward the photocathode 18 and an output face 30 toward the output screen. The output face 30 is spaced at a distance of about 40 mils from the output screen 20.

A section of a fragment of the channel plate 26 is shown in FIG. 2. Referring now to FIG. 2, the supporting structure of the channel plate 26 is a thin disc of lead glass 32 about 1 millimeter thick and about 1 inch in diameter. A large number of channels extend through the disc. The channels 34 are about 15 microns in diameter and are spaced from one another by a distance of about 1 mil. The channels are parallel to one another and are oriented at a bias angle .phi. of about 5.degree. with respect to the faces 28, 30 of the plate 26, as shown in the drawing of the FIG. 2 by dashed lines 35. The input face 28 is covered with an input electrode 36 which is a thin layer of chromium and extends a short distance into the channels. The output face 30 is also covered with an output electrode 38, also a layer of chromium. The output electrode 38, includes also annular endspoiling segments 40 which extend an average distance of about three channel diameters into each channel. The endspoiling segment ends 42 are slanted with respect to the output face 30 in the same direction as the channels are slanted with respect to the faces 14, 16. The slant angle .theta. of the endspoiling ends 42 with respect to the faces 14, 16 is shown in the FIG. 2 by dashed lines. The layer thickness of the output electrode 30 is on the order of several microns.

For operation of the tube 10, voltages of 0, +500, +1000, and +6000, are applied to the leads 46, 48, 50, 52 respectively, which connect to the photocathode 18, the input electrode 36, the output electrode 38, and the output screen 20 respectively.

The input and output electrodes 36, 38 may be deposited by off-axis evaporation from beads of a nickel-chrome alloy, or by rotation of the plate during the evaporation. To provide the slant to the endspoiling ends 42, the axis of rotation of the evaporation source is the axis of the plate itself rather than the axis of the channels. Therefore, the plate should not be tilted with respect to the axis of rotation for the evaporation to compensate for the bias. It may, however, be tilted in the opposite direction to increase further the angle .theta. to an angle greater than the bias angle .phi. of the channels with respect to the plate faces 28, 30.

The effect of the slanted endspoiling is that the angular distribution of the emerging electrons is primarily in a direction perpendicular to the output face, thereby resulting in a round, rather than an oval, spot on the output screen. It may be seen from the solid projection lines 54 in FIG. 2 that the aperture of the channels is asymmetrically limited by the slanted endspoiling segments extending into the channels so that the astigmatism due to the channel bias angle is compensated for. The dashed line 56 illustrates divergence of electrons due to astigmatism which would be present if the endspoiling were not slanted.

The dimensions of the channels and of the output electrode are too small to permit determination as to their precise relative angular orientations. However, it is believed that the endspoiling ends inside the channels lie approximately in a set of parallel planes which are at an angle .theta. with respect to the output face of the channel plate, as shown in the FIG. 2. For the preferred embodiment, this angle .theta. was found to be 42.degree.. However, smaller angles also reduce astigmatism. For plates with a greater channel bias, the optimum slant angle .theta. may be somewhat greater. In any case, the slant angle .theta. should be at least as great in magnitude as is the bias angle .phi. of the channels.

Although in the preferred embodiment the tube is a proximity focussed tube, it is to be understood that the slanted endspoiling at the output face of a channel plate is applicable to any tube in which the resolution of the electron image from the output face of a channel plate is important. Other types of image tubes, such as magnetically focussed and electrostatic lens focussed tubes, as well as the various intensifier type camera tubes are in this category.