Streak Camera Tube

The present invention relates to electron optical image tubes for use in providing direct measurement of luminous events having durations as short as one picosecond or less and with a time resolution in the picosecond range. More particularly the present invention is concerned with image tube streak cameras capable of providing direct linear measurements of ultra short radiant energy pulses, for example from a source such a laser beam, or a luminous or x-ray discharge from a plasma.

Image tubes for streak cameras heretofore known in the art have been required to compromise between resolution and speed of operation. High resolution tubes have employed a photocathode, upon which the target is imaged, and an anode phosphor screen placed fairly distant from the photocathode. Due to spherical sector electron pinhole optics, the space charge at the pinhole causes severe resolution loss. Such tubes, therefore, must be operated at low gain, requiring image intensifiers attached to the anode to provide the necessary gain. An extractor electrode placed adjacent the photocathode is employed to accelerate electrons rapidly to decrease velocity dispersion. The extractor electrode comprises a mesh at high voltage adapted to increase the velocity of the photoelectrons after they are emitted from the photocathode. However the extraction electrode tends to degrade the resolution of the tube by adding a lens effect between adjacent meshes, and decreases sensitivity by interrupting photoelectrons.

It is the object of the present invention to provide an electron optical image tube for use in an image tube streak camera which is capable of providing direct measurement of picosecond pulses of radiant energy, with a time resolution in the picosecond range.

This is achieved in accordance with the present invention by an electron optical image tube comprising a photocathode and a passive microchannel plate collimator arranged to convert an image of the target incident thereon to a collimated electron beam, with deflection electrodes arranged to receive the collimated photoelectrons and subject them to an electric field to produce a streak record. Employment of the microchannel plate enables transverse photoelectron velocity selection, and allows focusing by virtue of the proximity of the phosphor to the photocathode. The short distance decreases the effect of longitudinal photoelectron velocity dispersion, increasing time resolution and maintaining sensitivity. In addition optical alignment of the image tube with the target is simplified considerably since a knife-edge defining the portion of the image of the target of interest is not required in the optical path. The knife-edge is replaced by a narrow active area on the photocathode or a narrow collimator. As a result the electron beam which is deflected across the anode screen images only the wanted portion of the target.

The invention will be more fully understood by the following description of certain preferred embodiments which are given by way of example and with reference to the accompanying drawings.

FIG. 1 illustrates the streak image tube embodying the present invention;

FIG. 2 is a modification of the image tube of FIG. 1;

FIG. 3 illustrates the microtube collimator employed in the present invention; FIG. 4 is a cross section of the collimator of FIG. 3;

FIG. 5 illustrates the imaging of the target of the photocathode forming part of the image tube of the present invention; and,

FIG. 6 illustrates an x-ray version of the streak image tube of the present invention.

Referring now to the drawings and particularly to FIG. 1 thereof, there is shown a streak tube 2 having a photocathode 3 upon which white pulses are focused. The photocathode 3 emits electrons in direct response to the incident light, the electrons being emitted from the photocathode 3 with velocities of differing magnitude and direction.

In accordance with the invention, a microchannel plate collimator 4 is provided, spaced from and adjacent to the photocathode 3. Microchannel plate collimator 4 is more clearly illustrated in FIGS. 3 and 4. Microchannel plates 4 are fabricated of lead glass, making them efficient absorbers of charged particles such as electrons. They are fabricated of a plurality of soft glass tubes 5, clad with harder glass. The tubes are arranged in a hexagonal array, and etched through. Bore diameters are of the order of 8 microns on 12 micron centers. The hollow tube arrays may be on the order of 0.55 millimeters to 1 centimeter long. Arrays of square tubules are also available of similar dimensions. The interior surfaces of the tubules are then plated with a suitable conductive layer (not shown) which does not emit secondary electrons. The conductive plating, however, need not extend all the way through the bores of the tubes. The conductive layer minimizes surface charge buildup and the resultant effective closing of the holes to electrons. Since the time resolution of the tube depends upon field intensity at the emitting surface of the photocathode 3, the gap between the photocathode 3 and the facing surface of microchannel plate 4 is momentarily pulsed to a voltage that would break down the gap if sustained. The plane surface of photocathode 3 and microchannel plate 4 allow a higher field to be applied than grid structues structures the prior art, thereby resulting in greater resolution.

The electrons accelerated by the field between photocathode 3 and microchannel plate 4 pass through the tubes in the collimating microchannel plate 4 only if they are traveling substantially normal to the face of microchannel plate 4. Electrons passed through the tubes in the plate are traveling in parallel lines, and at substantially the same velocity.

Since the phosphor screen 6 and microchannel collimator plate 4 are at the same potential, the electrons are subject only to the field of deflection plates 8 and 9.

As illustrated in FIG. 1 electrons emitted by photocathode 3 are first accelerated by an intense electric field between the photocathode 3 and microchannel plate 4. Screen 6 is coated with a suitable phosphor to make the electron beam visible. The image on the screen is conveyed to a suitable camera or, if required, to an image intensifier, (not shown), by a fiber optic plate 7. The collimated electron beam is deflected by a ramp voltage applied to deflection plates 8 and 9 in synchronism with the voltage pulse between photocathode 3 and plate 4.

As illustrated in FIG. 5 photocathode 3 has imaged thereupon by means of a pinhole aperture 11, the target 12. Since tubule collimator 4 as illustrated in FIG. 1, is furnished with an array of tubules only 100 microns wide, only a thin slice of the image of target 12 is presented to the anode 6. Heretofore, it has been necessary to place a knife edge slit in the optical path between pinhole 11 and photocathode 3.

In the embodiment of the invention illustrated in FIG. 2 the active photocathode material is in the form of a narrow strip 13. Photocathode strip 13 enables the use of a wider collimator array of tubules 5. In this manner the narrow strip of active photocathode material 13 serves the same function as the previously recorded knife edge or the narrow collimator array illustrated in FIG. 1. The arrangement illustrated in FIG. 2 can also be employed in connection with the streak photography of an x-ray source.

Referring to FIG. 6, x-ray source 14 is imaged by pinhole 11 as in the opitcal case FIG. 5. The x-ray image impinges upon suitable x-ray sensitive photocathode 13. Such a photocathode is frequently fabricated of gold. Conveniently the x-ray source is imaged through the side of the tube rather than the backside of the photocathode as in the optical case, providing greater spectral sensitivity. As will be apparent, a suitable x-ray transparent window is provided in the side of the tube 2.