Magnetic channel cathode

Details Magnetic channel cathode Magnetic channel cathode

1998-11-23

2001-06-12

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

Cathode ray tube

Beam deflecting means

C313S431000

Reexamination Certificate

active

06246165

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to an area cathode suitable for use in a flat panel display and more particularly to an area cathode in which electrons are confined in magnetic channels and extracted by low voltage electrostatic fields.
BACKGROUND OF THE INVENTION
An area cathode of the present invention is particularly although not exclusively useful in display applications, especially flat panel display applications. Such applications include television receivers and visual display units for computers, especially although not exclusively portable computers, personal organisers, communications equipment, and the like.
All flat panel CRT technologies require an area cathode, that is a uniform planar source of electrons the same area as the display. There have been many designs developed over the years, based on technologies such as Field Emission Devices (FEDs), Metal-Insulator-Metal devices (MIMs) and the like. Probably the most successful types have been the virtual thermionic cathode from Source Technology, disclosed in European Patent Application 0 213 839, and the secondary emission channel hopping cathode developed by Philips for their Zeus display. All current designs, however, suffer from significant disadvantages of one sort or another. In particular the virtual thermionic type has high power and hence a major heat dissipation problem, and the channel hopping type has high and non uniform channel extraction voltages.
U.S. Pat. No. 5,227,691 discloses a flat tube display apparatus in which a row of many electron beam generators is arranged transversely in a thin flat vacuum tube body to generate a number of beams in parallel with each other which travel in parallel with an image screen and in which the electron beam generators are arranged to deflect the beams toward the image screen at a predetermined position. The beams are guided without being widely diverged due to the provision of a number of side walls arranged in parallel with each other to confine the beams and due to the provision of alternately strong and weak magnetic fields along the side walls forming periodic magnetic lenses. The electron beams are deflected electrostatically or using a magnetic field towards an electron beam multiplier and a phosphor screen.
It would be desirable to produce an area cathode that has:
1. An electron source based on known materials;
2. Generation of electrons at a low eV (hence low extraction voltages);
3. A narrow eV spread (hence low beam spreading);
4. A high degree of uniformity;
5. Low power and heat;
6. Isolation from external electric and magnetic fields;
7. Protection of the electron source from ion bombardment; and
8. Mechanical simplicity leading to low cost.
SUMMARY OF THE INVENTION
Accordingly, the invention provides an electron source comprising cathode means, a permanent magnet having a plurality of channels, extending between opposite poles of the magnet, parallel to a first surface of the magnet, the cathode means being located at a first pole of the magnet, the internal surfaces of each of the channels being conductive, each channel having a plurality of perforations located on the first surface of the permanent magnet, the surface extending between opposite poles of the magnet, wherein a potential applied between the cathode means and the conductive internal surfaces of the channels causes electrons to be received into the channels and wherein each perforation forms electrons received from the cathode means into an electron beam for guidance towards a target.
In a first embodiment, the cathode means comprises a line filament cathode. The use of a line filament cathode has the effect of providing a point thermionic cathode in each of the channels.
Preferably, the line filament cathode is indirectly heated. Use of an indirectly heated cathode means that the outer conductive sheath of the cathode can be held at a uniform 0 V, isolated from the internal heated core. This has the advantage that there is no variation in voltage along the length of the cathode and hence no change in eV of the emitted electrons.
In a second embodiment, the cathode means comprises a micromachined cathode. These cathodes have the advantage of a very low power and a low heat load.
Preferably, the permanent magnet comprises a first magnetic plate having grooves, extending between opposite poles of the magnet, along a first surface of the first magnetic plate, and a second magnetic plate having a plurality of perforations, said second plate being located so as to close the grooves to form channels, the channels having perforations located on a surface extending between opposite poles of the magnet. This allows the magnet to be constructed using standard mass production processes to form the grooved plate.
Preferably, the channels are arranged at a pitch corresponding to the pixel pitch of a display incorporating the electron source. This provides a single source of electrons for each of the pixels of the display incorporating the electron source.
Preferably, the magnet plane furthest from the perforations is at least twice as thick as the channel depth. This has the advantage that the flux density is increased within the channel, so increasing the isolation from external fields. This also has the advantage that null field points and non-linearities present in the channel are moved into the perforations. This provides an essentially linear field in the channels, with no field reversals.
Preferably, each channel has a depth greater than the width of the channel and wherein the portion of the channel furthest from the perforations is curved in cross-section. This has the advantage of increasing the volume of magnetic material on the non-perforated side of the magnet plate.
In a variation of the preferred embodiment, each channel is quadrilateral in cross-section, being either rectangular in cross-section or square in cross-section. This has the advantage of making the manufacture of a magnet plate having grooves particularly suited to conventional mass production techniques.
Preferably, the perforations are disposed in the magnet in a two dimensional array of rows and columns.
In a preferred embodiment, the perforations are arranged at a pitch corresponding to the pixel pitch of a display incorporating the electron source. This provides a single source of electrons for each of the pixels of the display incorporating the electron source.
Preferably, each of said channels is unperforated for a distance from the cathode means of ten or more times the pitch of the perforations. This unperforated distance means that the magnetic field is linear over a sufficiently long distance so as to allow collimation of the electrons to become established.
Preferably, the electron source further comprises a non-magnetic stainless steel plate located on the surface of the magnet furthest from the perforations. The use of a stainless steel plate gives the magnet assembly increased tensile strength.
In a variation of the preferred embodiment, the conducting surfaces associated with each of the channels are electrically separated. Since the current that enters each of the channels is all absorbed by the channel walls during the display blanking periods, by arranging for separate connection of each channel conducting surface, emission control on a channel by channel basis may be provided.
The invention also provides a display device comprising: an electron source as described above; a screen for receiving electrons from the electron source, the screen having a phosphor coating facing the side of the magnet having perforations; two perforated ceramic plates, each having a conductive surface, so as to cause a flow of electrons from the cathode to the phosphor coating via the channels and perforations thereby to produce an image on the screen.


REFERENCES:
patent: 5227691 (1993-07-01), Murai et al.
patent: 5844354 (1998-12-01), De Zwart et al.
patent: 5857883 (1999-01-01), Knickerbocker et al.
patent: 5959397 (1999-09-01), Lambert et al.
patent: 0 213 839 (1987-03-01), None
patent:

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