A conventional electron gun, using one or more thermionic cathodes with hot filaments has, after the cathode, a series of electrodes needed to form the electron beam then to focus it continually onto-the screen of the tube on which the images to be displayed are reproduced.

This series of electrodes means that the gun has a considerable length which contributes to the value of the final depth of the tube.

Because the angle of deflection of the electron beams which scan the screen of the tube remains substantially around 110°, this depth increases rapidly with the size of the diagonal of the said screen, while current consumer choice is changing towards large screens, but with a minimal depth.

The use of cold cathodes with microemitters, which emit by the field effect, in order to form the electron beams in combination with a suitable electrode structure may enable the length of the gun to be decreased slightly compared with a gun according to the prior art by virtue of a shorter electron beam formation region along the longitudinal axis of the gun.

Moreover, this type of cathode also has the advantage of instantly displaying the image on starting up the tube in which it acts as an electron source, of removing the colour instability during the minutes following start-up, and of reducing the power consumed by the cathodes.

However, replacing a thermionic electron source with a source consisting of field-effect emitters comes up against the problem of the current density likely to be supplied by this type of emitter, a density which is much lower than the thermionic sources. Thus, it is difficult for a source of electrons extracted by cold emission to reach a current density of around 2 to 3 A/cm2, while a thermionic cathode easily reaches a current density of around 30 A/cm2.

A first subject of the invention is a cold-emission cathode structure making it possible to obtain both a high current density and a beam whose fineness enables a cathode of this kind to be applied in applications such as high-resolution tubes.

To this end, the device for emitting an electron beam according to the invention comprises: - at least one cold-emission cathode, - a substantially pyramid-shaped electrode, covered with a layer of material having a secondary emission coefficient at least equal to 1, the said layer, being designed to transport secondary electrons from the base of the electrode to its tip, the said secondary electrons being generated by the incident electrons coming from the cold-emission cathode, - means for generating an electric field tangential to the electrode surface, which increases from the base of the electrode to its tip, - means for extracting the secondary electrons from the tip of the emissive electrode in order to generate an electron beam.

The invention and its many advantages will be better understood from the description below and the drawings among which: - Figure 1 illustrates an embodiment according to the prior art of an electron source using a device for generating an electron beam obtained by secondary emission.

- Figure 2 is a sectional view of a microtip emissive cathode.

- Figure 3 is a sectional view of an emissive structure according to the invention.

- Figure 4 shows, in section, the operation of an emissive structure according to one embodiment of the invention.

Figure 1 illustrates one embodiment of an electron source 1, described in Patent US 5 270 611, designed to generate a high-density electron beam. The source 1 comprises a thermionic cathode 2 with a heating filament 3, an emitting layer 5 supported by a dish 4. The electrons are emitted by the cathode in the direction of an elongate cavity 10, open towards the cathode and closed off by walls 7,8, 9. The cavity is made from an electrically insulating material having a secondary emission coefficient at least equal to 1. One of the walls has an opening 11 around which an anode 12 is placed. A potential difference is applied between the cathode and the anode 12 in order to generate a field large enough to transport the electrons into the cavity 10 and to make them pass through the opening 11. The electrons are transported by a secondary emission process in which, for each electron striking one of the walls of the cavity, at least one secondary electron is on average emitted.

This solution produces an electron source whose current density is greater than that of the initial cathode, but it has several drawbacks: some of the electrons which flow by secondary emission along the walls do not pass directly through the orifice 11, but are picked up by the anode 12; this is because the electrons have a path which hits this anode since the electrons progress in very small jumps along the walls and can therefore hit the anode during the next jump; consequently, the anode consumes part of the current emitted by the starting electrode.

if this drawback is solved by applying a sufficiently strong electrostatic extraction field, then another drawback appears: the electrostatic lens produced by this strong field has strong aberrations, therefore the emittance of the electron beam is degraded, which results in degraded resolution of the electron gun using this source. if a hot cathode, such as an oxide cathode or an impregnated cathode, is used as the initial electron source, barium gradually evaporates from the surface of this cathode and is deposited on the walls of the cavity; now these walls are prepared so that they have a suitable secondary emission coefficient, generally by covering them with a coating of a suitable material, such as magnesium oxide MgO ; while the source is in operation, there is therefore a risk of seeing the barium degrade the secondary emission properties by covering the walls.

The electron'source according to the invention uses at least one cold-emission cathode 20 as illustrated in Figure 2. A cathode of this kind comprises microemitters, for example microtips 21 emitting by the field : effect and two biasing electrodes 25 and 23 placed one above the other at a distance of about one micron. The electrode 25 or cathode enables the microtips 21, which are formed on the cathode, to be biased.

The cathode is deposited on a substrate 22, generally made of glass, providing mechanical rigidity for the cathode.

A resistive layer 27 may advantageously be deposited between the microtips and the cathode 25 in order to improve the uniformity of emission of each microtip. A second electrode, also called an extraction grid 23, is deposited above the cathode 25 from which it is insulated by an electrically insulating layer 28. The extraction grid is apertured above each microtip, which emits a microelectron beam 24 by applying a voltage which is positive by a few tens to a few hundreds volts with respect to the voltage of the cathode, to the grid.

One embodiment of the invention, suitable to be used as an electron source in a gun for a cathode-ray tube, is illustrated in section in Figure 3.

The electron source uses an emissive electrode in the form of a central, substantially pyramidal needle 31. The needle has a length of around 5 to 10 mm and a diameter of about 3 mm at its flared base and less than 1 mm at its tip. The needle consists of a hollow body 33 made of an insulator, whose resistivity is chosen so that by bringing its base 34 and its end 35 to different potentials, a potential increasing from its base 34 to its end 35 is created at the surface of the needle.

The material forming the body 33 may be glass or a ceramic, for example based on aluminium oxide A1203 or silicon oxide Si02. A thin layer 37 of resistive material having a secondary emission coefficient at least equal to 1 is placed on the outer surface of the needle. The material chosen to produce this layer 37 may advantageously be chosen from magnesium, calcium, strontium, barium or beryllium oxides. Alternatively, the layer 37 may be an organic film, for example a polyamide film.

The needle 31 is hollow and the inner recess 32 extends longitudinally from its base towards its end. A metal conductor 36 is inserted into the recess 32 to bring the potential of the end 35 of the needle to a value V greater than the potential of the base 34 using a power supply 50; the base 34 may, for example, be connected to earth or to a bias potential Vo. Thus an electric field tangential to the surface of the needle 31 will be created along the layer 37. Moreover, it is preferable not to use a long needle so that a very high potential difference does not have to be used between the base of the needle and its end.

An electrode 40 is placed close to the end 35 of the needle 31; this electrode 40 is provided with a central hole 45 having the same axis as the needle and which is contained in a plane perpendicular to the longitudinal axis of the said needle; this

electrode may be followed by a series of electrodes in order to form an electron gun. The electrode 40 is brought to a potential V'greater than the potential V of the tip using a source 51.

The primary electron source consists of at least one cold-emission cathode 20, for example a microtip cathode as described in Figure 2; this cathode is placed so that the microbeams 24 which it emits are directed towards the flared base of the needle 31. The operation of the source is illustrated in Figure 4: electron microbeams 24 are emitted by the cold cathodes in the direction of the base of the needle; these electrons hit the base and produce secondary electrons 42 during their journey from the base to the end of the needle; firstly, the primary electrons pass through a longitudinal electric field oriented along the perpendicular to the surface of the cold cathode, then a transverse field, oriented parallel to the surface of the needle; in the region close to the needle, the transverse component of the field predominates over the longitudinal component, such that the paths of the incident electrons are bent towards the tip of the needle, and the secondary electrons 42 have paths in the form of small jumps in the direction of the end. reflected electrons (those emitted with the same energy as the incident electrons) are partly picked up by the starting electrodes, more exactly by the grid for extracting or accelerating these electrons, and are partly transported like real secondary electrons; the reflected electrons represent a small fraction of the total of the secondary electrons (less than 10%); the reflected electrons, which are picked up by the starting electrodes, land thereon with an energy corresponding to the potential of the surface they impact upon, that is to say to the potential of the extraction grid of the cold cathode; this energy is from 50 to 100eV, which is a moderate energy; consequently, these electrons which are picked up by the

starting electrodes, are few in number, have little energy and therefore do not cause damage by heating. the real secondary electrons 42 (those emitted at a low energy, about 2eV), are subjected to the electric field which is tangential to the surface and are therefore transported at the surface of the needle to the tip; this involves gradual transport"in cascade", in which an electron which falls on the surface generates thereon on average one electron emitted by secondary emission such that the electron current is perpetuated along the needle. the tangential field is cancelled out close to the tip, but the extracting field produced by the electrode 40 takes over to create a tangential field in the region of the tip,-so it is the extracting field which causes the electrons to be transported up to the tip, and the electrons are extracted from the tip only since elsewhere the field is insufficient. the extracting field, shown by the field lines 52, causes an electron beam 41 to be emitted from the tip, since the extracting field reaches its maximum value at the said tip because of the field effect; here, the field amplifying effect produced by the tip, called the tip effect, is exploited; this extraction field is therefore very localized at the tip and the result of this is that the emissive surface is also very localized at the tip; this tip effect enables a highly-concentrated electron beam to be obtained and can be used as an electron source in an electron gun for a high-resolution tube, which gun then produces the image of the object point consisting of the emissive tip.

The invention is not limited to the embodiment of cathodes whose microemitters are microtips as described above. On the contrary, it can be used in the same way and with the same advantages in all other cases of microemitters emitting by field effect, especially flat carbon-based microemitters.

The cathode according to the invention, in all its embodiments, can equally be applied to a single-beam gun for a monochrome tube or to a three-beam gun for a colour tube.