Electric lamp and discharge devices – Electrode and shield structures – Cathodes containing and/or coated with electron emissive...
Patent
1997-11-05
1999-08-10
O'Shea, Sandra
Electric lamp and discharge devices
Electrode and shield structures
Cathodes containing and/or coated with electron emissive...
313458, 313346DC, 313293, H01J 2950
Patent
active
059363355
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is that of electron tubes and in particular those having longitudinal electron beams and having a grid, such as TOTs (Inductive Output Tubes).
2. Discussion of Background
An IOT has an electron gun which emits an electron beam, a resonant cavity through which the beam passes and a collector which receives the beam electrons as they leave the cavity.
The electron gun has a generally concave cathode in the form of a portion of a sphere, a control grid and an anode. The concave face of the cathode emits electrons when it is raised to a high temperature. The electrons pass through the control grid and are attracted by the anode and then enter the resonant cavity, forming a longitudinal beam.
The control grid serves to modulate the emission of electrons so as to group them into packets before they enter the resonant cavity.
The cathode is generally formed by a porous body impregnated with an emissive material. The porous body is made of tungsten and the emissive material of barium, calcium and strontium aluminates. It starts to emit electrons in the region of 900.degree. to 1100.degree. C.
The control grid is very close to the cathode. The gap between the grid and the cathode is about a few tenths of a millimetre. The emissive material has a tendency to evaporate and migrate in particular onto the control grid and onto the anode. The grid heats up, on the one hand, because of the proximity of the cathode and, on the other hand, because of the electrons which strike it. The emissive material which has migrated onto the grid causes spurious electron emission which disturbs the operation of the tube. Since the anode is relatively far from the cathode, it remains relatively cool, and the emissive material which covers it is not too much of a problem.
Solutions have been proposed for eliminating this spurious electron emission. One of them recommends bombarding the grid with electrons in order to heat it and cause the emissive material covering it to evaporate. The frequency of this heating may be daily, for example before each start-up. Cleaning the grid by heating is a severe constraint on an emitter of a television which may operate in a remote spot with remote control. Long term, this heating may also cause degradation in the operation of the electron gun.
Another approach taken for avoiding this spurious electron emission consists in decreasing as much as possible the temperature of the grid during operation of the tube. A known solution is to use a cathode working at temperatures lower than those of conventional cathodes, thereby lowering the temperature of the control grid. This solution does not give good results either. The spurious emission phenomenon is not eliminated, merely retarded.
Another drawback encountered is that the spurious emission from the control grid limits the size of the cathode and consequently the electron current produced.
SUMMARY OF THE INVENTION
The present invention is intended to remedy these drawbacks by proposing an electron gun whose cathode helps to cool the grid better and effectively prevents the spurious emission.
This cathode has an emissive part which delimits a more or less central hole passing right through it.
The cathode is advantageously heated by a heater which has a heating element on the opposite side from its emissive part, this heating element delimiting a hole facing the hole in the cathode.
The grid has a thermally radiative solid part intended to be placed lying opposite the hole in the cathode. The radiative heat dissipation of the grid is improved by this thermally radiative solid part since it can radiate towards the hole in the cathode and towards the anode region which is a cold region. Since the grid is cooled better, spurious electron emission is eliminated.
It is preferable to make the solid part from a material having a thermal radiation capacity close to that of a black body. Pyrolytic graphite is particularly advantageous.
The present invention also relates to an el
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O'shea Sandra
Smith Michael J.
Thomson Tubes Electroniques
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