Thermionic heater cathode assembly of electron-beam gun

A thermionic heater cathode assembly of an electron-beam gun, comprising a disc cathode made of a highly-emissive material and disposed near a U-shaped ribbon filament with a thermionic portion and current-carrying sections arranged in parallel, wherein the ends of said ribbon filament sections to be connected to a holder are arranged in a plane normal to the ends of the same sections, adjoining the thermionic portion of said ribbon filament.

The present invention relates to electron-beam engineering equipment 
wherein a high-energy electron beam is used as a working tool, and more 
particularly, to a thermionic heater cathode assembly of an electron-beam 
gun, the cathode whereof made of a highly-emissive material is heated by a 
filament. 
The term "electron-beam gun" is used to denote a plurality of electrodes 
producing and defining an electron beam, and also controlling its 
intensity. The electron-beam gun operates from a high-voltage DC power 
source. 
The thermionic heater cathode assembly of the present invention can most 
advantageously be employed for welding, fusion and other types of metal 
working based on electron-beam techniques. 
Experience has shown that operation of electron-beam guns, accompanied by 
waste gas pressure variations, requires that flat disc cathodes 
manufactured from a highly-emissive material be used. To achieve stable 
electron emission, the cathode must be heated to a temperature of 
1,800.degree. to 2,000.degree. C, with the heating temperature being 
maintained uniform and stable to a high degree. This requirement can be 
satisfied only by providing an efficient heater with such economy features 
as low power consumption, high resistance to deformation at elevated 
temperatures, and durability even when overheated. It has been found by 
experience that coil filaments with an axial lead meet these requirements 
to a certain extent. However, in high-power electron-beam guns, such 
filaments are not durable due to variations in turn-to-turn distances, 
non-uniform heating and turn-to-turn short-circuits. 
Owing to the high power of the beam produced by known electron-beam guns 
incorporating said filament, even minor changes in the initial shape of 
the coil and in the turn-to-turn distances occurring during manufacture 
and assembly of the cathode elements bring about non-uniform heating of 
the disc cathode, temperature variation during operating of said cathode, 
and non-uniform emission from the heated surface thereof. 
It has previously been proposed that coil holding insulators be employed to 
increase the resistance to deformation of the coil. However, this measure 
does not obviate all of the above disadvantages, impairs the cooling 
conditions of the thermionic cathode assembly, and complicates alignment 
of the coil in relation to the electrodes of said assembly. Besides, the 
heat energy cannot be focused in the center of the disc cathode because 
the area of the heated coil is large. 
In recent years, a ribbon filament has been proposed, the thermionic 
portion thereof is near the centre of the disc cathode. 
It is an object of the present invention to increase the power of an 
electron-beam gun. 
Another object of this invention is to provide a thermionic heater cathode 
assembly of an electron-beam gun, the ribbon filament whereof has a 
maximum efficiency. 
Still another object of this invention is to minimize the temperature of 
the ribbon filament current-carrying sections. 
These objects are attained by providing a thermionic heater cathode 
assembly of an electron-beam gun, comprising a disc cathode made of a 
highly-emissive material, and a U-shaped ribbon filament with a thermionic 
portion and current-carrying sections arranged in parallel, wherein the 
section ends connected to a holder lie, according to the invention, in a 
plane normal to the ends of the same sections, adjoining the thermionic 
portion. 
An important advantage of the present invention is that mutual heating of 
the parallel sections is decreased by 30 percent because they face each 
other only by the narrow sides over half the length thereof.

Referring now to FIG. 1, an electron-beam gun fitted to a metal welding 
machine (not shown) comprises a thermionic heater cathode assembly 1 and 
an anode 2 intended for producing an electron beam 3. An electromagnetic 
focusing lens 4, a centring system 5 and finally, a workpiece 6 to be 
welded are placed along the electron beam. 
The thermionic heater cathode assembly 1 includes a ribbon filament 7 given 
a U-shape, made of tantalum and disposed near a disc cathode 8 made of 
lanthalum hexaboride and axially aligned with a control electrode 9. The 
function of the control electrode is to regulate electron beam intensity. 
Each pair of parallel current-carrying sections 10 and 11 of the ribbon 
filament 7 is twisted in the centre thereof so that the cross-sections of 
both halves form an angle of 90.degree.. 
A thermionic portion 12 of the ribbon filament 7 adjoins, at 90.degree. 
ends of the parallel sections 10 and 11 and is parallel to the disc 
cathode 8, while the opposite ends of the sections 10 and 11 are connected 
to a holder (not shown). 
In operation, the workpiece 6, is placed in a vacuum chamber (not shown) 
and is exposed to a thin focused beam 3 of electrons moving at a velocity 
near to that of light. The electron beam interacting with the item 6 
causes heating and fusion of the metal being worked. 
The disc cathode 8 heated by the ribbon filament 7 constitutes a source of 
free electrons. The disc cathode 8 is heated under bambardment by electron 
emitted by the thermionic portion 12 of the ribbon filament 7. The heating 
temperature reaches 2,200.degree. C. The disc cathode 8 is at a positive 
potential in relation to the ribbon filament 7, which is essential for 
increasing the velocity of electrons and the intensity of bombardment of 
the disc cathode 8. The disc cathode is heated both by thermal radiation 
of the ribbon filament 7, and by the impact of the electron beam on the 
body of the disc cathode 8. Owing to the high temperature and electron 
bombardment, the material of the disc cathode 12 starts evaporating, 
reaches the thermionic portion 12 of the ribbon filament 7, activates said 
portion, and reduces its electronic surface work function. Since the 
cross-section of one half of each sections 10 and 11 of the ribbon 
filament is normal to that of the other half of the same section mutual 
heating of the sections 10 and 11 is reduced, with said cross sections of 
the sections 10 and 11 facing each other by the narrow sides thereof. 
Consequently, the thermal flux directed by the narrow side on the adjacent 
section is much less than that radiated by the wide side of the same cross 
section. Experience shows that the total heat exchanged between the 
sections 10 and 11 is reduced by 30 percent. Besides, the wide side of 
said cross section is cooled easily, with heat being radiated into the 
surrounding space. 
The decrease in temperature of the cathode assembly 1 contributes to a 
higher resistance to deformation of the ribbon filament 7, and also to 
stable heating of the disc cathode 8. 
The high electron energy is achieved by accelerating electrons by a 
high-voltage field produced by the anode 2. The energy is supplied by 
pulses with a duration of 10.sup.-4 to 10.sup.-6 sec. Upon impact of the 
electron beam on the surface of the workpiece 6, the electron energy is 
converted into heat. Thus, the temperature in the area where the beam 
contacts the workpiece (contact spots) rises to 3,000.degree. to 
4,000.degree. C. 
The invention permits increasing the service life of the ribbon filament to 
12 hours, and reducing the power drained by the filament to 70 W. The 
decrease in heat exchange due to rediation of the current-carrying 
sections of the filament results in a 3-fold increase in resistance to 
deformation of the filament in the course of operation.