Patent Publication Number: US-6984940-B2

Title: Electronic tube with simplified collector

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to amplifying electrode tubes operating at microwave frequencies. It applies more particularly to TWTs (travelling wave tubes), and it is therefore with regard to such a tube that the invention will be described. Such tubes are used, for example, for the transmission of telecommunication signals between Earth and satellites. They are also used as power transmitters in radars. 
   It will be briefly recalled that a TWT is a vacuum tube using the principle of interaction between an electron beam and a microwave electromagnetic wave in order to transfer part of the energy contained in the electron beam to the microwave so as to obtain, as output from the tube, a microwave of higher energy than that of the wave injected into the input of the tube. 
     FIG. 1  recalls the general principle of a TWT. The TWT shown is a helix TWT, but other types of TWT, such as TWTs with coupled cavities, TWTs with folded waveguides in the form of meanders, etc., are just as well covered by the invention. 
   TWTs comprise an elongate tubular sheath  10 , in which a vacuum is created, with, at a first end, an electron gun  11  that emits an electron beam  12  and, at a second end, a collector  14 ; the collector collects the electrons that have given up some of their initial energy to the electromagnetic wave that it is desired to amplify. The electron beam  12  is substantially cylindrical over almost the entire length of the tube between the gun  11  and the collector  14  along an axis  15 . This cylindrical beam shape is obtained, on the one hand, by the shape of a cathode  16  of the electron gun  11  (a cup-shaped convergent cathode) and, on the other hand, by magnetic focussing means provided over the entire length of the sheath  10  between the exit of the electron gun  11  and the entrance of the collector  14 . In the electron gun  11 , it is the cathode  16  that emits the electron beam  12 . These focussing means comprise, for example, annular permanent magnets  18  that are axially magnetized and of magnetization that alternates from one magnet to the next; these magnets surround the sheath  10  and are separated from one another by pole pieces  20  of high magnetic permeability. 
   In the case of a helix TWT, the electron beam  12  passes into a helical conducting structure  22  along which the microwave electromagnetic wave to be amplified flows; the amplification of microwave energy takes place by interaction between this microwave and the electron beam  12  that passes through the centre of the helix. The latter serves to decelerate the microwave in such a way that its velocity, along the axis  15  of the electron beam  12 , is approximately equal to that of the electron beam  12 . 
   A signal to be amplified of power Pe is injected at one end of the helical conducting structure  22  through a plug and a port  24  inside the sheath  10 . An amplified signal of power Ps is extracted at the other end of the helical conducting structure  22  through a plug and a port  26 . The amplification gain G of the electron tube is defined by the ratio G=Ps/Pe or, expressed in decibels, 10 log 10 (Ps/Pe). The efficiency η of the amplification is defined by:
 
η= Ps/V   o   xI   o .
 
V o  represents the voltage between the cathode  16  and the collector  14  and I o  represents the current flowing in the cathode  16 . The efficiency η is generally around 20 to 30%. It is often called the interaction efficiency ηi and it characterizes that part of the energy of the electron beam  12  converted into microwave energy in the amplified signal. The remaining energy, (1−ηi) V o xI o , in the electron beam  12  after the latter has passed through the helical conducting structure  22 , is then dissipated in the collector  14  where the electrons of the beam  12  bombard the walls of the collector  14  and convert their kinetic energy into heat. This heat is then discharged to the outside of the electron tube by conduction, convection or radiation. On the outside of the elongate tubular sheath  10 , the electron tube usually has, near the collector  14 , a heat sink (not shown in  FIG. 1 ). This heat sink is, for example, cooled by circulation of a liquid or gaseous fluid.
 
   In practice, one portion of the current I o , coming from the cathode  16 , flows in the helical conducting structure  22  as shown in  FIG. 2 . 
   In this figure, the collector  14  is connected to the positive pole  28  of a DC voltage source  30 . The helical conducting structure is also connected to the positive pole  28 . The negative pole  32  of the DC voltage source  30  is connected to the cathode  16 . The electron beam  12  develops between the cathode  16  and the collector  14 . In an experimental arrangement, using a 10 kV DC voltage source  30 , a current of 1 A output by the cathode  16  is obtained in the electron beam  12  and a power Ps of 2 kW is obtained as output from the helical conducting structure  22 . The return current between the collector  14  and the pole  28  is 0.99 A and the current between the helical conducting structure  22  and the pole  28  is 0.01 A. The efficiency is then expressed as: 
         η   ⁢       2   ⁢           ⁢   kW       10   ⁢           ⁢   kV   ×     (     0.99   +   0.01     )           =     20   ⁢           ⁢     %   .           
 
   The efficiency of an electron tube may be improved by using two voltage sources. This alternative arrangement is shown in  FIG. 3 . A first DC voltage source  34 , for example of 10 kV, is connected between the cathode  16  and the helical conducting structure  22  and a second DC voltage source  36 , the voltage of which is lower than that of the first voltage source, for example 6 kV, is connected between the collector  14  and the cathode  16 . Assuming the same current and power values as in the example given above in  FIG. 2 , the efficiency is then expressed as: 
       η   =         2   ⁢           ⁢   kW         (     10   ⁢           ⁢   kV   ×   0.01     )     +     (     6   ⁢           ⁢   kV   ×   0.99     )         =     33   ⁢     %   .             
 
   Advantageously, the collector  14  comprises several electrodes raised to various potentials. These various electrodes have the purpose of decelerating the electrons before they strike the walls of the electrodes. Thus, the heat dissipated in the collector  14  is less and the efficiency η increases. 
   An example of such a collector is shown in  FIG. 4 . In this example, the 10 kV DC voltage source  34  is connected between the helical conducting structure  22  and the cathode  16 . A current of 0.1 A flows in the voltage source  34 . 
   A DC voltage source  38 , for example of 6 kV, is connected between a first electrode  40  and the cathode  16 . A current of 0.4 A flows in the voltage source  38 . A DC voltage source  42 , for example of 4 kV, is connected between a second electrode  44  and the cathode  16 . A current of 0.48 A flows in the voltage source  42 . A second voltage source  46 , for example of 1 kV, is connected between a third electrode  48  and the cathode  16 . A current of 0.01 A flows in the voltage source  46 . The three electrodes  40 ,  44  and  48 , which belong to the collector  14 , are placed in such a way that the electrode  40 , subjected to the highest voltage relative to the cathode  16 , is the closest to the cathode  16  and the electrode  48 , subjected to the lowest voltage relative to the cathode  16 , is furthest away from the cathode  16 . Again assuming the power Ps is 2 kW, the efficiency is expressed in the following manner: 
       η   =         2   ⁢           ⁢   kW         (     10   ⁢           ⁢   kV   ×   0.01     )     +     (     6   ⁢           ⁢   kV   ×   0.40     )     +     (     4   ⁢           ⁢   kV   ×   0.48     )     +     (     1   ⁢           ⁢   kV   ×   0.01     )         =     45   ⁢           ⁢     %   .             
 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   This structure of the collector  14 , comprising several electrodes, is called a depressed collector. Of course, the number of electrodes and the numerical values of the currents, voltages and powers, have been given merely by way of example and the invention is not limited to these examples. 
   Although the final electrode has a low potential difference relative to the cathode  16 , the kinetic energy of the electrons that bombard it is still high and generates heat that has to be removed. The position on the end of the electron tube of the electrode  48  increases the difficulties in removing the heat that the electron bombardment generates since this position on the end of the tube is generally used to place means for creating the vacuum inside the electron tube, which vacuum is needed for establishing the electron beam  12 . To remove the heat generated within the electrode  48 , it is necessary to ensure heat transfer to the cooling means located in the immediately vicinity of the electrodes  40  and  44  on the side walls of the electron tube. This heat transfer is again difficult to achieve, especially because of the differential thermal expansion between electrically conducting elements, such as the electrodes  40 ,  44  and  48 , and insulating elements that separate these electrodes. It will be possible to reduce the heat generated within the electrode  48  by reducing the potential difference of the DC voltage source  46 . However, with this solution there will be a risk of reflecting a portion of the electron beam  12  bombarding the electrode  48  in the direction of the cathode  16 . This reflection runs the risk of destroying the helical conducting structure  22 . 
   SUMMARY OF THE INVENTION 
   Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: 
   The object of the invention is to alleviate this problem, by direct use of the means for creating the vacuum in the electron tube to repel a portion of the electron beam  12  towards the other electrodes  40  and  44  and not in the main direction of the beam indicated by the axis  15  in  FIG. 1 . 
   For this purpose, the subject of the invention is an electron tube comprising:
         a pump-out tube that allows the vacuum inside the electron tube to be created;   an electron gun that emits an electron beam inside the electron tube;   a collector that directly collects a first portion of the electron beam;
 
characterized in that the pump-out tube directly repels a second portion of the electron beam in the direction of the collector.
       

   In a preferred embodiment of the invention, the pump-out tube opens, inside the electron tube, along the axis of the electron beam. This simplifies the construction of the end of the tube. 
   The invention will be more clearly understood and other advantages will appear on reading the detailed description of one embodiment given by way of example, which embodiment is illustrated by the appended drawing in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: 
       FIG. 1  shows schematically the general operation of an electron tube; 
       FIG. 2  shows an electron tube using a single DC voltage source; 
       FIG. 3  shows an electron tube using two DC voltage sources; 
       FIG. 4  shows an electron tube having four DC voltage sources and one depressed collector; and 
       FIG. 5  shows one end of the electron tube with a depressed collector and part of means for creating the vacuum inside the electron tube. 
   

   To simplify the rest of the description, the same elements will bear the same reference numbers in the various figures. 
     FIGS. 1 to 4  have already been described above in order to introduce the invention. 
     FIG. 5  shows, in part, an illustrative example of an electron tube for implementing the invention. This electron tube comprises the tubular sheath  10  inside which the vacuum is created by means of a pump-out tube  50 , the open end  52  of which penetrates the inside of the sheath  10 . The other end of the pump-out tube is not shown in  FIG. 5  and is connected to a vacuum pump during the electron tube manufacturing operations. When a sufficient vacuum has been created inside the electron tube, the pump-out tube  50  is tipped off, for example by pinching it until the walls of the pump-out tube are cold-welded together to form a hermetic seal. 
   The electron tube includes an electron gun  11  (not shown in the figure), which emits the electron beam  12  inside the electron tube, and a collector  14  that directly collects a first portion of the electron beam  12 . The collector  14  has at least one electrode. It has three electrodes  54 ,  56  and  58  in the example shown. The three electrodes  54 ,  56  and  58  are axisymmetric about the axis  15  along which the electron beam  12  mainly runs. Each electrode  54 ,  56  and  58  has a cylindrical part, respectively  60 ,  62  and  64 , fastened to the inside of the cylindrical sheath  10 . The sheath  10  is also used about the axis  15 . The sheath  10  is, for example, made of ceramic and includes metallized parts  66 ,  68  and  70  that receive the respective electrodes  54 ,  56  and  58 . 
   The electrodes are, for example, based on copper and their cylindrical parts  60 ,  62  and  64  are brazed to the respective metallized parts  66 ,  68  and  70  of the sheath  10 . Between these metallized parts, the sheath  10  has grooves  72  and  74  which provide the insulation between the three electrodes  54 ,  56  and  58 . Each of all three of the electrodes  54 ,  56  and  58  is connected to a voltage source via respective connection means  76 ,  78  and  80 . 
   The three electrodes are pierced along the axis  15  with orifices, respectively  88 ,  90  and  92  that let the electron beam  12  pass, at least partly. 
   One end  81  of the sheath  10  is closed off by a cover  82  that is mechanically connected to the sheath  10  with sufficient elasticity to withstand any thermal stresses. This resilient connection between the sheath  10  and the cover  82  is, for example, achieved by means of a collar  84 . The cover  82  is axisymmetric about the axis  15 . Its centre is pierced so that the pump-out tube  50  penetrates inside the electron tube. The pump-out tube is electrically connected to a voltage source (not shown in the figure) via connection means  86 . The voltage thus delivered to the pump-out tube  52  is close to that of the cathode  16  forming part of the electron gun  11 . 
   When a portion of the electron beam  12  is not collected by one of the three electrodes  54 ,  56  or  58 , the pump-out tube  50  repels, directly, without an intermediary, this portion of the electron beam  12  in the direction of the collector  14  and more particularly to the electrode  58 . 
   Advantageously, the pump-out tube  50  has the shape of a nozzle, the end  52  of which, located inside the electron tube, is open. The pump-out tube  50  in fact repels that portion of electron beam  12  arriving in its vicinity. It may remain open in the direction of the axis  15  as no electron (or very few electrons) can penetrate into the pump-out tube  50 . There is therefore no risk of the temperature of the pump-out tube  50  rising owing to electron bombardment. 
   Advantageously, the end  52  of the pump-out tube  50  has a shape that is asymmetrical with respect to the axis  15 . This shape is, for example, obtained by bevelling the end  52 . The bevel thus formed is a cut made at the end  52  in a plane not perpendicular to the axis  15 . This asymmetrical shape allows the electrons arriving on the pump-out tube  50  along the axis  15  to be repelled along an axis other than the axis  15  and thus to reach one of the electrodes, especially the electrode  58 . The bevelled cut of the end  52  is very simple to produce, for example by cutting off the pump-out tube  50  at an angle. 
   It will be readily seen by one of ordinary skill in the art that the present invention fulfills all of the objects set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.