SYSTEM DEVISED TO PREVENT UNDESIRABLE OSCILLATIONS GENERATION WITHIN TRAVELLING WAVE TUBES BY DAMPENING UNDESIRED FREQUENCIES

Abstract: System to prevent undesired oscillations generation within coupled cell (or coupled cavity) travelling wave tubes, in essence consisting of one or more lossy termination waveguide sections, transparent to the undesired oscillation frequencies and non transparent to working frequencies; such sections include a dielectric filling (9) and a dissipative termination element (8). Such waveguide sections may be distributed in radial, longitudinal or both directions within the periodic structure of the travelling wave tube. Such waveguide sections are open towards the axis of the electron beam.

Description: "SYSTEM DEVISED TO PREVENT UNDESIRABLE OSCILLATIONS GENERATION WITHIN TRAVELLING WAVE TUBES BY DAMPENING UNDESIRED FRE¬ QUENCIES" The present invention is an improvement applicable to elec¬ tronic travelling wave tubes. Travelling wave tubes are devised to amplify RF oscillations in the GHz frequency range through interaction of a beam of charged particles, usually electrons, and an electromagnetic wave which propagates with suitable arrangements so as to amplify such electromagnetic wave. TWTS are particularly attractive and convenient because through their adoption, high amplification with low RF noise at relatively high powers and over extremely wide bands (of the order of hundreds of MHz) can be obtained. As a con'sequence these electronic tubes find wide application in radar equipment, microwave telecommunication systems, sa¬ tellites, etc. In use, these TWTS give way to the undesirable feature of pos- sible generation of spurious oscillations at the edges of SUBSTITUTE SHEET their operating bandwidth and particularly, close to the upper limit of their bandwidth. A further inconvenience connected with these oscillation instability phenomena is in that the anode voltage must be controlled accurately so that particular interaction modes do not generate, which would otherwise give way to these spurious oscillations. This required that, as an example, power modulation could be achieved only through the control grid. Many attempts have been made to reduce the impact of these inconveniences: 1) Bandwidth limitation: the problem of band edge oscillations has been avoided rather than cured, by reducing the TWT bandwidth to ensure synchronism between phase velocity of the circuit wave and the beam velocity close to inter¬ diction frequencies (Industrial Invention Patent IT 676.571, priority date USA 20 Nov. 1961). 2) Use of dissipating media which intervene also in the tube operating bandwidth. Oscillations are prevented by intro- ducing some attenuation (Power TWTS - J.F. Gitting - American Elsevies Publishing Co - N.Y. (c) 1965). 3) Use of low Q resonators coupled to the structure of interconnected cells or cavities. As for point 1) inconveniences are given by power limitation and by precarious stability, which impose strict control over anode voltage. As for point 2) inconveniences are given by gain reduction due to losses introduced, acting also within the operational bandwidth. As for point 3) inconveniences arise from the fact • that the SUBSTITUTE SHEET operating principles (resonating elements at single frequen- - cies) impose that the band of possible range of oscillation is covered by numerous elements, each tuned with great accuracy and therefore with great waste of work, bearing in mind that materials must be procured and tested to great accuracy, in the way of dielectric characteristics as regards permittivity. Therefore, in a few words, the most effective attempts aimed to the solving of this problem consisted in the introduction, within the tube structure, of elements resonating at the undesired frequencies, associated with dissipating elements, with the aim to attenuate the gain of the tube at these spurious frequencies. From an industrial viewpoint, the preparation of resonating elements is expensive because fine accuracy resonating fre- quency tuning of these elements is required. Scope of the present invention is that to provide a perfected system to obviate the inconveniences described above which show up in travelling wave tubes. According to the present invention, use is made of one or more waveguide sections with a lossy termination, transparent to the undesired frequencies and non transparent to the working frequencies. Still in accordance with the present invention, such waveguide sections include a dielectric filling, which on one side faces the electron beam interaction space, and on the other faces a dissipating termination element. Preferably, such waveguide sections are closed on the wall opposite to the election beam interaction space. Still according to the present invention, such waveguide segments with lossy termination, may be distributed in a SUBSTITUTE SHEET radial or longitudinal direction or both within the periodic structure of the electronic travelling wave tube. The present invention will now be described with reference to its presently preferred developments which are reported as an illustration, but are not limited to these, with reference to the figures and drawings enclosed. . Figure 1 shows the outline drawing of the TWT, where 1. is the magnetic focusing system of the tube 2. is the amplified signal output 3. is the signal input 4 & 5. are the cooling ducts 6. is the collector 7. is the cell coupling iris 8. is the attenuating element of the device object of this invention 9. is the device coupling & propagation element 10. is the cell (or cavity) spacer 11. polar expansion 12. electronic gun 13. cathode 14. focusing electrode Figure 2 shows three possible developments of the single atte¬ nuating device (the group of devices provide the attenuating system) (Figure 1 shows 2a only). Figure 3 shows some of the possible configurations of the accomodation of two or more devices per single cell. Figure 4 shows a) transmission band of the periodic structure without the attenuating device, while b) shows the periodic structure transmission band with the innovative device object . of this invention. SUBSTITUTE SHEET From figure 1 we may see the total longitudinal section of the ? TWT containing the group of devices object of the present invention. 12 shows the electron gun which contains a cathode 13 heated by a filament (not shown in the picture), an electron beam focusing electrode 14, a control grid 15. The electron beam, generated by the gun described, crosses the tube tunnel focused by means of the magnetic focusing system 1 which may be either a solenoid or a permanent magnet ones. Such electron beam interacts with an electromagnetic wave (input) through input circuit 3, the phase velocity of which is reduced within the periodic structure made up of cells (or cavities), coupler by means of the coupling irises 7. Of course, as can easily be seen in Figure 1, the amplified electromagnetic signal is picked up by port 2. The periodic structure formed of n cells is split into a given number of sections isolated from the RF viewpoint by means of suitable absorbing loads. At the end of their run, the electrons are picked up by col- lector 6 onto which they dissipate all their residual kinetic energy. Such collector 6 (which may be of the depressed type) is cooled by a liquid (or other means) which circulates within the apposite interspace through conducts 4 & 5. The collector may be cooled by forced air, through conduction etc. With reference to Figures 2 & 3, we shall now provide a detailed description of the structure according to the inven¬ tion presented. In it spacers 10 of the periodic structure are shown, of the known type for a TWT. Spacers 10 according to this invention are provided with waveguide sections loaded SUBSTITUTE SHEET with a dielectric 9 and terminated with a dissipating termina¬ tion 8. The proportioning of the waveguide sections shown in Figures 2 & 3 is made so that such sections are transparent to the spurious oscillation frequencies which could arise in par¬ ticular working conditions within the tube and non tran¬ sparent, i.e. below cut off, at working frequencies expected of the electronic TW tube so as not to modify, in a negative manner, normal operation of the tube within the expected frequency band. In particular the device, as can be seen in Figure 2a, is made up of a rectangular waveguide line filled with a dielectric 9 (such as Alumina or other) which faces the cell (or cavity) described and of a dissipating load 8 (made of Alumina, MgO or BeO loaded with conducting or semiconducting substances) placed in contact with dielectric 9. Within dielectric 9, the fundamental mode TE is excited, which propagates radially with reference to the TWT axis, to be then attenuated correspondingly to element 8. The dimension of the dielectric waveguide 9 is chosen as a function of cutoff frequency for mode TE starting from which frequency it is desired to introduce attenuation within the periodic structure. The device may be developed in different manners: - In manner 2a) , as just described. - In manner 2b), where the waveguide with dielectric has a circular section, in which case, within its body, mode TE is excited, with electrical field vibrating mainly in a direction parallel to the TWT axis, and where the waveguide diameter must be chosen as a function of such mode TE SUBSTITUTE SHEET cutoff frequency starting from which frequency it is desired that the device attenuates. - In manner 2c), where the dielectric waveguide is still rectangular and is still terminated with a dissipating load, while on its wider wall, a thin element is placed, as shown, consisting of lossy material which couples up wi^h the electric field propagating in the dielectric waveguide. It is worthwhile mentioning that, generally speaking, other configurations are possible, based upon the principle exposed above, i.e. that a line, dimensioned for a cutoff frequency, corresponding to the frequency above which it is desired to introduce attenuation into the tube, is coupled to the cell (or cavity) of the periodic structure and is terminated on the lossy load. It is not excluded that the dielectric line and the load may consist of one sole element performing both functions of frequency selection beyond which attenuation must begin and of attenuation itself. Figure 3 shows a few possible combinations of the attenuating devices within each cell (or cavity). The choice of number of devices to be housed within each cell (or group of cells) depends upon the desired attenuation which must be fixed from time to time. The results derived from this structure are shown in Figures 4a & b). Figure 4a) shows the shape of the periodic structure tran¬ smission curve without the attenuating devices and figure 4b) shows the shape of the same curve with attenuating devices inserted. As figure 4b shows, the device can attenuate effectively all SUBSTITUTE SHEET frequencies above 3.8 GHz in the example which refers to an S band structure, while no appreciable attenuation is introduced within the useful band of the TWT. By means of the inventions herein, results which are equal or better than those obtained by adopting other techniques to the same end, are attained with added advantages related to: 1. Simplicity of element build and tube assembly 2. Easily obtainable dielectric materials used 3. Less critical dimensional tolerance of materials 4. Lower cost by virtue of points 1, 2 & 3 above. As an example we may recall that a dielectric made of Al 0 and a dissipating element made of sintered 60% MgO and 40% SiC have been found convenient. This data should be taken as indicative, as it is left to an expert in the field to develop other composites which present the character stics needed to meet the effects of the technical guidelines as for this invention. This invention has been described with reference to some of its presently preferred developments, which are hereby pre- sented as illustrations, but are not limited to these, and it will be understood that in practice variations and modifica¬ tions may be introduced by an expert in the field without leaving the protected domain of the present industrial patent. SUBSTITUTE SHEET

Claims: CLAIMS 1. Travelling Wave Tube consisting of an electron gun for the generation of an electron beam, of a magnetic focusing system, of a collector to collect the exhausted electron beam and of the periodic structure of interconnected cells (or cavities), characterized by the fact that, such cells are coupled to devices which are lossy means and provide attenuation of the radio frequency energy and they select all frequencies above a prefixed frequency, with the end result of preventing oscillations (at such frequencies). 2. TWT as per claim 1 characterized by the fact that the device which prevents oscillations is made up of a section of waveguide line made of low tg material (such as Alumina) coupled to the cell and having cutoff frequency chosen to suit the desired propagation mode, and of a dissipating load having high tg , placed in contact with said waveguide made of low tg material. 3. TWT as per claims 1 & 2, characterized by the fact that the dielectric waveguide cutoff frequency is chosen so that it (guide) forms a high pass circuit starting from the frequency to be attenuated to prevent oscillations. 4. TWT as per claim 3, characterized by the fact that such dielectric guide is a high pass coupling line between cell and lossy load. 5. TWT as per Claim 2 characterized by the fact that the dissipating element may consist preferably of materials such as Alumina, Magnesium Oxide, Berillium Oxide, loaded with Carbon, Silicon Carbide or other so as to achieve a high tg dielectric. 6. TWT as per Claim 1 et alia, characterized by the fact that SUBSTITUTE SHEET the oscillation preventing devices may be one or more per periodic structure cell. 7. TWT as per Claim 1 characterized by the fact that the device may be made up (Figure 2) of dielectric elements in rectangular or cylindrical waveguide and the attenuating elements may be arranged so as to give way to such waveguide terminations, or along one or more walls of such dielectric waveguide. 8. System to attenuate undesidered frequency propagation at the upper boundary of the useful frequency band within a microwave structure, characterized by the fact that in the transit path of the electromagnetic energy one or more waveguide section are positioned, having a dissipating termination, transparent to the undesired frequencies and non transparent to the working frequencies, such sections, preferably, contain a dielectric filling and a dissipating termination, such waveguide sections may be distributed lenghtwise or radially or both within the microwave structure. 9. System, as per Claim 8 characterized by the fact that such microwave structure is a TWT as similar device. SUBSTJTUTE SHEET

Inventors: BUSACCA GUIDO, MELI VINCENZO, SPALLA MARIO, BUSACCA, GUIDO, MELI, VINCENZO, SPALLA, MARIO
Applicants: SELENIA IND ELETTRONICHE, SELENIA INDUSTRIE ELETTRONICHE ASSOCIATE S.P.A.
Application year: 1986
Main classification: H01J23
IPC codes: H01J23