US2151766A - Magnetron - Google Patents

Description

March 28, 1939. H, E, HQ| LMANVN 2,151,766

MAGNETRON Filed Sept. 8, 1938 Figs 1 INVENTORJ HANS ERIC/l HOLLMANN ATTORNEY.

Patented Mar. 28, 1939 MAGNETRON Hans Erich Hollmann, Berlin,

Germany, assignor to Telefunken Gesellschaft fiir Drahtlcse Telegraphic in. b. 11., Berlin,

tion of Germany Germany, a corpora- Application September 8, 1938, Serial No. 228,908

In Germany August 24, 1937 Claims.

My invention relates to electron discharge devices of the magnetron type, in which the electrode system is positioned within a magnetic field.

In the operation of the conventional magnetrons comprising an axial hot cathode and a cylindrical anode surrounding the cathode concentri'cally and consisting of a solid piece or split into a plurality of segments, it has frequently been observed that an abrupt rise of emission current occurs when the critical intensity of the magnetic field is approached or upon initiation of oscillations. The sudden rise of emission current is due to an additional or extra heating of the filament, in fact, this is readily discernible not only by a visible increase in the brightness, but also by a drop in the heating current occasioned by the incidental rise in the resistance of the filament. Under certain circumstances, the emission may gradually rise to such a point that the tube is caused to explode as a result of overload. Particularly where insufficiently evacuated tubes are concerned, or tubes exhibiting a tendency towards a sudden liberation of gases, the additional heating or extra heating eifect is dangerous, and this seems to indicate that the reheating is caused by bombardment of the cathode by gas ions. Another cause for such sudden reheating resides in the bombardment of the cathode by electrons returning to the cathode, a phenomenon which happens especially during the optimum working state of the tube, in other words, at a time when the energy yield is particularly great. It is known from practice that such extra heating effects in the case of oxide cathodes grow to proportions so that the use of cathodes of this kind is not feasible.

Various ways and means have been attempted in the past to either eliminate or suppress such extra heating efiects and their disturbing actions either entirely or else to at least diminish them. For instance, an attempt has been made to link the anode current of the tube with the magnetic field by causing the same to flow through one of the exciting coils. It has thus been feasible to meet a sudden and unexpected rise of the anode current by a practically simultaneous re-inforcement of the magnetic field. However, such a cure of the evil means always in practice a departure from optimum working conditions, in

fact, a magnetron arrangement utilizing the said idea operates under impaired conditions of efficiency. On the other hand, it has been suggested in the recent art to preclude from the outset all' risk of re-heating or extra heating by returning electrons by disposing the electron source itself outside the anode cylinder, the electrons being then attracted from the outside into the anode cylinder. As a consequence, they will never have a chance to fiy back to the cathode, indeed, they must always impinge upon the anode.

However, an arrangement of the cathode outside the anode cylinder is attended with the drawback that the distribution of the electrical field is essentially altered compared with what it is in the conventional magnetron tube of co-axial construction, and such unfavorable field distribution reacts disadvantageously upon the operation of tubes having an outside cathode.

The principal object of my invention is to provide an improved type of electron discharge device of the magnetron type in which undesirable heating by bombardment of the cathode by ions or electrons is substantially eliminated, and yet in which the distribution of the electrical field is not detrimentally modified.

The present invention discloses effective ways and means adapted to cure the undesirable heating evil by preserving the satisfactory concentric electrode mounting with radial field lines existing in the conventional type of magnetron, with this difference that the portion of the cathode which is located inside the cylindrical anode does not emit electrons and is cold, whereas the source proper of the electrons, that is to say, the heated cathode itself is at one or both ends of the cold cathode and lies on an extension of the axis of the cold section of the cathode. In order that the cold cathode may not be struck by returning electrons and thus be caused to give 01f secondary electrons, it will be found advantageous to impart to it a negative biasing potential with reference to the hot cathode so that reversing electrons are no longer able to strike it squarely.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which Figure 1 is a schematic section through one form of electron discharge device made according to my invention and its associated circuit and Figure 2 shows a schematic section of a modification of an electron discharge device made according to my invention.

Referring to Figure 1, an envelope G contains the cylindrical anode A which for the sake of simplicity is here shown unsplit, though it will. b evident that the anode cylinder could just as well comprise a plurality of segments. The hot cathode K is mounted outside the anode cylinder and extends in a direction along a prolongation of the axis of the tube. Now, according to the invention, adjacent the accelerator field directed from the cathode towards the anode cylinder, a well defined radial field is created by providing along the axis a rod or tubular cold cathode K which extends the full length of the anode cylinder, the cold cathode being kept either at the potential of the real cathode or the heated cathode K or else is impressed with a negative biasing potential referred to the hot cathode. It can be readily understood that all electrons which are drawn from the accelerative field extending from the heated cathode K to the anode cylinder into the space interiorly of the anode cylinder will, within the radial field between K and A be subject to the same or to similar kinetic laws as in the conventional type of magnetron tube. However, inasmuch as the paths of the electrons carried away from the heated cathode are spiraled, return of electrons to the heated cathode K is precluded and thus also the extra heating thereof is entirely avoided.

By choosing a suitable biasing potential for K it is an easy matter to make conditions so that optimum working conditions are realized. On the otherhandthecold cathodeK could be used to act as a control electrode for the purpose of modulating, for instance, the magnetron oscillations, and this is accomplishable in the absence of energy dissipation since it does not absorb any electrons. In order that in a similar Way the axial field component may be reinforced and suitably balanced, cathode K is conveniently surrounded by a metallic cylinder of the nature of the well-known Wehnelt cylinder or else rearward shielding is provided by means of a negatively biased terminal disk E. Further amplification of the axial field component, optionally, may be obtained by the aid of a terminal plate P which shuts the open end of the anode cylinder and which is kept at a positive potential. A coil M furnishes the magnetic field.

It will be understood that the basic idea underlying the present invention, namely, to divide the axial electrode into an electron-emissive part located outside the anode cylinder, and a central non-emissive portion is capable of a great many modifications Without departing from the fundamental idea of this invention.

An exemplified embodiment of such modification is shown in Figure 2 in the form of a symmetric magnetron tube in which the heated cathodes properly so-called surround the axial, cold cathode in the shape of concentric 0! coaxial spirals. The anode cylinder A has along its axis the negatively biased cold cathode K which, in this form of construction, extends at both ends beyond the anode cylinder. The projecting ends are concentrically enveloped by spirally heated wires, the latter constituting the heated cathodes K1 and K; at the two ends. As in the previous embodiment, both cathodes are shielded outwardly, that is to say, towards the end away from the anode cylinder, by means of terminal disks E1 and E2 which are also negatively biased, the said terminal disks, if desired, being fitted upon the axial electrode K. If necessary, the discharge spaces for the two cathodes may be separated by means of a radial disk or partition P mounted in the plane of symmetry. The action of the said partition disk is similar to the terminal disk in Figure 1. It will be obvious that the rotations of electrons must be in opposite senses in the two spaces because of the fact that the electrical fields are of contrary directions,

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is- 1. An electron discharge device having an envelope containing a straight cold cathode, an anode surrounding said cathode and co-axlal with said cathode, a thermionic cathode positioned adjacent one end but outside the space between said cold cathode and anode for supplying electrons to the space between said cathode and anode and lying along the axes of said cold cathode and anode and means for producinga magnetic field parallel to and between the cold cathode and said anode.

2. An electron discharge device having an envelope containing a straight cold cathode, an

anode surrounding and co-axial with said cold cathode, a thermionic cathode positioned adjacent one end and along the axis extended from said cathode and anode for supplying electrons to the space between said cold cathode and said anode and shielding means co-axial with said cold cathode and positioned outside of and adjacent the anode and the thermionic cathode.

3. An electron discharge device having an envelope containing a straight cold cathode, an anode surrounding and co-axial with said cold cathode, a thermionic cathode positioned adjacent one end and along the axis extended from said cathode and anode for supplying electrons to the space between said cold cathode and said anode and shielding means co-axial with said cold cathode and positioned outside of and adjacent the anode and the thermionic cathode, and a shield closing the other end of the anode.

4. An electron discharge device having an envelope containing a straight cold cathode, an anode co-axial with and surrounding said cathode, thermionic cathodes positioned adjacent each end of the anode and said cold cathode and co-axial with said cold cathode but outside of the space between said cathode and said anode, H

and shielding means adjacent each cathode outside of but adjacent said anode.

5. An electron discharge device having an envelope containing a straight cold cathode, an anode co-axial with and surrounding said cathode, thermionic cathodes positioned adjacent each end of the anode and said cold cathode and coaxial with said cold cathode but outside of the space between said cathode and said anode, and shielding means adjacent each thermionic cathode, and an electrode extending transversely of said cold cathode and anode and between said cold cathode and anode for confining electrons from each of said thermionic cathodes to the halves of the interior of said anode adjacent the respective thermionic cathodes,

HANS ERICH I-IOILMANN.

Images