WO2010044719A1

WO2010044719A1 - Vircator with variable frequency and polarisation

Info

Publication number: WO2010044719A1
Application number: PCT/SE2009/000461
Authority: WO
Inventors: Tomas Hurtig; Cecilia MÖLLER; Mattias Elfsberg
Original assignee: TotalFoersvarets Forskningsinstitut FOI
Current assignee: TotalFoersvarets Forskningsinstitut FOI
Priority date: 2008-10-16
Filing date: 2009-10-14
Publication date: 2010-04-22
Anticipated expiration: 2011-04-16
Also published as: SE0802209L; SE531894C2

Abstract

The present invention relates to a coaxial vircator where the cathode is arranged relative the anode in such a way that it is allowed to rotate around its own and the anodes cylindrical axis. In further embodiments are only parts of the cathode surface covered with an emitting material which leads to an angular dependence between the emitting cover and the anode. Alternatively, or as a complement, the anode and the cathode could be given asymmetrically arranged symmetry axes. This leads to a varying distance between the anodes midpoint and the cathodes midpoint.

Description

Vircator with variable frequency and polarisation

The present invention relates to a vircator that makes it possible for a user to manipulate the vircator in such a way as to generate radiation wherein both the polarisation and frequency of the radiation can be varied. More specifically the invention relates to a variation of a coaxial vircator.

Background to the invention

A vircator (from the English term "virtual cathode oscillator") is most easily described as a microwave generator where the radiation is generated by means of oscillations created between a cathode, a virtual cathode and an intermediate anode. The central concept with a vircator according to prior art is how the virtual cathode is created. In broad terms such a known vircator can be described as follows. First of all the vircator contains a cathode. In the case of a coaxial vircator said cathode possesses a cylindrical symmetry. Moreover said cathode is covered with an emitting material which, upon applying a voltage over the vircator, makes emission of electrons possible. The second part of the vircator is an anode which, as for the cathode, possesses a cylindrical symmetry in the case of a coaxial vircator and which furthermore is provided inside of the cathode (hence the name coaxial). To obtain a virtual cathode a necessary feature of the anode is that it is partly transparent for electrons. When a voltage is applied over the anode-cathode gap of the vircator, electrons will be emitted from the emitting material. These electrons will accelerate towards the anode and if this anode is transparent some of the electrons will propagate from the cathode pass the anode to an area beyond the anode. In this area the electrons will accumulate and create an electron cloud. This cloud of electrons can be viewed as a virtual cathode. Where in the vircator the cloud is created depends on the geometry of the vircator, in the case of a linear vircator (that is, the anode and the cathode is provided after each other in the linear direction of the vircator) the virtual cathode will be created behind the anode as seen from the cathode while, in the case of a coaxial vircator the virtual cathode will be created inside of the anode. It is the interaction between this virtual cathode, the anode and the real cathode that provides the vircator with the features that makes it suitable as a microwave generator. One drawback with currently known vircators (there exists, as is clear from above, a number of different vircators where the coaxial type is but one) is that the user lacks the possibility to steer or control the frequency and the polarisation of the generated radiation. Since the possibility to steer and control these entities - frequency and polarisation - would mean a profound enhancement with regard to the efficiency of the vircator it would be a great improvement to obtain a vircator that makes this possible. A specific variant of a vircator with these sought for features are disclosed in this application.

The vircator according to the present invention also provides the user with the possibility to quickly alterate the polarisation and the frequency of the pulses.

One of the advantages of this is that it becomes possible to assign every pulse in a pulse sequence its own frequency and polarisation. In the case of known vircators this is impossible since each vircator arrangement corresponds to one specific frequency and polarisation. To create pulses with different frequencies and polarisations it is therefore necessary to replace the parts making up the vircator after each generated pulse. This replacement of parts is a very time consuming process and it is therefore in practice impossible to create pulse sequences where each pulse corresponds to a specific polarisation and frequency.

Drawings

Fig. 1a discloses schematically a traditional and known coaxial vircator in side view. The cathode is given the numeral (11) and the anode the numeral (10). The whole surface of the cathode is covered by an emitting material (12). The common symmetry axis of the anode and the cathode is given the numeral (13). Fig. 1b discloses the cathode-anode arrangement for the same vircator as seen in cross-section. The common axis (13) of the cathode (11) and the anode (10) is directed into the paper.

Fig.2a, is a side view of a vircator version according to the present invention wherein the anode (10) and the cathode (11) are positioned asymmetrically, that is, they do not a share a common symmetry axis. The symmetry axis of the cathode is given the numeral (13) and the symmetry axis of the anode is given the numeral (23).

Fig 2b discloses the anode-cathode arrangement for the vircator in fig 2a, but as seen in cross-section. The symmetry axis of the anode as well as the symmetry axis of the cathode is directed into the paper. The axes are however asymmetrical. The symmetry point of the anode is denoted with an x corresponding to (14) and the corresponding symmetry point of the anode is denoted with a + corresponding to (24). In the figure are only parts of the inner surface of the cathode (11) covered with the emitting material denoted by (22).

Fig. 3 a-d discloses four different states for the case that the cathode (11) is allowed to rotate around the anode (10). In these drawings there is disclosed, in cross-section, a cathode-anode arrangement for a vircator with the features that the anode and the cathode are asymmetrically provided and that only parts of the cathode are covered with an emitting material.

Fig. 4a discloses a cathode-anode arrangement according to figure 2 b with a turning rod (15) provided on the cathode. There is also disclosed an electric motor (16) that acts turn the turning rod to thereby rotate the cathode (11) around the anode (10).

Fig 4b discloses the cathode-anode arrangement according to figure 2b or figure 4a but where it is also disclosed a magnet arrangement with an outer magnet (17) and an inner magnet (18) which, through magnetic interaction, is used to rotate the cathode around the anode. The magnets are symbolically disclosed as horse-shoe magnets in the figures to obtain clarity. Here the electric motor is used to magnetise the magnet (17).

Detailed description of the invention

In what follows the invention will be described with reference to the figures.

In figure 1a and 1b there is disclosed a conventional vircator according to prior art. As is clear from the figures the cylindrically shaped cathode is denoted (12) and the cylindrically shaped anode is denoted (10). The virtual cathode, which in reality comprises a number of electrons, is positioned in the cylindrically constrained area inside the anode. As has been mentioned, the whole inner surface of the cylindrically shaped cathode (11) is covered by a material (22) that emits electrons. Moreover, the symmetry axis (13) for the cathode and the anode coincides in this type of known vircator.

In figure 2 there is instead disclosed a vircator according to the invention. Here the cylindrically shaped cathode is provided relative the anode in such a way that their midpoints, or symmetry points, do not coincide. That is, they are asymmetrically provided relative each other. The advantages of this variant will be described in more detail in what follows.

In figure 3 there is disclosed how the co-axial vircator looks in different operative modes for the case that the cathode (11) is provided relative the anode (10) in such a way as to make it possible to rotate it around its own axis. This figure furthermore shows that only part of the cylindrically shaped cathode is covered with an emitting material (22).

In what follows the invention will be described by means of examples. In the first example there is considered a co-axial vircator wherein the cathode is both symmetrically provided relative the anode and is also allowed to rotate around its own axis. Furthermore there is assumed that only parts of the inner surface of the cathode, the surface directed towards the anode, is covered with an emitting material. Considering figures 3a-d (given that the asymmetry between cathode and anode is ignored, that is the cathode and the anode shares a common midpoint) it is clear that an angular dependence is obtained between the midpoint (symmetry point) of the anode and the part of the cathode that is covered with the emitting material. This angular dependence provides the user with the possibility to control the polarisation of the generated radiation by means of changing the angle between the emitting covering and the midpoint of the anode.

In another example we consider a vircator with the following features: i) the cathode of the vircator is only partly covered with an emitting material, ii) the cathode of the vircator is allowed to rotate around its own symmetry axis iii) the cathode of the vircator and the anode of the vircator are asymmetrically provided relative each other.

The feature that the cathode is asymmetrically provided relative the anode only means that the midpoints of the anode and the cathode are not common. In the case that the cylindrically shaped anode and the cylindrically shaped cathode share a common midpoint (symmetry point) they will clearly share a common symmetry axis (that is, a common cylinder symmetrical symmetry axis). In the case they lack a common midpoint this means that they possess non-common symmetry axes. It is this embodiment that is explicitly disclosed in figure 3a-d. As is clear from these figures this embodiment not only gives an angular dependence between the emitting surface and the midpoint of the anode it also means that the distance between the emitting surface and the midpoint of the anode will vary when the cathode is rotated around its own axis. Therefore the user obtains two variables to vary in this embodiment to influence/manipulate the frequency and polarisation of the radiation, namely the angle between the midpoint of the anode and the emitting surface as well as the distance between the midpoint of the anode and the emitting surface. A third embodiment of a vircator according to the present invention pertains to a vircator according to the preceding example with the difference that the whole surface of the cathode is covered with an emitting material. In this embodiment the cathode and the anode are asymmetrically provided and therefore a rotation of the cathode around the anode makes the distance between the midpoint, or symmetry point, of the anode and the emitting material vary. This will provide the user with a variable to vary during use, namely the distance between the anode and the surface of the cathode.

A vircator according to any of these described embodiments therefore provides the user with means to influence the polarisation and frequency of the generated radiation in such a way as to maximise the performance and usefulness of the vircator in different situations.

Concerning the rotation of the vircator cathode this can be achieved in a number of different ways. For example, with reference to figure 4a, by means of a turning rod (15) with an inner end (25) attached to the cathode and an outer end 26 reaching out of the vircator whereby a rotation of the cathode can be performed manually from a position outside of the vircator. If a turning rod is used it is possible to provide this turning rod with markings corresponding to different frequencies and polarisations for the radiation. The positions of these markings can be set by determining, experimentally, the polarisation and frequency of the generated radiation and then relate the position of the cathode to the markings on the turning rod. In the event that the intention is to provide a pulse train, wherein the pulses making up the pulse train discloses different frequencies and polarisations, the use of a manually rotatable cathode is not a preferred method. In these events it is necessary to rotate the cathode a lot faster, perhaps tens of revolutions a second. To obtain this type of rotation it is necessary to rotate the cathode with means of an electrical motor (16). One could, for example, connect an electrical rotation motor to a strictly mechanical turning rod whereby the cathode is rotated as in the above given example with the difference that the rotation is a lot faster. With reference to figure 4 b there is disclosed yet another method that can be used to rotate the cathode around the anode where the method is based on providing the outer surface of the cathode with an inner magnet (18). In this way it is possible to rotate the cathode by means of an outer magnet (17) provided on the outside of the vircator. In figure 4b the vircator casing is symbolically represented as a square. Through magnetic interaction between the inner and outer magnet it is possible make the cathode rotate by letting the outer magnet rotate around the symmetry axis of the cathode. It is possible to use more than one magnet on the cathode surface and more than one magnet provided outside the vircator. It is also possible to enclose the whole vircator in a material that can be magnetised. By inducing a magnet field in this material it is possible to rotate the cathode by means of magnetic interactions from this enclosing material. The use of magnets (17,18) to rotate the cathode provides for an advantage for those cases when a high degree of vacuum in the cavity of the vircator is needed. In those situations it is not suitable to use a turning rod since this would require that some sort of aperture is provided in the vircator wall to be able to direct the outer end of the turning rod out of the cavity. All sorts of apertures will of course influence the vacuum in a negative way.

Claims

1. Vircator comprising a cathode (11), provided with an emitting material (22), and an anode (10), the cathode (11) being arranged around the anode (10), characterised in that the cathode (11) is provided relative the anode (10) in such a way as to be rotatable around the anode (11).

2. Vircator according to claim 1 , characterised in that only parts of the surface of the cathode (11) is covered with the emitting material (22) whereby a rotation of the cathode (11) around the anode (10) leads to an angular dependence between the anode (10) and those parts of the cathode (11) that are provided with the emitting material.

3. Vircator according to claim 1 or 2, characterised in that the cathode (11) is attached relative the anode (10) in such a way that the midpoints (14, 24) of the anode (10) and the cathode (11) are not coinciding whereby a rotation of the cathode (11) leads to a varying distance between the anode (10) and the cathode surface with the emitting material (22).

4. Vircator according to claims 1-3, characterised in that it also comprises rotation means (15, 17, 18) for rotating the cathode.

5. Vircator according to claim 4, characterised in that said rotating means are driven by an electric motor.

6. Vircator according to claim 4, characterised in that said rotating means consists of a, to the cathode connected, turning rod (15) for a manual rotation of the cathode (11).

7. Vircator according to claim 4, characterised in that said rotation means consist of at least two magnets (17,18) where at least one inner magnet (18) is provided on the cathode (11) and where at least one outer magnet (17) is provided outside of the vircator whereby a rotation of the cathode (11) is obtained through magnetic interaction when the outer magnet (17) is rotated around the symmetry axis of the cathode.