Vircator

Field of the invention

The present invention relates to an improvement of an existing type of a micro-wave generator. More specifically it relates to a generator of a vircator type that is given such a configuration that it is possible to achieve an electromagnetical field coupling between the different resonance areas of the vircator.

Background to the invention A vircator is a type of radiation generator that comprises a vacuum pumped cavity and, usually, two electrodes, a cathode and an anode. There are a number of different vircator types wherein the differences resides in, among other things, the geometrical configuration of the cathode and the anode. There is for example a vircator type with two planar electrodes placed on a symmetry axis, a so called axial vircator, but there are also other types for example the co-axial vircator wherein the anode and the cathode are provided in the form of two concentric cylinders. In addition to these one can also mention the reflextriode and the reditrone. The functionality of the vircator is based on the fact that electrons accelerated from the cathode in the direction of the anode are allowed to pass through the anode through small apertures in the same. Those electrons that pass the anode through these small apertures will form an electron cloud in the area beyond the anode (in the direction taken from the cathode). The electron cloud formed in this way will constitute a virtual cathode, a fact that has given the device its name vircator (in English, Virtual cathode oscillator). The virtual cathode will reflect the incoming electrons and this will lead to an oscillation of electrons between the real and the virtual cathode. Not only will the electrons oscillate but also the electron cloud will oscillate with a given frequency to and fro in the direction of the radiation. The charge density of the electron cloud will also vary which will lead to a change of the self-oscillation frequency for the electron plasma in the cloud. The two electron oscillations, depending on variations of position and density, will couple to the surrounding cavity and will often lead to different radiation modes and different radiation frequencies. Beyond these there is also a further area wherein radiation is generated, the diode, that is, the area between the anode and the virtual cathode. This happens because the electron current in the diode will oscillate as a result of the fact that the electrons that have been reflected by the virtual cathode can push its way into the diode through the holes in the anode. In existing vircators

electromagnetic radiation is not allowed to pass through the anode since the dimensions of the holes in the anode are a lot smaller than the wavelength of the electromagnetic radiation.

As a consequence there are two separate radiation generating areas, the area between the cathode and the anode and the area around the virtual cathode. Each of these two areas constitutes two resonance cavities having different geometrical configurations and often different resonance frequencies. As a consequence the radiation generated in the vircator often shows a distribution in frequency, polarization and amplitude, and therefore the extracted radiation will lack a well defined radiation frequency or radiation mode, instead it will generally consist of a mixture of the different radiation modes and radiation frequencies. The present invention has as a purpose to provide a vircator with improved radiation characteristics regarding both frequency and mode. This object is achieved by means of a vircator according to claim 1. Preferred embodiments of the invention are disclosed in the dependent claims.

Brief disclosure of the invention

The fundamental idea behind the present invention is to provide the anode in the vircator arrangement with apertures or slots that makes it possible for electromagnetic radiation to at least partially pass the anode. Such an arrangement would yield the advantage that the two oscillation areas in the vircator were coupled through their electromagnetic fields. Each of these resonance areas would therefore resonate with the generated electromagnetic field and as a result one obtains a controllable coupling between the two areas. With partial passage through the anode is hereby meant that only certain polarizations of the electromagnetic radiation are allowed to pass. It is also possible to connect the anode to an externally placed regulation mechanism that can direct the anode so that one can manipulate the polarization direction from a position outside of the vircator cavity. It is, for example, possible to connect the frame of the anode to a turning rod that turns the anode in such a way that the apertures are turned which would give to a polarisation that depends on the turning angle. A possible arrangement to make a turning of the anode possible could use magnets.

Figures

Fig. 1 is a schematic disclosure of the configuration of a known axial vircator. Fig. 2 discloses, in cross-section, the configuration of a known axial vircator.

Fig. 3 discloses a possible geometric configuration for an anode used in the present invention. Here is disclosed a number of parallel metal rods that are attached to a circular frame is provided parallel with the frame holder.

Fig.4 discloses another possible geometric configuration for an anode used in the present invention. Here is disclosed a number of parallel metal threads that are attached to a circular frame and are arranged in angle relative the frame holder.

Fig. 5 discloses a possible geometric configuration for an anode that can be used in the present invention. Here is disclosed a piece with a number of cut-out slots, the metal piece is attached to a circular frame and the slots are orthogonal to the holder. Fig. 6 discloses schematically a cylindrical anode with axis-parallel conductors. The anode is intended to be provided within a cylindrical cathode. Only parts of the conductors are shown.

Embodiments In the following section the invention will be described with the help of preferred embodiments. These embodiments will be described with references to the appended drawings and in the case that the invention is part of an axial vircator. The fact that an axial vircator is used does not limit the applicability of the present invention; it is possible to use the invention in any type of vircator that generates two or more oscillation areas. Before we give explicit embodiments of the invention we will briefly describe the design of a conventional vircator. With reference to fig. 2 which discloses, in cross-section, a known design of an axial vircator, reference numeral 1 represents the cathode of the vircator 4. The cathode is constructed from, or lined with, a material or structure that can easily emit electrons. If a voltage is applied between the cathode 1 and anode 2 an electromagnetic field is generated that will rip electrons from the cathode 1 and accelerate these towards anode 2. The anode 2 is made from a thin foil or a fine-meshed net that partly let the accelerated electrons pass. The electrons that pass the anode will be collected in the area between the anode 2 and the window 3 and they will build up an electron cloud 5. As has been mentioned earlier this cloud could be construed as a virtual cathode. If the charge density of the cloud 5 is dense enough the electrons that closes in on the

cloud with a kinetic energy that is too low will get reflected back towards the anode 2. This will lead to a complicated coupling between addition of electrons through the anode 2 and losses of electrons from the virtual cathode 5 through the anode 2. This in turn will lead to oscillations in the area between the virtual cathode and the anode. Moreover, there will also arise oscillations between the real cathode 1 and the anode 2, as a consequence there will be two distinct oscillation areas in this type of vircator. The radiation generated through these oscillations will suffer from the earlier mentioned problems regarding different frequency modes and radiation amplitude variations. According to the invention these problems can be mitigated if the anode 2 is provided with slots or elongated apertures that make it possible for electromagnetic radiation to pass the anode. To obtain an acceleration field for the electrons that is homogeneous enough the apertures or slots cannot be to large. The reason behind this is the fact that the acceleration field will bend towards the direction of the conductors provided in the anode. The apertures can not be too small either since this would make the electrons collide with the conductors in the anode. The measure that is suited to regulate the choice of dimensions is the transparency of the anode which is defined to be the ratio between open area and anode area. The main purpose of having a well balanced transparency is to make it possible to quickly build up the electron cloud that defines the virtual cathode. In fig.3 the anode is constructed out of an arrangement of parallel extending metal rods attached to a circular frame. In the case the anode, in place of the fine-meshed net or the foil, is given the shape depicted in figure 3 the electromagnetic radiation from the two radiation generating areas will get coupled through the electromagnetic field that is polarized orthogonal to the direction in which the conductors of the anode runs, this arrangement will give a higher degree of coupling between the different electron oscillations then is the case with the use of known anode configurations. Figure 4 discloses another geometrically preferred shape wherein the anode comprises a number of metal threads oriented in an ambiguous angle relative the holder. Figure 5 gives yet another possible embodiment of the invention wherein the anode is constructed from a metal plate cast in one piece but where parallel slots have been cut out in a direction parallel to the holder.

One of the advantages of the above given anode configuration is that it is easy to control the geometric transparency of the anode, that is the total projected surface of the space between the threads/rods/slots as part of the total surface of the anode. It will be easy to adjust this transparency by means of changing the width of the slots or the space between the slots or, in the corresponding case, changing the

dimensions or the mutual distance between threads/rods. Another advantage that comes with the use of metal rods is that one can give their cross-section area such dimensions that they are capable of conducting the currents that can be necessary in case of large power output. The same advantage is obtained with an anode 2 that is constructed from a single piece but where slots have been cut out, in this case the remaining rods will be given the dimensions needed for the expected currents. An advantage with cut-out conductors instead of conductors attached to a frame holder is that a good contact is obtained between the rods and the rest of the anode. If there is a poor attachment between the threads/rods and the holder a contact resistance will be created that could impart an uneven anode potential and hence poorer performance. Yet another advantage obtained through the use of metal rods is the possibility to give the rods an ambiguous curved form. The nets that are usually used are often planar and cannot easily be given the form of a doubly curved surface, for example; instead they can only be given the form of simple curved surfaces such as a cylindrical shape.

The above described embodiments of the anode are well suited for use in a co-axial vircator. By using the present invention, which provides a partial transparency for micro-wave radiation, a coupling of oscillations of the electron plasma is obtained in the virtual cathode and in the volume between the cathode and the anode which provides the user with the possibility to control the generated modes.

Even in the case of a cylindrically formed anode arranged co-axially with the cathode is it possible to arrange a, from the vircator external, regulation mechanism that regulates the area of the anode. For example, in the case that the anode is constructed out of a number of symmetrically arranged conductors running parallel along an imagined cylindrical surface, the externally arranged regulation mechanism could be a squeeze mechanism that either squeezes the conductors together so that their mutual distance is reduced or release the conductors so that their mutual distance is increased. The cylinder surface that is spanned by these conductors would in these cases increase and decrease, respectively. As has been mentioned earlier such an arrangement could prove to be advantageous since one is able to control the vircator without having access to the vircator cavity by manipulating the transparency of the anode. Yet another such regulation mechanism could reside in the fact that the ring or the rings that the end or the ends of the conductors are attached to could be manipulated in such a way that the ring/rings get expanded or contracted in a radial direction as seen from the centre of the cylindrical surface. Since the ends of the conductors are attached to the ring or the rings their mutual

distance will be altered. As a direct consequence the user can influence the diameter of the anode and hence the cylindrical surface which will give a regulation of the vircator that makes it possible to vary the resonance frequency instead of the one or the ones that can be obtained with a fixed anode diameter.