The invention relates to an antenna producing a millimeter wave beam having a Gaussian-like distribution and using a high power source.

Such sources, which are called cyclotron resonance masers, for example gyrotrons, have successfully been used for plasma build-up, profile control and resonance heating in the field of thermonuclear fusion research at power levels in the MW- range. The oscillation of these sources is in axially symmet- ric modes TE O _n n or in high asymmetric modes (TE n) with >> 1.

As future fusion devices are planned to be larger and to ope¬ rate at higher magnetic fields, sources must be developped for higher frequencies (above 100 GHz) and higher microwave power. For these applications, it would be desirable to dispose of an axisy metric narrow pencil-like beam with well defined linear polarization, as for example a TEM _0f) Gaussian beam. In fact, this kind of beam is not only required for efficient electron cyclotron resonant heating of a plasma, but also for low-loss high power millimeter wave transmission.

From IEEE Transactions Microw. Th. Techn. , Vol. NTT-26 N° 5, 1978, page 332 to 334, a mode transducing antenna is known, having a corrugation by which a TE _β mode is converted into a linearily polarized beam. However, the microwave radiation issuing from such an antenna still presents considerable side- lobes which reduce the energy concentration in the main lobe and increase the losses of the system.

The main object of the present invention is thus to reduce the

energy dissipated in the sidelobes and to provide an axisym- metric, narrow, Gaussian-like main lobe.

According to a particular application of the present inven- tion, the antenna system provides for an output radiation of the linear TEM-.-mode type.

These objects are achieved according to the invention by the antenna as defined in the attached claims.

The invention will now be described in greater detail by means of several embodiments and in relation to the attached draw¬ ings.

Figure 1 shows schematically a preferred embodiment of the invention in a cross-sectional view.

Figure 2 represents a variant to the embodiment of figure 1.

Figure 3 shows schematically a partial mode converter used in the embodiments according to figures 1 and 2.

Figure 4 is a detailed view on an enlarged scale of the cor¬ rugated surface of a reflector used in the embodiments of figures 1 and 2.

Figure 5 shows an alternative configuration of reflectors for an antenna system according to the invention.

Although the invention applies indifferently to electric modes TE and magnetic modes TM, reference will be made hereafter in particular to TE modes.

The invention is based on the following common features for circular symmetric TEm,n modes, and in particular TE _n un modes:

a - all patterns have the same location of the minima between the sidelobes; b - the maximum of the main lobe is close to the angular posi- tion corresponding to the n-th zero of the Bessel function

J ₁ ( ^χ ) ; c - the phase of the main lobe in the TE _π mode series is alternating.

These properties allow the reduction of the inner and outer sidelobes of a given dominant TE _Q mode pattern by superim¬ posing a convenient percentage of power in the modes TE

^J 0,n-1 and TE _Q → (both with opposite phase with respect to TE. ) .

The same rule applies also to any TEm,n mode. Experimentally, it has been shown that at the centre of the pattern, there is practically no radiative power (the main lobe is down by more than 30 dB) and that the sidelobe level is down also by more than 30 dB. It has further been observed that there is practi¬ cally no cross-polarization which is a fundamental result, quite different from the starting rotating asymmetric modes.

Now reverting to figure 1, the system comprises a high power microwave source (e.g. gyrotron) including a gun and cavity portion 1 able to release for example a beam at 140 GHz and at a power level of 1 MW. The microwave beam output by the source

1 is of the type TE ♦ The values selected for m and n are ^J m,n for example 15 and 2 respectively. This microwave radiation is then converted in a partial mode converter 2 into a mixture of waves which still contains 70 to 85% of the dominant TEm,n mode and additionally small percentages between 10 and 20% of modes of the types TE → and TE , - . The mutual phase rela- m,n-l m,n+l tionship is such that the additional modes are both in coun- terphase with respect to the dominant mode. Although the mix¬ ture of these three modes provides best results, the micxture of only two of them, the dominant mode at 80% and one of the

additional modes at 20% power rate, provides beam characteris¬ tics which are only slightly worse.

The mixture of modes is then radiated to a collimation reflec- tor 3 which is provided with a central hole 4 allowing the electron beam of the gyrotron (issuing from the converter 2) to pass therethrough.

As it has been stated earlier, the mode mixture radiated by the converter 2 has an azimutal polarization with axial n- fold symmetry and the power is concentrated in a ring. Hence, this hole 4 does not increase the energy losses of the micro¬ wave radiation. Due to the fact that there are practically no sidelobes, the entire microwave energy is directed to the annular zone of the reflector 3. The reflector is shaped in such a way that the deflected radiation concentrates in a focus point 5 or, more generally, in a collimation zone around that point. The active surface of the reflector 3 is corruga¬ ted in such a way that the radiated mode mixture having a- circular electric field is converted into the linearily pola¬ rized TEM- ₀ mode, which can be directly used as the propaga¬ tion mode either in the free space or in an appropriate wave¬ guide which is not shown.

The entire system including the reflector 3 is enclosed in the gyrotron vacuum casing 8 and the microwave radiation issuing from the corrugated reflector 3 passes through a semicylindri- cal radiofrequency window 6 while the electron beam is pro¬ jected against a depressed collector 7 at the remote end of the casing 8.

An alternative configuration with respect to fig. 1 is shown in fig. 2. In this case, the corrugated reflector is a plane reflector 9 of an annular disk shape, whereas the collimation is achieved by an additional reflector 10 which is disposed in

the path of the microwave radiation downstream of the plane corrugated reflector 9 and outside of the casing 8 which is not shown here, but which is similar to that of fig. 1 and is equipped with a collector for the electron beam of the gyro- tron, and with a radiofrequency window.

Fig. 3 shows in more detail and at an enlarged scale the part¬ ial mode converter 2 in fig. 1. This converter receives on the left hand side of the figure the radiation issued by the gyrotron, the microwaves being of the TEm,n mode type. The converter is composed in series of two partial mode converters constituted by rippled wall waveguide portions 11 and 12 res¬ pectively. The shape of the ripples of the two partial con¬ verters is different and allows in the first converter 11 a portion of say 15% of the incident wave to be converted into the TEm,n-_ι. mode (in opposite phase with respect to the TEm,n mode) , whereas in the downstream converter 12, a similar amount of energy of the basic mode is transformed. into the mode TEm,n+,l_. (also in opposite phase) . The percentages of power converted in each converter depend on the length of the individual converters.

In order to reduce the size of the system, a compromise should be made between the oversizing of the partial mode converter and its length.

The. distance between the radiating surface of the partial mode converter 2 and the reflector having the hole is chosen in order to have a sufficiently low power density on the surface of the reflector to avoid radio frequency break-down, and to have a sufficiently narrow field radiation to reduce the size of this reflector.

Fig. 4 shows a detailed sketch of the corrugations of the reflector 3 or 9. The corrugations are shown here as straight

line corrugations for reasons of simplicity only. The pitch of the corrugations corresponds to about half the wavelength of the central operating frequency of the system. The depth of the grooves is about equal to a quarter wavelength at the same frequency.

The system as shown in fig. 1 and fig. 2 presents an expected efficiency, i.e. the ratio between the power collimated in the main lobe (in zone 5) having linear polarization and the power output of the gyrotron, of at least 95%.

Another possible use of the device is in a Cassegrain antenna of the type described in LTS-A-3 235 870. Figure 5 shows this application. The RF power radiated in a ring by a waveguide 13 is converted by a corrugated subreflector 14 into a linearly polarized beam and hence focussed by a main reflector 15. The latter has a hole in the center in order to allow the instal¬ lation of the waveguide 13.

The invention is not restricted to the embodiments described above.

In particular, it is possible to locate the casing 8 (fig. 1) together with the partial mode converter 2 outside the micro- wave source, either close to its exit or close to the place where the microwave energy is used, while the gyrotron is located remote therefrom and connected thereto by conventional guides. If, in particular applications, the conversion of the microwave energy into the basic mode TEM _Q0 is not required, of course, none of the reflectors (of fig. 1, 2, 5) must be a corrugated reflector.

If, in the course of the above description, reference has been made to a focus, this also includes the case of an infinite focus, or, in other words, of a parallel output beam.