EP0465845A2 - Microwave window - Google Patents

Abstract

A wide bandwidth microwave window has at least one pane which is inclined with respect to the propagation direction of the microwaves in accordance with the Brewster angle. The pane, which is transparent for microwaves, has plane-parallel, flat main surfaces. A plane formed by the propagation direction and the normal to the pane is in the same plane as the polarisation direction of the microwaves. Said window is suitable as a decoupling window for a quasi-optical gyrotron or as an inlet window of a fusion chamber or of a microwave calorimeter. <IMAGE>

Description

Technical field

The invention relates to a microwave window of high bandwidth for linearly polarized microwaves of high power, comprising at least one pane which is transparent to microwaves and has plane-parallel, planar main surfaces.

State of the art

When using microwaves for fusion research, it is desirable to be able to change the frequency of the source (transmitter) without having to accept a large reduction in power. This frequency change should also be able to happen quickly in order to possibly be able to react to physical processes in the plasma.

These conditions place high demands on both the microwave generation and the vacuum window of the microwave tube.

The quasi-optical gyrotron, as described, for example, in patent CH-664045 or in the article "The gyrotron, key component for High power microwave transmitter ", HG Mathews, Minh Quang Tran, Brown Boveri Review 6-1987, pp. 303-307, has already shown that it can be tuned in frequency over a relatively wide range.

The microwave window is required not to reflect for the desired frequency. In addition, it should also be low-reflection for neighboring frequencies in order to avoid unwanted oscillations.

For example, a coolable double-pane window is known from the report "Development of the Technological Basis of a Heavy-Duty Coupling Window for a 200 kW Long-Pulse Gyrotron at 140 GHz", Rudolf Bachmor, ITG Technical Report Vacuum Electronics and Displays of the ITG Conference from May 8-10, 1989. This double window consists of two ceramic panes that are perpendicular to the direction of propagation of the microwaves and e.g. be cooled by a coolant. It forms a Fabry-Perot resonator, the frequency of which depends on the distance between the panes. The thickness of the pane is usually chosen so that the window works for the main frequency without reflection. For every other desired frequency, the disc distance is set to optimal transmission.

This principle works for practically every fashion in the waveguide, to a limited extent even for fashion mixtures. The disadvantage of this solution, however, is that the frequency adjustment takes place mechanically and therefore slowly.

A broadband window is the so-called moth eye window. The ceramic disc forming the window is perpendicular to the direction of propagation of the microwaves. However, the surface of the ceramic is provided with pyramids that create an impedance matching. The height of these pyramids corresponds to at least a quarter of the wavelength of the desired center frequency. In this way it was possible to build a window with a relatively large bandwidth.

This principle also works regardless of the mode. However, the effort to manufacture such windows is very large. What is also unknown is the behavior of the ceramic tips at very high field strengths, where arcing is generally to be expected.

Presentation of the invention

The object of the invention is therefore to provide a microwave window of the type mentioned at the outset which has good transmission properties over a large frequency range and avoids the problems existing in the prior art.

According to the invention, the solution is that the disk is inclined with respect to a direction of propagation of the microwaves, and in such a way that the angle between the direction of propagation and the disk normal corresponds to the Brewster angle α _B and a plane formed by the direction of propagation and the disk normal is coplanar with the direction of polarization of the microwaves.

The essence of the invention is that the linear polarization of the microwaves, which is often already fundamentally dependent on the generation (such as, for example, in the case of the quasi-optical gyrotron), is used to optimally interact with the known Brewster principle. It is of course quite conceivable that the microwaves will only be polarized linearly after they have been generated. However, the advantage of the invention is particularly evident when the incoming waves are "naturally" linearly polarized, that is, if they are already previous component in the beam path or the microwave tube automatically brings about polarization.

An essential advantage of the invention is that there is in principle no frequency dependence of the transmission of the window. A maximum of one frequency dependence of the media's refractive indices can affect the bandwidth. However, this is negligibly small in the frequency range of interest.

The Brewster angle α _B = arctan n₂ / n₁ is typically between a good 50 ° and a good 75 ° for the materials of interest. Sapphire with a refractive index of n₂ = 3.4 requires (compared to vacuum) an angle of inclination of approximately 74 °. For Teflon with n₂ = 1.45, α _B = 55 °. Ceramics essentially move in this area.

The microwave window preferably comprises two parallel panes which are cooled by a cooling liquid. This embodiment is particularly suitable for high performance.

According to a particularly simple embodiment, the window has exactly one pane. This means that there is a variant that is suitable for low and medium capacities and that is less complex and less expensive.

If cooling is also desired for the single-pane window, cooling fins are preferably provided which are oriented perpendicular to the direction of polarization (see Swiss patent application CH-2314 / 89-2).

The disc can be elliptical or circular. Basically, the sectional figure between a cylinder (waveguide) and a plane inclined to the cylinder axis (disk plane) is an ellipse. From a design point of view, it may also be desirable to use a circular window. Then for optimal transmission the radius of the round disc of the large semi-axis corresponds to the sectional figure. In the quasi-optical case, ie when the microwaves are not guided by the waveguide, but spread almost under open space conditions, a smaller radius (preferably larger than the small semiaxis) may also suffice.

The microwave window according to the invention is particularly well suited as an exit window of a quasi-optical gyrotron. In principle, this produces linearly polarized microwaves in the millimeter and submillimeter range.

Further applications arise in connection with the consumer. In the field of fusion research, fusion chambers (e.g. tokamak) and microwave calorimeters are typical consumers.

Another application example is the window after a Vlasov converter. This converts a guided mode into a linearly polarized Gaussian wave.

Further advantageous embodiments result from the totality of the dependent claims.

Brief description of the drawings

• Fig. 1 eine Darstellung der Scheibe und der Brewster'schen Geometrie;
• Fig. 2 ein Einscheibenfenster mit Kühlrippen; und
• Fig. 3 ein Doppelscheibenfenster.

The invention is to be explained in more detail below on the basis of exemplary embodiments and in connection with the drawings. Show it:

• 1 shows a representation of the disk and the Brewster geometry;
• 2 shows a single-pane window with cooling fins; and
• Fig. 3 shows a double pane window.

The reference symbols used in the drawings and their meaning are summarized in the list of designations. In principle, the same parts are provided with the same reference symbols in the figures.

Ways of Carrying Out the Invention

Fig. 1 shows a window according to the invention, e.g. is used in a quasi-optical gyrotron. In such a gyrotron, an alternating electromagnetic field is excited in a quasi-optical resonator, which is essentially formed by two mirrors lying opposite one another. 1 shows one of the two mirrors 1 and the vessel wall 2 of the gyrotron. The remaining parts of the gyrotron are well known from the prior art cited at the beginning. A detailed description is not necessary at this point.

The microwaves coupled out of the resonator are linearly polarized. They run along a direction of propagation 3 towards the microwave window, which closes the evacuated tube vessel of the gyrotron from the outside (e.g. waveguide 5).

The microwave window comprises a pane 4 which is transparent to microwaves and which is inclined with respect to the direction of propagation 3. The angle α _B between a disc normal n and the direction of propagation 3 corresponds to the Brewster angle, that is, α _B = arctan n₂ / n₁. N₂ is the refractive index the disk material and n 1 that of the medium surrounding the disk, in the present case the vacuum, ie n 1 = 1.

The microwave window according to the invention is also characterized by the fact that the pane used has plane-parallel main surfaces. So there is no need for a surface structure like, for example, the moth eye window. No flashovers can occur.

The incident microwaves are linearly polarized. The electric field vector E is coplanar with a plane spanned by the disk normal n and the direction of propagation 3.

Sapphire with a refractive index n₂ ≈ 3.4 is suitable as the disc material. The angle of inclination is α _B = 74 °. A material which is generally known under the trade name Teflon is also preferred. Due to the refractive index of n₂ ≈ 1.45, α _B = 55 °. Ceramics can also be used in the invention.

Typically the angles are between 50 ° and 75 °, i.e. they are relatively large. In terms of shape, it is therefore advantageous to design the disk in an elliptical shape (corresponding to the intersection curve between the generally round vessel wall 2 and the disk plane). The area of the disc is therefore not larger than necessary, which has a positive effect on its mechanical stability.

Fig. 2 shows an inventive microwave window with cooling fins 6.1, ..., 6.3. The transparent pane breaks down into several sub-panes 4.1, .., 4.4. The cooling fins 6.1, ..., 6.3 are, for example, metal pipes and give the disc greater stability. They are aligned perpendicular to the electric field vector E (polarization direction). For this reason they are not "seen" by the microwave radiation.

Further advantageous embodiments for cooling fins can be found in Swiss patent application CH-2314 / 89-2. For this purpose, the said patent application is to be considered as included in this description.

3 finally shows a microwave window which has two parallel panes 7.1 and 7.2. They are inclined according to the Brewster angle. A coolant 8 flows between the two disks 7.1, 7.2.

In practice, the distance between the two parallel disks 7.1, 7.2 is less than their thickness. The advantage in close proximity is the correspondingly high flow rate of the coolant. What has already been said in connection with the single-pane window also applies to the double-pane window. In this case, there is of course no need for cooling by means of cooling fins.

The window according to the invention can advantageously be used wherever linearly polarized microwaves occur. In particular, it can be used as an entry window for consumers. The calorimeter is an example. Another application is the fusion chamber (e.g. tokamak), when a HE₁₁ wave is irradiated. Finally, the invention is also suitable for a Vlasov coupler, such as that e.g. from "An X-Band Vlasov-Type Mode Convertor", B.G. Ruth et al., 13th Int. Conf. on Infrared and Millimeter Waves, 5-9 Dec 1988, pp. 119-120.

In summary, it can be said that a real broadband window has been created with the invention. The bandwidth is limited at most by the frequency dependence of the refractive indices of the panes, which is negligibly small in practice. It should also be noted that the manufacturing effort is relatively low and the technical feasibility is ensured, since the known window technology can be used.

Label list

1 - mirror; 2 - vessel wall; 3 - direction of propagation; 4 - disc; 4.1, .., 4.4 - indexing disks; 5 - waveguide; 6.1, ..., 6.3 - cooling fins; 7.1, 7.2 - disc; 8 - coolant; E - electric field vector; n - disc normal; n₁, n₂ - refractive indices.

Claims (9)

High-bandwidth microwave window for linearly polarized high-power microwaves, comprising at least one pane which is transparent to microwaves and has plane-parallel, planar main surfaces, characterized in that the pane is inclined with respect to a direction of propagation of the microwaves and in such a way that the angle between the direction of propagation and the pane normals corresponds to the Brewster Angle corresponds to α _B and a plane formed by the direction of propagation and slice normal is coplanar with the direction of polarization of the microwaves. Microwave window according to claim 1, characterized in that the angle between the direction of propagation and the slice normal is between 50 ° and 75 °. Microwave window according to claim 1, characterized in that it comprises two parallel panes which are cooled by a cooling liquid. Microwave window according to claim 1, characterized in that it comprises exactly one pane. Microwave window according to claim 4, characterized in that the disc comprises cooling fins which are aligned perpendicular to the direction of polarization. Microwave window according to claim 1, characterized in that the at least one pane is circular. Microwave window according to claim 1, characterized in that the at least one pane is elliptical. Quasi-optical gyrotron, in which microwaves are excited in a quasi-optical resonator formed from two opposing mirrors, characterized by a microwave window according to claim 1. Microwave window according to claim 1, characterized in that it is the entry window of a consumer.

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