EP0940876A1

EP0940876A1 - Waveguide with a dielectric window assembly

Info

Publication number: EP0940876A1
Application number: EP99301201A
Authority: EP
Inventors: John Robert Brandon
Original assignee: De Beers Industrial Diamond Division Pty Ltd
Current assignee: De Beers Industrial Diamond Division Pty Ltd
Priority date: 1998-02-19
Filing date: 1999-02-18
Publication date: 1999-09-08
Also published as: RU99103339A; GB9803534D0; JPH11330807A; ZA991223B

Abstract

A waveguide for a beam of electromagnetic radiation includes a passage (12) and two windows (14, 16), each having a curved profile, spaced from each other and extending across the passage (12) and the space (12a) between the windows (14, 16) being at a pressure higher than that in the passage (12) outside the space (12a).

Description

BACKGROUND OF THE INVENTION

This invention relates to a waveguide for a beam of electromagnetic radiation.
Electromagnetic radiation, and more particularly microwave radiation, is generated in gyrotrons and other high power microwave sources. A beam of the radiation is generated and this passes along a waveguide to an exit port. The waveguide comprises a passage, generally of circular cross-section, having a window extending across it. The beam of electromagnetic radiation passes through the window. The window is used for the protection of the electromagnetic sources or detectors from environmental factors. The window may be made of a variety of materials such as sapphire, diamond and the like. Such windows are generally planar, although curved profiles have been suggested.

SUMMARY OF THE INVENTION

According to the present invention, a waveguide for a beam of electromagnetic radiation includes a passage and two windows, each having a curved profile, spaced from each other and extending across the passage, and the space between the windows being at a pressure higher than that in the passage outside the space.
Further according to the invention, the windows have convex and concave surfaces and it is the concave surface of one of the windows which is presented to the beam.
In one preferred form of the invention, the spaced windows each have convex and concave surfaces with the convex surfaces facing each other.
The space defined between the windows is at a pressure which is higher than the pressure in the passage outside the space. Typically, the passage will be under vacuum and the space pressure will be up to 5 atmospheres.
Further according to the invention, a further curved window, spaced downstream of the two windows and extending across the passage is also provided. This curved window preferably has a convex surface and a concave surface and it is the convex surface which faces the two windows.
The windows preferably have a shape consisting of part of a hemisphere. The optimum radius for the part hemisphere may be determined by two factors. First, the curvature should be sufficiently small to achieve a substantial mechanical advantage over a planar window. The mechanical advantage of the curved window should be sufficient to allow the thickness of the window to be reduced to a convenient level. For CVD diamond the thickness is preferably maintained at or below 2mm. This mechanical advantage can be used either to increase the area of the window, increase the pressure handling capability of the window or to reduce thickness and therefore manufacturing costs.
As the radius of curvature increases, the mounting of the edges of the window becomes more important as the lateral forces increase. However, reducing the radius to give a complete hemisphere presents other problems due to reflection of the beam caused by the large change in the angle of incidence from the normal.
The thickness of the windows is preferably an integral number of half-wavelengths of the electromagnetic radiation to minimise reflection of the beam.
The passage across which the window is located will generally be defined by a tube. The windows will have edges which will generally be supported in the tube.
The material from which the windows is made will typically be sapphire, diamond, germanium, zinc selenide, silicon, doped silicon, silicon nitride, aluminium nitride or boron nitride. The windows are preferably made of diamond having a low dielectric loss. Such diamond is preferably produced using chemical vapour deposition (CVD).
The invention has particular application to microwave radiation generated, for example, in a gyrotron.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 and 2 are schematic side views of different embodiments of a waveguide for the transmission of electromagnetic radiation.

EMBODIMENTS OF THE INVENTION

Two embodiments of the invention will now be described with reference to the accompanying drawings.
Referring first to Figure 1, a waveguide for electromagnetic radiation such as microwave radiation comprises a tube 10 defining a passage 12 of circular cross-section. Extending across the passage 12 are windows 14, 16 spaced from each other. A beam of electromagnetic radiation passes along the passage 12 in the direction of the arrow 18. The radiation is generated in a region which is upstream of the window 14. The beam passes through the window 14 and exits from the guide after passing through window 16.
The windows 14, 16 are curved and each defines part of a hemisphere. The windows each have a convex surface (14a and 16a) and a concave surface (14b and 16b). The convex surfaces of the windows thus face each other and define between them a space 12a which is under pressure, e.g. 5 atmospheres or less. The passage 12 outside of the space 12a is at a low pressure, typically a vacuum. The pressure difference maintains the convex surfaces in compression, i.e. the pressure on the convex surfaces should be higher than that on the concave surfaces.
The thickness of the windows 14, 16 is preferably an integral number of half-wavelengths of the electromagnetic radiation to ensure that no power is reflected.
The edges 14c and 16c of the windows are suitably mounted and supported in the tube 10. The mounting of the edges, particularly where the window is made of CVD diamond, should be such that the stress distribution at the edge is maintained in a stress state consistent with the equivalent region of a full hemisphere subject to the same loading. In the case of CVD diamond, this may be achieved by double-sided brazing with a metal such as molybdenum, tantalum or Inconel or a non-metal having approximately the same linear thermal expansion characteristics of diamond.
In a practical arrangement of a gyrotron and experimental nuclear fusion reactor, normal operating conditions are such that the gyrotron window is maintained at ultra high vacuum and the reactor itself is maintained at reduced pressure. In such a situation, the embodiment of Figure 1 is appropriate. In some special situations, e.g. in the event of the reactor component failure, the pressure inside the reactor can reach much higher levels of, for example, 5 atmospheres. It is thus preferable that the static pressure in the space 12a between the two convex surfaces 14a and 16a, is at a level somewhat higher than that which is to be anticipated in exceptional circumstances.
The space 12a between the convex surfaces 14a and 16a may be pressurised by use of a gas such as dry air, dry nitrogen or SF₆.
Where long sections of pressurised waveguide are unacceptable for any reason it is possible to locate the windows 14, 16 in close proximity as illustrated by Figure 2. In this figure, like parts carry like numerals to the embodiment of Figure 1. In this embodiment, it is the reactor end only which has double pressurised window 14, 16. A further window 20 is provided downstream of the double pressurised window. The edges 20c of the window 20 are suitably mounted and supported in the tube 10. The space 12b between the window 20 and the window 16 will typically be at a pressure of about 1 atmosphere, while the space 12a between the windows 14 and 16 will typically be at a higher pressure of typically 5 atmospheres.

Claims

A waveguide for a beam of electromagnetic radiation including a passage (12) and two windows (14, 16), each having a curved profile, spaced from each other and extending across the passage (12), the space (12a) between the windows (14, 16) being at a pressure higher than that in the passage (12) outside the space (12a).
A waveguide according to claim 1 wherein the windows have convex (14a, 16a) and concave surfaces (14b, 16b) and it is the concave surface (14b) of one of the windows (14) which is presented to the beam (18).
A waveguide according to claim 1 or claim 2 wherein the spaced windows (14, 16) each have convex (14a, 16a) and concave surfaces (14b, 16b) with the convex surfaces (14a, 16a) facing each other.
A waveguide according to any one of the preceding claims wherein the space (12a) defined between the windows (14, 16) is at a pressure of up to 5 atmospheres and the passage (12) is under vacuum.
A wave guide according to any one of the preceding claims wherein a further curved window (20), spaced downstream of the two windows (14, 16) and extending across the passage (12) is also provided.
A waveguide according to claim 5 wherein the further curved window (20) has a convex surface (20a) and a concave surface (20b) and it is the convex surface (20a) which faces the two windows (14, 16).
A waveguide according to any one of the preceding claims wherein the windows (14, 16, 20) have a shape consisting of part of a hemisphere.
A waveguide according to any one of the preceding claims wherein the thickness of the windows (14, 16, 20) is an integral number of half-wavelengths of the electromagnetic radiation.
A waveguide according to any one of the preceding claims wherein the passage (12) is defined by a tube (10).
A waveguide according to claim 9 wherein the windows (14, 16, 20) have edges (14c, 16c, 20c) which are supported in the tube (10).
A waveguide according to any one of the preceding claims wherein the windows (14, 16, 20) are made of a material selected from sapphire, diamond, germanium, zinc selenide, silicon, doped silicon, silicon nitride, aluminium nitride and boron nitride.
A waveguide according to any one of the preceding claims wherein the windows (14, 16, 20) are made of diamond having a low dielectric loss.
A waveguide according to claim 12 wherein the diamond is produced by chemical vapour deposition.
A waveguide according to any one of the preceding claims wherein electromagnetic radiation is microwave radiation generated in a gyrotron.