Patent Application
20100171423
This application claims priority to GB 0900153.8 filed in the United Kingdom on Jan. 6, 2009, the disclosure of which is incorporated herein by reference in its entirety.
This invention relates to output windows of vacuum electron devices.
For vacuum electron devices with circular multi-mode output waveguide designs the output window normally consists of one or more layers of dielectric at least one of which will be joined to the device's output waveguide in a vacuum-tight bond, usually achieved by brazing the dielectric to the metal waveguide. Whilst a single layer window gives excellent transmission at a sequence of defined wavelengths, to achieve broadband performance a multi-layered window should be used.
Theoretically the bandwidth performance of a single half-wavelength-thick window design can be improved via the use of quarter wavelength transformers abutting the two faces of the window in order to match the window impedance to the free space impedance. To space the triple layers of the window takes this concept one step further, and spaced triple windows have been proposed for electron tubes. Thin ceramic layers may be spaced from the central half-wavelength-thick window to form “matching” cavities. These cavities enable the bandwidth of the window to be significantly extended beyond 20% (a range extending from 10% below the centre frequency to 10% above the centre frequency) with a return loss of better than −25 dB. This is true for both single and multi mode circular waveguides.
The invention is especially concerned with output windows for gyrotron-travelling wave tube electron devices, although it is also applicable to other broadband vacuum electron devices.
Referring to
The interior of the waveguide 1 may be provided with a helical corrugation (not shown)—“Gyro-TWT with a Helical Operating Waveguide: New Possibilities to Enhance Efficiency and Frequency Bandwidth”, Gregory G. Denisov, Vladimir L. Bratman, Alan D R Phelps and Sergei V Samsonov, IEEE Transactions on Plasma Science, Vol. 26, No. 3, June 1998. In view of the broadband nature of the output, a spaced triple layer window is used. For optimum performance the design should be symmetrical about the central window. The performance of such a design is very sensitive to dimensional variations, with spacing and ceramic thickness tolerances of tighter than ±0.05 mm necessary to ensure 20% bandwidth performance does not degrade beyond −20 dB return loss.
The conventional approach to manufacture such a spaced triple layer window is similar to that employed for pillbox windows used to vacuum seal rectangular waveguides, i.e. each ceramic disc is first brazed into a copper tube which is then lapped to the desired length. For the triple layer window, which is shown on an enlarged scale in
An alternative approach that has been employed is to sandwich accurately machined copper cylinders between the ceramic layers and place the entire assembly within an outer copper tube, such that the ceramic layers appear to the microwave signal as being set in recesses in a copper tube. Unfortunately for waveguides with a large number of possible propagating modes the differential expansion between the copper and ceramic materials require significant recess depths to be employed, which degrades the microwave performance of the window assembly such that the 20% bandwidth with a return loss of better than −20 dB cannot readily be achieved.
The present invention provides an output window for a vacuum electron device, comprising an output waveguide, an intermediate layer of dielectric material joined to the interior of the output waveguide with a vacuum-tight seal, layers of dielectric material spaced apart from the intermediate layer and which, in an orientation of the output waveguide in which the seal was made, were upper and lower relative to the intermediate layer, supports being provided extending inwardly into the outer waveguide and openings being provided in the upper and lower layers such that, in the orientation in which the sealing of the intermediate layer took place, the upper and lower layers were supported by the supports and the intermediate layer was able to be supported through the openings in the lower layer by pillars, while at the same time the regions between the intermediate layer and the upper and lower layers were able to be vented.
The invention also provides a method of making an output window for a vacuum electron device, comprising the steps of supporting upper and lower layers of dielectric material on supports extending into an output waveguide, supporting an intermediate layer of dielectric material between the upper and lower layers in spaced relationship therewith on pillars extending through openings in the lower layer, and making a vacuum-tight seal between the intermediate layer and the interior of the output waveguide.
The openings in the layers of dielectric material in conjunction with the supports allows the three dielectric layers to be secured simultaneously, thus simplifying the manufacturing process.
In the following description, like parts have been given like reference numerals throughout all the drawings.
The output window shown in
The window consists of three layers of dielectric material mounted in an output waveguide 7, namely, an intermediate layer 10, and an upper and a lower layer 9, 11. The thickness of the intermediate layer is approximately one quarter of the wavelength of the centre frequency of the band of frequencies transmitted by the window, and the thickness of the upper and lower layers approximately one twentieth of the centre frequency. The spacing between the upper layer and the intermediate layer, and between the intermediate layer and the lower layer, is approximately one eighth of the centre frequency.
These dimensions satisfy the 20% bandwidth requirement for a particular waveguide size and ceramic material, and if the waveguide diameter were to be changed, the thickness and spacing ratios would be different.
Referring to
Thus, the output window is manufactured with the output waveguide 7 in the vertical orientation shown in
In order to locate the intermediate layer in the correct position during manufacture, it is supported on a jig 18 which includes four upstanding pillars 19 (one of which cannot be seen in the section of
With the layers 9, 11 supported on the sets of corbels 14, 15, and the intermediate layer supported on the pillars 19, the three layers are simultaneously brazed to the interior of the output waveguide. The openings 17 are of greater diameter than the pillars, and thus the regions above and below the intermediate layer are vented during the brazing process.
The venting holes within the two thin ceramic layers eliminate trapped volume problems which would otherwise arise and allow pillars to pass through the ceramic and hold off the intermediate layer at the desire spacing during brazing. With careful design the corbels, even though non-symmetric, do not degrade the microwave performance and in particular do not cause mode conversion, thus enabling the introduction of corbels on to the inner wall of the output waveguide to support the upper thin layer. By careful consideration of the material expansion coefficients, the use of corbels and pillars the spaced triple layer window assembly can be brazed in it entirety, achieving the required dielectric layer spacing without the need for subsequent mechanical adjustment and hence the desired microwave performance whilst reducing the likelihood of assembly induced mode conversion.
The output waveguide can be a copper tube. The layers of dielectric material can be discs of ceramic such as alumina. While the output waveguide is part of a gyrotron-TWT as described, it could be used with other broadband electron tubes such as coupled cavity TWTs.
The invention has been described in detail with respect to various embodiments, and it will now be apparent from the foregoing to those skilled in the art, that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications that fall within the true spirit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| GB 0900153.8 | Jan 2009 | GB | national |
