Circular electric mode microwave window

#1. Field of the Invention

#The invention pertains to high power microwave transmission. A waveguide window is often needed to get the power into or out of a vacuum device such as an electron tube or plasma chamber or a pressurized section of waveguide.

#2. Prior Art

#FIG. 1 is an axial cross-section of the inventive window assembly.

#FIG. 2 is an axial section of a slightly different embodiment.

#FIG. 1 shows an example of the inventive window assembly between two sections of circular waveguide 10 whose inner surfaces are right circular cylinders with axis 12. At one end is a waveguide flange 14 for connection to other components. The other end 16 may be the output waveguide of a microwave generating electron tube, for example. The actual vacuum-tight windows are two circular plates of dielectric 18 perpendicular to axis 12. The dielectric may be high-alumina or beryllia ceramic or single-crystal sapphire. Plates 18 are separated by a small spacing so that cooling fluid may flow between them. Near the periphery of plates 18 are metallized circular bands 20 by which they are brazed to the flanges 22 of thin metallic cylinders 24, 25, as of iron-nickel-cobalt alloy. Cylinders 24, 25 are brazed to waveguide sections 10 and form electrical continuations of them. Waveguide sections 10 are attached to mounting flanges 26 which are bolted to a common support ring 28 to hold the sections 10 firmly aligned and spaced. Support ring 28 has grooves 30 containing 0-rings 32 to make the window assembly gas-tight.

#A cooling fluid having low dielectric loss, such as a fluorocarbon gas or liquid is pumped in through a coolant pipe 34 at the top of the figure. It circulates through a channel 36 bounded by a dielectric cylinder 38 as of fluorocarbon polymer. It flows over the surface of thin cylinder 24, thereby cooling it. Cylinders 24, 25 are thin so that they have enough raidal flexibility to take up the thermal expansion differences from plates 18 when they are brazed together. They thus have poor thermal conductivity so that fluid cooling is advantageous. At the bottom 40 of channel 36, as shown by the flow arrows, the cooling fluid passes through a row of gaps 42 through a projecting flange 44 on the flow-confining dielectric cylinder 38. It then flows upward between window plates 18 to cool their entire area. At the top, flange 44 is impervious but the other flange 46 has a series of gaps 48 through which the fluid passes to a second circular channel 50, flowing over the second thin cylinder 25 to cool it. At the bottom of channel 50 the fluid flows through a hole 52 into an outer circular channel 54. Inside channel 54 is wave absorbing material 55, such as water contained in plastic tubes 56. The cooling fluid flows around channel 54, removing heat from tubes 56 caused by any microwave energy they absorb, to the top where it leaves the window assembly via an outlet tube 58. Heat is also removed by causing the fluid 55 to flow through the tubes 56.

#In operation, very little wave energy of the circular-electric-field mode, such as TE.sub.01, flows out of the waveguide 10 through the small gap 60 between flanges 22, because the electric currents in the wall have no axial components crossing gap 60 to induce fields in the outer wave-confining channel bounded by flanges 26 and ring 28. However, many other undesired modes do involve axial currents which couple into the outer channel where their energy is absorbed by the lossy material 55.