RELATED APPLICATIONS This application claims priority from U.S. provisional patent application 60/014,957 filed 8 April 1996, which is hereby incorporated by reference.

FIELD OF THE INVENTION This invention relates to the damping of unwanted standing waves in a waveguide circuit, and, more particularly, to the damping of unwanted modes that can oscillate in an extended interaction output circuit of a high power klystron when the klystron is coupling energy to a narrow band device such as a mode transducer.

BACKGROUND OF THE INVENTION

A klystron is a type of electron beam microwave amplifier. A klystron comprises a number of cavities divided into essentially three sections: an input section, a buncher section, and an output circuit. An electron beam sent through the klystron is velocity modulated by an input RF signal that is provided to the input section. In the buncher section, the faster electrons gradually overtake the slower electrons, resulting in electron beam bunching. The traveling electron bunches represent an RF current . The RF current induces electromagnetic oscillations in the output circuit of the klystron as the bunched beam passes through the output circuit, and the electromagnetic energy is extracted from the output circuit as an electromagnetic wave.

The development of high powered klystrons has necessitated the use of extended interaction output circuits. Conventional single gap output circuits will suffer breakdown at high power levels (100 MW) and long pulse widths (1.5 μs) due to high surface electric fields . The use of extended interaction output circuits provides a means of reducing these fields by dividing among several gaps the voltage necessary to extract energy from the electron beam. As power levels increase, the output circuit must be made longer with more gaps. Longer output circuits have an increased susceptibility to resonating at undesired frequencies, diverting energy from the desired frequency and possibly causing damage. Often, more than one unwanted frequency is generated.

The risk of unwanted oscillations is increased when the klystron output is directed into a narrow band device such as a mode transducer, pulse compressor, or half-wave window. These devices typically present a high reflection coefficient to frequencies outside the narrow frequency range for which the device was designed. The reflected waves propagate back to the klystron where they are amplified and returned to the narrow band device. The trapped waves quickly overwhelm the desired signal. The result is a powerful standing wave between the klystron and narrow band device oscillating at an undesired frequency.

What is needed is a simple means by which the undesired oscillations can be damped that does not attenuate the desired signal.

OBJECTS AND ADVANTAGES OF THE INVENTION

Accordingly, it is an object of the present invention to provide a means for attenuating the undesired standing waves that can result when a klystron with an extended interaction output circuit is connected to a narrow band device.

More generally, it is an object of the present invention to provide a means for attenuating undesired standing waves in a waveguide.

It is a further object of the present invention to provide standing wave attenuation with a simple, inexpensive apparatus .

SUMMARY OF THE INVENTION These objects and advantages are attained by coupling a resonant cavity made of a lossy material to the side of a waveguide. The coupling is provided by an iris cut into the side of the waveguide at the location of an antinode of the standing wave. The cavity is resonant at the unwanted frequency. The lossy material can be stainless steel or other metals and may be coated on the inside with lossy coatings, e.g. iron particles. The lossy material can dissipate energy through resistive effects or magnetic hysteresis effects or both. The Q ₀ of the cavity and the Q _e of the iris are equal. The Q's are made as low as possible without causing excessive attenuation of the desired frequency.

The resonant cavity can be made cubic, spherical, cylindrical or any other shape.

The oscillating standing wave couples energy into the resonant cavity through the iris. This energy is dissipated in the walls of the cavity, thereby attenuating the standing wave.

DESCRIPTION OF THE FIGURES

Fig. 1 is a top view of the present invention used in conjunction with a klystron. A graph of the standing wave electric field pattern is included.

Fig. 2 is a graph of attenuation vs. frequency for two resonant cavities with different values of Q ₀ ■

DETAILED DESCRIPTION

A preferred embodiment of the invention is shown in Fig. 1. Here, the present invention is used to attenuate the unwanted oscillations that can occur in a klystron output circuit 2. ? The output 2 feeds into a pair of rectangular waveguides 4 which join at a coupler to form a single rectangular waveguide 6. The waves propagate to a narrow band device 8. In Fig. 1 narrow band device 8 is a TE]_o to TEoi mode transducer. The mode transducer 8 is tuned to the desired operating frequency 0 of the klystron and so only these frequencies pass through the transducer 8. All other frequencies are reflected back to the klystron output circuit 2. Other narrow frequency band devices such as pulse compressors or half wave windows will similarly reflect frequencies outside their designed range of 5 operation. The present invention can be used in any situation where undesired frequencies establish a standing wave pattern in a waveguide. Since counter propagating waves comprise a standing wave, the present invention can be used in any situation where undesired frequencies are preferentially 0 reflected.

The reflected wave sets up a standing wave pattern in the waveguide. The amplitude of the standing wave electric field with respect to position along the length of waveguide 6 is 5 shown in a graph 10. A resonant cavity 12 is coupled to the waveguide 6 through an iris 14 which is located at a maximum in the standing wave electric field pattern (an antinode) . The positions of the antinodes are indicated in Fig. 1 as dashed lines 16. Typically, the antinodes will be an odd 0 number of quarter wavelengths from narrow band device 8. The resonant cavity 12 is resonant at the frequency of the undesired standing waves. Energy from the standing wave enters the resonant cavity 12 through the iris 14. The energy- is then dissipated in the walls of the cavity 12 , which are 5 made of a lossy material.

When used between a klystron 2 and a narrow band device 8, the present invention can prevent the klystron from oscillating at an unwanted frequency.

The resonant cavity 12 can be any shape. In the embodiment of Fig. 1 the cavity 12 is a pillbox shape. The design and construction of resonant cavities is well known in the art.

The material of the resonant cavity 12 can be lossy through surface current losses or through magnetic hysteresis losses or a combination of both. 400 series stainless steel, for example, is lossy through both mechanisms. The lossy material may also comprise, for example, kovar or monel .

The Qo of the cavity 12 will depend upon the loss characteristics of the material of which the cavity 12 is made. Lossier materials will lower the Q _Q - A stainless steel cavity, for example, may have a Q _D of approximately 300-350 at approximately 10 GHz . The use of lossy surface coatings such as flame sprayed iron filings can also be used to achieve a desired Q ₀ . Such techniques are well known in the art.

The Qo of the cavity 12 and the Q _e of the iris 14 are made equal in order to provide the most effective coupling between the resonant cavity 12 and the unwanted standing wave in the waveguide 6. Circular and rectangular irises 14 are the most commonly used in the art, but the iris 14 can be any shape.

The Qo of the cavity 12 and the Q _e of the iris 14 are made as low as possible without attenuating the desired signal. The lower the Qs of the cavity 12 and iris 14, the less precise the position and construction of the cavity 12 and iris 14 needs to be. Fig. 2 illustrates the mechanism that places a lower limit on the Q's of the cavity 12 and iris 14. FI is the unwanted frequency to which the cavity 12 is tuned, hence the peak of the attenuation curve is located at FI. F2 is the

desired signal frequency. Attenuation curve 18 corresponds to a relatively large cavity/iris Q. If the cavity/iris Q is lowered, as shown by the curve 20, then the attenuation curve broadens and the cavity 12 will absorb energy at the desired signal frequency, F2, which is, of course, undesirable. It will be obvious to one skilled in the art how to adjust the Q ₀ of the cavity 12 and Q _e of the iris 14 to achieve the optimum attenuation of unwanted frequencies while leaving desired frequencies relatively unaffected.

It will be obvious to one skilled in the art that the present invention can be applied to circular waveguides and rectangular waveguides .

It is understood that if two or more unwanted frequencies exist, two or more resonant cavities 12 can be used, each individually tuned and located to produce damping at a chosen frequency.

It is also understood that two or more cavities 12 can be used together to attenuate the same unwanted frequency. In this case, identical cavities 12 and irises 14 can be placed at separate antinodes .

It will be clear to one skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents .