1. Field of the Invention
This invention relates to low-noise, crossed-field devices such as microwave magnetrons, microwave ovens utilizing same, crossed-field amplifiers and methods of converting noisy magnetrons to low-noise magnetrons.
2. Background Art
The noise generation mechanisms of linear electron beam devices are well known. Generally, fluctuations of cathode electron emission excite space charge waves, which propagate along the electron beam. Calculations and computations of noise figures in linear devices agree with experiments. Methods of noise suppression in linear tubes are at a very advanced stage. On the other hand, noise generation mechanisms in cross-field devices are not presently understood and predictive computational calculations do not exist. Methods of noise suppression in crossed-field devices have not previously been practically realized.
Existing magnetrons and crossed-field amplifiers use an azimuthally-symmetric, axial magnetic field, shown in
As described by J. M. Osepchuk in the 1995 article entitled “The Cooker Magnetron as a Standard in Crossed-Field Research,” PROCEEDINGS OF THE FIRST INTERNATIONAL WORKSHOP ON CROSSED-FIELD DEVICES, Ann Arbor, Mich., Aug. 15-16, 1995, University of Michigan, “The existence of magnetron noise is assuming a very practical aspect. There are over 200 million microwave ovens in the world operating at 2.45 GHz. There also are plans for a wide variety of new ‘wireles’ services to operate with frequency allocations ranging from 1.5 GHz to 3.0 GHz and possibly even higher, especially at 5.8 GHz. There are some serious questions about the potential that some of these systems will encounter unacceptable interference from microwave ovens—i.e., the sideband noise. Thus the characteristics of microwave oven noise are being studied extensively and there are plans for interim and final (tighter) specifications to limit such noise through regulations originating in current activities of the CISPR community within the IEC (International Electrotechnical Commission). Because the noise is predominantly at low anode currents most of the time, microwave oven noise shows up as sub-millisecond pulses of noise. Some experts believe modern digital and spread-spectrum communication techniques can live with this. On the other hand, if discrete spurious signals show up especially at close to peak current, the RFI might not be tolerable. The magnitude of the peak noise or spurious in the worst cases is of the order of 100 dB above a pW as measured in a 1 MHz bandwidth or even higher (or similar numbers in units of μV/m as measured at 3 meters from the oven). At present some authorities are investigating peak limits near such levels along with limits 30 to 40 dB lower when using narrow video bandwidths (e.g. 100 Hz) to yield ‘average’ measures of the noise.”
As further described in the above-noted article, “Cooker magnetron noise, therefore, will attract regulatory pressure in the future at the same time that others, i.e., the DOE in the U.S., are pressuring for higher oven efficiency which is, in principle, associated with higher noise. At the same time there are other magnetron-driven ISM devices that may amplify the concern about noise, e.g., the microwave ‘sulfur’ lamps, that are very efficient light sources that some day may operate for many hours per night illuminating large areas in buildings and parking lots, etc. One can presume that users of magnetrons may be forced to find ways of reducing such noise. Otherwise, competing devices might for the first time in history pose a threat to the magnetron as the power source of choice for ovens and other power applications. In the past year there was the preliminary report of an efficient (67%), low voltage (600 Volts) multi-beam klystron suitable for microwave oven use. Its developers estimate that in three years problems of cost, size and weight might be resolved. The klystron poses no noise problems and has other advantages. One can expect controversial discussions of competing power sources at meetings such as those of IMPI (the International Microwave Power Institute).”
Since the above-noted article was written, several communications systems have developed in the unlicensed, 2.4 GHz radio spectrum:
U.S. Pat. No. 4,465,953 issued to Bekefi uses a magnetic configuration which modulates the radial magnetic field by an azimuthally, spatially-periodic array of magnets in a magnetron to generate free electron laser radiation.
U.S. Pat. No. 3,932,820 issued to Damon et al. discloses how the noise in a crossed-field amplifier output is reduced by providing a non-uniform magnetic field across the surface of a cathode. A curved magnetic field may be provided across the cathode or by providing a concave shaped cathode. Additionally, the cathode may be tilted with respect to the crossed magnetic field.
U.S. Pat. No. 4,709,129 issued to Osepchuk discloses a typical microwave power source for a microwave oven in which a microwave magnetron is supplied simultaneously with filament heater power and with anode voltage through an inductive reactance power supply.
U.S. Pat. No. 6,437,510 issued to Thomas et al. discloses a crossed-field amplifier or magnetron which has a cathode body portion and an anode which cooperates with a crossed magnetic field to maintain emitted electrons on cycloidal paths and amplify an input signal or develop a microwave or millimeter wave output signal in an interaction space.
U.S. Pat. No. 4,310,786 issued to Kumpfer discloses a magnetron electron discharge device preferably for use in microwave heating or cooking apparatus which has a cylindrical resonant anode structure surrounding a concentric electron emitting filament.
An object of the present invention is to provide low-noise, crossed-field devices such as a microwave magnetron, a microwave oven utilizing same, crossed-field amplifiers, and a method for converting a noisy magnetron to a low-noise magnetron by the use of an azimuthally varying, axial magnetic field.
In carrying out the above object and other objects of the present invention, a low-noise, crossed-field device is provided. The device includes an electrical circuit for generating a radial electrical field, and a magnetic circuit for generating an axial magnetic field substantially perpendicular to the radial electric field. The axial magnetic field is azimuthally varying to substantially eliminate noise in the device.
The device may be a microwave magnetron including a cathode for emitting electrons and an anode having a plurality of resonant cavities. The cathode and anode may define an interaction space therebetween wherein interactions between electrons emitted from the cathode and the electric and magnetic fields produce a series of space charge spokes that travel around the space in an azimuthal direction.
The microwave magnetron may be a plasma processing magnetron, an oven magnetron, a lighting magnetron, or an industrial heating magnetron.
The device may be a crossed-field amplifier including an input for receiving an input signal to be amplified within the device and an output for carrying an amplified signal from the device.
The amplifier may be a radar amplifier.
The magnetic circuit may include at least one perturbing magnetic field source for causing azimuthally varying perturbations in the axial magnetic field.
The at least one perturbing magnetic field source may include at least one permanent perturbing magnet, at least one shaped magnetic pole piece, or at least one shaped coil or multiple coils.
The magnetic circuit may includes a pair of spaced magnets and at least one perturbing magnet coupled to at least one of the spaced magnets for causing azimuthally varying perturbations in the axial magnetic field.
The magnetic circuit may further include a plurality of perturbing magnets.
The device may be a microwave magnetron having startup and peak power phases, and the noise may be substantially eliminated independent of magnetron current.
The device may be a linear crossed-field amplifier including a cavity region, and the magnetic field may vary in a direction of electron drift in the cavity region.
The device may be a microwave magnetron including one of a plurality of mode control devices such as strapping and rising sun geometries, or a coaxial cavity magnetron.
A typical magnitude of azimuthal variations of the axial magnetic field may be approximately 50%.
Further in carrying out the above object and other objects of the present invention, a microwave oven is provided. The microwave oven includes a compartment, and a low-noise, oven magnetron for generating microwaves in the compartment. The magnetron includes an electrical circuit for generating a radial electrical field. The circuit includes a cathode for emitting electrons and an anode having a plurality of resonant cavities. The cathode and the anode define an interaction space therebetween. The magnetron further includes a magnetic circuit for generating an axial magnetic field substantially perpendicular to the radial electrical field in the interaction space wherein interactions between electrons emitted from the cathode and the electric and magnetic fields produce a series of space-charge spokes that travel around the space in an azimuthal direction. The axial magnetic field is azimuthally varying in the interaction space to substantially eliminate noise in the device.
Still further in carrying out the above object and other objects of the present invention, a method of converting a noisy magnetron to a low-noise magnetron is provided. The noisy magnetron includes an electrical circuit for generating a radial electric field and a magnetic circuit for generating an axial magnetic field substantially perpendicular to the radial electric field. The method includes azimuthally varying the axial magnetic field to substantially eliminate noise in the noisy magnetron.
The magnetic circuit may include a pair of spaced magnets, and the step of azimuthally varying may include the step of coupling at least one perturbing magnet to at least one of the spaced magnets for causing azimuthally varying perturbances in the axial magnetic field.
A typical magnitude of azimuthal variations of the axial magnetic field may be approximately 50%.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.
a is a side schematic view of a prior art oven magnetron including its magnetic configuration;
b is a top view of the magnetron of
a is a side schematic view of an oven magnetron including magnets for generating an azimuthally varying axial magnetic field in its magnetic configuration;
b is a top view of the magnetron of
a is a side schematic view of an upper (or lower) magnet of a magnetron including magnetic pole pieces constructed in accordance with a third embodiment of the present invention;
b is a bottom view of the magnetron magnet of
In general, low-noise, crossed-field devices such as a microwave magnetron and microwave oven utilizing same are disclosed. In a first embodiment of the invention, at least one permanent magnet is added to the existing magnetron magnets to cause the axial magnetic field to vary azimuthally (exterior dashed line in
a also shows a cathode, an anode and an electrical circuit for generating a radial electric field.
The perturbing magnets 10 perturb the axial magnetic field of the magnetron or crossed-field amplifier (i.e. FIG. 8).
a-4b show alternative apparatus of generating azimuthally varying axial magnetic field for a magnetron (or crossed-field amplifier).
In general, in order to generate an azimuthally varying axial magnetic field, a number of different embodiments are possible, including, but not limited to:
a and 4b are side and bottom views, respectively, of a third embodiment of the present invention wherein magnetic pole pieces 40 generate an azimuthally varying axial magnetic field. The pole pieces 40 are coupled to an upper (or lower) magnetron magnet 42.
The low-noise, crossed-field devices have application to reducing interference with telephone and computer communications by microwave magnetrons in microwave ovens.
Magnetrons are also used for lighting and industrial heating and the noise-free magnetrons of the present invention are applicable in these areas.
The efficiency of magnetrons would also be improved for applications which require a precise microwave frequency, such as plasma processing.
Another important application of the invention is the reduction of noise in crossed-field amplifiers utilized for the Department of Defense. This could lead to higher signal-to-noise ratios and better resolution for radars.
The invention reduces the noise in magnetrons, both during the critical startup phase and in the peak power phase. The reduction of noise is independent of magnetron current. Microwave noise is reduced in both new magnetrons and older, noisy magnetrons.
This invention extends to a linear crossed-field amplifier in which the transverse magnetic field varies in the direction of the electron drift in the cavity region.
This invention also applies to magnetrons that employ mode control devices such as strapping and rising sun geometries, as well as coaxial cavity magnetrons.
The typical magnitude of the azimuthal variations of the axial magnetic field are in the range of 50%.
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
This invention was made with Government support under Grant Nos. F49620-99-1-0297 and F49620-00-1-0088, awarded by the AFOSR. The Government has certain rights in the invention.
Number | Name | Date | Kind |
---|---|---|---|
3932820 | Damon et al. | Jan 1976 | A |
3958148 | Wilbur et al. | May 1976 | A |
4310786 | Kumpfer et al. | Jan 1982 | A |
4465953 | Bekefi | Aug 1984 | A |
4668924 | Van De Sande | May 1987 | A |
4709129 | Osepchuk | Nov 1987 | A |
4855645 | Kinuno et al. | Aug 1989 | A |
4928070 | MacMaster et al. | May 1990 | A |
5412281 | Farney et al. | May 1995 | A |
5635798 | Ogura et al. | Jun 1997 | A |
5798602 | Gopanchuk et al. | Aug 1998 | A |
6437510 | Thomas et al. | Aug 2002 | B1 |
Number | Date | Country |
---|---|---|
6-5211 | Jan 1994 | JP |
9-129149 | May 1997 | JP |
11-283517 | Oct 1999 | JP |
Number | Date | Country | |
---|---|---|---|
20040206754 A1 | Oct 2004 | US |