This invention relates to magnetron microwave vacuum tube devices used to generate radio-frequency (RF) electromagnetic energy, and which find applications in microwave heating. More particularly, the invention relates to components used in magnetrons to suppress spurious radio-frequency energy transfer and to provide electrical insulation of electrodes; and further relates to designs and methods of construction to reduce failure rates of these components.
The magnetron is a well known vacuum tube electronic device used to generate radio-frequency (RF) electromagnetic energy. The magnetron was invented by Hull in 1921, and came into rapid development during the Second World War as a high-power microwave generator for radar transmitter applications. Currently, magnetrons are in widespread use for microwave cooking, thawing, tempering, drying of materials such as textiles and lumber, and other industrial and laboratory heating processes such as waste remediation and chemical vapor deposition.
Magnetrons are made up of machined or formed metal parts, some of which function as electrodes. The electrodes are appropriately separated by electrically insulating elements, and arranged and sealed to form an evacuated enclosure. The electrodes include a heated cathode that emits electrons, and an anode that is shaped to form the essential resonant cavities needed to generate high-frequency (several hundred megahertz to several gigahertz) electromagnetic radiation. Referring to
Ideally, all of the electromagnetic energy generated by the magnetron would be coupled into a waveguide or antenna, or else focused as a directed, collimated beam. However, it is practically inevitable that some of the radiation generated by the magnetron will be transmitted to surrounding areas of the magnetron where it is neither utilized nor wanted. It has proven useful, and often necessary, to reduce this spurious electromagnetic radiation that permeates into the surroundings of an operating magnetron. This leakage radiation can interfere with electronics in the vicinity of the magnetron. Adequate suppression of leakage radiation can be achieved by a device termed an RF suppressor. The RF suppressor component is an approximately toroidal-shaped element, resembling a collar or split lug, made of a material that absorbs radio-frequency electromagnetic energy. The RF suppressor is mounted on the magnetron and absorbs radiant energy, thus avoiding or lessening many of the problems associated with electromagnetic interference from the magnetron. According to the design of many typical magnetrons, the RF suppressor is most effective at reducing problematic leakage radiation when it partially shrouds the magnetron tube in close proximity to the cathode connector for the cathode voltage bias. On account of this, it is convenient and effective to make the RF suppressor component integral to the fixture used to make electrical connections to the cathode. In the current practice of utilizing RF suppressors in magnetrons, one side of the RF suppressor collar is machined to accommodate the cathode voltage supply connecter fixture that clamps the cathode and provides a terminal post for electrical leads. Two threaded holes receive screws with washers to hold the RF suppressor to the cathode connector fixture. The cathode connector is also used to tie a lead of the filament heater circuit to the cathode. Thus, the combined component, comprising the RF suppressor mated to the cathode connector fixture, serves multiple functions.
Many applications of magnetrons benefit from increased power capacity, and there is incentive to increase the radio-frequency electromagnetic power that can be produced by magnetrons. Increased power levels imply higher operating voltages, specifically a negative voltage, of higher than normal magnitude, is applied to the cathode 102. The relationship between magnetic field strength, cathode bias voltage, filament current, and magnetron geometry needed to establish a stable operating point of a specified RF output power and frequency is complex. However, the present design trend is that high output power requires a higher magnitude cathode voltage bias between the cathode and the grounded anode. As a consequence of greater operating power levels, more stringent demands are imposed on the ability of the magnetron design and materials to withstand relatively high electric fields. The total electric field is the sum effect of the static electric field due to the dc voltage bias applied between the cathode and system ground, and the time-varying RF electric fields generated in the resonant cavity structure of the magnetron related to the cycloidal motion of the electrons. The grounded elements of the magnetron include the anode, coolant tubes, the enclosure housing, external magnet pole piece, and mounting brackets. The exact spatial distribution of this resultant electric field is difficult to predict, but it is evident, both intuitively and from experiment, that the sites that bear the largest magnitudes of the electric field, and hence are the most problematic with regard to high-voltage associated failures, occur near the terminal where the high-negative-voltage bias is applied to the cathode. As evidence, it is noted that as the output power levels of some industrial magnetrons have increased from 30 kilowatts to 80 kilowatts in recent years, there has been a significant increase in the failure rate of the RF suppressor component of magnetrons due to the increased power levels and associated higher cathode bias voltages. In many cases, the RF suppressor has proven to be the magnetron component that is most susceptible to high voltage breakdown effects, and is implicated as the dominant cause of failure in magnetrons operated at high power levels.
The present inventor has analyzed the failure mechanisms of RF suppressors and has identified the specific sites of magnetron RF suppressor failure and the specific nature of the failure. As a result of those investigations, a new insulated RF suppressor which is adapted to high-voltage operation has been developed. The invention disclosed herein provides an improved RF suppressor, the design of which ameliorates the main causes of component failure, i.e., electrical arcing. Laboratory testing of magnetrons utilizing these insulated RF suppressors has indicated that significantly reduced failure rates can be anticipated.
An example of a commercial magnetron used for industrial heating, such as in food processing, is shown in
A magnetic pole piece 316 disposed around the upper portion of the magnetron 302 serves as an electrical connection to ground. An RF gasket 318 disposed around the lower portion of the magnetron adjacent the waveguide 304 seals the base of the magnetron to the waveguide. An air inlet 320 for the tube output ceramic dome 306 is provided on the bottom of the waveguide to provide cooling air to the ceramic dome. Most of the generated RF radiation (arrow 322) is directed down the waveguide, away from the magnetron in the known manner.
The known RF suppressors are formed in the shape of an annular collar piece. A molded RF suppressor piece is shown in
The known RF suppressor is made of a molded epoxy binder material having iron particles suspended therein. This material is chosen primarily for its excellent RF radiation absorbing properties. There are a number of commercially available RF and microwave absorbing materials, such as, for example, those supplied by the Emerson & Cuming Co. (Randolph, M A) under the trademark ECCOSORB®, that are suitable for the fabrication of RF suppressor components. Material selection can be used to optimize these absorbers for a particular application. The known materials can be molded into various shapes and sizes. After molding the basic shape of the RF suppressor component, the top end is machined with a groove and other features to accommodate mating with the cathode voltage supply connector fixture of the magnetron.
The effectiveness of an RF suppressor can be quantified in a number of ways. A suppression ratio can be defined as
and measured in decibels (dB). In the above equation, Pleakage detected is the RF power detected at some reference location with respect to the magnetron, and Pcoupled is the RF power coupled through the waveguide to a load. The suppression ratio is somewhat arbitrary since it depends on the detector reference location and the magnetron operating conditions. Nevertheless, the suppression ratio can be used to compare various RF suppressors: the utility of a particular RF suppressor can be evaluated by measuring the suppression ratio with and without the suppressor installed under identical operating conditions. The standard RF suppressor provides about −3 dB additional attenuation compared to magnetron operation with no RF suppressor installed.
The identification of common failure modes of a magnetron, i.e., the locations and mechanisms of phenomena that result in sub-optimal performance, malfunction, or damage to the device, is necessary in order to design a better magnetron. The present invention is concerned with failure modes associated with the effects of high electric fields on the RF suppressor. Virtually all materials exhibit some type of failure or breakdown when immersed in an electric field of sufficiently high field strength. The failure phenomena, in some cases classified as dielectric breakdown, often involve a combination of arcing and avalanche effects resulting in irreversible changes in materials properties, and invariably rendering the material unsuitable for continued use. As a result of this potential for permanent damage, materials are rated according to a maximum tolerable electric field strength. Since the electric field is due mainly to voltage differences imposed across the material, this maximum field strength criteria can also be expressed in terms of a voltage-hold off capability. For magnetrons, the voltage-hold off capability implies a maximum cathode voltage bias that should not be exceeded in order to safeguard the magnetron. Failure and damage due to high electric fields can be prevented by either selecting materials with higher voltage-hold off capability, or by employing designs which avoid the occurrence of excessive electric fields in parts of the device that are vulnerable to electric field-induced break down. In order to design improved RF suppressors and assess their potential, the cause and mechanism of field-induced failure must be identified and analyzed.
In the context of magnetron RF suppressors, the susceptibility to electric field-induced damage has been investigated by the present inventor and prominent failure mechanisms have been identified. In the normal course of operation of a magnetron, a high voltage is imposed between the metal cathode terminal and other electrically grounded metal surfaces, including the anode, casing, cooling tubes, etc. This dc cathode voltage bias sustains the thermionic electron emission current from the cathode to the anode that is necessary for operation of the magnetron. The resulting static electric field distribution depends on the geometric details of the magnetron including the boundary conditions imposed by metal surfaces, and the relative dielectric constants of the component materials. The static electric field is supplemented by a radio-frequency electric field caused by the cycloid motion of electrons in the magnetron cavity. Further, when the magnetron is turned on, there is a transient electric field due to overshoot of the power supply used to bias the cathode. The resultant electric field distribution from all of these contributions can be complex, but it is generally true that regions of comparatively high electric field strength occur in the vicinity of the cathode connection to its voltage bias supply. These regions of concentrated electric field strength, should they occur within or near materials with relatively low breakdown-voltage characteristics, are susceptible to damage. The RF suppressor component is one such component that is both prone to high electric field breakdown effects and deployed at a location where the electric field strength is expected to be relatively high.
Once voltage breakdown has been initiated in the composite RF suppressor material, it contributes to an avalanche effect in which a small electric arc travels through the suppressor, and a plasma is formed in the air surrounding the suppressor. The arc enlarges, ionizing the air, and forms a conducting channel that extends from the cathode terminal on the magnetron to a grounded surface in the vicinity of the suppressor that may include the external magnet pole piece, coolant water tubes, or some other grounded structure in the RF shield cabinet where the magnetron is stationed. Although the arc is eventually extinguished when the over-current protection device on the cathode power supply shuts the cathode voltage supply off, significant damage will still have occurred to the suppressor material. Failed suppressors are frequently charred or otherwise burned in an area where the suppressor contacts the high-voltage cathode supply terminal, or else along the inner surface of the suppressor annulus in the vicinity of the magnetron cathode contact. The damage to the RF suppressor will also typically include a punch through characterized by a perforation of the RF suppressor along the radial direction. A hole may be completely burned through the RF suppressor from its inner surface to its outer surface, or there can be a partial punch-through hole where material is visibly ablated mostly from the outer surface of the RF suppressor.
The damage to the RF suppressor due to cathode supply arcing is almost always irreversible. At minimum, the damage almost always requires replacement of the RF suppressor part for continued operation of the magnetron. Moreover, the magnetron itself is often damaged. The choke ceramic often sustains severe arcing characterized by a blackened area of several square centimeters in extent. The ceramic-to-metal seal on the magnetron choke is often damaged to a degree that results in loss of magnetron tube vacuum. When a vacuum tube loses its vacuum seal, it is no longer viable and must be rebuilt at considerable cost. The economic costs associated with RF suppressor component failure has made the development of improved industrial magnetron RF suppressors, able to sustain higher electric fields without damage, a pressing priority and motivate the present invention.
The present invention is directed to overcoming the problems associated with the known RF suppressors by use of an insulated RF suppressor that provides significant improvement with regard to tolerating higher cathode bias voltages. The insulated RF suppressor according to the present invention reduces magnetron failure rates and permits safer and more reliable operation at high microwave power levels.
The insulated RF suppressor according to the present invention is formed as a two-layered annular structure including an inner insulating sleeve and a coaxial outer RF absorbing shell. The inner sleeve of the RF suppressor is fabricated, by for example, machining or molding, from an electrical insulating material such as polytetrafluoroethylene (PTFE) and has a thickness of approximately 100 mils (about 2.5 millimeters). The outer shell is molded from the same or similar RF-absorbing material used in conventional magnetron RF suppressors. The PTFE sleeve provides a high degree of resistance to electrical breakdown at precisely the sites of the RF suppressor that are most susceptible to the adverse effects of high operating voltages. The use of the insulating inner sleeve realizes voltage break down characteristics that are significantly superior to those exhibited by conventional RF suppressors. At the same time, the molded outer cladding layer shell provides an RF absorbing function nearly equivalent to that attained in conventional RF suppressors that have no inner insulating sleeve. Thus, RF suppression is not unduly sacrificed in order to gain higher operating voltages.
The insulating inner sleeve may be fabricated, by for example, machining or molding, with a groove to seat the metal ring fixture that clamps the cathode for electrical contact and present a terminal post for can connecting the cathode voltage bias circuit and one lead of the filament circuit that heats the cathode. The screws, washers, and threaded holes used to fasten the cathode contact fixture to the RF suppressor are replaced with tabs in the insulating sleeve that hold the seated fixture in a groove formed on the edge of the RF suppressor insulated sleeve. The elimination of machined surfaces and the associated metal hardware is expected to provide further improvements in the voltage-hold off capability of an RF suppressor. Further, machined surfaces that absorb moisture and sharp edges that promote acing are eliminated in the RF absorber shell of the insulated RF suppressor.
The insulated RF suppressor according to the present invention is shaped and configured to be completely compatible with any arrangement for an industrial magnetron, and thus can be immediately incorporated into the manufacture of new magnetrons. The insulated RF suppressor can also be used to replace damaged conventional RF suppressors, or serve as a substitute component to retrofit magnetrons in the field with insulated RF suppressors as part of a preventative maintenance program.
Electrical testing of insulated RF suppressors indicates higher breakdown voltages are achieved with the insulated RF suppressor compared to conventional RF suppressors. In one series of tests, the insulated RF suppressor demonstrated a 30% higher voltage needed to initiate arcing, compared to a commercially-available, currently-used RF suppressor. Measurement of RF suppression performance showed that the insulated RF suppressor performed comparably to, or in some cases even outperformed, several commercial RF suppressors currently in use. Further, magnetrons using insulated RF suppressors were operated for prolonged, failure-free periods at RF power levels of 80 kilowatts. By contrast, some experience with the same magnetrons using conventional (non-insulated) RF suppressors under similar operating conditions showed a marked higher rate of failure.
A new type of RF suppressor is described herein. By fabricating the RF suppressor component from two functionally distinct materials, the performance of the RF suppressor, particularly with respect to its high-voltage tolerance, can be enhanced compared to that of RF suppressors made from only one type of material. The present invention is an insulated RF suppressor that incorporates an inner sleeve of highly electrically resistive material that can withstand the application of very high electric fields. The insulated RF suppressor component is fabricated as a bilayer composite of two parts: an insulating member shaped from a polymer material such as PTFE, and an RF-absorbing member comprised of a suspension of iron particles in an epoxy resin and shaped by using the insulating member as part of a form to mold the RF-absorbing material. The resulting RF-suppressor is then a single-piece comprised of an annular-shaped insulating polymer sleeve with a molded RF-absorbing shell formed as a cladding layer on the outer surface of the insulating sleeve member.
Referring now to
The insulated RF suppressor performs basically the same function as the conventional RF suppressor described above in connection with
An insulating RF suppressor according to the present invention is preferably made as follows. The insulating sleeve is machined or molded from PTFE or other suitable polymer. A mold is made up of two cylinders of differing diameters. The inner insulating sleeve is slipped snugly over the outside of the smaller-diameter cylinder. The smaller-diameter cylinder with insulating sleeve is then placed co-axially inside the larger-diameter cylinder. The RF suppressor material, such as ECCOSORB®-CR, comprising two components, an iron-powder-filled resin and an activator/hardener, is mixed and filled into the annular spaces between the two cylinders and the insulating sleeve. The molded mixture is then cured in an oven according to the process specifications provided by the manufacturer of the RF absorbing material.
An important difference between the known RF suppressor and the insulated RF suppressor according to the present invention is that in the insulated RF suppressor, the machined surfaces used to form the groove for the metal cathode connector fixture are restricted to the insulating sleeve, whereas in the conventional RF suppressor, the molded RF absorber material is machined. In fact, in the insulated RF suppressor, there is no machining of the molded RF absorber material. This aspect has important significance for the high-voltage tolerance of the insulated RF suppressor relative to the conventional RF suppressor because it is believed that machining of the molded RF absorber material causes suspended iron particles to be exposed at the machined surfaces. Such exposed metal particles act as point radiators or can concentrate the electric field and promote arcing effects. Thus, the elimination of machined surfaces, as well as the general avoidance of any sharp geometric features, in the molded RF absorber material contributes to the improved high-voltage tolerance of the insulated RF suppressor. Further, machined surfaces of the ECCOSORB® materials are believed to have a higher propensity to absorb moisture which degrades the electrical performance of the material such as its RF radiation absorption characteristics and voltage-holdoff capabilities.
Several tests were conducted to evaluate both the ability of an insulated RF suppressor according to the present invention in attenuating RF energy in a magnetron and in reducing failure associated with high-voltage, high-power operation of a magnetron. The tests were performed with a working example of the insulated RF suppressor according to the present invention.
Arcing Test
Using a high electric potential test, the arcing properties of a working example of the insulated RF suppressor according to the present invention were compared to those of a non-insulated RF suppressor of the type currently used in commercial industrial magnetrons. In this particular high potential test, as depicted in
In the comparative testing, the non-insulated RF suppressor was observed to arc at 24 kilovolts applied potential whereas the insulated RF suppressor was observed to arc at 30 kilovolts applied potential. Therefore, the insulated RF suppressor according to the present invention provided a 6 kilovolt improvement in the hold-off voltage under the test conditions specified.
High-Voltage Test on a Magnetron
An insulated RF suppressor according to the present invention was installed on a magnetron and the cathode bias voltage was snapped from 0 volts to −35 kilovolts in 2 seconds. This test was repeated five times with no failure of the RF suppressor. The leakage current measured through the RF suppressor was 80 microamps, well below the normal allowable leakage current of 2 milliamps.
RF Suppression Test
The insulated RF suppressor according to the present invention was evaluated for RF radiation suppression effectiveness in a magnetron unit using a Burle Model S94604F magnetron under operating conditions typical of its customary use in service. A magnetron having no RF suppressor was tested to provide a baseline reference for RF suppressor performance. The comparative setups included a magnetron having a standard (i.e., non-insulated) RF suppressor made by Burle Industries (part CR116VAC-2) and a magnetron using a standard (non-insulated) RF suppressor of the type used in commercial industrial magnetrons made by a U.S. manufacturer of microwave heating equipment.
The RF suppression is assessed by comparing the amount of leakage RF power measured relative to that measured when no RF suppressor is used. The suppression or emission power ratio is a figure of merit for comparing the efficacy of RF suppressors. The RF suppression ratio is defined as the leakage power emanating from the magnetron and measured by an RF power meter with its receiving antenna situated at a defined reference point with respect to the magnetron to the RF powered delivered to a load, as measured by the change in temperature of a water heating load that terminates a waveguide coupled to the magnetron.
The insulated RF suppressor provided about 4 to 6 dB of attenuation, with respect to a baseline case of a magnetron operating with no suppressor, and also outperformed a commercial RF suppressor used by at least one U.S. magnetron microwave heating manufacturer. Further, the insulated suppressor attenuation was almost comparable to that provided by the standard Burle CR116VAC-2 RF suppressor. Therefore, it is evident that the insulated RF suppressor design has not greatly sacrificed RF attenuation capability in order to achieve improved high-voltage resistance. The insulated RF suppressor was tested with and without a HUMISEAL® coating; with the coating provided a small but perceptible improvement in RF attenuation. This observation is in accordance with the expectation that such coatings would not significantly affect magnetron operating performance with respect to RF absorption. The purpose of such coatings is instead to merely provide additional resistance to moisture absorption and thus help reduce certain degradation phenomena associated with moisture.
Life Testing
Beginning-of-Life testing was initiated for the example of the insulated RF suppressor according to the present invention with the following cycle sequence: (1) high-voltage cathode bias OFF, (2) high-voltage cathode bias ON, (3) snap on RF power from 0 to 75 kilowatts, (4) snap RF power OFF, (5) high-voltage cathode bias OFF. This cycle was repeated ten times in a typical industrial microwave heating unit where the insulated RF suppressor was installed on the magnetron. No circuit breaker tripping nor arcs were evident at any time. In a further test, an industrial heating magnetron employed an insulated RF suppressor in continued use for several hundred hours without failure.
Alternative Embodiment of the Insulated RF Suppressor and Cathode Connection Fixture
Alternative embodiments of the insulated RF suppressor that conform to and improve upon features prescribed by the basic design described hereinabove are possible. Such alternate embodiments of the invention may include additional insulating coatings, shrink tubing or shrink wrapping, or other types of encapsulants to provide additional insulating protection and/or moisture barriers. An RF suppressor made of two machined insulating members with an intervening layer of RF absorbing material is one possible alternative embodiment of the present invention.
Referring now to
Further, the design of the insulated RF suppressor according to the present invention provides an opportunity to improve the design of the metal cathode connector fixture that mates the RF suppressor to the magnetron cathode. The connector fixture shown in