WO2000024228A1

WO2000024228A1 - Microwave apparatus and method for heating thin loads

Info

Publication number: WO2000024228A1
Application number: PCT/US1999/024344
Authority: WO
Inventors: Per O. Risman
Original assignee: Rubbright Group Inc
Current assignee: Rubbright Group Inc
Priority date: 1998-10-19
Filing date: 1999-10-19
Publication date: 2000-04-27
Anticipated expiration: 2001-04-19
Also published as: AU1121600A

Abstract

Apparatus and method for coupling microwave energy to a relatively thin load where the load has a thickness less than a half wavelength in the load and a penetration depth greater than the thickness of the load using an evanescent mode for the microwave energy where the mode is a TM type having a predominantly capacitive reactance and the load has a predominantly inductive reactance such that the mode is at least partially resonant in the system of the load and applicator space. The capacitive reactance evanescent mode is achieved by making the normalized wavelength be greater than 1.0.

Description

MICROWAVE APPARATUS AND METHOD FOR HEATING THIN LOADS

Background of the Invention This invention relates to the field of microwave heating, more particularly, to applicators adapted to heat relatively thin loads.

In the past, it was found difficult to heat thin loads using known microwave techniques, in part because prior art microwave modes, or field patterns, did not couple well to thin loads. The present invention overcomes this deficiency by significantly increasing the coupling of microwave energy to such thin loads. Another difficulty was that the frequency bandwidth for good impedance matching to the generator was very small for some load materials having low loss tangents (tan δ). The present invention also has applicability to other loads such as fluidized bed applications where the characteristic thickness of individual load elements is both: i) less than about a half a wavelength in the load, and ii) smaller than the power penetration depth.

Brief Description of the Drawings Figure 1 is a perspective view of a computer-generated three- dimensional representation of an envelope of a vertically directed electric field in a vertical cross-section through a vertical centerline of an applicator and load of the present invention.

Figure 2 is a side section view of the representation shown in Figure 1. Figure 3 is a profile of the envelope of the vertical E field of Figure 1 taken along a centerline of the applicator.

Figure 4 is a perspective view of a computer-generated three dimensional representation of a heating pattern in a 3x40x40 mm load. Figure 5 is a top plan view of the representation shown in Figure

3.

Figure 6 is a is a simplified perspective view of an outline of an applicator useful in the practice of the present invention.

Figure 7 is a simplified perspective view of an outline of an alternative embodiment applicator useful in the practice of the present invention.

Figure 8 is a perspective view of a computer-generated three dimensional representation of a microwave field pattern of the applicator of Figure 7. Figure 9 is a top plan view of a computer-generated representation of a heating pattern in the applicator of Figure 7.

Figure 10 is a polar plot of the matching characteristics of the applicator of Figure 7.

Figure 11 is a top view of another applicator useful in the practice of the present invention when it is desired to irradiate a fluidized bed with evanescent resonant microwave energy.

Figure 12 is a side view of the applicator of Figure 10. Figure 13 is an exploded perspective view of the applicator of Figures 11 and 12. Figure 14 is a side view of still another applicator useful in the practice of the present invention to apply evanescent resonant microwave energy to a sheet or web of thin material as a load.

Figure 15 is a bottom view of the applicator of Figure 14. Figure 16 is a perspective view of the applicator of Figures 14 and 15.

Figure 17 is a top plan view of an. applicator system (with microwave feed parts removed) for processing loads such as baking cookies using the present invention. Figure 18 is a perspective view of the system of Figure 17.

Figure 19 is a side section view of a single applicator from the system shown in Figure 17, illustrating certain aspects of this embodiment of the present invention.

Figure 20 is a top plan view of the single applicator of Figure 19 showing the microwave feed system.

Detailed Description of the Invention As is conventional, a parameter of importance in waveguide and applicator analysis is the normalized wavelength, v (greek "nu"):

where f is the so-called cutoff frequency, and f is the operating frequency. In a rectangular waveguide with side lengths a and b having a mode with indices m and n in these respective directions, the solution to the boundary value problem gives: v² = (λ₀/2)²[(m/a)² + (n/b)²] (2)

This equation has a limited number of integer solution pairs (m;n) in each interval of v. As a consequence, all possible combinations of (m;n) modes for given values of a and b are represented by a finite set of v values. A normally propagating mode must have 0 < v < 1. Modes with v = 1 are often called zero index modes, and modes with v > 1 are often called cutoff modes. For cutoff or evanescent modes, there is a gradual decay of mode energy along the waveguide (or in a cavity as the microwave energy progresses away from the feed structure i.e. in the "downstream" direction). If there is a load which can absorb power located in a waveguide within a distance to the source of microwave energy of the same order as that over which the mode intensity decays, power will be absorbed by the load. As an example, if v = 1.01 for a waveguide mode at 2450 MHz, the real part of the vertical wavenumber kz [which becomes 2π/λ₀( 1 -ζ²) with the function exp(-jkzz) determining the decay] becomes 1 at 137 mm; the field intensity has decayed by 1/e (the base of the natural logarithms) over that distance. The energy decay distance (dao) is: d_do = λ₀/(4π(v²-l)^1/2) (3) where λ₀ i_s the free space wavelength. This is the distance in an empty and constant cross section waveguide over which a unidirectional evanescent mode field amplitudes decay by a factor oHe and the energy density of the field by e.

The present invention is particularly suited to heat relatively thin loads, where the penetration depth, d_P is much greater than the thickness of the load. d_p = λ₀Vε'/(2πε") (4)

Using the present invention, microwaves are passed multiple times through the load using multiple reflections in a resonant system which includes the applicator and the load. One application of the present invention is to puff or pop foodstuffs such as cereal or snack food. In such circumstances, the load is typically a cluster of unpuffed or unpopped items (such as kernels) initially, changing to a widely distributed collection of individual items upon puffing or popping caused by microwave heating. Even if the material parameters and ε" are difficult to define in microwave physical terms, it is clear that the effective permittivity ε' and the loss factor ε" are low. The volume of the (unpopped) mass is also small. In order for analysis to be possible, one must assume some numerical data. Certain measurements yielded the following range: ε = (2.3- jO.15) to ε = (2.3-J0.30), consistent with knowledge of the water and ionic content and literature data for similar products. These values can thus be used as the dielectric data for one typical load for evanescent heating. The size may typically be like small spheres with a 5 mm diameter. Such individual bodies are inductive in the 2450 MHz ISM operating microwave frequency band. It has been found convenient to model the load as located on a low block-shaped rectangle made of polytetrafluorethylene (PTFE). Using an unpopped mass of 3 mm thickness (height) and 40x40 mm horizontal dimensions, the load volume then becomes 4.8 cm ). Larger and smaller volumes were also used in the modeling, for sensitivity testing. Once individual items are popped or puffed, it has been found desirable to transport them away from the remaining unpopped items by a fluidized bed technique using air levitation and transport. The popped items are not expected to influence the microwave properties of the physical space in the applicator. Relatively few such items will remain there, since they are quickly blown out. In addition, popped items typically will have lost water and therefore interact much less with the microwaves. Finally, the applicator is preferably designed so that there is no large applicator volume above the support for the unpopped items.

Microwave transfer to the load

Some basic considerations are needed. The first is the microwave penetration depth d_P of the load. It becomes 197 mm and 99 mm, respectively, for the two end point values of the permittivity range (ε) mentioned above. However, this is only valid in free space with perpendicular incidence. The absorption depth d_a is given by: d_a = - λ₀[l/(4πlm(ε-v)^1/2)] (5)

It is to be understood that the absorption depth must be used instead of the penetration depth when v ≠ 0. A reasonably efficient applicator must couple as strongly as possible to the load and will then have a normalized wavelength v close to 1. With this, the absorption depth da (which is valid for specified mode characteristics) will be between 148 mm and 74 mm, for the permittivity range specified previously. These shorter da than d_P values also show that modes with v close to 1 have improved absorption characteristics.

If the load is reasonably large horizontally, there will be a defined equivalent angle of incidence towards it. Using the optical analogy for waveguide modes, one obtains sinθ' = θ, where θ' is the angle of incidence with a normal to the load surface as a reference. One must then apply the formula for absorption depth, which also considers θ' (this is set to 0°i_n the definition of the penetration depth d_P)- Using another example, for an ε = (2.5 - jθ.25), the penetration depth d_P becomes about 125 mm using Equation (4). Insertion of v=l into Equation (5) gives an absorption depth da of about 95 mm, which is significantly shorter than the penetration depth of 125 mm for the same substance. It is to be understood that for v > 1, the equivalent incidence angle may be specified as complex. The real part becomes π/2 and

2 1/2 the imaginary part becomes In [v + (v - 1) ], in radians.

Another consideration in the practice of the present invention is that the load thickness is typically much smaller than da, by as much as 25 to 50 times smaller. This makes it necessary that microwaves pass through the load many times, by multiple reflections in the system of the applicator with the load. Thus a high efficiency is obtained by constructive interference and resonance. The applicator system (which may include its feed structure) must thus be resonant with the load.

Resonant Applicators and Quality Factor (Q value)

The need for constructive microwave interference requires quite exact conditions when many retroreflections are needed, as here. This leads to a narrow frequency bandwidth of the system. The so-called unloaded Q value (Qu) of the load itself is the lowest achievable for a weakly coupled applicator. The value is:

Q_u = ε'/ε" - 1/tan δ (6) and becomes 15 for the low-ε" load. (For other loads of interest, Q_L may take on values in the range of 3 to 20.) However, this represents the oscillating energy in the load only, and a larger applicator volume than that is needed for providing an even heating and for practical considerations - both mechanical (space is needed for feeding the microwave energy into the applicator, and room is needed to allow the physical insertion and removal of the load) and electrical (space is needed for providing enough distance from the feed region to avoid spot heating, and to avoid arcing between adjacent metal parts). In addition, the design must not be complicated and expensive, which ordinarily leads to designs with characteristic applicator dimensions on at least the order of one half of the free space wavelength λ_{0 a}t the nominal frequency of operation.

The total Qu value must then increase over that of the load only, by the quotient between the equivalent electrical volume of the applicator and that of the load. Using a cube-shaped applicator with λ₀/2 sides thus leads to a total Qu value of 229/4.8 times the inherent Q_u of 15, i.e. about 700. This is reduced to half due to a balancing with feed properties at matching (no reflected power), i.e. to a loaded Q value QL of about 350.

The QL value can now be used to assess the frequency bandwidth of the system. This simply becomes the operating frequency f (2450 MHz in the normal case) divided by QL. The result is 7 MHz ±3.5 MHz - which is too small to be practical in a system where a reasonably high efficiency is needed. By adjustment of the system coupling so that it becomes overcoupled, an efficiency better than about 60% may then be achievable over a range of ±5 MHz. However, this is still quite small and not satisfactory, since the system becomes too sensitive.

In this case, it becomes imperative to use a small electrical volume of the applicator, and at the same time try to maintain other design criteria. The first item to consider is the microwave feed. This must be through physically and electrically small openings. The second item is to consider is a method of reducing the size of the applicator. This reduction is achieved using an evanescent hybrid mode resonator. The evanescent mode uses a "special" nearfield (in a region near the feed area) having an excess capacitive energy which is compensated for by induced inductive properties of the flat load, so that resonance occurs. There are several advantages with this approach:

1. The electrical volume becomes very small in a direction away from the applicator feed area. 2. The load may be positioned close to the feed (while avoiding higher-order nearfϊelds causing hot or cold spots) thus further reducing the electrical volume.

3. Using an evanescent mode results in a lower resonance frequency change with load size and properties than other single-mode systems. This means that the resonance frequency remains more stable than in other systems, so that motor-driven matching devices or other expensive means to maintain a high efficiency can be avoided.

4. The applicator region above the load becomes a leakage- preventing zone, which is easy to design due to the evanescence of the applicator mode.

However, it is desirable to avoid heating with conventional nearfields (i.e., modeless direct coupling, to be contrasted to the "special" nearfield of an evanescent mode) to avoid disturbing the operation of the system. One approach is to use at least two feed slots in a surface of the applicator cavity parallel and adjacent to the load. Typically, 50 mm or less vertical distance (i.e., perpendicular to the principal surface of the load) is achievable even for applicators with more than 200 cm horizontal cross sectional area (parallel to the principal surface of the load) and operating in the 2450 MHz ISM band. It is also necessary to address the distance from the load to the metal plane opposite the feed. The horizontal (parallel) E component will vanish at the metal plane close to a thin, low-permittivity load. Making the distance between the load and this metal plane short, the applicator volume decreases, resulting in a larger frequency bandwidth for resonance. The v (nu) value for resonance also increases somewhat, but system resonance is somewhat insensitive to variations in the distances between the principal surfaces of the load and the applicator cavity walls parallel thereto. Nevertheless, the evanescent resonance will become weaker with a metal plate close to the load, since the parallel E field component in the load will be partially shorted out by the plate.

In order to reduce the electrical volume of the system (the applicator and load combination), it is desirable to increase v (nu) (i.e., increase evanescence) of the mode. The field energy is bound to the load and the total energy typically decreases away from it, in spite of the decreasing amplitude of the primary wave that impinges on it. Decreasing the cross sectional area of the applicator cavity downstream of the load (taken with respect to the microwave power applied to the load) will cause a major decrease in the electrical volume of the system, and reduce da. For example, increasing v from 1.02 [corresponding to (90-jl 1)°] to 1.05 [corresponding to (90-jl 8)°] requires a 3% linear reduction in the horizontal cross section of the applicator, and results in dd being reduced from 48 to 30 mm at 2450 MHz.

Other advantages of the cross section reduction (in addition to reducing the electrical volume) are as follows:

1. The reduction can be in the form of a frustrum of a pyramid, which is a practically realizable shape, causes less disturbance of the field pattern at the wide end of the frusteum which is closer to the load, and results in very strong evanescence (energy decay) moving away from the load in the direction of the decreasing cross sectional area. 2. The decreasing cross sectional segment of the applicator increases the relative strength of the parallel E field component in the load, since a more evanescent mode gets an increasingly dominant capacitive (perpendicular E) field pattern, resulting in adjustment of the relative strength of the parallel and perpendicular components of the E field in the load, thus enabling adjustment of the evenness of the heating pattern.

3. The decreasing cross sectional area region provides a convenient shield or enclosure for exhausting air in drying processes and assists in evacuation of the product in fluidized bed applications such as in popping or puffing foodstuffs, where the load is airborne away from the applicator after microwave treatment.

It has been found achievable and preferable to use resonant evanescent modes with a v (nu) corresponding to at least (90-j 5)° up to about (90-j 10)°. These modes are useable for load thicknesses about and preferably less than the energy decay distance dd of the load. Advantages then include:

1. Generally permittivity-independent system impedance matching, allowing high efficiency for a variable permittivity load, such as often occurs during drying.

2. Independence of impedance matching with respect to load thickness within an appropriate range.

3. Only weak dependence on load position with respect to the feed region, due to lack of standing wave phenomena, permitting treatment of non-planar or particulate type loads without degradation of the microwave heating effect. 4. For loads having a horizontal principal surface, obtaining the strongest possible heating by the perpendicular E field component (Ex), resulting in a heating pattern more evenly distributed in the horizontal direction. This is because the pattern caused by losses due to currents induced by the horizontal H fields is complementary to the pattern caused by the direct action of the vertically directed E field.

5. There is only a weak dependence on the distance between the load and the underlying (downstream) metal surface. It is even possible to have the load contact the metal surface since the E _ component is maximum at the bottom surface of the load and the E|| component is at a minimum there.

6. Open-ended applicators can be designed and used without appreciable leakage, since the acting mode is evanescent and has a limited action distance in the free space. One disadvantage, however, is that the very low mode impedance results in high wall currents in the cavity surfaces, relative to power transferred to the load. This can be addressed by suitable choice of a wall material to have high conductivity, such as aluminum. Other high conductivity materials may be used, such as by plating cavity surfaces with silver or gold or other highly conductive material. Another disadvantage is that the dimensions of the applicator become quite sensitive and thus require close control of dimensional tolerances. Typically a tolerance of the resonant frequency of 15 MHz or less is desirable in 2450 MHz systems. This means that the tolerance of some inner dimensions must be less than 0.6% (i.e., 1.2 mm on a 200 mm dimension). However, the same stringent tolerance requirement does not apply to the load; a deviation of 20% in overall load volume will typically cause insignificant changes of system efficiency, and even a very significant change in its permittivity will not change it much. In effect the system sensitivity has been moved from the load to the applicator, where it may be easier and simpler to control.

It is to be understood that evanescent resonant modes can only exist with a load. Since resonance is defined as the equality of electric and magnetic oscillating energies in the system, the load is needed to supply the inductive energy component to match the inherently capacitive energy characteristic of TM type evanescent modes. The particular characteristic is that the intensity of the fields is larger near the load than elsewhere in the applicator space (with the exception of conventional nearfields in the immediate vicinity of the feed port(s). Thus, if wave energy density at a distance from the feed port(s) increases near the load, a clear and unambiguous indication that an evanescent resonant mode exists in that region.

Another characteristic of both resonant and non-resonant evanescent modes is that the mode field impedance (i.e., the vectorial sum of the impedances of the forwards and backwards waves in a specified point or plane) in the feed region is very low. Typically, low impedance feed structures, such as narrow slots, are then preferred. Depending upon the Q value of the applicator and load combination, the slots may be about equal to or shorter than half a free-space wavelength (λo/2). In optical terms, the mode or modes used in the present invention may be said to be "beyond Brewster conditions" and of the total reflection type (as that term is used in optics). They may also be classified as surface waves. The simplest mode type in a Cartesian applicator (i.e., one having only walls with right angles to each other, such as a rectangular applicator with rectangular cross section) is a simple TM type, lacking an H field parallel to the principal surface of the load. It is to be understood to be within the scope of the present invention, however, to have or utilize hybrid TM type modes. One or more hybrid TM modes are, in fact, preferred in the practice of the present invention because:

1. They can be fed by slots, which are easier and less expensive to manufacture than coaxial antennas needed for pure (simple) TM mode excitation.

2. Hybrid TM modes lack one E field component parallel to a principal surface of the load (i.e., they lack the E field parallel to the long direction of the slot), making it possible to substantially eliminate edge overheating of load sides parallel to the feed slot long dimension.

A further characteristic of evanescent resonant applicators is that the distance from the feed area to the load is comparable to (i.e., between about 1/4 and about 4 times) the decay distance dao in the applicator space. For applicator-load systems operating in the 2450 MHz; ISM band and fed from a single, dual or multiple slots, typical distances between the feed and load are in the order of 30 to 100 mm. The v value in the space between the feed and the load then typically corresponds to about (90-j 10)° which means that v is about 1.015, giving a dao of about 80 mm.

In a 2450 MHz open-ended Cartesian applicator system using the lowest mode, TEyπ_e, (where the subscript "y" stands for "transverse electric to y,"the subscript e stands for evanescent, and the third subscript position indicates the z direction) the load may be placed or located 30 mm away from the feed area. This distance has been found to be sufficient to avoid conventional nearfield effects with their consequent uneven heating by coupling directly to the load.

Data from Modeling

With the low-loss load described above, an applicator allowing substantially even heating over about 65x65 mm load surface area has a resonant frequency of 2448 MHz and works well within ±12 MHz. This corresponds to an effective electrical volume of only about 200 cm , whereas the mechanical volume exceeds 400 cm . The resonant frequency becomes the same with the higher ε" load. With a 3 mm thick but 60x60 mm large low ε" load (twice the mass) the resonant frequency becomes 2436 MHz. In addition, the frequency bandwidth becomes larger: about ±20 MHz.

Referring now to the figures and.most particularly Figures 1-3 a vertically directed electric field 10 in a vertical cross section through the vertical centerline is illustrated (Figure 3 shows the envelope 12 of the field). It shows that the field 14 in the waveguide 16 below the applicator is much weaker than the field 18 in the applicator 20 (as is typical for well/designed high-Q systems). It also shows the field 22 decay upwards, above the load 24. The level is so low there that no substantial leakage occurs. Almost all leakage that does occur is caused by diffraction phenomena of the load resulting in the so-called normal mode (of a TE_Z type in this case) which would require a longer distance to be choked.

The notch 26 in the envelope 12 in Figure 3 indicates where the field 18 couples to the load 24, with the width 28 of the narrow end of the notch 26 corresponding to the thickness 30 of load 24. Figures 4 & 5 show the heating pattern in a horizontal cross section of a 3x40x40 mm load 24. Figure 6 shows the outline of the applicator 20 with its underlying waveguide 16 and the two narrow slot feeds 30, 32 into the bottom of the applicator. Dimensions are relative (the overall height of the applicator is about 140 mm) and it is to be understood that figure scale factors are slightly distorted. In this embodiment, load 24 is supported on a microwave transparent shelf 34. Applicator 20 has a pyramidal frustrum section 36 reducing the cross sectional area 38 at the downstream end 40 of the applicator 20.

The present invention makes use of TM modes with v > 1. In the practice of the present invention, it is desirable to have such modes dominate the system operation.

The reflection of an evanescent TM mode at the surface of a thick dielectric is quite low, because the mode has a very low impedance in the cavity, but a partial cancellation of the reactive field parts further reduces the reflection. However, evanescent mode resonances become pronounced only if the dielectric is so thin relative to d_P that retroreflections occur in it, and not so thick that the first internal halfwave resonance is approached since the inductivity will then decrease. The field pattern in the load achieved with the present invention will have a strong E field component perpendicular to the major surface of the load (Ej_). Other E components can also be made strong in the load. Since the external horizontal E field components are in phase quadrature to the vertical (perpendicular) one and in addition are horizontally located a quarter wavelength apart, an extremely important and advantageous property of the present invention is achieved: two substantially arithmetically added, complementary heating patterns are achieved by the same mode. Normally (in non-evanescent systems), quadrature of the induced horizontal current, causing heating of a (typically thick) load, and the directly influencing perpendicular E component heating the load is not significant since the perpendicular E component is weakened by a factor of ε (epsilon) in the load. For propagating modes having v (nu) close to 1, the two mechanisms heat a load equally only for permittivities ε < 3. However, using a strongly evanescent mode to heat a very thin load, the perpendicular E field component heats more strongly. Adjusting the evanescence so that the decay distance is increased, will increase the coupling to the load of the evanescent mode and decrease the coupling of the perpendicular E field component. Increasing either the load thickness or the value of the dielectric constant, ε', (or both) will also reduce the relative heating from the perpendicular E field. It is thus possible to optimize the system by adjusting the system to have equal heating by induced current and the direct action of the perpendicular E field.

Referring now to Figures 7-10, an alternative embodiment applicator 44 may be seen, along with its microwave field pattern 46 and its heating pattern 48. Figure 10 gives a polar plot 50 of the matching characteristic 52 of the applicator 44. Applicator 44 has a top feed 54 delivering microwave energy to a containment chamber 56. A load (not shown) may be placed on a shelf (also not shown) located in plane 58.

Examples of Evanescent Resonant Applicators

A. Fluidized Bed Applicator for Small Particles

Referring now to Figures 1 1, 12, and 13 an alternative applicator 60 for a fluidized bed operation may be seen.

Since the particles move around in the applicator, there is no need to optimize the system for equal heating by the E_± and the E|| components. However, both ends of the applicator need to be open to admit both the load material and the levitating gas, typically air. A magnetron 74 powers a forked waveguide feed apparatus 62 has a pair of slots 64, 66 coupling microwave energy into the cavity 68 having a pyramid frustrum 70 above the waveguide structure. An air inlet duct 72 is coupled to the applicator 60 for levitation and transport of particles (not shown) forming the load. The main microwave cavity space of the applicator is as small as possible in relation to the volume needed for mass transfer of the particulate or granular product load. In this system, the applicator mode is substantially a square TMiie type, with a strong vertical (perpendicular) E field, centered horizontally, which is the dominant heating component.

To size an applicator according to this aspect of the present invention, a suitable decay distance, (e.g., 60 mm) is chosen, related to the vertical distance between the feed and the load. The corresponding v (nu, normalized wavelength) is then calculated using Equation (3), and the resulting value is then introduced to Equation (2) using m=n=^;:l and a=b. A value for the dimension a=b is thus obtained. The vertical distance from the feed to the load is preferably chosen to be as small as possible, while avoiding nearfield effects directly on the load, and simultaneously providing the desired evanescent mode pattern.

B. Evanescent Resonant Applicator for Thin Sheet or Web Loads

Referring now to Figures 14, 15 and 16, an alternative applicator 80 according to the present invention is shown for use with thin, usually continuous, loads such as a sheet or web material 82, typically moving in a continuous fashion through a pair of openings 84 in the applicator 80. In this embodiment, the load 82 is flat, thin and horizontal. It may be unevenly wetted and needs to be evenly dried. For this reason, this embodiment is dimensioned for equal heating by the vertical and one horizontal E component. Operating in the 2450 MHz ISM band, the width 86 of the thin load is about 300 mm. For a load made up of a sheet or ribbon of paper, the preferable mode is TM2ie, with index 1 in the 300 mm (width) direction. Choosing dd to be about 70 mm, v = 1.04 and the remaining horizontal dimension 88 is 120 mm.

The feed from the waveguide to the applicator is via a hole pattern in a metal wall shared by the waveguide and the cavity. The microwave mode in the waveguide is TEio, (with the horizontal direction as a reference). This results in an applicator mode of TM2ie with a dominating vertical E field. The feed type causes minimal conventional nearfields, so the distance from the feed area to the load can be as close as 30 mm. It is preferable to make the applicator wider than the load, since the field intensity falls off near the shorter side walls. To improve the two strong vertical E field components, the applicator is not closed at the lower end 90, but is provided with a pyramidal frustrum 92 open at both ends. To prevent physical entry and reduce microwave leakage, the open lower end 90 may be covered with a metal mesh or screen. The field amplitude at this plane is attenuated sufficiently that the mesh will have negligible effect on the heating pattern.

C. Evanescent Resonant Applicator for Processing Loads such as Cookies Referring now to Figures 17 - 20, an applicator system 94 for processing loads such as baking cookies may be seen. The applicators in this example have the same mode, TM₂ie, as in the previous example, but it is to be understood that other modes may be chosen, consistent with the needs dictated by product belt width, microwave generator power, and desired power flux density in the load. In the views shown in Figures 17 and 18, the microwave feed system is omitted for clarity. In this system, a plurality of individual applicators 96 are arranged in a staggered relationship offset by a predetermined amount, preferably one quarter of the free space wavelength, between successive applicators. Each applicator has a feed slot 98 in a top wall 100. The applicators are surrounded by a choking flange 102 which has overlapping portions 104, 106 extending downward at opposing sides thereof. System 94 has a metal belt 108 for supporting the cookies 110 as they are moved past the plurality of applicators 96. Capacitive flanges 102 act to couple vertically directed current (as displacement current) to the metal belt 108. The overlapping portions 104 and 106 prevent leakage along the sides of the metal belt 108.

Referring now most particularly to Figures 19 and 20, each applicator 94 preferably has a waveguide 112 and magnetron 114 to provide microwave power through feed slot 98. Since high power is needed over a large surface area, a multi- applicator and load system with staggered applicators is preferable for the invention embodiment of this example. The mode is preferably TM2ie with the system 94 operating in the 2450 MHz ISM band.

The evanescent mode used in this embodiment preferably has a weak horizontal E field component relative to the vertical component, because the metal belt effectively shorts out those components, and thus will reduce or eliminate edge overheating of the cookies forming the load. The cookies forming the load are typically porous, with a height of about 8 mm, and an initial permittivity of about (16-J8) , decreasing to about (2-J0.5) as drying takes place as a result of the microwave irradiation used in this embodiment.

Because of the mode used, there will be 2 elongated areas 1 16 of higher intensity Ez field irradiation (so-called "hot-spots") in each applicator, as shown in the leftmost applicator 96 in Figure 17 and in Figure 19. There will be a "cold spot" or region 118 between areas 116 in each applicator 96. Because the mode intensity will fall off near the short sidewalls 120,122 of each applicator, it is preferable to space cookies to be processed from the sidewalls

To design an applicator 96 for system 94, one must first decide on a desired mode, chosen to avoid arcing at lateral edges of the conveyor belt 108. Typically, and preferably, one would select a mode index 1 (for the "n" index) to simplify exclusion of unwanted modes. Next the other ("m") index is chosen, usually selected to be of a low order, such as "2" or "3" (mode index 1 is considered to result in too narrow of coverage) to get a desired power flux density in the cavity of the applicator. Finally, the applicator is sized to avoid undesirable modes and to get the evanescent mode that is desired, according to the principles of the present invention. In this embodiment, it is apparent that the evanescence must exist between the feed and the load, because of the desirability of using a metal conveyor belt — to withstand the temperatures and meet the sanitation requirements of baking cookies, which may include the use of direct thermal energy (such as by way of forced hot air convection) in addition to the microwave irradiation. If the load is relatively lossy and large (as cookies typically are) it is preferable to use a relatively higher cavity height with the consequent higher Q and with v close to 1. If a thin load is to be processed, it is preferable to use a support such as ceramic to locate the load away from the metal belt, in which case the support may be chosen to have a similar ε to the load, resulting in an ability to lower applicator height and operate with a lower Q value. Furthermore, it is desirable to make the ratio of b/n larger than the ration of a/m where "b" is the distance between the sidewalls 120, 122 and "a" is the width of a sidewall 120 or 122. Making b/n greater than a/m by 3 times or more, will cause the x-directed H field to be come much weaker than the y-directed field. Hence this will reduce the risk of arcing or leakage at the sidewalls.

As to the choice of v (nu), choosing v=l (zero index mode) may under some circumstances provide an acceptable result, in particular if the load fills the belt rather well and is quite lossy. However, the system with a semi- dry load would then become rather mismatched, resulting in poor efficiency. As a compromise, an evanescent mode provides a higher efficiency for semi- dried loads, but v (nu) is now chosen between 1.006 and 1.010 and the applicator height is chosen so that the matching as a function of the changing dielectric properties becomes suitable; 70 to 120 mm may be found suitable in this specific example. The invention is not to be taken as limited to all of the details thereof as modifications and variations thereof may be made without departing from the spirit or scope of the invention. For example, it is to be understood that the present invention includes applicators having a zero order mode between the feed and a dielectric type load, provided that there is also a TM type evanescence in the applicator resonantly interacting with the load. One example is an applicator having an open-ended frustrum section downstream of the load where the evanescence at least partially resonates with the load.

Claims

What Is Claimed Is:

1. A microwave applicator for heating loads having both a thickness less than a half wavelength of a propagating microwave field in the load and a penetration depth in the load larger than the load thickness, the applicator comprising: a) a microwave containment chamber formed of material impervious to the passage of microwave energy and having a free space region therein; b) means for generating a TM type evanescent mode having a reactance that is predominantly capacitive in free space in the containment chamber; and c) a relatively thin dielectric load having a predominantly inductive reactance and located in the chamber such that the microwave energy is at least partially resonant with the load to heat the load.

2. The applicator of claim 1 wherein the chamber has generally planar sides, and the evanescent mode has a normalized wavelength in the free space region of the chamber in the range of greater than 1.0 and about 1.1.

3. The applicator of claim 1 wherein the free space region of the chamber has at least a portion thereof characterized by a changing cross section.

4. The applicator of claim 3 wherein the portion of the free space region characterized by a changing cross section is formed by wall portions in the shape of a pyramidal frustrum.

5. The applicator of claim 1 wherein the containment chamber has a feed port for delivering the evanescent microwave energy in the 2450 MHz ISM band, and wherein the load is located at a distance from the feed port such that the normalized wavelength, v, is in the range of 1 < v < 1 J, where the normalized wavelength is obtained by selecting an energy decay distance ddo and solving the formula v = [1 +

(λ₀/4πd_do)²]^1/2.

6. The applicator of claim 5 wherein the load is spaced a distance from the feed port sufficient to avoid nearfield effects from directly influencing the load.

7. The applicator of claim 1 wherein the containment chamber has a surface impervious to the passage of microwave energy generally perpendicular to the direction of the microwave energy evanescence and downstream of the load and positioned a distance from the load to obtain a desired balance between orthogonal components of the microwave energy in the load.

8. The applicator of claim 1 wherein the load has at least one edge and the containment chamber has a surface impervious to the passage of microwave energy generally perpendicular to the direction of the microwave energy evanescence and downstream of the load and positioned a distance sufficiently close to the load to reduce edge overheating by reducing an E field component of the mode oriented parallel the edge of the load.

9. The applicator of claim 8 wherein the distance between the load and the downstream chamber surface is sufficient to enable the microwave field to be resonant with the load.

0. The applicator of claim 1 wherein the containment chamber has an input port having a nearfield region adjacent thereto delivering microwave energy to the interior of the containment chamber and the field intensity of microwave energy in the containment chamber is larger near the load than elsewhere in the containment chamber other than in the nearfield region.

1 1. The applicator of claim 2 wherein the planar sides of the containment chamber are perpendicular to an x-y plane in the chamber and the load is positioned parallel to the x-y plane in the chamber and the dimensions a,b of the planar sides in the x and y directions are selected with relatively low integer indices for m and n to result in a normalized wavelength in the free space region of the chamber in the range of greater than 1 to about 1.1 according to the formula: v² = [(λ₀/2)²][(m/a)² + (n/b)²] where v is the normalized wavelength and λo is the free space wavelength.

12. The applicator of claim 11 wherein the m and n indices are each selected from among the group of integers of 1 and 2.

13. The applicator of claim 1 1 wherein m is chosen to be 1 and the ratio of m/a is made substantially less than the ratio of n/b.

14. The applicator of claim 11 wherein the microwave energy is delivered to the containment chamber via a feed port and the load is spaced from the feed port by a distance sufficient to avoid near field irradiation of the load by the energy delivered via the feed port.

15. The applicator of claim 14 wherein the feed port includes multiple apertures, and the load is spaced from each aperture sufficient to avoid near field irradiation of the load by energy emanating from each aperture.

16. The applicator of claim 14 wherein the feed port is located so that the desired evanescent mode is field matched and any propagating mode is made anti-resonant by choice of containment chamber height to avoid coupling undesired modes into the chamber.

17. The applicator of claim 1 wherein the chamber has a generally cylindrical sidewall, and the evanescent mode has a normalized wavelength in the free space region of the chamber in the range of greater than 1.0 to about 1 J .

18. The applicator of claim 17 wherein at least a portion of the cylindrical sidewall of the containment chamber is a right circular cylinder extending about a cylindrical axis at a radius a, and wherein the load is positioned in a plane perpendicular to the cylindrical axis in the chamber and the length of the radius is selected with relatively low integer indices for m and n to result in a normalized wavelength in the free space region of the chamber in a range of greater than 1.0 and about 1.1 according to the formula: v = λnχ_mp/2-πa where v is the normalized wavelength and λo i_s the free space wavelength, xm_P is the p:th zero of the Bessel function J_m(x).

19. The applicator of claim 17 wherein free space region of the chamber has at least a portion thereof characterized by a changing cross section.

20. The applicator of claim 19 wherein the portion of the free space region characterized by a changing cross section is formed by a wall portion in the shape of a conical frustrum.

21. The applicator of claim 18 wherein the portion of the free space region characterized by a changing cross section is formed by a wall portion in the shape of an elliptical frustrum.

22. The applicator of claim 18 wherein the normalized wavelength in the free space region of the applicator is in the range of greater than 1.0 to about 1 J .

23. The applicator of claim 22 wherein the microwave energy is delivered to the containment chamber via a feed port and the load is spaced from the feed port by a distance sufficient to avoid nearfield irradiation of the load by the energy delivered via the feed port.

24. The applicator of claim 1 wherein the load is located at least partially within the energy decay distance of the evanescent mode, dao, where ddo = λ₀/[4πV(_v ² - 1)], and λ₀ is the free space wavelength and v is the normalized wavelength in the free space of the chamber.

25. The applicator of claim 1 wherein the resonant frequency of the empty chamber is separated from the frequency of the resonance with the load.

26. The applicator of claim 25 wherein the resonant frequency of the empty chamber is separated by about at least 20 MHz from the frequency of the resonance of the evanescent mode in the load when the applicator is operated in the 2450 MHz ISM band.

27. A method of using microwaves to heat loads having a thickness less than a half wavelength in the load and a penetration depth in the load larger than the thickness, the method comprising: a) placing a relatively thin load having a predominantly inductive reactance in a microwave containment chamber formed of material impervious to the passage of microwave energy; b) providing a TM type evanescent mode in the containment chamber with the mode having a reactance that is predominantly capacitive in a free space region of the chamber; and c) applying the evanescent mode microwave energy to the load such that the microwave energy is at least partially resonant in the combination of the free space region and the load.

28. The method of claim 27 wherein the evanescent mode has a normalized wavelength in free space in a range of greater than 1.0 to about 1.1.

29. The method of claim 28 wherein the normalized wavelength is about 1.015.

30. The method of claim 27 wherein substantially all the microwave energy in the containment chamber is evanescent.

31. The method of claim 27 wherein the load is located substantially outside any nearfield region in the chamber.

32. The method of claim 27 wherein the containment chamber has a feed port for delivering microwave energy in the 2450 MHz ISM band and the load is located at a distance greater than about 50 mm from the feed port.

33. The method of claim 27 wherein step b) further comprises delivering microwave energy through a feed port to the chamber and the load is located within an energy decay distance, ddo, of the feed port, as determined by ddo = λ₀/[4π /(v² - l)], and λ₀ is the free space wavelength and v is the normalized wavelength in the free space of the chamber.

34. Apparatus for high-intensity microwave popping of foodstuffs of the type having a thickness substantially smaller than a microwave absorption depth of the material of the foodstuff, the apparatus comprising an evanescent hybrid mode resonant cavity for microwave irradiation of a generally thin load such that the microwaves are reflected repeatedly through the load wherein the capacitive energy is compensated by induced inductive properties of the load so that resonance occurs.

35. A method of irradiating foodstuffs of the type having a thickness substantially smaller than a microwave absorption depth of the material of the load, the method comprising the steps of: a) placing a relatively thin load in a microwave applicator; b) energizing the applicator with an evanescent hybrid mode such that the capacitive energy of the mode is compensated by the induced inductive properties of the load so that resonance occurs; and constructively reinforcing the thermal energy in the load by retroreflections of the microwave energy in the applicator.