Electromagnetic power source

Abstract

An electromagnetic power source (100, Fig. 1), e.g. a cylindrical or co-axial cavity gyrotron, surface wave source, klystron or travelling wave tube, configured to generate microwave or terahertz power modes for heating plasma in a nuclear fusion reactor (300, Fig. 6) is described. The power source comprises an excitation cavity 108 configured to generate an electromagnetic power mode 112, wherein the excitation cavity comprises a first, inner layer 122 thermally coupled to a second, outer layer 124, wherein the first layer comprises electrically conducting metal, e.g. copper, silver or gold, and the second layer comprises diamond. The diamond of the second layer may be a metal composite diamond or may be formed by chemical vapour deposition (CVD). The second layer may, for example, comprise: a unitary structure; a plurality of coaxially stacked, hollow cylindrical or toroidal segments (130, Fig 8a); or a plurality of rectangular bars joined using a brazing or bonding material (123, Fig. 4).

Description

Field of the Invention

[01] The present disclosure relates to electromagnetic power sources. In particular, the disclosure is concerned with electromagnetic power sources which comprise cavity structures. Yet more specifically, the disclosure is concerned with microwave or terahertz power sources which incorporate cavity structures such as cylindrical cavity gyrotrons, co-axial cavity gyrotrons, surface wave sources, klystrons, travelling wave tubes, and the like. Background

[02] Current high power microwave or terahertz sources - herein power sources operating on the order of magnitude of ~1 MW - are limited in the cavity region due to high thermal loading (~20 MW per square metre for current cavity designs). To increase output power it is typically required to increase the cavity diameter in order to reduce thermal load, however doing so allows for the cavity to operate at higher order cavity modes. The increased density of power modes makes creates detrimental mode competition and presents a challenge for exciting the correct mode. Excitation of the wrong mode results in the wrong frequency of microwaves, and the majority of the power trapped in the source resulting in damage to the source.

[03] Microwave and terahertz power sources have recently become a front runner for plasma heating in fusion applications due to their efficiency compared to other heating routes such as neutral beams. Here exciting the correct frequency in the power sources is of particular importance in order to locally frequency match the plasma for efficient heating.

[04] It can also be desirable in fusion applications to access higher frequencies f, however this also presents a problem as ohmic losses in the cavity wall increase on the order of / 5 / 2, thereby reducing the available power. Byway of example, existing cavity designs which allow 1 MW output power at 170 GHz would, if operated instead to output power at 220 GHz, only do so at 0.5 MW.

[05] Therefore, it is now desired to provide an alternative cavity design which allows for high power high frequency operation, as an alternative to previously available designs. Summary

[06] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[07] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current cavity structure electromagnetic power sources, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein.

[08] Suitably, in one aspect of the invention there is provided an electromagnetic power source such as a cylindrical cavity gyrotron, co-axial cavity gyrotron, surface wave source, klystron, travelling wave tube, and the like, which is configured to generate microwave or terahertz power modes. Nominally power source frequency output may be between about 20 GHz and 5 THz, however output between 100 GHz and 400 GHz is expected to be sufficient for many fusion applications. The output power may be from about 0.5 MW to about 4 MW. The power source comprises an excitation cavity configured to generate a desired electromagnetic power mode (TEm.n), wherein the excitation cavity comprises a first, inner, layer comprising conducting metal preferably selected from one of copper, silver, and gold. The first layer is thermally coupled to a second, outer, layer, comprising diamond. Diamond provides much greater thermal conductivity than metal which therefore allows for greater thermal load on the cavity compared to existing metallic cavities, in turn increasing the possible power output. Recent improvements in EM power source design have allowed for 5 to 10 % gains in thermal evacuation techniques. In comparison, the diamond based design discussed herein offers an over 500% thermal conductivity gain.

[09] In one example, the diamond may be chemical vapour deposition diamond, which has a thermal conductivity of ~2,200 W / m °K. In another example the diamond may be as a metallic diamond composite, examples of which may achieve thermal conductivities >700 W / m °K.

[10] Preferably the cavity is substantially cylindrical, and comprises a unitary structure of diamond. In another example, the diamond layer may be formed from a plurality of hollow cylindrical segments stacked coaxially. In another example, the diamond layer may be formed from a plurality of rectangular shaped rods aligned parallel with the cylindrical cavity axis. Forming the diamond layer from smaller segments has manufacturing advantages and allows for easy production of differently sized cavities.

[11] In an example, the cavity may be from about 0.5 cm to about 6 cm in length, be from about 4 mm to about 60 mm in diameter (although ultimately the diameter is dependent on frequency and power), the first layer may be about 0.5 micro metre to about 500 micro metres in thickness, and the second layer may be from about 1 mm to about 6 mm thick, preferably about 2mm.

[12] In a related aspect of the present invention there is provided a nuclear fusion reactor comprising a vacuum vessel and an aforementioned EM power source configured to heat plasma within the vacuum vessel. The power source may be one in a set of power sources each configured to output different frequency (between from about 100 GHz to about 400 GHz) and different powers (between from about 0.5 MW to about 4 MW, preferably about 2 MW). Suitably each power source may be arranged to heat a different region within the vacuum vessel, or overlapping regions. A nuclear fusion reactor comprising the technology discussed herein will be cheaper to produce and more efficient to operate.

[13] Although example embodiments herein focus on the application of the aforementioned cavity structure to nuclear fusion, the improvements herein may be readily applied to electromagnetic power sources used in other industries. In particular, the techniques herein open up the feasibility filling in the so called “THz gap”, which would therefore allow microwave sources from ~200GHz to >1THz for multiple applications including telecommunications, space, microwave imaging, military, and so on.

[14] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word "about", even if the term does not expressly appear. The term “about”, or substantially, when used herein means + / - 5% of the stated value. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein, and the terms “from” and “to” a pair of values are intended to indicate such values are included in the range. Singular encompasses plural and vice versa. Additionally, although the present invention has been described in terms of “comprising”, the processes, materials, and coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’. Brief Description of the Drawings

[15] For a better understanding of the present disclosure reference will now be made by way of example only to the accompanying drawings, in which:

[16] Fig. 1 shows an example gyrotron for power sources up to 1 MW in the range of 25 GHz to 170GHz; and

[17] Fig. 2 shows a cut through a prior art cavity for a gyrotron;

[18] Fig. 3 shows a cut through an improved example cavity for a gyrotron;

[19] Fig. 4 shows a cut through another improved example cavity for a gyrotron;

[20] Fig. 5 shows a graph of example output power vs frequency curves;

[21] Fig. 6 shows a schematic of an example fusion reactor and a gyrotron system;

[22] Fig. 7 shows a graph of output power and frequency for desirable gyrotron usage in a variety of technology fields;

[23] Fig. 8 shows an example of an improved cavity with incorporated diagnostics to assess the cavity performance;

[24] Fig. 9 shows an example use of an improved cavity for >0.4THz sources. Detailed Description

[25] At least some of the following example embodiments provide for improved electromagnetic power generation via an improvement in cavity design for cavity structure type power sources. The example cavity allows for a much greater thermal load, thereby increasing the power that may be achieved at a given frequency without the usually required increase in size. Other advantages and improvements will be appreciated from the discussion herein.

[26] Figure 1 shows an example of a cavity structure based electromagnetic power source. In particular, Fig. 1 shows an example gyrotron 100, which generates high-frequency electromagnetic radiation by stimulated cyclotron resonance of electrons moving through a strong magnetic field.

[27] Here, an electron gun (cathode) 102 generates a cylindrical beam of electrons 104 which are accelerated by a high voltage electric field through a vacuum tube 106. The electron beam 104 travels through an excitation (resonant) cavity 108 which is penetrated by a strong magnetic field supplied by a toroidal super conducting magnet 110. The cavity 108 is cooled by a suitable coolant fluid to draw heat away from the cavity when in use; in a basic example, the coolant fluid may be water.

[28] As electrons travel helically along a magnetic field line through the cavity 108 (substantially along the z direction shown in the Figure) they radiate electromagnetic ‘EM’ power 112 at the cyclotron resonance frequency, which falls in resonance in the cavity structure 104. The mode of EM power generation is determined by a combination of the cavity 108 diameter, the electron beam 104 diameter and the local magnetic field intensity. By way of example, the gyrotron may be configured to stimulate EM power in a TEm.n mode at a frequency of about 170 GHz and 1 MW output power. This arrangement represents converting about 30% of the electron beam 104 energy into microwave power.

[29] The spent electron beam continues to a collector 114, while the generated power mode 112 is directed to a mode converter 116 configured to reshape the EM power into a Gaussian beam profile 118. A set of reflectors are arranged to direct the Gaussian mode power 118 to exit the gyrotron sideways (i.e., orthogonal to the original direction of the electron beam 104) through a diamond window 120.

[30] Figure 2 shows an example of a prior art cavity which is approximately 4 cm in diameter and around 2 cm in length (z direction). The cavity wall is formed from a copper alloy (such as glidcop) which is approximately 3 mm thick. Outside of the cavity may be cooling channels used to evacuate the >20 MW / m2 thermal energy. The 3mm thick wall is required to avoid deformations from the coolant pressure, thus limiting the thermal conductivity by the combination of thick wall and Copper conductivity of ~440W / m°K.

[31] Figure 3 shows an improved example cavity 108 which represents a major improvement in evacuating thermal load compared to previous designs such as those of Fig. 2. Notably, the cavity 108 comprises a two layered design of an inner wall layer and an outer sheath layer. A first, inner, layer 122 comprises electrically conducting material so as to form a resonant chamber from the conductive surface for the EM power emitted by the electron beam 104. High electrical conductive materials such as copper, silver, gold (preferably), ora combination thereof, may be suitable materials for the first layer 122.

[32] A second, outer, layer 124 comprises a thermally dissipative material in order to shed heat from the cavity. The two layers are thermally coupled so as to allow the transfer of heat from the first to second layer. Thermal coupling maybe provided by, e.g., having the layers in direct contact with each other via bonding, brazing, or equivalent. An additional benefit of the second layer 124 is to provide structural rigidity from the pressure of coolant fluid that can implode the cavity if the thickness of the two layers are insufficiently rigid.

[33] Here, the second layer 124 comprises diamond. In particular, the second layer 124 may comprise chemical vapour deposition ‘CVD’diamond or a metallic diamond composite. Integrating diamond into the cavity structure improves thermal conductivity and allows for higher power loadings exceeding 100 MW / m2. In particular, CVD diamond may achieve thermal conductivities of approximately 2,200 W / m °K, while metallic diamond composites may achieve thermal conductivities >700 W / m °K. While diamond composites don’t offerthe same magnitude of benefit to thermal conductivity, they have advantages for simplifying cavity integration compared to CVD diamond.

[34] To form the cavity including diamond, the diamond second layer 124 may be grown into the desired cavity shape, and then a coating of metal conductor 122 may be applied to the inner surface of the diamond. Techniques for growing CVD or metallic composite diamond are known in the art. The metal (first) layer 122 may then be machined to the desired thickness, which also has the benefit of precision control of the inner layer 122 to improve EM excitation, although it should also be appreciated that in some examples the first layer 122 coating may be applied to the inside of the second layer 124 without any post machining.

[35] Preferably, a total diameter of the cavity may be from about 4 mm to 60 mm (though preferably less than 40 mm), with a length from about 4 mm to about 40 mm, and the first layer (inner) metal coating being between 0.5 pm and 200 pm, and a second (outer) layer 124 thickness from about 1 mm to 6 mm, preferably 2 mm. The cavity opening is of course determined by the EM frequency modes it is desired to excite.

[36] In the example of Figure 3, the diamond layer 124 is provided in a substantially cylindrical shape, having been grown as a unitary structure. Here the metal inner layer 122 may be about 1 pm and the outer diamond layer 124 about 2 mm.

[37] As an alternative to a unitary structure, the second layer 124 in this example may be provided as a series of hollow cylindrical, or toroidal, segments (essentially like washers) which are grown individually and then arranged proximate to each other to form a cylindrical cavity; more specifically, the cylindrical segments may be identically sized and stacked along the central major axis of the resulting cylindrical structure (see e.g., Fig 8A). In this case the length of the diameter of the resultant cavity is set by the shape of the segments but its length may be set by the number of segments used and therefore adjusted based on requirements, making this approach more attractive from a manufacturing perspective. A bonding or brazing material may be used to seal the segments together preventing gaps between individual segments. A thicker inner metallic coating is applied and post machined to achieve the desired cavity tolerances.

[38] Figure 4 shows an alternative arrangement for the cavity 108. Here the second layer 124 is formed from a plurality of rectangular bars which may be assembled together using a brazing or bonding material 123. The bars are elongate along the major Z (i.e., straight) axis of the resulting cylinder. That is, the bars are arranged adjacent to each other azimuthally to form the cylindrical shape, so that the diameter of the cavity is determined by the width and number of bars used while the length of the cavity is a set parameter. Here the inner layer 122 may be about 100 pm and the outer diamond layer 124 about 2mm. In this example, a thicker inner metallic layer 122 (compared to Fig. 3) is desired to fill in polygonal structure and then post machined to achieve the cylindrical surface. In contrast to Fig. 3, a rectangular bar arrangement may provide greater surface area on the outer part of the cavity 108, so that heat may be more readily transported away from the cavity 108 by a suitable coolant (e.g., a fluid such as water in contact with the diamond layer 124). The efficiency of the heat extraction largely depends on the increased wetted surface of the diamond, relative to the area forming the cavity inner surface. It will be appreciated that other rod shapes may also be used while achieving similar effect, for example wedge, trapezoidal, or circular. The diamond layer may also be built up from combinations of different cross-section bars in order to provide the coverage, strength and / or heat dissipation area properties, as required.

[39] In general, any replacement of metallic cavity with diamond material will enhance the cavities ability to deal with thermal load, and therefore it will be appreciated that a great many alternative arrangements, not shown here, could exist.

[40] Figure 5 shows a graph of gyrotron output power with frequency for different cavities at typical frequencies of interest for nuclear fusion applications. Line 126 shows the power with frequency characteristics for a cavity as described above (i.e., incorporating a diamond outer layer 124), while line 128 shows power with frequency characteristics fora prior art purely copper based cavity. Here we see the output characteristics of existing gyrotrons for fusion projects 140 and targeted frequency range 142 for STEP, the expected output powers would fall on or beneath line 128, as is to be expected. Range 146 shows the targeted range for other potential fusion projects utilising higher magnetic fields than existing devices.

[41] Compared to a (copper) prior art cavity which corresponds to line 128, line 126 assumes a (conservative) 5 times improvement in thermal conductivity, a linear decrease in diameter to wavelength, gold coating, a wall thickness of 2 mm and a 1.5 times enhancement in longitudinal cooling. The graph neatly shows that with a cavity designed according to the present disclosure, not only are higher power outputs achievable - specifically, a two fold gain from 1 MW to 2 MW, and potentially as high as a four fold gain to 4 MW - but that such a power gain is also achievable at much higher frequencies. Of particular interest is power output >200 GHz corresponding to the “THz Gap”, which the presently described cavity would allow for operation across the board at higher power.

[42] Figure 6 shows an example of a nuclear fusion reactor 300 configured to heat plasma using a gyrotron 302 comprising cavity 108 as described above. The gyrotron 300 may be one gyrotron or a set of gyrotrons 300a-d. In this example four gyrotrons are shown although it will be appreciated the set could have a greater or lesser number. The gyrotrons are connected to a transmission lines 302 which transmit the microwaves 304 to the tokamak 300. At the end of the transmission lines, the microwaves are launched 304 toward the plasma 310. The power is deposited locally 306a-d dependent on the gyrotron frequency and the magnetic field strength.

[43] The reactor comprises a vacuum vessel 300 in which plasma 310 is contained and heated. Preferably plasma containment may be by magnetic field, such as in a tokamak type reactor, although the present techniques may be equally applied to other types of reactors.

[44] Preferably, each gyrotron 302a-d in the set may be configured to heat a different region 306a-d within the plasma 310. Moreover, each gyrotron in the set may be configured to operate at a different frequency, for example a frequency between 100 GHz and 200 GHz, inclusive. In this way the set of gyrotrons may be adapted to frequency match the plasma at different radial locations and thereby provide more efficient localised heating. Also, each gyrotron in the set 302a-d may be configured to output a different power, although it is preferred they all output the same power (e.g., between 1 MW and 4 MW, preferably 2 MW).

[45] In some examples, one or more gyrotrons 302a-d in the set may be configured to heat the same radial region 306a-b within the vacuum vessel 300 but to do so at similar frequencies. In some examples the reactor may be considered to comprise multiple sets of gyrotrons, each set having gyrotrons operating at different frequencies, but with different frequency gyrotrons from each set having overlapping heating regions 306c-d. Suitably, different gyrotrons, or sets of gyrotrons, may be controlled to adaptively heat the plasma in the vacuum vessel 300 based on currently measured plasma conditions.

[46] In the context of the STEP project, it is envisaged that around 95 gyrotrons with improved cavities as discussed herein could be utilised to provide suitable plasma heating, with each gyrotron operating at about 2 MW output power (and at variable frequency as per Fig. 5). By contrast, utilising current technology at an average output of 0.8 MW (across all frequencies) will require 231 sources. Thus the present invention allows for significant costs savings for producing next generation fusion power plants, as well as offering more efficient operation.

[47] Figure 7 shows a similar graph to that of Fig. 5 showing frequency ranges that are desirable for an electromagnetic power source in a variety of fields and which are achievable by a power source designed according to the techniques herein. Notably, the presently described power sources may be usable for military, telecoms, and medical applications which current power sources are not (i.e., compare the diamond 154 and diamond composites 152 lines, which correspond to the present invention, with the copper line 150 of existing cavities). Notably, an EM power source with a cavity structure as discussed herein fills in the THz gap from 0.3 THz to as much as high 5 THz.

[48] Regions 160 to 168 show frequency ranges which may be desirable for certain industries, such as medical applications 160, telecommunications 162, military applications 164, homeland security 166, and fusion applications 168.

[49] Figures 8 and 9 show potential applications of an example improved EM power source beyond plasma heating.

[50] Figure 8 shows an example of a “washer stack” arrangement configured to profile the electric field within the resonant cavity 108. “Washer stack” arrangement refers to the cavity being formed from plurality of hollow cylindrical segments as described above. Suitably, each washer / segment 130 may be provided with a thermocouple 132 on its edge. Adjacent thermocouples are suitably insulated from each other by a bonding material 134 provided in between adjacent stacked segments 130. The temperature experienced by the thermocouple is a direct result of the microwave power being generated within the cavity, and so the corresponding electric field can be determined, as shown. This provides a real-time measurement of the resonant electric field for assessing the cavity performance which would not be feasible in a traditional copper cavity.

[51] Figure 9 shows an example of a cavity design for a THz source which has a much smaller cavity diameter of just a few millimeters. The design may use a single crystal grown of CVD diamond 500 with the cavity 510 drilled out along with the neighbouring taper sections equivalent to the standard gyrotron cavity 108. The single crystal CVD diamond growth technology is compatible with the smaller size assembly that may be up to 2cm in length and ~8mm in outer diameter. The inner surface would be coated with gold, silver or copper 520 to form the resonant cavity. The CVD diamond body provides the improved thermal conductivity to access higher power microwave sources >1 OkW consistent with Figure 7.

[52] In summary, exemplary embodiments of an EM power source, such as a gyrotron, and an improved cavity structure therefore have been described. The described exemplary embodiments provide for improved thermal conductivity of the excitation cavity allowing for higher output power and frequency regimes to be achieved. Additionally, the described exemplary embodiments are convenient to manufacture and straightforward to use. In particular, the exemplary cavity may be readily used as a replacement cavity in many existing EM power sources.

[53] The described cavity may be manufactured industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[54] Although preferred embodiments) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims.

[55] For example, although the above discussion has focussed on a gyrotron and cavity thereof, it will be appreciated that the cavity discussed herein may be readily applied to other types of electromagnetic power source that incorporate an power mode excitation cavity. For example, the cavity structure herein may be applied to co-axial cavity gyrotrons, surface wave sources, klystrons, travelling wave tubes, and the like.

[56] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[57] All of the features disclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[58] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[59] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. An electromagnetic power source configured to generate microwave power modes, comprising:an excitation cavity configured to generate an electromagnetic power mode,wherein the excitation cavity comprises a first, inner, layer thermally coupled to a second, outer, layer, wherein the first layer comprises electrically conducting metal and the second layer comprises diamond.

2. The power source of claim 1, wherein the diamond of the second layer is chemical vapour deposition diamond.

3. The power source of claim 1, wherein the diamond ofthe second layer is metallic composite diamond.

4. The power source of any preceding claim, wherein the cavity is substantially cylindrical.

5. The power source of any of claims 1 to 4, wherein the second layer comprises a unitarystructure.

6. The power source of any of claims 1 to 4, wherein the second layer comprises a plurality of hollow cylindrical segments stacked coaxially.

7. The power source of any of claims 1 to 4, wherein the second layer comprises a plurality of rods with elongate axis arranged parallel to a major central axis ofthe cavity.

8. The power source of claim 7, wherein the rods are rectangular shaped.

9. The power source of any preceding claim, wherein the first layer is from about 0.5 micro metre to about 500 micro metres in thickness.

10. The power source of any preceding claim, wherein the cavity is from about 0.5 cm to about 6 cm in length.

11. The power source of any preceding claim, wherein the cavity is from about 2 mm to about60 mm in diameter.

12. The power source of claim 11, wherein the cavity is less than about 40 millimetres in diameter.

13. The power source of any preceding claim, wherein the second layer is between 1 mm and 6 mm thick.

14. The power source of claim 13, wherein the second layer is about 2 mm thick.

15. The power source of any preceding claim, wherein the electromagnetic power source is configured to output a power from about 0.5 MW to about 4 MW.

16. The power source of any preceding claim, wherein the first layer comprises at least one of copper, silver, and gold.

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