Multistage depressed collector

Abstract

A multistage depressed collector (MSDC) 200 is disclosed for recovering energy from an electron beam 204 in, for example, an electromagnetic power source generating stimulated Cherenkov radiation. The

Description

Field of the Invention

[01] The present disclosure relates to a multistage depressed collector, and apparatus incorporating the same. Background

[02] Devices which generate high-frequency electromagnetic radiation by stimulated cyclotron resonance of an electron beam have become a front runner for plasma heating in fusion applications due to their efficiency compared to other heating routes such as neutral beams. Example devices include cylindrical cavity gyrotrons, co-axial cavity gyrotrons, gyro-klystrons, gyro-travelling wave tubes, and the like.

[03] One of the important elements of an electron beam microwave device is the collector. This component helps to recover and recycle the electron beam energy that has not been used in the interaction region (where the microwave radiation is generated) to enhance the total efficiency of the device.

[04] However, current devices generating stimulated cyclotron resonance radiation have difficulty with efficiently recovering electron energy from electron beam geometries with large proportion of its energy accumulated in gyration motion of the electrons, such as the electron beam of gyrotron. The conventional energy recovery system can efficiently and reliably extract energy from electron beam translational motion. The research in recovery of energy from gyrational beam has already been underway for more than 10 years but it is still far away from practical applications outside the research laboratories. The overall efficiency of a modern gyrotron cannot be above 55%.

[05] The alternative to the cylindrical cavity gyrotron is the family of slow-wave Cherenkov oscillator type electromagnetic sources which generate stimulated Cherenkov radiation through the interaction with non-gyrating electron beam. Such devices use a linear electron beam, an external magnetic field, and a slow-wave interaction zone to produce radiation through the Cherenkov effect and includes devices such as travelling-wave tubes (TWTs), backward-wave oscillators (BWOs) or gyrotron. Similar devices operating at low power (below 100W) in the frequency range below 300GHz and using solid cylindrical (pencil) electron beam are used for satellite communications with reported efficiency around 80%. Therefore, it is now desired to provide a more efficient energy recovery collector to increase the overall efficiency (above 60%) for high power (up to 2MW) source capable generating radiation up to 1THz, as an alternative to previously available designs. More recent devices using periodic structures or meta-materials on the walls of the waveguide in the slow-wave interaction zone utilise a hollow-pencil beam (or annular) electron beam. Traditional multi-stage depressed collector designs do not operate efficiently in devices using such an annular electron beam. Summary

[06] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current electron beam multistage depressed collectors, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein.

[07] 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.

[08] Suitably, in one aspect of the invention there may be provided a multistage depressed collector configured to recover energy from an electron beam. The collector comprises a set of stages each configured to collect electrons at different energy ranges. Each stage of the collector comprises an annular inner surface configured to receive a first voltage, an annular outer surface configured to receive a second voltage (the first and second voltages can be the same but can also be different), and an annular gap separating the inner surface and outer surface. The inner and outer surfaces are suitably arranged at least partly orthogonal to a direction of electron travel through the respective stage in order to collect electrons of suitable energy from the electron beam (according to the potential(s) applied to the inner and outer surfaces). The collector provides improved efficiency of electron recovery for annular electron beams, such as a surface wave source coaxial type electron beam or annular beam formed by a continuous beam or a small radius solid beamlets.

[09] In an example where the first voltage and second voltage may be different, the voltages may be different by up to about 50% of an average (between two voltages) value which can improve efficiency of secondary beam suppression. Each stage is insulated to maintain the voltage difference between the stages.

[10] In an example, the set of stages may comprise a second stage further along the electron beamline than a first stage, and at least one of a thickness and radius of the annular gap of the second stage may be greater than the thickness / radius (as appropriate) of the annular gap of the first stage.

[11] In an example, the collector may comprise a cavity around which an inner wall of each of the stages is arranged. In particular, the cavity may be arranged co-axially with a central major axis of the collector. Such a cavity may reduce the mass of the collector, improving thermal management and magnetic field control, while making it easier to manufacture and deploy.

[12] In an example, the collector may comprise, disposed within the cavity, means to shape a magnetic field within the collector. For example an electromagnetic coil, permanent magnet, or superconducting magnet. In this way the electrons may be better steered onto the first and second surfaces of the set of stages for more efficient energy recovery.

[13] In a related aspect of the invention, there is provided an electromagnetic power source comprising means to generate electromagnetic radiation by stimulated Cherenkov resonance of electrons moving along a strong magnetic field lines, and the aforementioned collector. In an example, the power source may generate an electron beam by a surface wave source. Brief Description of the Drawings

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

[15] Fig. 1 shows an example electromagnetic power source; and

[16] Fig. 2 shows a slice through an example multistage depressed collector; in this figure, numerals appended with a, b, orc indicate a feature of a particular stage a, b, orc of the multistage depressed collector, such a feature otherwise being common to all stages unless otherwise stated in the text. Detailed Description

[17] At least some of the following example embodiments provide an improved electron collector; specifically, an improved form of multistage depressed collector. The example collector device is more efficient than existing designs, in particular where the electron source generates a hollow beam of electrons. Many other advantages and improvements will be discussed in more detail herein.

[18] By way of introduction, Figure 1 is a schematic drawing of an apparatus 100 configured to generate electromagnetic (EM) radiation 102. The generated EM radiation 102 may comprise a frequency component in a frequency range of 10 GHz to 10 THz. The apparatus 100 comprises an electron source 104 (such as an electron gun) configured to produce a hollow electron beam 106. The apparatus 100 comprises a magnetic field generator 108 configured to produce a magnetic field to condition and guide the electron beam 106 within an interaction region where the EM radiation 102 is generated. One or more magnetic field generators 108 (e.g., coils) may be included as part of the apparatus 100, in order to provide the conditioning and guiding functionality.

[19] The apparatus 100 also comprises a waveguide 110 which is substantially cylindrical in structure. The cylindrical structure is coaxially aligned with the electron beam 106 in the interaction region. Here, an inner surface of the cylindrical structure is configured to facilitate a Cherenkov interaction between the electron beam 106 and an electromagnetic field excited and supported inside the waveguide 110, in order to generate the EM radiation 102. A beam profile of the electron beam 106 through the apparatus 100 may be an annular continuous beam (i.e., having laminar flow).

[20] The apparatus 100 further comprises an output coupler 112 configured to output the electromagnetic radiation 102 from the apparatus 100. For example, the output coupler 112 may comprise a reflector configured to reflect at the operating wavelength of the apparatus 100. The output coupler 112, specific for a specific operating mode, may be positioned to reflect the EM radiation 102 while also allowing the electron beam 106 to pass through to a latter section of the apparatus 100. For example, the reflector may be an annulus which allows the electron beam 106 to pass through its centre and reflect an annular beam profile of the stimulated EM radiation 102.

[21] The apparatus 100 further comprises an electron beam collector 114 configured to collect the electron beam 106 and recuperate energy from the electron beam 106 after the interaction region. In an example, the electron beam collector 114 may comprise a depressed collector such as a multi-stage depressed collector (MDC). The amount of energy recuperated from the electron beam 106 plays a role in determining the energy efficiency of the apparatus 100, since any energy recuperated may be used to decrease energy input to the apparatus 100 in order to generate a given EM radiation 102 power.

[22] It will be appreciated that the above is a brief overview of the operation of a surface wave source type apparatus (or similar device), the general configuration and operation of which may be familiar to those in the art. In the following, an improved design for the collector 114 will be discussed, however the described collector is not limited to use only with surface wave sources; rather, it will be appreciated that the described collector may be readily applied to other forms of power sources which generate electromagnetic power from an electron beam. Further details of an exemplary surface wave source type apparatus are described in GB2620215, the entirety of which is incorporated herein by reference.

[23] Figure 2 shows a slice (z, 0 plane) through an example collector 200 designed to be used with the apparatus of Figure 1 (i.e., in place of collector 114) and other electromagnetic power sources. That is, the collector 200 may be incorporated into an electromagnetic power source comprising means to generate electromagnetic radiation by electrons moving along magnetic field lines, and similar forms for electromagnetic power sources.

[24] In particular, the collector 200 is a multistage depressed collector. Suitably, the collector 200 comprises a set of stages 202. The stages 202 are arranged sequentially with respect to a central (z) major axis 206 of the collector 200; the central axis 206 being aligned with the principal direction of entry 208 of a hollow electron beam 204 into the collector 200. In the example shown, the central axis 206 is coaxial with the central axis of the hollow (cylindrical) electron beam 204. The hollow electron beam 204 may be of a coaxial type (annular) as generated by a surface wave source, or an annular beam formed by small radius beamlets generated by many types of power sources known in the art.

[25] In the example shown, the collector 200 comprises three stages 202a-c in the set 202, followed by an end cap 203. The cap 203 may also be thought of as a final stage, but for the purposes of the following, is not considered one ofthe set 202. It should be appreciated, however, that the collector may comprise fewer or more stages than shown. As will be appreciated by those in the art, each stage in the set 202a-c is configured to collect electrons in a predetermined energy range. Put another way, the stages are configured to collect electrons according to different energy ranges, or bins.

[26] As shown, each stage in the set 202 comprises a pair of surfaces: an inner surface 210 and an outer surface 212. Here, inner and outer are defined with respect to the central (z) axis 206, such that the inner surface 210 is closerto the central axis 206 and the outer surface 212 further from the central axis 208. The inner surface 210 and outer surface 212 are arranged substantially radially around the central axis 206, each forming an annulus at least partly orthogonal to a direction of travel of electrons at that stage of the collector 200. Put another way, the inner surface 210 may extend between radial distances rl and r2 from the central axis 206, and the outer surface 212 may extend between radial distances r3 and r4 from the central axis, where r4 >r3 >r2 >rl.

[27] As can be seen, the inner surface 210 extends outwards (rl -> r2) from an inner wall 214 of a stage, while the outer surface 212 extends inwards (r4 -> r3) from an outer wall 216 of a stage. The inner and outer walls 214, 216 may be parallel and may be angled with respect to the central axis 206, and in general define a substantially cylindrical internal area of each stage. Preferably, each of the inner surface and outer surface are unitary structures, but in some examples the surfaces 210, 212 may be formed from a plurality of individual segments.

[28] In order to decelerate and capture primary electrons of different energies, each stage may be set to different voltages than other stages in the set 202, and each stage is electrically insulated from an adjacent stage. That is, the first stage 202a may be set to operate at a voltage VI (or thereabouts), the second stage 202b to operate at a voltage 72 (or thereabouts), and the third stage 202c to operate at a voltage 73 (or thereabouts). Preferably, the voltage increases in the direction of electron travel, so that later stages capture higher energy electrons - i.e., 73 >72 >71.

[29] Suitably, the voltage of each stage is applied to the inner and outer surfaces 210,212. That is, the inner surface 210 and outer surface 212 are configured to receive voltage from a power source (not shown); more generally, the inner surface 210 is configured to receive a first voltage and the outer surface 212 is configured to receive a second voltage. The first and second voltages may be the same, both corresponding the voltage 71...73 for the respective stage of the collector 200.

[30] Alternatively, the inner surface 210 and outer surface 212 may be set to different voltages. For example, an inner surface 210 of a stage 202 may be set to voltage V (e.g., one of VI to 73, as appropriate), while the corresponding outer surface 212a may be set to voltage V'; here V and V may differ by a some amount, such as for example 20% of an intermediate value between V and V (e.g., the mean value). Here, the inner surface 210a may be electrically coupled to the outer surface 212a by a resistor 218 in order to allow for different voltages on the two surfaces from a common voltage source. It will be appreciated that each of the stages 202a-c may be provided with a corresponding resistor, where that stage is configured to have different voltages on its inner and outer surface.

[31] As will be familiar to those in the art, the collector 200 suitably comprises one or more magnets 220, arranged at the periphery (i.e., outside the outer walls 216a-c), which are configured to provide a magnetic field which drives primary electrons onto the inner and outer surfaces 210, 212, of a particular stage 202 of the collector 200 based on the voltages of the respective surfaces 210, 212 of the set of stages 202. Additionally, having slightly different voltages applied to the inner and surfaces has been determined to improve efficiency of secondary beam suppression.

[32] It will also be appreciated that each of the inner surface 210 and outer surface 212 are configured to capture secondary electrons produced by the multistage depressed collector 200, and suppress the formation of a secondary beam. Capturing / suppression of secondary electrons is achieved in large part by the shape and size of the inner and outer surface 210, 212 - e.g., the annular thickness and angle with respect to the Z axis 206 - in correlation with the potentials being used at each stage, as will be familiar to those in the art of multistage depressed collectors.

[33] As shown, each stage comprises an annular gap 222 separating the proximal edges of the inner surface 210 and outer surface 212 - i.e., the gap exists between r2 and r3, where r2 r3. The annular gap 222 allows electrons of suitably high energy (i.e., too high to be captured by a given stage) to pass through to a subsequent stage in the set 202. That is, the gap 222 allows electrons of too high energy to continue on a trajectory caused by the magnetic field of the magnet(s) 220 and the deceleration force caused by the voltages of any earlier stages in the set 202.

[34] Suitably, the thickness of the annular gap 222 of each stage may increase from the first stage in the set 202 to a last stage in the set 202 (with respect to the direction of travel of electrons through the collector 200). That is, the thickness of the gap Tabc = r3a b c - r2abc may be larger for later stages so that, for example, Tc >Tb >Ta (where the subscript a,b,c indicates the parameter for n example stage 202a,b,c). In this way the collector 200 may be improved by accounting for a broader energy spread of spent electron beam including higher energy electrons having a wider flux angle at later stages due to a wider range of possible deceleration forces experienced. Put another way, where the set of stages 202 comprises a first stage 202a and a second stage 202b further from the electron entry 208 / further along the beam line 204 than the first stage 202a, the second stage 202b may comprise an annular gap 222b with greater width than the annular gap 222a of the first stage 202a.

[35] Similarly, a central radius of the annular gap 222 may also increase from the first to the last stage. That is, Rabc = (r3 + r2) / 2 may be larger at later stages so that, for example, Rc >Rb >Ra. In this way the collector 200 may account for higher energy electrons having a greater angle in respect with axis 206 caused by the magnetic field configuration at later stages. Put another way, where the set of stages 202 comprises a first stage 202a and a second stage 202b further from the electron entry 2081 further along the beam line 204 than the first stage 202a, the second stage 202b may comprise an annular gap 222b with greater radius than the annular gap 222a of the first stage 202a.

[36] In an embodiment, the collector 200 comprises a cavity 224 penetrating into the cap end 226 (i.e., the end opposite electron entry 208). The cavity 224 may be provided coaxially about the central axis 206. Put another way, each of the set of stages 202 (including the final stage 203) may be hollow, with cylindrical inner walls 214. Suitably, having such a cavity 224 within the collector 200 reduces the mass of the collector (compared to a version without a cavity) and allows for improvement of thermal management by allowing access to the inner walls 214 of the set of stages 202.

[37] Moreover, the collector 200 may utilise the space within the cavity 224 for additional components. In an embodiment, the collector 200 may comprise means 226 for shaping magnetic fields within the collector 200. That is, the fields which are used to shape the trajectory of the electron beam 204 for more efficient deceleration of the electrons and minimisation of the secondary electron generation. Such shaping of the magnetic field is not possible in prior art devices, which instead must rely only on the outer magnets 220.

[38] In one example, such means 226 may take the form of an electromagnetic coil. In another example, such means 226 may take the form of a permanent magnet or ferrimagnetic material, ora superconducting magnet or combination of several i.e., elements which can control magnetic field structure.

[39] In summary, exemplary embodiments of an improved electron collector for electromagnetic power sources have been described. The example embodiments are particularly suited for a coaxial type electron beam or annular beam formed by a small radius solid beamlets. Additionally, the described exemplary embodiments are convenient to manufacture and straightforward to use.

[40] The exemplary collector may be manufactured industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[41] Although preferred embodiment(s) 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.

[42] 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.

[43] 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.

[44] 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.

[45] 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. A multistage depressed collector for recovering energy from an electron beam, comprising:a set of stages configured to collect electrons at different energy ranges, each stage comprising:a substantially annular inner surface configured to receive a first voltage,a substantially annular outer surface configured to receive a second voltage,the inner and outer surfaces being are arranged at least partly orthogonal to a direction of electron travel through the stage, anda substantially annular gap separating the inner surface and outer surface.

2. The collector of claim 1, wherein the first voltage and the second voltage are the same.

3. The collector of claim 1, wherein the first voltage and second voltage are different.

4. The collector of claim 3, wherein each stage further comprising at least one resistorarranged to electrically couple its annular inner surface to its outer surface.

5. The collector of claim 3 or 4, wherein a difference in the first and second voltages is up to 50% of an intermediate value between the first and second voltage.

6. The collector of any preceding claim, wherein a second stage in the set of stages comprisesan annular gap having same or greater radius than an annular gap of a first stage in the set ofstages.

7. The collector of any preceding claim, wherein a second stage in the set of stages comprises an annular gap having same or greater width than an annular gap of a first stage in the set of stages.

8. The collector of claim 6 or 7, wherein the second stage is further from a point of entry of an electron beam to the collector than the first stage.

9. The collector of any preceding claim, further comprising a cavity around which an inner wall of each of the stages is arranged.

10. The collector of claim 9, wherein the cavity is arranged co-axially with a central major axis of the collector.

11. The collector of claim 9 or 10, further comprising, disposed within the cavity, means to shape a magnetic field within the collector.

12. The collector of claim 11, wherein the means comprise an electromagnetic coil.

13. The collector of claim 11, wherein the means comprise a permanent magnet or ferrimagnetic material.

14. The collector of any preceding claim, wherein the inner surface is formed from a plurality of segments.

15. The controller of any preceding claim, wherein the outer surface is formed from a plurality of segments.

16. An electromagnetic power source comprising means to generate electromagnetic radiation by stimulated Cherenkov radiation, and the collector of any preceding claim.

17. The power source of claim 16, wherein an electron beam generated by the power source is substantially annular in cross-section.

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