Cathode-in-cathode type high power microwave (HPM) vacuum tube source and alignment method

Abstract

A method for precise coaxial alignment of a high-power microwave (HPM) vacuum tube source and its field emission (FE) cathode, cylindrical RF generator tube, and magnetic field includes positioning a low-power thermionic emission (TE) cathode within the FE cathode in a "cathode-in-cathode" configuration. When the HPM source is under vacuum and the FE cathode is deactivated, the TE cathode emits a substitute electron beam through the generator tube. A measurement circuit measures the position of the substitute electron beam relative to the front-to-rear longitudinal axis of the generator tube. The measurement circuit can be, for example, a repositionable fluorescent target or electric field sensor embedded in the cylindrical RF generator tube. The coaxial alignment of the primary cathode, cylindrical RF generator tube, and magnet is adjusted until the position of the substitute electron beam meets the coaxial alignment tolerance.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Patent Application No. 18 / 121,937, filed March 15, 2023, the entire contents of which are incorporated herein by reference.

[0002] This disclosure relates to high power microwave (HPM) vacuum tube sources, and more particularly to a cathode-in-cathode structure and method for precise coaxial alignment of a field emission (FE) cathode, a cylindrical RF generating tube, and a magnetic field. [Background technology]

[0003] HPM vacuum tube sources can be configured as oscillators or amplifiers that output high-power RF pulses. Such sources include a field emission (FE) cathode configured to emit a pulsed electron beam within a vacuum chamber, and a cylindrical RF generator tube, such as a slow wave structure (SWS) or resonant cavity, that interacts with the pulsed electron beam to generate RF pulses. A magnet, such as an electromagnet (EM), is configured to generate a magnetic field within the vacuum chamber with field lines parallel to the tube's longitudinal axis, confining the electron beam and accelerating the electrons to a high enough velocity to interact with the generator tube. Coaxial alignment of the FE cathode, RF generator tube, and magnetic field within close tolerances is critical to ensure proper operation and to prevent damage to the RF generator tube.

[0004] Referring now to FIG. 1, a known configuration of an HPM vacuum tube source 100 includes a vacuum chamber 102. One end of the vacuum chamber terminates in a flange 104 connected to a high-voltage power supply 106. The flange 104 terminates in an O-ring 108, which allows for positioning along the X and Y axes. A window 110 through which high-power RF pulses 112 are emitted is located at the other end of the vacuum chamber 102. An EM 114 is positioned around a portion of the vacuum chamber 102 and generates a magnetic field 116 with field lines parallel to a longitudinal axis 118 (or Z-axis). A cathode stalk 120 serves to position an FE cathode 122 within the magnetic field 116 and to provide high-voltage pulses from the high-voltage power supply 106 to drive the FE cathode 122 to emit a pulsed circular electron beam 124. An SWS 126, such as that described in U.S. Patent No. 9,819,320, issued November 14, 2017, entitled "Coaxial Amplifier Device," is secured to the vacuum chamber 102 and positioned so that the annular electron beam 124 is concentric with and closely spaced from the inner surface of the cylindrical SWS 126. As shown, the HPM vacuum tube source is configured as an oscillator that generates RF pulses 112. Alternatively, the source can be configured as an amplifier, in which case the RF input can be injected through the vacuum chamber and enter the SWS near the location where the primary electron beam enters the SWS.

[0005] Adjustment mechanisms 130 are located toward the opposite end of the vacuum chamber 102 and allow translational movement of the vacuum chamber and SWS 126 along the X and Y axes. The FE cathode 122 is fixed in the X, Y, and Z directions. Adjustment mechanisms 132 are located toward the opposite end of the EM 114 and allow angular adjustment of the magnetic field 116 in the XZ or YZ planes. These mechanisms can be used to approximately align the FE cathode 122, SWS 126, and magnetic field 116 coaxially. A technician can "eyeball" the setup to roughly align the components. However, at typical power levels, it is important that the FE cathode 122, SWS 126, and magnetic field 116 be coaxially aligned to close tolerances to ensure proper source operation and prevent damage to the SWS. 2, if this tolerance is not maintained, the electron beam 124 may strike the inner surface 128 of the SWS 126, generating a plasma 202 that may puncture or burn the SWS at the beam impact point 200. This may not only damage the SWS, but may also contaminate the SWS and the vacuum chamber.

[0006] 3A-3B, a known method for aligning an HPM vacuum tube source 100 to meet tight coaxial alignment tolerances involves placing sacrificial targets 300 and 302 alternately in positions in front of (toward the FE cathode 122) and behind the SWS 126, firing the FE cathode 122 to burn circles 304 and 306 onto the targets, extracting positional information from the targets, and controlling adjustment mechanisms 130 and 132 to make the translational and angular adjustments necessary to meet the coaxial alignment specifications. This is a time-consuming and expensive process that itself carries the risk of damaging the SWS and contaminating the vacuum chamber.

[0007] For each data point, a clean sacrificial target (e.g., steel or plastic) must be placed in the SWS, a vacuum applied, the FE cathode fired to mark the target, and the target removed to extract positional information. Each target is used once and then discarded. A pair of front and rear data points is measured and used to calculate translational and angular adjustments. These adjustments are made, and the entire process is repeated multiple times to meet alignment specifications. The number of sacrificial targets is costly, as is the time required to perform the alignment. Breaking and pumping the vacuum after each measurement is time-consuming and poses a contamination risk. Scratching the sacrificial target causes contamination. Poor rough "eyeball" alignment can cause the FE cathode's high-power electron beam to strike and damage the interior walls of the SWS, creating further damage, debris, and contamination. Summary of the Invention

[0008] The following is a summary to provide a basic understanding of some aspects of the disclosure. This summary is not intended to identify key or critical elements of the disclosure or to delineate the scope of the disclosure. Its sole purpose is to present some concepts of the disclosure in a simplified form as a prelude to the more detailed description and claims that are presented later.

[0009] The present invention provides a method for precise coaxial alignment of an HPM vacuum tube source, a high-power FE cathode, a cylindrical RF generator tube (e.g., a slow wave structure (SWS) or resonant cavity), and a magnet. A low-power thermionic emission (TE) cathode is coaxially positioned inside the high-power FE cathode, resulting in a "cathode-in-cathode" configuration. When the HPM vacuum tube source is under vacuum and the FE cathode is deactivated, the TE cathode emits a continuous, low-power, substitute electron beam. A measurement circuit measures the position of the substitute electron beam relative to the front and rear longitudinal axes of the cylindrical RF generator tube. The measurement circuit is preferably reusable and capable of real-time continuous measurement, thereby enabling alignment adjustments. The measurement circuit may include, for example, a repositionable fluorescent target or electric field (E) sensor embedded in the cylindrical RF generator tube. The coaxial alignment of the FE cathode, cylindrical RF generator tube, and magnet is adjusted manually or by computer control until the position of the low-power surrogate electron beam meets the coaxial alignment tolerance. This approach not only reduces the time, number of vacuum cycles, and cost required to align the HPM vacuum tube source, but also reduces the risk of damage or contamination to the HPM source, especially the cylindrical RF generator tube.

[0010] In one embodiment, the FE cathode emits a circular high-power primary electron beam in discrete pulses that interact with the cylindrical RF to generate or amplify RF pulses. The FE cathode emits discrete pulses that are suitably less than 100 microseconds in duration and spaced at least five times the pulse duration apart. When activated, the TE cathode emits a continuous low-power replacement electron beam, facilitating real-time continuous measurements for alignment. The diameter of the low-power replacement electron beam is suitably no more than 1 / 10 of the diameter of the high-power primary electron beam, and the average power of the replacement beam is less than 1 / 1000 of the peak power of the primary beam.

[0011] In one embodiment, multiple measurements and adjustments are made at each instance in each of the forward and rearward measurement positions to improve coaxial alignment tolerances. This "in-the-loop" adjustment reduces the number of times the vacuum must be cycled, thus reducing the total time required for alignment and the opportunity for contamination. In certain embodiments, the forward and rearward measurement positions alternate. In the forward measurement position, at least a translational adjustment is made between the FE cathode and the cylindrical RF generator tube perpendicular to the longitudinal axis. In the rear measurement position, at least an angular adjustment of the magnet relative to the longitudinal axis is made. In other embodiments, forward and rearward measurements are made simultaneously and are used to make both translational and angular adjustments.

[0012] In one embodiment, measurements are made by repositioning the fluorescent target forward and backward. At each instance, a vacuum is drawn, the TE cathode is activated, and an alternate electron beam is emitted, striking the fluorescent target and causing fluorescence within the spot. The spot is visualized, and translational or angular adjustments are made to center the spot on the fluorescent target. This process is repeated until the spot position meets the coaxial alignment tolerance.

[0013] In another embodiment, measurements are made by electric field sensors embedded in the front and rear positions of the cylindrical RF generator tube. This configuration allows for alternate or simultaneous measurements at the front and rear positions. Furthermore, this configuration is well suited for periodic realignment of the source. Additionally, the electric field sensors can be used to measure and characterize the primary electron beam. [Brief explanation of the drawings]

[0014] These and other features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings.

[0015] [Figure 1] As mentioned above, this is a diagram of an HPM vacuum tube source. [Figure 2]As mentioned above, this indicates the risk of SWS damage when using a high-power circular electron beam from an FE cathode. [Figure 3A] As mentioned above, we have shown how the annular high power electron beam of the FE cathode can be used to achieve coaxial alignment of the primary cathode, SWS and magnetic field of the HPM source. [Figure 3B] As mentioned above, we have shown how the annular high power electron beam of the FE cathode can be used to achieve coaxial alignment of the primary cathode, SWS and magnetic field of the HPM source. [Figure 4] FIG. 1 is a diagram of an embodiment of a cathode-in-cathode HPM vacuum tube source in which a low-power TE cathode is placed within the FE cathode for alignment purposes. [Figure 5] FIG. 1 is a diagram of an embodiment of a cathode-in-cathode configuration. [Figure 6] FIG. 10 is a diagram of an embodiment for measuring the position of a low-power alternative electron beam at positions before and after the SWS to align the HPM source. [Figure 7A] FIG. 10 illustrates aligning the HPM source using fluorescent targets alternating before and after the SWS and visual readout of alternate electron beam positions. [Figure 7B] FIG. 10 illustrates aligning the HPM source using fluorescent targets alternating before and after the SWS and visual readout of alternate electron beam positions. [Figure 8] 10 is an embodiment of an alignment method using a low-power TE cathode and a repositionable fluorescent target. [Figure 9] FIG. 10 illustrates an alternative method for measuring electron beam position using electric field sensors embedded before and after the SWS. [Figure 10] 1 is an embodiment of an alignment method using a low-power TE cathode and an electric field sensor. DETAILED DESCRIPTION OF THE INVENTION

[0016] This disclosure describes an HPM vacuum tube source and a method for precise coaxial alignment of the FE cathode, cylindrical RF generator tube, and magnet. The method involves placing a low-power TE cathode within the FE cathode in a "cathode-in-cathode" configuration. When the HPM source is under vacuum and the FE cathode is deactivated, the TE cathode emits a substitute electron beam. A measurement circuit measures the position of the beam relative to the front and rear longitudinal axes of the cylindrical RF generator tube. The measurement circuit can be, for example, a repositionable fluorescent target or an electric field sensor embedded in the cylindrical RF generator tube. The coaxial alignment of the FE cathode, cylindrical RF generator tube, and magnet is adjusted until the position of the substitute electron beam meets the coaxial alignment tolerance.

[0017] The "cathode-in-cathode" configuration includes a TE cathode placed deep within the apex of the FE cathode. The FE and TE cathodes are never in operation simultaneously. During alignment, the FE cathode is inactive, so it does not affect the operation of the TE cathode and there is no risk of damage or contamination from the high-power primary electron beam. During normal operation, the electric field during FE cathode operation is low, and the presence of the cold TE cathode does not affect FE cathode operation.

[0018] In one embodiment, the FE cathode emits a circular high-power primary electron beam in discrete pulses, suitably less than 100 microseconds in duration and spaced at least five times the pulse duration. By comparison, when the TE cathode is activated, it emits a continuous, lower-power, alternative electron beam, typically a solid "pencil" beam. The diameter of the low-power alternative electron beam is suitably no more than 1 / 10 of the diameter of the circular high-power primary electron beam, and the average power of the alternative electron beam is less than 1 / 1000 of the peak power of the discrete pulses.

[0019] FE cathodes use an electric field to push electrons from atoms in the cathode material into the vacuum space just outside the cathode, where they are accelerated by a magnetic field. FE cathodes have a very sharp shape, causing the applied potential to be concentrated at a sharp emitting edge. This results in an electric field strength that can force electrons in the cathode material to concentrate at the sharp knife-edge to the extent that their quantum wave function begins to partially reside outside the material at the cathode edge. At that point, the electrons begin tunneling through a barrier from the cathode into the adjacent vacuum, where they are repelled by the cathode's strong negative charge. Once this process begins, very large currents can flow from the cathode, depending on the power source.

[0020] TE cathodes operate by heating the cathode. Depending on the cathode material (and its characteristic work function), temperatures typically range from 900 to 2000 °C (orange to yellow / white heat). At these temperatures, the thermal agitation of the atoms in the cathode material is sufficient to strip electrons from the atoms in the cathode material. At this point, applying a negative charge to this hot cathode ejects the stripped electrons from the cathode into the vacuum space within the tube. TE cathodes typically cannot deliver as much current as FE cathodes. However, compared to larger FE cathodes, they require a much lower voltage to operate and emit electrons.

[0021] The cathode-in-cathode configuration can be used with any HPM vacuum tube source and facilitates alignment of the FE cathode, cylindrical RF generator tube, and magnetic field where the FE cathode and generator tube are located within a vacuum chamber. The source can be configured as an oscillator or amplifier. The cylindrical RF generator tube can be an SWS or resonant cavity. Generally, the alignment mechanism must be capable of translational movement in the X and Y axes sufficient to coaxially align the FE cathode, generator tube, and magnetic field. In one embodiment, the vacuum tube and integral generator tube can be translated along the X and Y axes, and the magnet can be angularly adjusted in the XZ and YZ planes. In other embodiments, components can be positioned to allow adjustment of the FE cathode.

[0022] Referring now to FIG. 4, one embodiment of an HPM vacuum tube source 400 includes a cathode-in-cathode configuration for alignment and normal operation. One end of a vacuum chamber 402 (an integral part of the vacuum tube) terminates in a flange 404 connected to a high-voltage power supply 406. The flange 404 terminates in an O-ring 408, which allows for positioning along the X and Y axes. A window 410 through which high-power RF pulses 412 are emitted is located at the other end of the vacuum chamber 402. An EM 414 is positioned around a portion of the vacuum chamber 402 and generates a magnetic field 416 with field lines parallel to a longitudinal axis 418 (or Z-axis).

[0023] The cathode stalk 420 serves to position the cathode-in-cathode structure 450 within the magnetic field 416. The cathode-in-cathode structure 450 includes an FE cathode 422 and a TE cathode 452 positioned deep within the apex 454 of the FE cathode 422. The TE cathode 452 includes a heating element 455, a cathode 456, and a wire 458 connecting the heating element to the cathode 456. When activated, a high-voltage pulse from the high-voltage power supply 406 is sent along the outside of the cathode stalk 420 to the FE cathode 422, causing it to emit a pulsed, circular electron beam 424. When activated, a low voltage is supplied via a wire through the interior of the cathode stalk 420 to the heating element 454, causing the cathode 456 to emit a continuous, low-power, alternating electron beam 460. The diameter of the low-power substitute electron beam is suitably no more than 1 / 10 the diameter of the annular high-power primary electron beam 424 and less than 1 / 1000 of its peak power.

[0024] An SWS 426, such as that described in U.S. Patent No. 9,819,320, issued November 14, 2017, entitled "Coaxial Amplifier Device," is secured to the vacuum chamber 402 and positioned so that the annular electron beam 424 is concentric with and closely spaced from the inner surface of the SWS 426. An alternative electron beam 460 generally travels along a longitudinal axis 418. As shown, the HPM vacuum tube source is configured as an oscillator that generates RF pulses 412. Alternatively, the source can be configured as an amplifier, in which case RF input can be injected through the vacuum chamber and enter the SWS near the location where the primary electron beam enters the SWS.

[0025] The adjustment mechanism 430, located toward the opposite end of the vacuum chamber 402, allows translation of the vacuum chamber and SWS 426 along the X and Y axes. The FE cathode 422 is fixed in the X, Y, and Z directions. The adjustment mechanism 432, located toward the opposite end of the EM 414, allows angular adjustment of the magnetic field 416 in the XZ or YZ plane. These mechanisms can be used to approximately align the FE cathode 422, SWS 426, and magnetic field 416 coaxially. A technician can "eyeball" the setup to roughly align the components. However, at typical power levels, it is important that the FE cathode 422, SWS 426, and magnetic field 416 be coaxially aligned to close tolerances to ensure proper source operation and prevent damage to the SWS. A cathode-in-cathode structure 450, specifically the TE cathode 452, is used to achieve precise coaxial alignment.

[0026] 5, the cathode stalk 420 extends the cathode-in-cathode structure 450 into the magnetic field and provides the necessary drive voltages for the FE cathode 422 and the TE cathode 452. A high-voltage power supply distributes high-voltage pulses along conductors 470, through a cathode insulating plate 472, and along the outside of the cathode stalk 420 to the FE cathode 422. An isolation transformer 474 is located outside the vacuum chamber within insulating oil 476 along with the conductors 470 and is coupled to an external power source via a plug 478, such as a 110V plug. Electrical wires 480 from the isolation transformer pass through an airtight seal 482 in the insulating plate 472 along the inside of the cathode stalk 420 and provide drive voltage to the heater 455 of the TE cathode.

[0027] The FE cathode 422 and the TE cathode 452 are never operating simultaneously. During source alignment, the FE cathode 422 is deactivated, and no high-voltage pulses are applied to the FE cathode. During normal source operation, the isolation transformer 474 is disconnected from the external power source, and the isolation transformer 474 and TE cathode 452 float at the voltage supplied to the FE cathode 422. The TE cathode 442 is located just below the apex of the FE cathode 422. There, the electric field during FE operation is low, and the presence of the TE cathode 452 does not affect FE cathode operation. The wire 480 connecting the transformer 474 to the TE cathode 452 runs inside the FE cathode stalk 420 and is virtually invisible to FE operation. When alignment is required, a detachable power cord is connected to the isolation transformer 474, which is then turned on and begins emitting the alternate electron beam 460 used for alignment. Conversely, during normal tube operation, TE cathode 450 receives no power and remains cool. When the power cord is removed from isolation transformer 474, the transformer and TE cathode simply float harmlessly, regardless of the voltage supplied to the FE cathode for tube operation.

[0028] Referring now to FIG. 6 , to precisely align the HPM vacuum tube source 400 with the cathode-in-cathode structure 450 to meet tight coaxial alignment tolerances, measurement circuits 500 and 502 are alternately or simultaneously positioned at the front and rear positions of the SWS 426. When the FE cathode 422 is deactivated, the TE cathode 452 is activated and driven to emit a substitute electron beam 460. The measurement circuits 500 and 502 measure the position of the substitute electron beam 460 relative to the longitudinal axis (Z-axis) 418 at the front and rear positions. Alignment mechanisms 430 and 432 are used to adjust the alignment of the FE cathode 422, SWS 426, and magnet 414 until the position of the substitute electron beam 460 at the front and rear positions meets the coaxial alignment tolerances. The measurement circuits typically perform multiple measurements to facilitate multiple adjustments. In various configurations, the position measurements may be visually read by a human or an automated camera system, or may be rendered electronically by controller 504. Similarly, the adjustment mechanism may be controlled by a human or an automated system coupled to controller 504.

[0029] Because the forward measurement is closest to the FE cathode 422, it is more likely to indicate a translation error between the FE cathode 422 and the input to the SWS 426. A tilt error between the FE cathode 422 and the SWS 426 is less likely due to simple machining tolerances. While the forward measurement may indicate a very large magnet alignment error, the relatively short travel distance from the FE cathode 422 to the forward measurement circuit makes it difficult to detect the magnet tilt error. Meanwhile, the rearward measurement is much farther away, making it easier to detect and correct any angular misalignment with the magnetic field. Therefore, a common approach is to use alignment mechanism 430 to translate the vacuum tube, and therefore the SWS 426, in the X and Y axes to improve coaxial alignment, and alignment mechanism 432 to adjust the angle of the magnet 414 in the XZ and YZ planes to improve coaxial alignment.

[0030] 7A, 7B, and 8, in one embodiment of the HPM vacuum tube source 400 and alignment method, fluorescent targets 700 are alternately positioned in front and behind the SWS 426. The fluorescent targets 700 are very similar to those found on older CRT televisions. When an alternating electron beam of the correct energy strikes the target 700, it fluoresces, forming a visible beam spot 702. A camera 704 is positioned to view the interior bore of the SWS 426 through the window 410. The camera 704 can visualize and display an image 706 of an alignment reticle 708 and the beam spot 702. By making translational and angular adjustments, the beam spot 702 can be centered at the front and rear positions of the reticle 708.

[0031] In a specific embodiment, the TE cathode is recessed at the apex of the FE cathode, forming a "cathode-in-cathode" configuration (step 800). To align the HPM vacuum source, the FE cathode, SWS, and magnet are first roughly aligned visually. More specifically, the FE cathode and cylindrical RF generator tube are aligned approximately coaxially so that the cathode face appears visually centered within the internal bore of the SWS when viewed from the far end of the SWS (step 802). The magnet is then aligned approximately so that the magnetic field is approximately coaxial with the FE cathode and cylindrical RF generator tube by visually adjusting the magnet until the exterior of the cylindrical RF generator tube appears concentric within the bore of the magnet (step 804).

[0032] The fluorescent target is positioned forward within the SWS toward the FE cathode (step 806). A vacuum is drawn in the vacuum chamber (step 808), and the TE gun is activated to emit an alternating electron beam, which strikes the fluorescent target and fluoresces within the beam spot (step 810). A camera visualizes the position of the beam spot through a transparent window at the opposite end of the SWS (step 812). An adjustment mechanism is controlled to translate the vacuum chamber, and therefore the SWS, in the X or Y axis to center the beam spot on the target (step 814). Steps 812 and 814 are repeated until the beam spot is within tolerance of the reticle center or until a set number of iterations have occurred (step 816).

[0033] The vacuum is broken (step 818), and the fluorescent target is removed and placed against a rear window in the SWS (step 820). Equivalently, a different fluorescent target can be used. A vacuum is drawn in the vacuum chamber (step 822), and the TE gun is activated to emit an alternative electron beam, which strikes the fluorescent target and fluoresces within the beam spot (step 824). A camera visualizes the position of the beam spot through a transparent window at the opposite end of the SWS (step 826). An adjustment mechanism is controlled to adjust the angle of the magnet in the XZ or YZ plane to center the beam spot on the target (step 828). Steps 826 and 828 are repeated until the beam spot is within tolerance of the center of the reticle or until a certain number of iterations have occurred (step 830).

[0034] On the first pass through the front and rear measurement positions, there is a significant chance that the beam spot will not converge to the final coaxial alignment tolerance. This can be handled by repeating the measure and adjust steps a certain number of times before moving, or by starting with a larger tolerance and decreasing the tolerance with each pass until the final coaxial alignment tolerance can be achieved. On the next pass, break vacuum (step 831) and repeat from step 806.

[0035] Once the final coaxial alignment tolerances are achieved in the forward and aft positions, power is removed from the TE cathode (step 832) and the fluorescent target is removed from the source (step 834). A vacuum is pulled (step 836) and normal operation of the HPM vacuum source is initiated by activating the FE cathode (step 838).

[0036] 9 and 10, in an embodiment of the HPM vacuum tube source 400 and alignment method, electric field sensors 900 are embedded in the front and rear positions of the SWS 426. The TE cathode 452 emits a substitute electron beam 460 through the internal bore of the SWS 426. When the beam is perfectly aligned with the longitudinal axis 418 of the SWS, the electric field sensor 900 in the front position should measure the same electric field strength, indicating that the substitute electron beam is perfectly coaxially aligned with the SWS. If the substitute electron beam is translated or tilted from the longitudinal axis or the magnetic field is misaligned, the electric field sensors 900 in the front and rear positions will reflect this by measuring different electric field strengths. The position of the substitute electron beam can be calculated from the various electric field strengths by a controller 902, and appropriate translational and angular adjustments can then be determined.

[0037] In a specific embodiment, the TE cathode is recessed at the apex of the FE cathode to form a "cathode-in-cathode" configuration (step 1000), and electric field sensors are embedded in the walls of the SWS at the front and rear positions (step 1002). To align the HPM vacuum source, the FE cathode, SWS, and magnet are first roughly aligned visually. More specifically, the FE cathode and cylindrical RF generator tube are aligned approximately coaxially so that the cathode face appears visually centered within the internal bore of the SWS when viewed from the far end of the SWS (step 1004). The magnet is then aligned approximately so that the magnetic field is approximately coaxial with the FE cathode and cylindrical RF generator tube by visually adjusting the magnet until the exterior of the cylindrical RF generator tube appears concentric within the magnet's bore (step 1006).

[0038] A vacuum is drawn in the vacuum chamber (step 1008), and the TE gun is activated, emitting a replacement electron beam through a window into the internal bore of the SWS (step 1010). An electric field sensor is used to measure the position of the replacement electron beam relative to the longitudinal axis at the forward and rearward positions (step 1012). An adjustment mechanism is controlled to translate the vacuum chamber, and therefore the SWS, in the X or Y axis and adjust the angle of the magnet to center the replacement electron beam at the forward and rearward positions. Steps 1012 and 1014 are repeated until the replacement electron beam is within the coaxial alignment tolerance (step 1016).

[0039] Once coaxial alignment is achieved, power may be removed from the TE cathode (step 1018) and the electric field sensor may be deactivated (step 1020). Normal operation of the HPM vacuum tube source begins with activation of the FE cathode (step 1022).

[0040] Embedding the electric field sensor in the SWS has several potential advantages. First, there is no need to break or pump vacuum to perform the alignment. This reduces the time required to perform the alignment and eliminates the risk of contamination due to vacuum breaking. Second, the electric field sensor can be reactivated and used to measure and characterize the primary electron beam (step 1024). Finally, the FE cathode can be deactivated and the TE cathode reactivated (step 1026) without the need to cycle vacuum and periodically realign the HPM vacuum tube source.

[0041] While several exemplary embodiments of the present disclosure have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Such variations and alternative embodiments are contemplated and can be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. 1. A method of aligning a high-power microwave (HPM) vacuum tube source, the HPM vacuum tube source including a magnet configured to generate a magnetic field, a field emission (FE) cathode configured to emit an annular high-power primary electron beam, and a cylindrical RF generator tube configured to interact with the annular high-power primary electron beam to generate or amplify an RF signal, the method comprising: disposing a thermionic emission (TE) cathode within the FE cathode, the TE cathode configured to emit a low-power alternative electron beam coaxially with the annular high-power primary electron beam; By evacuating the HPM vacuum tube source and deactivating the FE cathode, activating the TE cathode to emit the low power alternative electron beam; measuring a position of the low power alternative electron beam relative to a longitudinal axis of the cylindrical RF generator tube at positions before and after the cylindrical RF generator tube; adjusting the alignment of the FE cathode, cylindrical RF generator tube, and magnet until the position of the low power alternate electron beam at the front and rear positions meets a coaxial alignment tolerance.

2. Before activating the TE cathode, substantially coaxially aligning the FE cathode and a cylindrical RF generator tube such that a cathode face appears visually centered within an internal bore of the RF generator tube when viewed from a distal end of the cylindrical RF generator tube; 10. The method of claim 1, further comprising: roughly aligning the magnet so that the magnetic field is approximately coaxial with the FE cathode and the cylindrical RF generator tube by visually adjusting the magnet until the outside of the cylindrical RF generator tube appears concentric within the bore of the magnet.

3. 2. The method of claim 1, wherein the FE cathode emits the annular high-power primary electron beam as discrete pulses having a pulse duration of less than 100 microseconds and a duration between pulses of at least five times the pulse duration, and the TE cathode emits the low-power replacement electron beam with a diameter no greater than 1 / 10 of the diameter of the annular high-power primary electron beam and an average power less than 1 / 1,000 of the peak power of the discrete pulses.

4. the HPM vacuum tube source further comprising a cathode stalk configured to position the FE cathode and the TE cathode therein within the magnetic field; During source alignment, a drive voltage is coupled from an isolation transformer to the heater of the TE cathode via a wire along the interior of the cathode stalk; 2. The method of claim 1, wherein during normal source operation, a high voltage pulse is coupled to the FE cathode along the exterior of the cathode stalk, and the isolation transformer and the TE cathode float at the voltage supplied to the FE cathode.

5. The method of claim 1 , wherein multiple measurements and adjustments are made at each instance at each of the forward and aft measurement positions to improve the coaxial alignment tolerance.

6. The forward and rearward measurement positions are alternated to adjust the alignment. In the forward measurement position, at least a translational adjustment is performed between the FE cathode and the cylindrical RF generator tube perpendicular to the longitudinal axis; The method of claim 5 , further comprising: at least angularly adjusting the magnet relative to the longitudinal axis in the rearward measuring position.

7. the forward and rearward measurements are taken simultaneously and the alignment is adjusted; 6. The method of claim 5, comprising making at least a translational adjustment between the FE cathode and the cylindrical RF generator tube perpendicular to the longitudinal axis, and making at least an angular adjustment of the magnet relative to the longitudinal axis.

8. The position of the low power substitute electron beam is (a) positioning a fluorescent target on the cylindrical RF generator tube toward the FE cathode, drawing a vacuum with the HPM vacuum tube source, activating the TE cathode to emit the low-power alternate electron beam to strike the fluorescent target and cause it to fluoresce in a spot, visualizing the spot through the cylindrical RF generator tube to measure the position of the low-power alternate electron beam, and adjusting the alignment of the FE cathode, cylindrical RF generator tube, and magnet to position the spot at the center of the fluorescent target; (b) breaking vacuum and removing the fluorescent target; (c) placing the fluorescent target behind the cylindrical RF generator tube, drawing a vacuum with the HPM vacuum tube source, activating the TE cathode to emit the low-power alternate electron beam to strike the fluorescent target and cause it to fluoresce in a spot, visualizing the spot to measure the position of the low-power alternate electron beam, and adjusting the alignment of the FE cathode, cylindrical RF generator tube, and magnet to center the spot on the fluorescent target; 2. The method of claim 1, wherein the measured position of the alternate electron beam is measured forward and backward by repeating steps (a)-(c) until the measured position of the alternate electron beam satisfies coaxial alignment.

9. 9. The method of claim 8, wherein, when the fluorescent target is positioned toward the FE cathode, alignment includes at least adjusting a translational position of the FE cathode or a cylindrical RF generator tube perpendicular to the longitudinal axis, and when the fluorescent target is positioned behind the cylindrical RF generator tube, alignment includes at least adjusting an angular position of the magnet relative to the longitudinal axis.

10. 10. The method of claim 1, further comprising embedding electric field sensors at front and rear positions of the cylindrical RF generator tube to measure the position of the low power alternative electron beam.

11. 11. The method of claim 10, wherein alignment includes at least adjusting a translational position of the FE cathode or cylindrical RF generator tube perpendicular to the longitudinal axis and at least adjusting an angular position of the magnet relative to the longitudinal axis.

12. 11. The method of claim 10, further comprising, once aligned, periodically deactivating the FE cathode and realigning the HPM vacuum tube source using the TE cathode.

13. The method of claim 10 , further comprising activating the FE cathode once aligned and measuring the annular high-power primary electron beam using the embedded electric field sensor.

14. 1. A high power microwave (HPM) vacuum tube source comprising: a magnet configured to generate a magnetic field; a field emission (FE) cathode configured to emit a circular high-power primary electron beam in discrete pulses; a cylindrical RF generator tube along a longitudinal axis configured to interact with the annular high-power primary electron beam to generate or amplify a pulsed RF signal; a thermionic emission (TE) cathode coaxially disposed inside the FE cathode, the TE cathode configured to emit a low-power alternative electron beam as a continuous beam; an adjustment mechanism responsive to position measurements of the alternate electron beam before and after the cylindrical RF generator tube, the adjustment mechanism adjusting the alignment of an FE cathode, a cylindrical RF generator tube, and a magnet until the position measurements meet a coaxial alignment tolerance.

15. 15. The HPM vacuum tube source of claim 14, wherein the FE cathode emits the annular high-power primary electron beam as discrete pulses having a pulse duration of less than 100 microseconds and a duration between pulses of at least five times the pulse duration, and the TE cathode emits the substitute electron beam with a diameter no greater than 1 / 10 of the diameter of the annular high-power primary electron beam and an average power less than 1 / 1,000 of the peak power of the discrete pulses.

16. The HPM vacuum tube source is a cathode stalk configured to position the FE cathode and the TE cathode therein within the magnetic field; a conductor coupled to the cathode stalk and along the exterior of the cathode stalk for supplying a high voltage pulse to the FE cathode; an isolation transformer for coupling to an external power source; an electrical wire coupled from the isolation transformer along the inside of the cathode stalk to provide a driving voltage to the heater of the TE cathode; During source alignment, the high voltage pulse is not applied to the FE cathode; 15. The HPM vacuum tube source of claim 14, wherein during normal source operation, the isolation transformer is disconnected from an external power source, and the isolation transformer and TE cathode float at the voltage supplied to the FE cathode.

17. 15. The HPM vacuum tube source of claim 14, wherein in each of the forward and rearward measurement positions, the adjustment mechanism improves the coaxial alignment tolerance in response to a plurality of measurements.

18. 15. The HPM vacuum tube source of claim 14, further comprising fluorescent targets arranged alternately before and after the cylindrical RF generating tube, the fluorescent targets fluorescing within a spot in response to the alternate electron beam to measure the position of the alternate electron beam.

19. 15. The HPM vacuum tube source of claim 14, further comprising a plurality of electric field sensors embedded at front and rear positions of said cylindrical RF generator tube for measuring the position of said alternate electron beam.

20. 1. A method of aligning a high-power microwave (HPM) vacuum tube source, the HPM vacuum tube source including a magnet that generates a magnetic field, a field emission (FE) cathode configured to emit an annular high-power primary electron beam, and a cylindrical RF generator tube configured to interact with the annular high-power primary electron beam to generate or amplify an RF signal, the method comprising: substantially coaxially aligning the FE cathode and a cylindrical RF generator tube such that a cathode face appears visually centered within an internal bore of the cylindrical RF generator tube when viewed from a distal end of the cylindrical RF generator tube; roughly aligning the magnet so that the magnetic field is approximately coaxial with the FE cathode and the cylindrical RF generator tube by visually adjusting the magnet until the outside of the cylindrical RF generator tube appears concentric within the bore of the magnet; Precise coaxial alignment of an FE cathode, a cylindrical RF generator tube, and a magnetic field, wherein the FE cathode: (a) disposing within the FE cathode a thermionic emission (TE) cathode configured to emit a low-power replacement electron beam coaxially with the annular high-power primary electron beam, the replacement electron beam having a diameter no greater than 1 / 10 of the diameter of the annular high-power primary electron beam and an average power less than 1 / 1,000 of the peak power of the annular high-power primary electron beam; (b) disposing a fluorescent target between the FE cathode and the cylindrical RF generator tube, drawing a vacuum with the HPM vacuum tube source, activating the TE cathode to emit the alternate electron beam to strike the fluorescent target and cause fluorescence in a spot, visualizing the spot through the cylindrical RF generator tube to measure the position of the alternate electron beam, and adjusting at least a translational position of the primary cathode or the cylindrical RF generator tube to position the spot at the center of the fluorescent target; (c) breaking vacuum and removing the fluorescent target; (d) positioning the fluorescent target behind the cylindrical RF generator tube, drawing a vacuum with the HPM vacuum tube source, activating the TE cathode to emit the alternate electron beam to strike the fluorescent target and cause it to fluoresce in a spot, visualizing the spot to measure the position of the alternate electron beam, and adjusting the angular position of at least the magnet to center the spot on the fluorescent target; repeating steps (b) through (d) until the measured position of the replacement electron beam meets a coaxial alignment tolerance; and deactivating said precisely coaxial alignment by

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