N221-037 TITLE: Compact Electron Beam Focusing System for Millimeter Wave Sources

OUSD (R&E) MODERNIZATION PRIORITY: Directed Energy (DE)

TECHNOLOGY AREA(S): Sensors

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Develop a low-voltage, high-current, round-beam electron gun that significantly reduces the size and weight of high-power W-band traveling-wave tube amplifiers.

DESCRIPTION: Traveling Wave Tubes (TWTs) are the primary radiofrequency (RF) power amplifiers used in applications that require both high output power and wide bandwidth (large power-bandwidth product). However, systems that employ these critical components at high millimeter-wave (mmW) frequencies, up to and including W-band (75-110 GHz), are especially difficult to realize in low size, weight, and power (SWaP) form factors. They are also extremely costly. This is due to the extremely small size of the high-frequency RF components which impose electrical and thermal limits on device operation and result in demanding manufacturing tolerances. The state of the art for such devices at mid W-band frequencies (near 94 GHz) is approximately 100 W output power with an electron beam current of 100-250 mA and a beam voltage of 20-25 kV. In comparison, for practical applications in W-band, the Navy desires TWTs producing much greater power (5X nominally) and wide bandwidth. Such TWTs do not currently exist in acceptable form factors.

The fundamental enabling technology for such a TWT is the electron beam generation, focusing, and transport system. The formation of the electron beam, in terms of focusing, transport, and current density, ultimately determines the output power and instantaneous bandwidth of a TWT. While higher beam current allows more RF power to be generated in the amplifier circuit, a high magnetic field is required to confine and transport such a beam through the small beam tunnel transiting the circuit. In the present state of the art, the magnet required to generate the high quality and very intense magnetic field needed to confine such a beam is large and heavy. For example, at W-band a conventional magnet (either electromagnet or permanent magnet) used for high-current beams (> 500 mA) of appropriately tight focus typically weighs 50-100 pounds. Lower-current beams can be focused by more compact configurations, such as conventional periodic permanent magnets (PPM), but these TWTs produce significantly less RF power than required. Spatially-distributed beams, such as sheet beams, are effective at producing high device power density (electron beam power per device weight). However, at high frequencies, these devices are prone to beam-defocusing instabilities and parasitic RF oscillations due to limited fabrication precision that causes non-uniformities in the magnetic field and RF circuit. Therefore, compact electron beam generation, focusing, and transport systems suitable for high power W-band TWTs are a critical enabling technology for future millimeter wave systems.

The Navy needs a novel electron beam focusing system for generation and transport of high-power (10 kW peak) electron beams of round cross-section. Ultimately, the electron beam focusing system will be integrated with a broadband beam-wave interaction circuit and an electron beam collector to form a complete W-band TWT. Development of the complete TWT is beyond the scope of this effort and details of the intended interaction circuit need not be specified as multiple device concepts require this technology.

To achieve the required beam current while minimizing the overall volume and weight (including the size and weight of any power supplies necessary to operate the device), a solution utilizing PPM based focusing and precision fabrication methods is anticipated. The design (including the integral electron gun), the fabrication techniques for the magnet structure, the magnetic materials, and the methods for integration of the magnets with appropriate RF circuits should result in designs that produce higher transport magnetic fields for a given magnet volume than is possible with conventional PPM based focusing. Magnetic materials should be capable of stable operation at temperatures up to 200�C. The magnetic focusing system should maintain the transverse dimensions of the electron beam over the entire beam transport distance of 5 cm for 100% beam transport (no RF applied) and consistent with efficient beam-wave interaction at W-band. However, no RF circuit is required of this effort and the technology shall be demonstrated as a beam-stick (i.e., with a copper "blank" containing only the beam tunnel in place of the circuit). The beam-stick shall have a uniform 4 mm by 4 mm square cross-section extending along the entire 5 cm tunnel length, consistent with the expected size and shape of the envisioned W-band amplifier interaction circuit.

The electron gun shall operate at a voltage of 25 kV or less with a minimum peak beam current of 0.4 amperes and be capable of pulse repetition rates of 10-50 kHz with a minimum duty factor of no less than 3%. The round electron beam should be transported through a tunnel no larger than 0.5 mm in diameter, with a maximum average beam diameter of 0.3 mm. The pulse voltage required to turn the beam on and off is another key design consideration, as it affects the size and weight of the power supply and pulser circuit required to operate the device. Consequently, the electron gun should be designed to require the lowest possible voltage swing necessary for device operation. The electron gun should also be designed for maximum operational life.

Vacuum device performance is determined by mechanical dimensions and precision alignment of the electron beam, magnetic field, and RF circuit. Consequently, manufacturing, including yield, is the overwhelming cost driver for any vacuum device. For W-band devices, the small feature sizes and tight machining and assembly tolerances required for stable operation are extremely challenging. Therefore, advances in fabrication, machining, alignment, fixturing, and joining techniques are required to reduce the cost of manufacturing the electron beam focusing system. Consequently, designing critical components, such as the electron gun and magnet assembly, to take advantage of advanced fabrication techniques is also an important part of this effort.

Acceptable solutions must meet the mechanical and electrical requirements described above. The key goal is then to optimize the power density of the device where power density is defined as the peak beam power divided by the combined weight of the gun, beam transport system (including magnets), beam tunnel "blank", and collector. A minimum power density of 500 W/lb is the goal of this effort. The solution should have some degree of scalability to accommodate different RF circuit lengths and widths (with proportional increases in beam focusing system size and weight).

Demonstration of a beam-stick prototype at the company facility is required to verify performance and assess power density. In order to confirm beam transmission, the collector should be isolated, though collector depression is not required. The physical interface of the electron gun should avail itself to integration with a W-band beam-wave interaction structure (circuit) according to standard industry manufacturing practice without compromise of performance or reduction in power density. Therefore, a technical data package (TDP) sufficient to facilitate replacement of the beam-stick "blank" with a future circuit, including modelling, simulation, assembly, processing, quality conformance, and test instructions, shall be delivered with the prototype to the Naval Research Laboratory at completion of the effort.

PHASE I: Develop a concept for a compact electron beam focusing system that meets the objectives stated in the Description. Demonstrate the feasibility of the proposed approach by some combination of analysis, modelling, and simulation. The feasibility analysis should confirm the compatibility of the solution for integration with an appropriate W-band traveling-wave interaction structure (circuit) and beam collector through prediction of the expected performance from a complete device. The Phase I Option, if exercised, will include the initial design specifications, initial interface description, test specification, and capabilities description to build and test a prototype beam focusing system (with beam-stick and collector) in Phase II.

PHASE II: Develop, demonstrate, and deliver a prototype compact electron beam focusing system (with collector and beam-stick) that meets the requirements in the Description. Note that this effort is iterative by nature and more than one prototype (or partial prototype) may be developed. At the conclusion of the effort, test, seal, package for vacuum integrity, and deliver to the Naval Research Laboratory the best performing prototype. Test data shall also be delivered with the prototype as well as operating instructions and the TDP as noted in the Description.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology for Government use. Transition will include assisting with integration of beam-wave interaction circuits and scale the beam focusing system to produce specific device designs (for example, W-band TWTs). The technology will be validated by providing fabrication, process, and test support in manufacturing and demonstrating specific devices incorporating the electron beam focusing system.

The technology resulting from this effort is anticipated to have commercial application in the telecommunications industry; for example, as amplifiers in 5G backhaul transmitters.

REFERENCES:

1. Zhang, X., Feng, J., Cai, J., Wu, X., Du, Y., Chen, J., Li, S., and Meng, W. "Design and Experimental Study of 250-W W-band Pulsed TWT With 8-GHz Bandwidth." IEEE Transactions on Electron Devices 64 December 2017: 5151-5156. https://ieeexplore.ieee.org/document/8093750.
2. Theiss, A. J., Meadows, C. J., True, R. B. "Experimental Investigation of a Novel Circuit for Millimeter-Wave TWTs." IEEE Transactions on Electron Devices 54 May 2007: 1054-1060. https://ieeexplore.ieee.org/document/4160141.
3. Leupold, H. A. and Potenziani, E. A Permanent Magnet Circuit Design Primer. Army Research Laboratory Technical Report ARL-TR-946, July 1996, DTIC accession number ADA311457; https://apps.dtic.mil/sti/citations/ADA311457.
4. Borchard, P., Appert, S., and Hoh, J. "Fabrication of Split-Section X-band Structure Using Elastic Averaging." Journal of Physics, Conf. Series 1067, 2018: 082002. https://iopscience.iop.org/article/10.1088/1742-6596/1067/8/082002.

 

KEYWORDS: Traveling Wave Tube; periodic permanent magnets; PPM; PPM focusing; W-Band; Electron Beam Focusing; Power Amplifiers; Electron Gun