TECHNOLOGY AREA(S): Battlespace, Electronics, Sensors

ACQUISITION PROGRAM: PEO IWS 2.0, Above Water Sensors Program Office, Advanced Offboard Electronic Warfare (AOEW) program

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 section 3.5 of 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 and demonstrate an electron gun with integral beam transport system capable of high pulse repetition rates that produce a low-voltage, high-current, ribbon-like electron beam.

DESCRIPTION: The efficient amplification of millimeter-wave (MMW) power over broad bandwidths is a difficult problem under the best of circumstances. In applications where size and weight are at a premium, it has proven largely impossible except through the employment of vacuum electron devices incorporating novel interaction structures. Even then, the most highly sophisticated interaction structure is rendered useless unless coupled to an electron beam of exacting dimensions, precise uniformity, and strict confinement. Exacerbating these already stringent requirements, many Navy applications require small size, low weight, and the most efficient use of power. These size, weight, and power (SWaP) considerations drive vacuum amplifier designs toward the lowest possible operating voltage, as high voltage power supplies are naturally bulky. The need to maintain high power output therefore requires a corresponding increase in operating current. Taken together, these requirements can only be met by an electron gun employing a spatially distributed emitting surface. Presently, such an electron gun, suitable for application in a broad bandwidth Ka-band amplifier, does not exist.

In particular, the Navy needs a novel electron gun capable of generating a high-power electron beam with a ribbon-like (sheet) cross-section at high pulse repetition rates. Ultimately, the electron gun will be integrated with a broadband Ka-band beam-wave interaction circuit and collector to form a complete vacuum electronic amplifier. Details of the intended interaction circuit need not be specified as multiple device concepts require such an electron gun.

The electron gun should operate at a voltage of 25 kV or less with a peak beam current of at least 1.0 A. The electron gun should be capable of pulse repetition rates of 10 kHz or greater with a duty factor of no less than 3%. The sheet electron beam, at the entrance to the beam transport structure, should have a beam-width to transverse-height ratio of at least 5:1 and the allowable transverse height of the beam is 0.5 mm maximum. The electron gun design should balance trade-offs in areas such as beam convergence, cathode loading (current density), maximum electric field gradients in the gun region, and required modulating voltage, to achieve acceptable electron beam transport. Acceptable beam transport is considered to be 100% beam transmission over a longitudinal distance of at least 10 cm while minimizing overall volume and weight and maximizing the operational lifetime of the electron gun. The peak beam current of 1.0 A is the minimum requirement – higher currents are desired.

To minimize the overall volume and weight (including the size and weight of the system power supplies necessary to operate the device), periodic permanent magnet-based focusing (or some variant thereof) is desired. Magnetic materials should be capable of stable operation in ambient temperatures up to 200 degrees C. The magnetic focusing system should maintain the broad and transverse dimensions of the electron beam over the entire beam transport distance, consistent with the need for efficient beam-wave interaction. The minimum pole gap (magnet bore) is 5 mm x 10 mm, consistent with the expected size of the Ka-band interaction circuit.

Successful designs must meet the mechanical and electrical requirements outlined above. A key metric is the power density of the device, which is defined as the peak beam power divided by the combined weight of the gun, beam transport system (including permanent magnets), and collector. A minimum power density of 100 W/lb is the goal of this effort. The minimum-to-maximum voltage swing required to turn the beam on and off is another key design consideration as it affects the size and weight of the power supply required to operate the device. Consequently, this voltage should be as low as possible. The key criterion for success is the demonstration of 100% non-intercepting beam transport under zero-drive conditions (no RF input) over the entire longitudinal beam transport distance.

Demonstration of a beam-stick prototype is required to verify performance. The physical interface of the electron gun should avail itself to integration with a Ka-band beam-wave interaction structure according to standard industry practice. Therefore, a technical data package sufficient to facilitate joining the electron gun to the amplifier body, subsequent processing, and testing should also be delivered.

PHASE I: Propose a concept for an electron gun and beam transport system as described above. Demonstrate the feasibility of the proposed approach by some combination of analysis, modelling, and simulation; and predict the utility of the concept in developing an electron gun, beam transport system, and collector optimized for integration with a Ka-band sheet beam interaction structure. Develop a Phase II plan. The Phase I Option, if exercised, will include an electron gun specification and a test plan to develop the full electron gun prototype and demonstrate it in Phase II.

PHASE II: Develop and demonstrate prototypes of the electron gun with its integral beam transport system and collector proposed in Phase I that meets the requirements in the Description. Also develop a beam-stick prototype that can be used to verify beam transmission and power density. Test, seal, package for vacuum integrity, and deliver to the Naval Research Laboratory.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology for Government use. Provide fabrication, process, and test support in demonstrating the electron gun in the sheet-beam amplifier application.

Support transition of the technology to the vacuum electronics industry for application in the telecommunications market as replacements for conventional (high-voltage) travelling wave tubes.

REFERENCES:

1. Pasour, J., Nguyen, K.T., Wright, E.L., Balkcum, A., Atkinson, J., Cusick, M., and Levush, B. “Demonstration of a 100-kW Solenoidally Focused Sheet Electron Beam for Millimeter-Wave Amplifiers.” IEEE Trans. Electron Devices 58(6), June 2011, pp. 1792-1797. https://ieeexplore.ieee.org/document/5741717/

2. Liang, H., Ruan, C., Xue, Q., and Feng, J. “An Extended Theoretical Method Used for Design of Sheet Beam Electron Gun.” IEEE Trans. Electron Devices 63(11), November 2016, pp. 4484-4492; https://ieeexplore.ieee.org/document/7572094/

3. Booske, J.H., McVey, B.D., and Antonsen Jr., T.M. “Stability and confinement of nonrelativistic sheet electron beams with periodic cusped magnetic focusing.” J. Appl. Phys. 73(9), 1993, pp. 4140-4155. https://aip.scitation.org/doi/10.1063/1.352847

4. Booske, J.H., Basten, M.A., and Kumbasar, A.H. “Periodic magnetic focusing of sheet electron beams.” Phys. Plasmas 1(5), 1994, pp. 1714-1720. https://aip.scitation.org/doi/abs/10.1063/1.870675

KEYWORDS: Electron Gun; Sheet Electron Beam; Millimeter-wave; mmW; Power; Vacuum Electron Devices; Periodic Permanent Magnet; Beam-Wave Interaction