Spatially Distributed Electron Beam Gun for High Pulse Repetition Rate Operation
AREA(S): Battlespace, Electronics, Sensors
PROGRAM: PEO IWS 2.0, Above Water Sensors Program Office, Advanced Offboard
Electronic Warfare (AOEW) program
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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.
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
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.
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.
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.
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.
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/
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/
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
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
Electron Gun; Sheet Electron Beam; Millimeter-wave; mmW; Power; Vacuum Electron
Devices; Periodic Permanent Magnet; Beam-Wave Interaction
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