TECHNOLOGY AREA(S): Sensors
OBJECTIVE: To develop high current, high brightness and long lifetime electron amplifiers based on diamond cathodes.
DESCRIPTION: Stable and efficient electron emitters are critical for a wide range of applications such as high power vacuum electronic microwave/millimeter-wave/terahertz power amplifiers, coherent x-ray sources, electron diffraction and microscopy, electron-beam lithography, flat panel displays, and thermionic energy conversion (TEC) through thermal electron emission for renewable energy generation. Traditional thermionic electron sources operate at cathode temperatures over 1000o to produce appreciable electron emission. Such high temperature have serious consequences in terms of lifetime and reliability.
As an electron emitter, diamond offers several advantages over conventional electron emitters. These advantages include a wide bandgap, large breakdown field, high electron mobilities, and high thermal conductivity. Its ability to control electron affinity through surface termination and doping is also extremely important for electron emission. Negative electron affinity (NEA) has been demonstrated through hydrogen termination of the diamond surface. This has resulted in superb electron emissivity even at room temperature. Recent advances of diamond thin film growth based on techniques such as chemical vapor deposition and thermodynamic growth under high-pressure high-temperature have resulted in commercially available large-size, single-crystal, and high-purity synthetic diamond substrates. Furthermore, post growth processing techniques such as surface polishing and atomic layer etching have also significantly reduced surface roughness of these diamond films. All of these new developments now open the door for realizing practical diamond-based applications including efficient and low temperature field-emission electron sources.
In a diamond electron amplifier (DEA), electrons are generated as secondary emission from a hydrogen terminated surface of a diamond film after excitation by a primary electron beam. It has demonstrated the ability to amplify an electron beam current by several orders of magnitude while at the same time yielding high current and high electron beam quality with ultralow emittance and energy spread while maintaining relative low cathode temperatures. All of these are desirable characteristics for the aforementioned applications. However, key scientific and technical challenges still need to be addressed for DEAs to realize their full potential. Issues such as hydrogen desorption under high current and elevated temperature and DC shielding by surface charge build-up due to surface dangling bonds and impurities have been observed and resulted in reduced electron emission efficiency. The origin of these surface degradation processes need to be investigated and eventually compensated in order to recover the reduced emission efficiency. New surface processing techniques for surface termination with molecules other than hydrogen and incorporating dopants into diamond can also be investigated and developed to achieve higher NEA and further improve electron emission efficiency. The goal of this topic is to investigate electron emission process from diamond, develop new surface processing techniques for diamond to improve electron emission efficiency, and create DEA prototypes which incorporate these new techniques to achieve high current, high brightness and long lifetime operation.
PHASE I: During the Phase I effort, a numerical model and design methodology for diamond electron amplifiers (DEAs) will be developed. A prototype DEA will be designed and tested to verify the model and design methodology. Technical risks will be identified and plans for minimizing these risks will be devised. The prototype devices should have the following specifications: electron energy of 10 KeV, average current of 0.5 µA, bunch charge of 200 pC, diamond amplifier gain of ~200. New techniques for surface termination and doping to improve emission from diamond and related materials will be investigated.
PHASE II: A prototype diamond electron amplifier (DEA) will be designed based on the numerical model and design methodology developed in Phase I. The prototype device will be built, assembled, and tested. Target specifications for the Phase II design are as follows: electron energy of 100 keV, average current of 0.3 mA, bunch repetition frequency of 3 MHz, thermal emittance of 0.2 µm, maximum peak current of 100 mA, diamond amplifier gain of >200 and a lifetime of at least one year. Technical risks will be identified and plans for minimizing these risks will be devised. New techniques for surface termination and doping to improve emission from diamond and related materials will be investigated, and incorporation of these new techniques into the Phase II prototype will be explored.
PHASE III: Diamond electron amplifiers (DEAs) would be highly beneficial for applications requiring high current, high brightness and stable electron beams, e.g., high power, high frequency vacuum electronic power amplifiers for radar and directed energy applications, coherent x-ray generation, and thermionic energy conversion (TEC) through thermal electron emission for renewable energy generation. Phase III effort will explore opportunities for integrating DEAs with suitable electron beam parameters into these systems for improved performance in both defense and commercial sectors. An example of a potential Phase III product demonstration will be a high power microwave source such as a traveling wave tube with an integrated DEA cathode. The targeted frequency and power level should be in the range of X-band (8-12 GHz) and ~10s-100 KW which would be suitable for insertion into existing radar and/or directed energy systems.
REFERENCES: 1: J.Y. Tsao, et al., "Ultra-wide-Bandgap Semiconductors: Research Opportunities and Challenges," Adv. Electron. Mater. 4, 1600501 (2018).2: X. Chang, et al., "Electron beam emission from a diamond-amplifier cathode," Phys. Rev. Lett. 105, 164801 (2010).3: W.F. Paxton, et al., "Thermionic Emission from Diamond Films in Molecular Hydrogen Environments," Front. Mech. Eng. 3, 18 (2017)4: M.C. James, et al., "Negative electron affinity from aluminium on the diamond (1 0 0) surface: a theoretical study," J. Phys: Condens. Matter 30, 235002 (2018)5: K.M. O'Donnell, et al., "Extremely high negative electron affinity of diamond via magnesium adsorption," Phys. Rev. B 92, 035303 (2015)KEYWORDS: Ultrawide-bandgap Semiconductors, Diamond Thin Films, Electron Sources, Negative Electron Affinity, Hydrogen Termination, Field Emission, Secondary Electron Emission, Diamond Electron Amplifiers
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