Cold Field Emitter Arrays for HPM

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Directed Energy (DE) 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: Establish a modular field emitter array that can be implemented in wide array of devices for generating electron beams and be a reasonable alternative to both thermionic and explosive cold emitting cathodes. These arrays yield consistent performance over hours of on time without the need for a heating element, giving predictable current densities when exposed to known field strengths. The beams are low emittance and compressible using electrostatic and magnetic focusing to give high current density without significant scalloping. These emitters arrays can be scaled to deliver 100s of amperes of total current and provide a variable level field enhancement versus emitting area depending on the application. DESCRIPTION: Develop Spindt type field emitter arrays at small scale that can be mounted to a cathode and have voltage applied to initiate a beam. The geometry of the field emitter array should be consistent across individual arrays and between multiple arrays. The resulting turn on field value and emitted current density as a function of voltage should be predictable. Preferably, several array geometries should be developed that explore the trade space between field enhancement, emission area, and durability. A minimum lifetime of arrays should be evaluated as a function of the applied field and resulting current density. Once field emitter arrays have been established at small scale, they should be increased in size or arrayed to create a cathode that can deliver HPM relevant total currents. Proposals should include guidance for mounting emitters and making proper electrical connections. The large area cathode will be evaluated for total current, uniformity of current, and longevity. A cathode composed of an array of field emitter arrays where individual emitters are modular and can be exchanged when necessary is desirable. PHASE I: Proposals should have demonstrated the ability to create small scale field emitting arrays and measure relevant metrics such as the current versus applied field and the lifetime of the array. The field strength needed to initiate current draw should demonstrate field enhancement at the emitter tips. The magnitude of field enhancement should be known for a given field emitter array geometry. Current densities in excess of 1 Ampere per square centimeter should be demonstrated. Active time for the emitters should demonstrate run times greater than 1 hour with less than 5% change in current draw over that time. Proposals should have plans for scaling the manufacturing capability to deliver the total currents expected in the phase II while keeping costs competitive with existing high performing electron gun technology. PHASE II: Awardees will fabricate a number of small arrays and internally test the conformity between arrays. These tests will evaluate the statistical variance in field enhancement factor, current density as a function of applied field, and likelihood of exceeding a minimum lifetime. If multiple geometries of field emitter array can be fabricated, only a few (2 3) of these will be chosen to undergo conformity analysis. With small scale emitters validated, awardees will propose plans to construct a cathode capable of delivering 100 300 Amperes of total current. Awardees will provide the field emitter arrays necessary for this, either an array of small scale arrays or a single large array. They will also either recommend a procedure and provide any of the materials necessary for mounting the emitters to a cathode substrate, or supply the emitters already mounted to a metallic cathode that can be incorporated into an AFRL test stand. AFRL facilities will be used to pulse the cathodes at high rep rates. The cathode will be investigated for compatibility with HPM sources measuring the total current as a function of applied field, emission pattern, lifetime under pulsed operation, and vacuum degradation. The technical and financial feasibility of implementing these emitters as an alternative to thermionic and explosive emitters will be evaluated and discussed in a final report. PHASE III DUAL USE APPLICATIONS: Technology readiness level shall be level 4 at entry to phase III. Cathode arrays relevant to high power microwave sources are likely to be of continued interest and modular, source agnostic electron guns would be a valuable asset that a phase III could deliver. Proposals should additionally mature cathode technology and standardize operating parameters for a range of applications. Pursue commercialization and identify applications beyond directed energy research. Identify commercial partnerships and collaborators. REFERENCES: 1. Whaley, D.R., et al., 100 W operation of a cold cathode TWT, IEEE Trans. Electron Devices 56, 896 (2009); 2. R. J. Barker and E. Schamiloglu, High Power Microwave Sources and Technologies. IEEE Press, 2001. KEYWORDS: Cold Field Emitter Array; Spindt Array; High Power Microwaves; Electron Beams; Directed Energy