Direct Generation of Electromagnetic Radiation with a Compact, Air-Stable, Optically Modulated Electron Emitter
Abstract
The modern day congested and contested electromagnetic spectrum has placed stringent demands on electronic systems. A single electromagnetic (EM) source that change transmission bands, multiple frequencies, or even frequency bands, and be able to quickly switch frequencies when the desired communication channels becomes contested is of value in a contested environment. Additionally, directed EM sources which can temporarily or permanently deny communications are of significant value. One approach to this may be the directgeneration of EM radiation from a modulated electron source. This approach could enable compact, high-frequency, high-power HPM sources which could serve the needs of directed energy systemsThe first goal of this proposal is to continue to develop and demonstratea simple planar silicon-insulator-graphene structure to create an air-stable, electrically tunable, negative electron affinity surface by applying a bias between the graphene and silicon, termed Hot ElectronLight Assisted Cathode (HELAC). A continuous or pulsed photon source may then be used to excite electrons in the silicon, which will then be emitted into vacuum when a small (4-10 V) bias is applied across the device. This electron source can be compact, environmentally robust, and use low energy photons in the near-IRto visible range. Based on initial simulations, modulation frequencies of >250 GHz and 1 A/cm^2 emission currents should be achievable through device optimization. Initial experimental prototypes have already demonstrated 1 mA/cm^2, despite a relatively unoptimized structure. Using this electron source, we will then design and build a device similar to an inductive output tube, where an electron beam is density modulated by incident light, then accelerated to a desired voltage, and finally RF power is extracted from the beam using an output cavity. We refer to this device as a Light Modulated IOT (LM-IOT). Unlike IOTs, which have frequency and total current limitations due to the grid-cathode spacing and uniformity, this approach is expected to overcome those issues by eliminatingthe need for a grid and input cavity. We will build a prototype LM-IOT, where the beam generation, confinement, and collector segments are fixed, and the output frequency will be determined by swapping output cavity. Specifically, we will carry out simulations toestablish basic operation principles, design the overall device, and then fabricate and assemble the device. This will be used to establish the proof of principle operation for LM-IOTs as well as make projections about device parameters.Finally, we aim to exploremulti-frequency devices, where we simultaneously excite the electron emitter with multiple modulation frequencies, creating an electron beam with multiple frequencies, and then use multiple output cavities to extract and emit those frequencies separately. In thistask, the goal is to understand interaction between multifrequency beams and multiple cavities, the limitations in power distribution between frequencies, and any non-linear effects due to the electron source itself that could take two input frequencies and generate different output frequencies. These limits would be driven by a combination of the physics of the electron source and the beam-cavity interactions. Approved for Public Release
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 09, 2021
- Source ID
- N000142112634
Entities
People
- Rehan Kapadia
Organizations
- Office of Naval Research
- United States Navy
- University of Southern California