Laser-based diagnostic for plasma-surface interactions

OUSD (R&E) MODERNIZATION PRIORITY: Directed Energy TECHNOLOGY AREA(S): Air Platform OBJECTIVE: We seek novel laser-based non-destructive evalution diagnostics for surface-plasma interactions that can be integrated into devices with materials that are both under vacuum and are energetized by high-field and high-power drivers. DESCRIPTION: The highly energetic species in plasma environments that interact with surfaces lead to a range of kinetic processes that exist over several lengths and time scales. These processes can lead to heating, structural modifications, surface etching, emission, chemical reactions, etc that are intimately related to the irradiating plasma characteristics and structural, chemical and thermal properties of the materials being irradiated. Novel laser-based metrologies have recently emerged as unique diagnostic tools to provide real-time surface characterization of materials being irradiated with energetic plasmas. These metrologies have the advantage of providing rapid and highly sensitive NDE measurements of surfaces while achieving relatively spatially localized measurements with micron scale resolution. Further, the spatial manipulation of the laser focal plane can generate surface maps of thermal and chemical gradients on a plasma irradiated surface. Here we seek an advanced plasma diagnostic tool capable of spatially resolving the thermal and chemical gradients of a plasma exposed surface with real time NDE functionality. This diagnostic approach should be able to measure local thermal and chemical property variations on a plasma irradiated surface, and be adaptable for in situ testing under vacuum and other high field environments. Spatial resolution of surface thermal and chemical dynamics < 10 microns is desired. This diagnostic capability will need to be integrated into both vacuum systems as well as high-power drivers up to and including typical high-power electroamgnetic sources. PHASE I: Phase I will work to research and asssess if a novel fiber optic based diagnostic system with the following capability: -Thermoreflectance in integrated fiber optic assembly capable of detecting thermal property changes of materials exposed to plasma -Spatial control of focused laser location on sample surface via integrated fiber optics and modulated piezo mirrors instead of physically moving sample (making this tool assessable and integrable into vacuum assemblies -Temporal resolution of continuous wave reflected probe intensity through electronic detection scheme of reflected probe light trigged with modulated laser or plasma source -Detection of both thermoreflectance and Raman signals from focused laser spot on samples surface using single element detection and all-in-fiber spectroscopy This assessment will including vacuum and field/power parameters and constraints for the systems with which the diagnostic can be integrated. PHASE II: Based on the Phase I assessment, the Phase II effort will impliment and test a novel laser-based diagnostic system for plasma-surface interaction. PHASE III DUAL USE APPLICATIONS: The fundamental nature of the AFOSR programs reflects the potential to extend beyond directed energy applications, and re-vector this diagnostic for any kind of energetic surface-plasma systems such as pulsed power, plasma processing, advanced space thrusters, radar/communication/electronic warfare sources. and plasma combustion ignition. REFERENCES: Tomko et. al., "Plasma-based Surface Cooling." Arxiv: 2018.02047, 2021; KEYWORDS: laser diagnostics; plasma-surface interaction; thermoreflectivity; and Raman;