[Under construction]2017 AFOSR MURIPO: Dr. Jason Marshall, Plasma and Electro Energetic PhysicsPI: John Verboncoeur, Michigan State UniversityMURI Website
Multipactor onset, growth, associated space charge effects, and transition to ionization breakdown due to ambient or desorbed gases represent key stages of single and multi-frequency RF-driven phenomena that inhibit performance in space-based and terrestrial vacuum electronics devices. Performance degradation through space charge detuning and interference with gain is expected for medium duration pulses, and ion generation and damage for longer pulses.
Despite decades of research on multipactor and breakdown, fundamental research questions remain. Multipactor materials data is poorly characterized, including energy and angular dependent secondary electron emission yields, and emission distributions. Models often exclude key effects such as space charge, which, however, are predicted to play an important role in multipactor growth, saturation, and interference with device operation. Indeed, the multipactor characteristics, baseline performance data, and geometric effects data are lacking and thus deserve detailed research attention. These include the impact of seed electron generation from sources including cosmic rays, field emission, photoemission, triple points, volumetric charged dust particles, metastables, and photons, among others.
Technical approaches include the development and deployment of theoretical, computational, and experimental tools. Specifically, three experimental test cells are proposed to provide common platforms for planar, cylindrical, and stripline configurations. The test cells allow study of multipactor susceptibility, as well as novel material, geometric, and electrical mitigation techniques in isolation or as a system. Test cell designs allow for variability of gap spacing, driving frequency, waveform shape, ambient and desorbed gas, and many others. Diagnostics are developed and deployed, such as direct multipactor electron detection, optical/VUV emission spectroscopy, and X-ray imaging to provide characterization of the multipactor process over ns timescales and micron spatial scales. Analytic and computational modeling tools are developed to address the fundamental research questions. Models include abinitio models to study secondary emission processes at the materials-vacuum interface, zero-dimensional kinetic global-Boltzmann models for rapid modeling of gas reactions, fluid-Boltzmann, and particle-in-cell models provide full modeling capabilities. Furthermore, density functional theory and Monte Carlo modeling will guide the study of novel materials with low secondary electron yield.
Potential impact on DoD capabilities of this work includes improved understanding of the susceptibility and mitigation of multipactor in RF space technologies, including an understanding of the role of space charge, and the interaction with single and multi-tone signals. Characterization of test cells, and validation of models, provide a full suite for studying multipactor, RF loss and phase noise in systems and components under real world conditions. The suite of tools will go beyond current normalized multipactor tables to provide transformational physics based capabilities for design and testing. The impact extends beyond space-based devices, to terrestrial RF and microwave devices, to accelerators, pulsed power, and other high voltage devices.