UV emission and probe diagnostics and computational modeling of a low pressure microwave excited microplasma source

Summary form only given. Low-pressure microwave-excited microplasmas have a wide variety of potential applications, including their use as UV photoionization sources for mass spectrometry. Resonant microwave microstrip architectures can be used to initiate and sustain these microplasmas. When operated in a windowless configuration, the plasma plume exiting from an aperture in the plasma confinement structure contains a complex mixture of particle species (UV photons, groundstate neutrals, metastables, and plasma electrons and ions)1. Understanding the makeup of this plume is critical to optimize parameters for photoionization, or any other application where exposure to the plasma plume takes place. The microplasma source under investigation consists of a resonant microstrip pattern on alumina substrate, with an elongated 1.5 mm x 6.5 mm plasma confinement structure. 2.5 GHz microwave power is delivered at up to 5 W. Argon and helium/krypton mixtures are flowed through the confinement region at flow rates up to 10 sccm producing confinement region pressures near 1 Torr, with the plasma plume exiting into high vacuum through a 300μm-by-600μm aperture. The effect of confinement region and microstrip geometries, net absorbed microwave power, and source region pressures on the properties of the plasma in the source region and plume are investigated. Ultraviolet emission and Langmuir probe diagnostics are used to diagnose the plasma. The spatial distribution of the plasma density in the plume is measured for a range of microwave powers and flow rates. We also present computational modeling results of this microplasma using the Hybrid Plasma Equipment Model (HPEM)2 with comparisons to the experiments. The angular distribution of UV flux is observed, revealing a highly directional radiation pattern owing to an elongated source region. This flux magnitude and directionality is additionally a function of source region pressure, due to variation in the spatial- distribution of the plasma and resonant UV photon absorption and quenching.