Hope College Physics Department Research Experiences for Undergraduates Summer 2012 Project Summary

In the presence of a strong electric field, gas molecules become ionized, forming a plasma. The study of this dielectric breakdown at microwave frequency has important applications in improving the operation of radio frequency (RF) devices, where the high electric fields present in small gaps can easily ionize air. A cone and tuner resonant structure was used to induce breakdown of nitrogen in adjustable micro-gaps ranging from 50〈m to 1.2 mm. The electric field for plasma formation exhibited strong pressure dependence in the larger gap sizes, as predicted by previous theoretical and experimental work. Pressure is proportional to the frequency of collision between electrons and molecules(v_c) when the gap is large. In the micro-gap region, when the pressure increases, v_c levels off. A separate model of the breakdown electric field based on the characteristic diffusion length of the plasma also fit the data poorly for these smaller gap sizes. This may be explained by the hypothesis that dielectric breakdown at and below the 100 μm gap size occurs outside the gap, an argument that is supported by the observation of very high breakdown threshold electric fields in this region. Optical emissions revealed that vibrational and rotational molecular transitions of the first positive electronic system are suppressed in micro-gaps, indicating that transitions into the molecular ground state do not occur in micro-gap plasmas.

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