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


Project Title: Modeling Plasma Formation in a Micro-Gap at Microwave Frequency
Student Name: Arthur Bowman III
Student's Home Institution: Wayne State University
Research Advisor: Dr. Stephen Remillard
Source of Support:

This research was made possible by funding from the National Science Foundation under NSF-REU Grant No. PHY/DMR-1004811, the Collegial Faculty Development award endowed by the Provost’s Office of Hope College, and the Hope College Division of Natural and Applied Sciences.

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(vc) when the gap is large. In the micro-gap region, when the pressure increases, vc 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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