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This work was also supported, in part, by NSF REU Grant #0452206.

Microwave Discharges in Gases

High amplitude electric fields will cause an avalanche breakdown current in a gas. We use a resonant structure, which supports a very high electric field, to introduce high power microwaves and establish a high electric field. Electrons are accelerated and ionize molecules in the gas. The acceleration occurs over a very small distance and depends on the distance between gas molecules and the scattering cross-section of the molecules. These properties determine the mean free path of the electrons, which can be adjusted independently by changing the gas presure.

microwave resonator

An example of a microwave resonator. The 5 cm circumference corresponds to one half of a wavelength. The high electric field region is in the small gap. The electric field exists predominantly in the gap and the gap is where the avalanche breakdown occurs.

 

Using a re-entrant cavity resonator, free electrons accelerate in the high electric field and knock bound electrons from atoms and molecules in the gas, creating a plasma, or a gas of ionized particles. The ions eventually capture lower energy free electrons. As the captured electrons settle into the lowest energy state, photons are emitted. Since many of these photons are visible, a glow discharge can be viewed with the naked eye.

 

. Perturbation theory is used to calculate the electric field from the microwave source power at breakdown. There is a pressure where the necessary electric field is a minimum. We measure this minimum for various gases using microwaves, and use the results to understand the threshold electric field for the gas. A model developed by our group fits the data well and produces information about ionization rate in the gas. Breakdown electric fields determined from perturbation theory are compared to breakdown measurements made in an RF capacitive gap. The model developed in our group is shown fit to our data.

Shown above, a discharge in helium is observed to undergo a visible color change as the pressure varies from a collisionless plasma, at 0.3 torr, to a collisional plasma, at 6 torr.

Photos by Isaac Angert.

The spectral content of an argon discharge was measured using an SBIG spectrometer as the pressure is swept through the Paschen minimum. The legend shows the wavelength in nm, and the line color corresponds roughly to the wavelength's visible color. The blue line (in bold) clearly becomes more prominant at low pressures.


A nitrogen plasma in the collisional regime was observed by Cameron Recknagel to undergo spectral changes as the driving power incident upon the cavity was varied. Cam took spectra using the SBIG spectrometer and observed at high power the emergence of the 1st positive system along with some unidentified peaks and the suppression of the 2nd positive system.

 

Research Student's Bibliographic Reference

Results from Summer/Autumn 2008
Results from 2008-2009 academic year

Results from Summer 2011

Jun Xue and Jeffrey A. Hopwood, Microwave-Frequency Effects on Microplasma, IEEE Transactions on Plasma Science, vol 37, no. 6, June 2009, pp. 816-822.

A.V. Gurevich, et al., Intense Growth of Ozone Concentration in Subcritical Fields in Oxygen Plasma, Physics Letters A, vol 201, 22 May 1995, pp. 234-238.

A.V. Gurevich, et al., Artificially Ionized Region as a Source of Ozone in the Stratosphere, Physics Uspekhi, vol 43, no. 11, 2000, pp. 1103-1123.

D. Anderson, et al., Microwave Breakdown in Resonators and Filters, IEEE Trans. on Microwave Theory & Techniques, vol 47, no. 12, December 1999, pp. 2547-2556.

W. McColl, et al, Electron Density and Collision Frequency of Microwave-Resonant-Cavity Produced Discharges, J. Appl. Phys, vol 74, no. 6, 15 September 1993, pp. 3724-3735.

D.J. Rose and Sanborn C. Brown, “Microwave Gas Discharge Breakdown in Air, Nitrogen, and Oxygen,” J. Appl. Phys, Vol. 28, No. 5, 561-563 (1957).

 

Spectroscopy Reference Data:

NIST atomic spectra database

The Spectrum of Molecular Nitrogen by Alf Lofthus