Accurately Measuring Plasma With Interference

Researchers have developed a method to precisely measure the electron density of plasma even at atmospheric pressure.

AsianScientist (Jun. 15, 2015) – A team from the National Institutes of Natural Sciences, National Institute for Fusion Science and The University of Tokyo Graduate School of Frontier Sciences Department of Advanced Materials Science research group has developed an electron density measurement method for atmospheric pressure low-temperature plasma. Their results, published in the Journal of Physics D, mean that plasma—the fourth fundamental state of matter—could one day be harnessed for environmental protection, medicine and biology.

Their results were made possible by borrowing the technique, originally developed in fusion plasma research, of using an interferometer to probe the electron density around a plasma rapidly and high resolution, which in the past had been difficult due to the influence of atmospheric pressure.

Atmospheric pressure low-temperature plasma shows research promise for decomposing hazardous materials containing potentially dangerous microbes, and as such is potentially useful for environmental protection, medicine and biology.

In order to produce plasma with the appropriate properties for each field, it is necessary to precisely measure the plasma’s physical properties and to control the parameters used to generate it. Electron density is one of these fundamental physical properties, and to date various measurement methods have been used.

Interferometry, which passes a laser beam through the plasma, is a standard electron density measurement method. But in the case of atmospheric pressure low-density plasma, the rapid fluctuations of atmospheric pressure in as well as around a plasma makes it difficult for the technique to be used accurately.

On the other hand, development of high precision electron density measurements is also becoming an important research topic in magnetic field confinement plasmas intended for fusion reactors. By utilizing the Large Helical Device at the National Institute for Fusion Science to produce and measure samples of atmospheric pressure low-density plasma, the researchers developed a dispersion interferometer, which utilizes interference between a laser and its higher harmonics to measure the electron density of a plasma with high accuracy.

Using this method developed by high-temperature plasma diagnostics, they succeeded in greatly reducing the influence of atmospheric pressure on the output signal. The measurement also does not depend upon the exact gas composition used to produce the plasma, as compared to other electron density diagnostic methods, allowing for it to be applied generally. Furthermore, this new technique is able to measure the electron density with far greater accuracy than before.

These new high-accuracy measurement techniques will reduce the reliance of plasma experimenters on experience and trial and error, aiding in the production of plasmas that are optimal for application in the fields of medicine and biology.

Furthermore, these measurements will also provide valuable data for computer simulations that can improve our understanding of the dynamics of active ion species as they interact with living organisms and materials hazardous to the environment.

The article can be found at: Murari et al. (2015) Application Of Phase-modulated Dispersion Interferometry To Electron-density Diagnostics Of High-pressure Plasma.

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Source: National Institutes of Natural Sciences.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Shern Ren is studying towards a PhD degree in physics at the National University of Singapore. When he isn't working on the statistical mechanics of nanomachines and single-molecule systems, you may find him scratching his head over politics, education and the mathematics of Threes.

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