Plasma is studied not only for fusion energy, but also for applications in various other fields. In particular, recently, application research in environmental and medical fields has flourished. In the field of hazardous materials that pose harmful effects to the environment, and also in the sterilization of medical equipment and, moreover, the treatment of illnesses and injury, research utilizing plasma is being conducted. Not limited to use for application in fusion energy, the environment, and medicine, in studying plasma it is necessary to understand accurately the condition of the plasma. The quantities that express a plasma’s condition vary. For example, the electron density in a plasma is the quantity that expresses the “denseness” of the plasma. This is extremely important for clarifying the physical phenomenon.
The electron density in a plasma can be measured using electromagnetic waves. The electromagnetic waves in the vacuum travel at the same speed even if the frequency differs. However, in a plasma, when the frequency of the electromagnetic waves differs, the speed of the wave differs, too. Then, when the electron density increases, the difference in the speeds of the waves with different frequencies grows. In order to measure electron density using the qualities of the electromagnetic waves inside a plasma, at the National Institute for Fusion Science we have developed a high-performance Harmonic Interferometer. This device uses a particular laser light in which are mixed the frequency of a particular electromagnetic wave and a “double harmonic wave,” which is a wave that is exactly two times the frequency wave. When we inject this particular laser light into a plasma, because the speeds of the waves of the two different frequencies are different, there appear differences in the time of arrival at the detector. From this time difference we can know the electron density of a plasma. We have installed this harmonic interferometer to the Large Helical Device (LHD), and we have succeeded in taking highly accurate measurements of electron density in plasmas.
On the other hand, because plasmas used in medical and environmental fields are generated in the atmosphere, such plasma are called “atmospheric-pressure plasma.” Due to effects from the air around the plasma, it was difficult to accurately measure the electron density. Research groups at The University of Tokyo and Tokyo Metropolitan University, and the National Institute for Fusion Science found that the harmonic interferometer is useful not only for LHD plasma but also for measuring atmospheric-pressure plasma. Then, undertaking joint verification experiments, we succeeded in taking highly accurate measurements of electron density in atmospheric-pressure plasma. By using the harmonic interferometer we can advance plasma research for environmental and medical applications more effectively.
These results from this joint research were published in the academic journal of the Institute of Physics in England, and this paper was selected by the Institute as an especially significant paper. Further, in April 2015 we were invited to deliver a lecture at the “First European Physics Conference for Plasma Diagnostics.” We will continue to advance further in this joint research. And seeking to reduce the size of the diagnostic and to further enhance its accuracy, we plan to design a diagnostic that will be easy to utilize in various places and for various purposes.