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August 24, 2015
Microwave Imaging: New Image Diagnostic Techniques Born of Fusion Research

“Seeing is believing.” In hospitals, too, observation of lesions through image diagnosis methods such as CT and echo is being conducted. And observation also is extremely important for fusion research. In the Large Helical Device (LHD), we heat a plasma to extreme high temperatures that exceed one hundred million degrees. From such extreme high temperature plasma, light that may be seen by the eye, that is, visible light, is hardly emitted. For that reason, it has been extremely difficult to observe complicated phenomena occurring in an LHD plasma. Among the numerous electromagnetic waves, microwaves are reflected by plasma. Thus, by injecting microwaves into the plasma we considered observing the waves reflected from the plasma. The frequency of the reflected microwaves changes depending upon the density of the plasma. Because the plasma’s density has a profile within the plasma, if we inject microwaves of varying frequencies into the plasma and observe the microwaves reflected, we should be able to observe the inside of the plasma three-dimensionally. However, for such observation, a camera that can photograph microwaves did not exist until now.

At the National Institute for Fusion Science, together with researchers and technicians at Kyushu University, Kansai University, Tokyo Institute of Technology, Tokyo University of Agriculture and Technology, Fukuoka Institute of Technology, Ube National College of Technology, and the Institute for Molecular Science, we have made advances in developing a microwave camera. Through the development of cellular phones, semiconductor parts for microwaves have entered the market, and in 2014 we succeeded in developing a camera installed with a high-sensitivity and high-speed microwave image sensor of 8x8 pixels. This is a great advance, for there was no two-dimensional microwave image sensor. Using this newly-developed microwave camera, we attempted plasma observation in the LHD. We simultaneously injected microwaves with four frequencies from 25-35GHz in an LHD plasma, and by capturing microwaves thus reflected using a microwave camera, we succeeded in observing plasma three-dimensionally. Next, by combining this observation with data analysis using a computer, we can anticipate that clarification of the complicated phenomena that occur in a plasma will advance.

Microwaves penetrate through clothing and fog. Application of the microwave camera to discovering weapons hidden in clothing and to safe passage through thick fog by airplanes and ships is anticipated. Further, low frequency microwaves penetrate to a certain degree through concrete and the human body. If this can be used for CT, new image diagnosis without exposure to X-ray radiation will become possible. The microwave camera used in the LHD and the microwave camera used for CT are equipped with 90% of the same parts. Thus, the group developing the microwave camera has begun research development for microwave CT diagnostics together with Nagasaki University, which is undertaking theoretical research on microwave CT.

Development of the microwave camera, which began from observations of fusion plasma, is expected to contribute to the growth of new imaging diagnostic technology in medical and industrial fields.