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February 2, 2015
High-speed Photography of Changes in Plasma: High-speed Soft X-ray Camera Measurements

The Large Helical Device (LHD) confines high-temperature plasmas by making them float in the container that is ringed by magnetic lines of force. Confined plasma is controlled so that its density and temperature are constant, but when examined closely, in actuality, density and temperature are fluctuating at high speeds. When the fluctuations are small there are no problems. But when fluctuations become larger due to some particular cause, the plasma changes shape, and in some instances the plasma escapes to outside the magnetic field container. For that reason, we are conducting research that investigates causes of the growth of fluctuations through comprehending the circumstances of the change in the plasma’s shape. Here, we introduce camera diagnostics for high-speed observation of plasma movement.

The camera to be introduced here differs greatly in two points from the camera that one knows. First, this camera does not photograph visible light. Rather, the camera photographs what is called soft X-ray light. Light is one type of electromagnetic wave, and when considered as a wavelength it is from 400 to 800 nanometers (one nanometer is 1/1,000,000 of a millimeter). Moreover, when the wavelength becomes short, this enters in the region of ultraviolet radiation and of X-ray light at a wavelength of some several nanometers. Because a typical camera photographs visible light, the image that the human eye sees will become the photograph. And because there is much light irradiated from a high-temperature plasma that is beyond visible light, most light cannot be seen with the human eye. On the other hand, from plasma, because light of various wavelengths is irradiated in response to that energy level, we can know the condition and the quality of the plasma from observing that light. Because the strength of the soft X-ray light that is irradiated from the LHD’s high-temperature and high-density plasma is greatly dependent upon the plasma’s density and temperature, through measurements and photography we can comprehend plasma fluctuations in a wide range.

Second, in order to accurately measure fluctuations in a plasma we make it possible to take photographs at a high speed. A typical camera uses a light detection semiconductor called CCD and measures light. However, in order to take photographs at a high speed, we must shorten the exposure time. In that case, we are not able to obtain sufficient sensitivity. Thus, instead of CCD, we lined up several large diameter photo diodes in order to heighten the sensitivity of the measured light and achieved high-speed measurements and high-speed photography.

Using a high-speed soft X-ray camera that has been developed, we are conducting research for understanding phenomena related to plasma movement. From research conducted to date, we found the phenomenon in which large fluctuations were born when the pressure in the plasma core suddenly increased like a spire, and the pressure in the core fell in a short amount of time. In order to control this type of phenomenon and to generate and maintain plasma, it is necessary to clarify causes of the growth of plasma movement. Because those movements occur at high speeds and are predicted to have a spatial structure, we are advancing with research by applying measurement methods developed. In the future, utilizing this high-speed soft X-ray camera, we will clarify the circumstances of changes in plasma movement and the structure of those movements, and continue to advance in our research on generation and maintenance of high-performance plasma.