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November 11, 2013
Viewing Plasma through an Infrared Pinhole Camera: Developing a Method for Measuring Radiation Three-dimensionally

In order to achieve fusion energy it is necessary to confine high-temperature plasma that exceeds 120,000,000 degrees in its core. Conversely, outside the plasma confinement area, the temperature of the plasma that has transferred to that area is sufficiently cooled and forced to terminate at a pre-determined place. At this time, because the plasma at the edge area loses energy through radiation and the temperature has fallen, in order to examine the plasma’s behavior it is necessary to minutely measure the condition of the radiated heat. Development is currently advancing so to examine the condition of the heat radiation emitted from the plasma edge in the Large Helical Device (LHD). Here we will introduce an advanced three-dimensional radiation measuring method.

High-temperature plasma is confined in a magnetic field container and floats apart from the wall. However, plasma that has leaked from the container moves along the magnetic field lines that were pulled from the container’soutermost surface and is led to the pre-determined place called the “Divertor.” For that reason, in the Divertor is concentrated the plasma’s heat. In some instances, the Divertor is damaged. In order to mitigate the Divertor’s heat load, there is a method of increasing “radiation loss” by which the heat is scattered through radiation when plasma moves at the peripheral region. Readers probably have heard of the use of radiation in infrared stoves and other appliances. When heat is forced to be lost through radiation, similar to heating from a stove because the heat spreads in all directions, we can disperse the concentration of heat moving to the Divertor.

In order to increase the loss of the plasma’s radiation, typically, we introduce a small amount of impurity gas into the edge area near the Divertor. However, because the central part of the plasma will become thermally unstable if we increase the radiation loss by too much, we must control the amount of radiation loss. (For details, please see “Reducing the Heat Load on the Wall: Controlling the Peripheral Plasma,” Research Updates No. 175, May 7, 2012.) For this reason, it is necessary to measure three-dimensionally from where radiation is emerging and how much radiation is emerging. When considering three-dimensional measurement, we think of the CT scan used for examining the inside of the human body. Through the CT scan a device moved around the outside of the body provides measurements. Differing from this method, however, based upon the LHD’s three-dimensional radiation measurements, infrared pinhole cameras are placed at two locations each for the horizontal direction and for the vertical direction, or a total of four places, and the plasma is measured.

Regarding the pinhole camera, perhaps you have made one by opening holes in the bottom of a milk carton or a box of sweets and placing photograph film on the opposite side. The basic structure of the pinhole camera used for measuring radiation is the same, but the size of the aperture is to be no larger than 8 millimeters square. And the size of the platinum thin film to be used as a screen for the film is no larger than 11 centimeters by 15 centimeters. The platinum thin film is coated with carbon and then blackened. This heightens the absorption efficiency of the plasma heat. The radiation from the plasma is projected as temperature distributions on the platinum thin film. The radiation distribution can be measured by observing the back side of the thin film using an infrared camera.

Using an infrared camera developed for this purpose, upon measuring the plasma radiation from areas near the Divertor we have found strong radiation distribution from places predicted through calculations. The three-dimensional measurement of fusion plasma is challenging research, and has not yet been achieved elsewhere in the world. By further enhancing this measurement technology being developed through the LHD and heightening its performance, fusion plasma research can be expected to move forward.

Images of LHD plasma composed by computers (see the image to the left) and temperature distribution on platinum thin film measured by an infrared pinhole camera (see the image on the right). In the area called “X point of helical divertor,” which is shown here as pink-colored lines, as seen in the image on the left, we can see the temperatures growing higher on the platinum thin film. This image shows that plasma radiation in this area is growing stronger.