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Fig 2-7 Examples of the outermost surface of the plasma cross section In addition to the ordinary position control of the outermost magnetic surface, it is possible to use the configuration of the magnetic divertor, as well as the oval and/or triangular components as feed back control parameters. (a) a plasma column still touching the wall at a time t=1 s after the beginning of the discharge (limiter mode) (b) a case with a large oval component at t=3 s (c) the standard configuration with the divertor at t=5.5 s (d) a case with a large triangular component at t=6 s

High temperature plasmas are confined by closed magnetic surfaces in a fusion reactor. To ensure that the plasma surface does not touch the wall, a feedback control system is needed for the plasma shape and position. Magnetic sensors are used to detect the field outside of the plasma, but to estimate the plasma shape and position we usually need to make some assumptions regarding plasma quantities such as the plasma current profile. Recent experiments show that exact control of the oval and/or triangular components of the plasma shape can result in improved plasma confinement. To capitalize on this, we have developed a method to obtain exact information on the outermost magnetic surface, using only the information from the external magnetic sensor, and without using any kind of assumption. The magnetic field including plasma is described by a number of differential and integral equations, which are difficult to solve for a complex shape without using some simplifying assumptions. It is possible to obtain exact solutions on the outermost closed magnetic surface, without assumptions, when there is convergence of the boundary integral equation. We have developed a high-speed computer code for these solutions, and as a result, the plasma shape can be obtained at a rate of 10 frames per second, without any assumptions.

Reference K. Kurihara, Development of the Real-time Plasma Shape Visualization in JT-60, JAERI-Research 97-055 (1997).

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