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Visible Fluctuation Measurements on the TST-2 Spherical Tokamak

The four-beam correlation method has a potential of measuring fine-scale fluctuations locally. In the method, many lines of sight are measured and the coherences among them are calculated. As a proof of principle experiment, a visible fluctuation measurement system has been fabricated and applied to the TST-2 spherical tokamak plasma. Using this system, fluctuations were measured and high-frequency components (100±12.5 kHz) were analyzed. Because of a long parallel correlation length, local information was not obtained directly. However, comparison of the experimental results and those of a model calculation suggests that (i) the fluctuating emission is relatively localized in the peripheral region of the plasma, (ii) the perpendicular correlation length is of the order of 1 cm, while the parallel one is much longer than it and (iii) the magnetic field angle in the peripheral region agrees with that estimated from magnetic measurements.