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(PDP/03) Core Density Fluctuations in Reverse Magnetic Shear Plasmas with Internal Transport Barrier on JT-60U

R. Nazikian¹, K. Shinohara, R. Yoshino, T. Fujita, H. Shirai, G. J. Kramer, and the JT-60 Team

Japan Atomic Energy Research Institute, Naka-machi, Ibaraki-ken, 311-0193 Japan
¹ Princeton Plasma Physics Laboratory, Princeton NJ 08543-0451 USA

Abstract.  First measurements of the radial correlation length of density fluctuations in JT-60U plasmas with internal transport barrier (ITB) is reported. The measurements are obtained using a newly installed correlation reflectometer operating in the upper X-mode. Before transport barrier formation in the low beam power current ramp-up phase of the discharge, reflectometer measurements indicate density fluctuation levels $\tilde{\mathrm{n}}$/n $\sim$ 0.1 - 0.2% and radial correlation lengths 2-3 cm ( k_r$\rho_{\mathrm{i}}^{}$ $\leq$ 0.5) in the central plasma region ( r/a < 0.3). A rapid increase ( $\tilde{\mathrm{n}}$/n $\sim$ 0.5%) in the density fluctuation level is observed within 50 ms of the turn on of high power neutral beam injection during formation phase of the internal transport barrier (ITB). Inside the ITB, the radial correlation length of density fluctuations is short ($\sim$ 0.5cm at r/a $\sim$ 0.57) corresponding to k_r$\rho_{\mathrm{i}}^{}$ $\sim$ 3. However, fluctuation levels are considerably higher than measured near the magnetic axis. Reflectometer measurements obtained at the foot of the ITB also indicate high fluctuation levels compared to measurements in the central region of the discharge.

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