Studies on MHD Fluctuations through Direct Measurements of Eigenfunctions for Reduced MHD Equations Using Heavy Ion Beam

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Reduced MHD equations are commonly used as a powerful tool for the study of MHD instabilities both in theoretical researches and in computer simulations. In the framework of reduced MUD equations, two eigenfunctions are a stream function (Φ) and poloidal flux function (Ψ). In heavy ion beam probe diagnostic for axi-symmetric tori like tokamaks, observables are directly connected with the two eigenfunctions through energy and angular momentum conservations of the injected heavy ions. Those observables are a scalar potential (f) and toroidal component of vector potential (A_ζ). The heavy ion beam probe has been powerful in measurement of scalar potential, but measurements of A_ζ have been rare so far. This is because the observable in this method is a path-integrated one and does not give a local value.The purpose of this research program is to clarify the critical conditions for the observable to be connected with local measurement of A, and to provide experimental approach for MHD fluc … More tuation studies that can directly linked with theoretical approach using reduced MHD equations. Our method is as follows. Various types of MHD oscillations (spatial location and mode numbers) are assumed as model fluctuations. Then fluctuations in toroidal displacement of the secondary beam at the detector plates are numerically calculated by orbit analysis. The results are as follows:(1)When MHD oscillations are localized near the center of the plasma, locality of the measurements is fairly good(2)When MHD fluctuations are localized near the edge region of the plasma, some amount of spurious signal appears in the central region where fluctuations are absent in the model.(3)Locality of the measurements is improved when the ion gun and the detector are located far from the torus.These results suggest that locality of the measurements of A_ζ is rather good in a wide range of applications to axis-symmetric tori. Next step is to expand our investigation to non-axis-symmetric tori, where angular momentum conservation cannot be preserved. Less