Study of magnetic fourier components effect on ion viscosity in Tohoku University Heliac

Neo-classical theories explain that the non-linearity of the viscosity plays the important role in the bifurcation phenomena of the L-H transition observed in large tokamaks and stellarators. In the Tohoku University Heliac (TU-Heliac), a helical axis stellarator, the effects of the viscosity maxima on the improved mode transition has been investigated by electrode biasing experiments using a hot cathode made of LaB6. The driving force J x B for a plasma poloidal rotation was externally controlled and the poloidal viscosity was successfully estimated from the driving force. It was experimentally confirmed that the local maxima in the viscosity play the key role in the L-H transition. One of further extended works is to clarify the effect of magnetic Fourier components on the neo-classical viscosity. The minimization of helical ripples allows the reduction of viscosity, which is expected to bring good accessibilities to the improved confinement modes. It is also important to find out the effect of low mode rational surfaces or magnetic islands on the viscosity, because magnetic islands, which have low poloidal mode numbers and locate along a plasma periphery, have the possibility to become a control knob on a radial electric field and a radial particle flux. In a heliac device toroidal ripples, helical ripples and bumpiness can be changeable by selecting ratios of coil currents, even though these flexibilities are not independent each other. The hot cathode biasing above-mentioned can be one of the useful methods for the evaluation of viscosities in the particular magnetic configurations. In TU-Heliac the viscosities in various magnetic configurations were evaluated experimentally by the hot cathode biasing method. The toroidal ripples, the helical ripples and the bumpiness were changed in the range of about 30, 20 and 80% respectively by shifting the magnetic axis under the fixed rotational transform profile. The preliminary experimental results showed that the viscosity maxima qualitatively agreed with neo-classical predictions, demonstrating the usefulness of the hot cathode biasing method in the viscosity evaluation. TU-Heliac has local vertical field coils which produce external perturbation fields to resonate the magnetic Fourier components of (n, m) = (3, 2), (5, 3) and to grow m = 2 and 3 magnetic islands. Increasing the width of magnetic islands located along the plasma periphery, the electrode current J required for the improved mode transition was increasing. It suggested that the ion viscosity increased according to the increase of the magnetic island width. In order to estimate precisely poloidal Mach numbers in the biased plasmas ion temperature measurements were carried out by a high-resolution spectrometer and the measurement of poloidal flow velocities was tried out by a multi-Langmuir probe to compare with the E x B drift velocities. (author)