Characteristics of quasi-single-helicity (QSH) states have been studied in a low-aspect-ratio reversed field pinch (RFP) machine RELAX mainly by magnetic diagnostics. Internal profiles of the fluctuating radial, poloidal and toroidal magnetic fields have shown good agreement with eigenfunctions of a single helical mode. The edge magnetic fluctuation spectra are somewhat broader than what are expected from internal magnetic field profiles. In spite of these slight discrepancies, the usual measure for the QSH, the spectral index N S lower than 2, still provides a reasonable measure for QSH states in RELAX. The QSH persistence has been improved in RELAX by the reduction of the poloidal resistance of flanges at poloidal gaps, mainly due to the improved axisymmetry of the toroidal magnetic field. QSH persistence more than 30% of the flat-topped current phase has been realized with current density lower than in other RFP, and probability of spontaneous QSH is 12.8 ± 7.3% which is higher than high-aspect-ratio RFP. It suggests the advantage of low-aspect-ratio configuration in attaining to the QSH. Comparison with recent experiments using active resonant perturbation may suggest the importance of further reduction of field errors to improve the quality of QSH.

A fast successive soft-X ray (SXR) imaging system where SXR camera and high-speed camera for the study of time evolution of magnetic islands has been constructed. A preliminary experimental result in which we observed tangential SXR images from quasi-periodic quasi-single helicity (QSH) state in low-aspect-ratio (low-A) reversed field pinch (RFP) is presented. We successfully obtained time evolution of SXR images from tangential port. By comparison obtained images with simulated images, we may conclude that the evolution of experimental SXR image suggests the rotating QSH RFP configuration. The filamentous configuration tends to be observed in QSH state rather than in multi-helicity (MH) state.

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The rock mass properties such as deformability and strength are highly dependent on the existence of cracks. The crack propagation directly influences the decrease of effective Young's modulus and rock mass failure, and the analysis of crack propagation is indispensable for the evaluation of mass properties.In this study, crack propagation analysis was conducted by applying SC-DDM (stress compensation-displacement discontinuity method) to the two-dimensional multi-crack model. Since it is difficult to consider mechanical interaction between all cracks which has arbitrary arrangements, the homogenization method was introduced in the analysis. The model, crack homogenization model, consists of infinite numbers of unit cells and each unit cell has the same crack distribution. Consequently, all cracks behave the same and the mechanical properties of the entire model can be estimated from the analysis of representative unit cell. Here, the alternative model of a single crack set is assumed. The procedure of crack propagation analysis was introduced and the optimization method of crack propagation angle was discussed. In this study, the accuracy of the analyses was verified through the evaluation crack propagation angle and crack profiles, then the characteristics of the crack propagation analysis was discussed. It was proved that the results have enough accuracy and that this method is suitable for the crack propagation analysis. It was also found that this model has a peak strength during crack propagation process, and the relation between loading angle and crack propagation stress was discussed. KEY WORDS : Crack Propagation, Multi-Crack Set, Rock, SC-DDM, Fracture Mechanics

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