Quasi-single helicity state by a small positive pulse of toroidal magnetic field in TPE-RX reversed field pinch experiment

By applying a small positive pulse (Delta B-ta) in toroidal magnetic field, the quasi-single helicity (QSH) state can be obtained with a controllable and reproducible manner in a reversed-field pinch (RFP) experiment on the large RFP machine, TPE-RX [Y. Yagi , Fusion Eng. Des. 45, 421 (1999)]. The QSH state in RFP is one of the states where the improved confinement can be observed, and is important for development toward the pure single helicity (SH) state. In the SH state, the dynamo-action for sustaining the RFP configuration will be driven by a single helical mode and its harmonics, and the anomalous plasma loss can be avoided which is caused by the multi-helicity dynamo action in ordinary RFPs. In the operating condition presented here, the reversal of toroidal magnetic field (B-ta) is maintained at a shallow value (similar to-1 mT) for a certain period (similar to 20 ms) after the setting up of the RFP configuration and then the positive Delta B-ta (less than or similar to 5 mT magnitude and similar to 2 ms width) is applied to the B-ta, which is usually negative during the sustaining phase of RFP. Just after applying the pulse, the m/n=1/6 mode (m and n being the poloidal and toroidal Fourier mode numbers, respectively) grows dominantly and the configuration goes into QSH state. This QSH state can be sustained for a long period (up to similar to 45 ms) almost until the end of discharge by applying a delayed reversal of B-ta with appropriate timing and magnitude. The setting up of the QSH states shows a reproducibility of almost 100% with the same timing corresponding to the applied positive pulse. This observation can confirm the interpretation in the former report [Y. Hirano , Phys. Plasmas 12, 112501 (2005)], in which it is claimed that the QSH state is obtained when a small positive pulse in toroidal magnetic field spontaneously appears. (c) 2006 American Institute of Physics.