Overview of Quasi Single Helicity experiments in reversed field pinches

We compare Quasi-Single Helicity (QSH) states obtained in RFX with those observed in other three RFPs: EXTRAP T2R at Royal Institute of Technology, Stockholm, TPE-RX at AIST, Tsukuba, MST at the University of Wisconsin-Madison. QSH is the closest experimental approximation of the theoretical SH state, where the RFP dynamo is driven by an individual helical m=1 perturbation. The SH state would naturally be resilient to the magnetic chaos implicit in the standard multi-mode dynamo. Experimental QSH differs from SH because of the quality of magnetic spectra of m=1 modes. While one m=1 mode dominates the QSH spectra, the other modes still have non-zero amplitude. We analyse QSH observed in the four devices to study it in a variety of regimes and to get information on the experimental parameters and operating conditions that facilitate the transition to QSH spectra and their optimisation. The results show that the QSH regime can be considered as a general feature of the RFP. Several parameters influence the probability of obtaining QSH spectra and their quality. Experiments agree on the importance of the mode spectrum purity for the exploitation of QSH potential in terms of confinement. (author)