VOLUME 85, NUMBER 8 PHYSICAL REVIEW LETTERS 21 AUGUST 2000 Quasi-Single-Helicity Reversed-Field-Pinch Plasmas D. F. Escande, 1,2 P. Martin, 1,3 S. Ortolani, 1,4 A. Buffa, 1,5 P. Franz, 1,5 L. Marrelli, 1,4 E. Martines, 1,4 G. Spizzo, 1,5 S. Cappello, 1,4 A. Murari, 1,4 R. Pasqualotto, 1,4 and P. Zanca 1 1 Consorzio RFX-Corso Stati Uniti, 4, 35127 Padova, Italy 2 CNRS-Université de Provence, Marseille, France 3 Istituto Nazionale Fisica della Materia and University of Padova, Physics Department, Padova, Italy 4 Istituto Gas Ionizzati del CNR, Padova, Italy 5 University of Padova, Electrical Engineering Department-INFM, Padova, Italy (Received 15 November 1999) The reversed field pinch (RFP) is a configuration for plasma magnetic confinement. It has been tradi- tionally viewed as dominated by a bath of MHD instabilities producing magnetic chaos and high energy transport. We reportexperimental results which go beyond this view. They show a decrease of magnetic chaos and the formation of a coherent helical structure in the plasma, whose imaging and temperature profile are provided for the first time. These quasi-single-helicity states are observed both transiently and in stationary conditions. The last case is consistent with a theoretically predicted bifurcation. Our results set a new frame for improving confinement in high current nonchaotic RFP’s. PACS numbers: 52.55.Hc, 05.45. – a, 52.30. – q The magnetic confinement of toroidal plasmas for ther- monuclear fusion is presently sought in several types of devices. Among these the largest are the stellarator, the tokamak, and the reversed-field pinch (RFP). This order corresponds to systems with increasing self-organization. While in the stellarator the highly nonaxisymmetric con- figuration can be entirely produced by external windings, in both the tokamak and the RFP the poloidal magnetic field is due to a toroidal current. However, in contrast to the tokamak where the toroidal field is essentially produced by external coils, in the RFP this component is also mainly self-generated by the plasma and has an amplitude similar to the poloidal one [1]. This feature makes the plasma ring prone to magnetohydrodynamic ( MHD) instabilities which break the toroidal symmetry of the magnetic field. These instabilities, which can be characterized by poloidal and toroidal mode numbers m and n, are the drive, through a solenoidal effect, of the reversed toroidal field regeneration mechanism. The ratio mn gives the pitch or helicity of a corresponding perturbation. Until now most of the experi- mentally studied RFP configurations showed the presence of an MHD turbulence: there is a wide time-fluctuating spectrum of m  0 and m  1 modes in nonlinear inter- action. Such a spectrum also induces chaos in magnetic field lines, hereafter referred to as magnetic chaos. This weakens energy confinement and also produces, through mode phase locking [2], a localized bulging of the plasma with strong plasma-wall interactions. This plasma state is dubbed the multiple helicity (MH) state. On the basis of a wide experimental database, this magnetic topology was considered to be intrinsic to the configuration, although some edge magnetic measurements of temporary transi- tions to a narrow m  1 spectrum have been reported in several RFPs [3–7]. However, from a theoretical point of view [8–11] a single m  1 helicity can be sufficient to drive the poloidal current necessary for the plasma to gen- erate its toroidal magnetic field, and there is a bifurcation to a single pitch state, which provides good magnetic sur- faces and a less localized plasma-wall interaction. In this Letter we report the first experimental results which, by means of a direct tomographic imaging of the plasma and of highly spatially resolved temperature measurements, show the existence of quasi-single-helicity (QSH) states, where a single (m  1, n  n 0 ) mode is dominating the n spectrum. We show that in these states a coherent helical structure appears in the plasma core. With direct temperature measurements we observe that this helical structure is hotter than the plasma nearby. It acts as an insulating barrier and it has better confinement properties. Finally we report the first experimental results where the QSH state is consistent with a bifurcation: it exists during the whole duration of the plasma. The results presented in this Letter open a path beyond the standard paradigm that a bath of magnetic turbulence is intrinsic to the RFP configuration. The experiments have been performed in RFX [12], an RFP with minor and major radii, respectively, equal to a  0.46 m and R  2m. QSH states are obtained either in transient or in stationary conditions. In the first case they last for a few ms, whereas in the second case they last for the entire pulse length. A key measurement is the tomo- graphic imaging of the plasma soft x-ray (SXR) radiation, which was first used for an RFP in the ZT-40M device [13]. SXR isoemissive surfaces can, in fact, be considered representative of magnetic surfaces in the magnetohydro- dynamic framework. Figure 1 shows the n spectrum of the m  1 magnetic modes, measured with pickup coils located at the plasma edge, taken in a MH [Fig. 1(a)] and in a QSH [Fig. 1(c)] plasma with similar global parameters and the corresponding images of the SXR emissivity. The emissivity is poloidally symmetric in the MH state [Fig. 1(b)], where it has been shown that 1662 0031-9007 00  85(8)  1662(4)$15.00 © 2000 The American Physical Society