In the quest for new energy sources, the research on controlled thermonuclear fusion 1 has been boosted by the start of the construction phase of the International Thermonuclear Experimental Reactor (ITER). ITER is based on the tokamak magnetic configuration 3, which is the best performing one in terms of energy confinement. Alternative concepts are however actively researched, which in the long term could be considered for a second generation of reactors. Here, we show results concerning one of these configurations, the reversed-field pinch 4,5 (RFP). By increasing the plasma current, a spontaneous transition to a helical equilibrium occurs, with a change of magnetic topology. Partially conserved magnetic flux surfaces emerge within residual magnetic chaos, resulting in the onset of a transport barrier. This is a structural change and sheds new light on the potential of the RFP as the basis for a low-magnetic-field ohmic fusion reactor.The main magnetic field configurations studied for the confinement of toroidal fusion-relevant plasmas are the tokamak 3 , the stellarator 6 and the reversed-field pinch 4,5 (RFP). In the tokamak, a strong magnetic field is produced in the toroidal direction by a set of coils approximating a toroidal solenoid, and the poloidal field generated by a toroidal current flowing into the plasma gives the field lines a weak helical twist. This is the configuration that has been most studied and has achieved the best levels of energy confinement time. Thus, it is the natural choice for the International Thermonuclear Experimental Reactor, which has the mission of demonstrating the scientific and technical feasibility of controlled fusion with magnetic confinement.The RFP, like the tokamak, is axisymmetric and exploits the pinch effect due to a current flowing in a plasma embedded in a toroidal magnetic field. The main difference is that, for a given plasma current, the toroidal magnetic field in a RFP is one order of magnitude smaller than in a tokamak, and is mainly generated by currents flowing in the plasma itself. This feature is underlying the main potential advantage of the RFP as a reactor concept, namely the capability of achieving fusion conditions with ohmic heating only in a much simpler and compact device. In the past, this positive feature was overcome by the poorer stability properties, which led to the growth and saturation of several magnetohydrodynamic (MHD) instabilities, eventually downgrading the confinement performance. These instabilities, represented by Fourier modes in the poloidal and toroidal angles θ and φ as exp [i(mθ − nφ) were considered as an unavoidable ingredient of the dynamo self-organization process 4,8,9 , necessary for the sustainment of the configuration in time. The occurrence of several MHD modes resonating on different plasma layers gives rise to overlapping magnetic islands, which result in a chaotic region, extending over most of the plasma volume 10 , where the magnetic surfaces are destroyed and the confinement level is modest. This conditi...

Recent experiments on the Type I ELMy H-mode regime performed at JET with improved diagnostics have expanded the range of parameters for the study of Type I ELM energy and particle losses. Deviations from the standard behaviour of such losses in some areas of the Type I ELMy H-mode operating space have revealed that the ELM losses are correlated with the parameters (density and temperature) of the pedestal plasma before the ELM crash, while other global ELM characteristics (such as ELM frequency) are a consequence of the ELMdriven energy and particle flux and of the in-between ELM energy and particle confinement. The relative Type I ELM plasma energy loss (to the pedestal energy) is found to correlate well with the collisionality of the pedestal plasma, showing a weak dependence on the method used to achieve those pedestal plasma parameters: plasma shaping, heating, pellet injection and impurity seeding. Effects of edge plasma collisionality and transport along the magnetic field on the Type I ELM particle and energy fluxes onto the divertor target have also been observed. Two possible physical mechanisms that may give rise to the observed collisionality dependence of ELM energy losses are proposed and their consistency with the experimental measurements investigated: collisionality dependence of the edge bootstrap current with its associated influence on the ELM MHD origin and the limitation of the ELM energy loss by the impedance of the divertor target sheath to energy flow during the ELM event.

Divertor detachment is explored on the TCV tokamak in alternative magnetic geometries. Starting from typical TCV single-null shapes, the poloidal flux expansion at the outer strikepoint is varied by a factor of 10 to investigate the X-divertor characteristics, and the total flux expansion is varied by 70% to study the properties of the super-X divertor. The effect of an additional X-point near the target is investigated in X-point target divertors. Detachment of the outer target is studied in these plasmas during Ohmic density ramps and with the ion ∇B drift away from the primary X-point. The detachment threshold, depth of detachment, and the stability of the radiation location are investigated using target measurements from the wall-embedded Langmuir probes and two-dimensional CIII line emissivity profiles across the divertor region, obtained from inverted, toroidally-integrated camera data. It is found that increasing poloidal flux expansion results in a deeper detachment for a given line-averaged density and a reduction in the radiation location sensitivity to core density, while no large effect on the detachment threshold is observed. The total flux expansion, contrary to expectations, does not show a significant influence on any detachment characteristics in these experiments. In X-point target geometries, no evidence is found for a reduced detachment threshold despite a Nuclear Fusion Results from recent detachment experiments in alternative divertor configurations on TCVInternational Atomic Energy Agency a See the author list of 'Overview of progress in European Medium Sized Tokamaks towards an integrated plasma-edge/wall solution' by H. Meyer et al, to be published in the Nuclear Fusion

The transition to a new magnetic topology, characterized by a quasi-single-helicity state with a single helical magnetic axis has been experimentally observed for the first time in a reversed-field-pinch plasma. The occurrence of the new state, which has been dubbed a single-helical-axis state, was found to provide magnetic chaos healing and enhanced thermal content of the plasma. The helical structure extends on both sides of the vessel geometric axis, and is related to exceeding a threshold in the ratio between the amplitude of the dominant MHD mode and the amplitude of the secondary ones.

We describe plasma profiles evolution during pulsed poloidal current drive experiments performed with the RFX reversed field pinch device. With external drive of edge poloidal current, magnetic fluctuations are reduced suggesting a concomitant reduction of the spontaneous dynamo action. The electron temperature profile is seen to peak in the plasma core, consistently with a reduction of the heat conductivity due to a substantial decrease of MHD dynamo fluctuations. Our results also indicate that the magnetic turbulence due to these fluctuations, which dominates heat transport in the core of the reverse field pinch configuration, does not drive an appreciable heat flux at the edge.[S0031-9007(98)08371-9] PACS numbers: 52.55. Hc, 52.25.Fi, 52.25.Gj, 52.35.Py A growing body of experimental and theoretical work is indicating that in the reverse field pinch (RFP) configuration [1] for magnetic confinement of a thermonuclear plasma a large fraction of the energy losses is due to transport mechanisms linked to MHD turbulence. In fact, magnetic fluctuations are intrinsic to the RFP, since the configuration is usually maintained by a strong dynamo mechanism, which converts part of the energy externally supplied by the toroidal transformer to the poloidal magnetic field component B u into energy associated with the toroidal component B f . This process regenerates toroidal magnetic flux lost by resistive diffusion.Many of the mechanisms (see [2], and references therein, and [3,4]) proposed to explain the dynamo are based on the nonlinear interaction of global, low-m resistive MHD modes resonant inside the toroidal field reversal surface. According to the most developed theory, the MHD dynamo (see [2] for a review), they produce coherent magnetic fieldb and velocityỹ fluctuations which combine to generate an electromotive electric field, E d ͗ỹ 3b͘. This dynamo electric field is found in three-dimensional numerical simulations [5-7] and has been directly measured [8]. Magnetic fluctuations in the RFP are then intimately connected to the dynamo, and in standard conditions they are somewhat unavoidable. This leads to the stochasticization of the magnetic field lines over a large part of the core plasma and, as a consequence, thermal isolation is only provided by a thin layer located at the plasma edge. The mechanism controlling the edge energy transport in a RFP is still a matter of debate, as there are not yet definite conclusions on whether electrostatic or magnetic turbulence is mostly responsible for it. Recent measurements [9][10][11] have shown the existence of a strongly sheared plasma flow in the edge region of a RFP discharge, suggesting a quenching of turbulent transport with a mechanism similar to the one identified in tokamaks [12].Reducing the impact of magnetic turbulence on transport has become one of the major challenges of RFP research. In particular, external poloidal current drive has been proposed as a tool for improving confinement properties of the RFP configuration by alleviating the need of the dyna...