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...

The frequency spectral broadening of lower hybrid (LH) waves injected into tokamak plasmas is extensively analysed with reference mostly to experimental data from the ASDEX tokamak. The link between the magnitude of the pump spectral width and the degradation of the LH current drive efficiency (up to a factor of 2), pointed out in previous works, is explained. The experimental behaviour of LH power absorption is also well reproduced, even in situations when the access of the launched LH waves to the core plasma should be largely forbidden. Experiments are described that are aimed at determining whether parametric decay instabilities (PDIs) or scattering of LH waves by density fluctuations in the plasma edge are causes of the broadening of the LH pump frequency spectrum. Fluctuations emerge as the largely dominant process, while no signature of PDI processes is observed. Careful measurements of the density fluctuations in the ASDEX scrape-off layer plasma allow the analytical description given by Andrews and Perkins (1983) to be assumed as the appropriate model for LH scattering. Indeed, it supplies the correct magnitude for the frequency spectral width of the LH pump, and explains quantitatively, together with a ray tracing code, why the CD efficiency decreases with the broadening of the pump spectrum. It can also account for the observed LH power absorption coefficient

This paper presents results obtained on the Reversed Field eXperiment, RFX ͓G. Rostagni, Fusion Engineering and Design 25, 301 ͑1995͔͒, on the study of local transport properties of the RFP configuration both in the standard regime and during improved confinement phases obtained with poloidal current drive. The plasma core is dominated by magnetic turbulence, as indicated by both energy and particle transport analysis. Diffusivity decreases toward the edge and is reduced in the core at high Lundquist number. The edge region is apparently not affected by dynamo mode turbulence and determines global confinement in standard operation. Particle and heat diffusivity in this region show a favorable dependence with increasing density. The most interesting feature of the edge region of RFX consists of a sheared radial electric field with shearing rate comparable to that observed in tokamaks. Indications of turbulence reduction in the shear layer have been obtained. Enhanced energy confinement has been produced by pulsed poloidal current drive ͑PPCD͒. A clear reduction of the dynamo activity has been observed with a decrease of core thermal diffusivity to level comparable to the edge and global confinement improvement up to a factor of 3.

In this work we describe the heating results in the LH frequency range (f = 2.45 GHz, Pw 5 250 kW, P < 6 k_W/cmz) both in the electron and ion regimes. Efficient electron heating was observes f o r n 5 5 ~1 0 ' ~ c w 3 . Increases of the peak electron temperature up to 700 eV were measured. At higher densities the interaction with electrons disappears. In the ion regime fast neutral tails and neutron enhancement were observed. The influence of plasma boundary conditions on the penetration of the wave is demonstrated. The principal physical problems are pointed out and some possible conclusions are given.

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