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A method to estimate the plasma boundary and global parameters such as βp+li/2 and the edge safety factor q95 is described. The method is based on poloidal flux extrapolation in the vacuum region between the plasma and the magnetic measurements, and it is efficient and accurate even if a limited set of sensors is used. The discrepancy between the plasma boundary provided by this method and the boundary computed by the Grad-Shafranov solver MAXFEA is lower than 8 mm in all the considered cases. Moreover, the method is robust against the noise level present in the RFX-mod measurements. The difference between the estimated global parameters and the MAXFEA simulation results is lower than 4%. The method was finally implemented in the RFX-mod shape control system, working at 5 kHz cycle frequency, to provide a reliable set of plasma-wall distances (gaps) used as feedback signals. Experimental results obtained in one year of RFX-mod operation are shown.

This paper describes a novel and simple concept of plasma source, which is able to produce a radiofrequency magnetized discharge with minimal power requirements. The source is based on the magnetron concept and uses a permanent magnet as an active electrode. The dipolar field produced by the magnet confines the electrons, which cause further ionization, thus producing a toroidally shaped plasma in the equatorial region around the electrode. A plasma can be ignited with such scheme with power levels as low as 5W. Paschen curves have been built for four different working gases, showing that in Helium or Neon, plasma breakdown is easily obtained also at atmospheric pressure. The plasma properties have been measured using a balanced Langmuir probe, showing that the electron temperature is around 3–4 eV and higher in the cathode proximity. Plasma densities of the order of 10^{16} m^{-3} have been obtained, with a good positive scaling with applied power. Overall, the electron pressure appears to be strongly correlated with the magnetic field magnitude in the measurement point.

The reversed field pinch (RFP) is a magnetic configuration alternative to the tokamak that can be considered for a second generation of reactors. In this paper new remarkable results obtained in the RFP experiment RFX-mod are presented, showing that an internal transport barrier delimitates a large fraction of the plasma volume in a RFP when the current is raised to ∼1.5 MA. The formation of this transport barrier is related to a profound, spontaneous modification of the magnetic topology. Due to the occurrence of a saddle node bifurcation the plasma enters in the single helical axis state, which is theoretically known to be more resilient to chaos. This bifurcation is driven by the amplitude of the helical perturbation which dominates the mode spectrum. © 2009 American Institute of Physics.

A series of issues with toroidally confined fusion plasmas are related to the generation of 3D flow patterns by means of edge magnetic islands, embedded in a chaotic field and interacting with the wall. These issues include the Greenwald limit in Tokamaks and reversed-field pinches, the collisionality window for ELM mitigation with the resonant magnetic perturbations (RMPs) in Tokamaks, and edge islands interacting with the bootstrap current in stellarators. Measurements of the 2D map of the edge electric field E-r (r = a, theta, phi) in the RFX reversed-field pinch show that E-r has the same helicity of the magnetic islands generated by a m/n perturbation: in fact, defining the helical angle u = m theta - n phi + omega t, maps show a sinusoidal dependence as a function of u, E-r = (E) over tilde (r) sin u. The associated E x B flow displays a huge convective cell with v(a) not equal 0 which, in RFX and near the Greenwald limit, determines a stagnation point for density and a reversal of the sign of E-r. From a theoretical point of view, the question is how a perturbed toroidal flux of symmetry m/n gives rise to an ambipolar potential Phi = (Phi) over tilde sin u. On the basis of a model developed with the guiding center code ORBIT and applied to RFX and the TEXTOR tokamak, we will show that the presence of an m/n perturbation in any kind of device breaks the toroidal symmetry with a drift proportional to the gyroradius rho, thus larger for ions (rho(i) >> rho(e)). Immediately, an ambipolar potential arises to balance the drifts, with the same symmetry as the original perturbation.

An investigation is presented of the edge turbulence in the National Spherical Torus Experiment [M. Ono, M. G. Bell, R. E. Bell et al., Plasma Phys. Control. Fusion, 45, A335 (2003)] based on the optical gas puff imaging (GPI) diagnostic. First of all, the edge fluctuations are characterized for the low confinement mode (L-mode) discharges as a function of the radius. The probability distribution function of the fluctuations is shown to be non-Gaussian for all the radial positions studied, but the deviation from the normal distribution is greater outside the separatrix; in this region the area occupied by the edge structures ("blobs") is greater than inside the separatrix, and this is correlated with the decrease of the logarithmic radial derivative of the pressure gradient. Then the difference between the L- mode and the high confinement mode (H-mode) is studied. With the continuous wavelet transform technique the intense bursts in the GPI signal can be detected; their number decreases in the H-mode with respect to the L-mode, with no significant change in the poloidal velocities. The difference between the two regimes is also observed in the poloidal wavenumber spectra: L-mode and H-mode have two different injection scales for the energy, and different cascades take place. Only in the L- mode the energy flows toward the small wavenumber feeding the bigger blobs. (C) 2007 American Institute of Physics.