• Energy Research

Abstract Reconnection events in high current reversed field pinch plasmas are often associated to the partial or total loss of the helical magnetic topology. The electron temperature collapse during these phenomena is investigated in RFX-mod thanks to high time resolution soft-x-ray diagnostics; these data are used, together with magnetic energy reconstructions, for energy balance analysis. The paper shows that the energy released during reconnection events, similarly to astrophysical plasmas, might be involved in ion heating, the latter being estimated by the energy distribution function of neutral atoms, a rather interesting feature in a reactorial perspective. These issues will be further investigated in RFX-mod2 , an upgrade of the present device starting its operations from 2022, where the modified boundary conditions are expected to increase the helical states duration and reduce the frequency of reconnection events.

In this paper, the pressure behavior is characterized for helical states in the Reversed Field eXperiment-mod plasmas; using the helical coordinates as a reference frame to match data coming from different diagnostics, pressure profile variations have been correlated to magnetic topology changes in plasma. The use of helical frame to integrate diagnostics is also exploited as a tool to identify changes in the electronic heat transport, within a single shot or a reduced shots ensemble, confirming via 1-D and 2-D power balance models the improvement in the plasma performances.

The edge of the RFX-mod (R=2m, a=0.46m) Reversed Field Pinch device is characterized by weak magnetic chaos affecting ion and electron diffusion. Edge particle transport is strongly influenced by a toroidal asymmetry caused by magnetic islands and an ambipolar radial electric field ensures local neutrality, in a way similar to the stochastic edge of tokamaks when resonant magnetic perturbations (RMPs) are applied. The H? emission and floating potential Vf measured in different poloidal and toroidal positions shows a helical shape of the Plasma Wall Interaction, fitting the spatial periodicity of the innermost resonant tearing mode (m/n=1/7) [1]. However, detailed measurements, along the poloidal (parallel) direction, of the electron density and temperature with the Thermal Helium Beam, and of the floating potential Vf with electrostatic probes, show that the response of the edge plasma depends on the poloidal angle, in a more complicated way than a pure 1/7 harmonic. In particular, multiple poloidal harmonics can be recognized in the measurements. The results are robust, because data analysis has been performed with different techniques: in terms of correlations between Vf signals and the corresponding local flux-surface displacement, by the conditional average technique applied at Vf signals, and finally also in terms of a travelling helical angle frame as reference of the measurements. The interpretation of the results is not obvious, but it highlights the fact that the correlation between magnetic islands and kinetic properties of the edge plasma is not a simple one-to-one causal relationship, as it is often assumed in RMP studies in tokamaks.

Reconnection events in high current reversed field pinch plasmas are often associated to the partial or total transitions from a helical topology with conserved flux surfaces to a configuration characterized by a chaotic magnetic field. The electron temperature dynamic together with the magnetic energy reconstructions are used to evaluate the energy balance during these events and to quantify the associated dissipated power and released energy. A fraction of the energy released during reconnection events is involved in ion heating, as estimated by the energy distribution function of neutral atoms, a rather interesting feature in a reactorial perspective.

The Frascati Tokamak Upgrade (FTU) device can operate at high electron density regimes of the order of 1020 m- 3, where the MARFE instability is present at various plasma current and magnetic field values. When the MARFE is well developed and oscillating, its movement causes continuous density fluctuation, contaminating the integral density measurements. The amplitude and frequency of these density fluctuations are well revealed by the high resolution interferometer available on FTU, the dependence of the frequency versus basic plasma parameters is investigated in this paper. A specific experimental session on FTU, including some discharges with reversed toroidal magnetic field, and pushing the plasma column towards the internal or external side of the vacuum chamber, respectively, has shown that, when the plasma column is distant from the toroidal limiter, the MARFE is stable and does not oscillate around the mid plane. For these last cases the MARFE localization with respect to the ion drift direction, which can influence the stable and unstable positions, is also discussed.

Reconnection events in high current reversed field pinch plasmas are often associated to the partial or total transitions from a helical topology with conserved flux surfaces to a configuration characterized by a chaotic magnetic field. The electron temperature dynamic together with the magnetic energy reconstructions are used to evaluate the energy balance during these events and to quantify the associated dissipated power and released energy. A fraction of the energy released during reconnection events is involved in ion heating, as estimated by the energy distribution function of neutral atoms, a rather interesting feature in a reactorial perspective.