Sito personale di Emilio Martines

Highlights - Ten selected publications

This paper was published on the August 2009 issue of Nature Physics, which also featured it on the cover. It marks a reappraisal of the RFP configuration as the basis for a fusion reactor. In the article we show, through a reconstruction of the helical equilibrium, obtained superposing the dominant mode eigenfunction to an axisymmetric equilibrium, that the temperature and density measurements obtained in SHAx states are actually function of the magnetic surface label only. This demonstrates the existence, at least partial, of the surfaces themselves, which is an indication that SHAx states are not dominated by magnetic chaos any more. Furthermore a clear trend of reduction of the amplitude of the secondary modes, responsible for the residual chaos, as the plasma current is increased is shown. Also the SHAx state duration is shown to increase as the plasma current is raised. These observations allow to conclude that future, high current RFP experiments could operate in stationary SHAx states, with confinement levels similar to the tokamak ones.

  • E. Martines, N. Vianello, D. Sundkvist, M. Spolaore, M. Zuin, M. Agostini, V. Antoni, R. Cavazzana, C. Ionita, M. Maraschek, F. Mehlmann, H. W. Müller, V. Naulin, J. J. Rasmussen, V. Rohde, P. Scarin, R. Schrittwieser, G. Serianni, E. Spada, the RFX-mod team and the ASDEX Upgrade team, Current filaments in turbulent magnetized plasmas, Plasma Physics and Controlled Fusion 51, 124053 (2009).

Review of different observations of current filaments in magnetized plasma turbulence. In particular, emphasis is placed on the presence of coherent structures, identified as "drift-Alfvén vortices", both in the outer region of a plasma confined in RFP configuration and in the Earth magnetosphere. A similar comparison is made for current sheets formed in reconnection events, measured both in RFP plasmas and in the magnetosheath. Finally, direct evidence of the presence of current filaments in the Edge Localized Modes (ELMs) observed in tokamaks operating in H-mode is shown. This paper follows the invited talk given at the 36th EPS Conference on Plasma Physics, held in Sofia (Bulgaria) from 29th June to 3rd July 2009.

Description of a low power, atmospheric pressure plasma source, specifically designed for the treatment of living tissues. In the paper we show its ability to kill different kinds of bacteria with decimal reduction times of the order of one minute, without appreciable damage to the cell which compose the substrate. The plasma source has been the object of an international patent request.

First experimental observation of Single Helical Axis (SHAx) states in a RFP plasma. These states, which had already been theoretically predicted, are a subclass of Quasi Single Helicity (QSH) states, which occurs when the dominant mode amplitude exceeds a threshold, and the main magnetic axis collapses onto the dominant mode magnetic island X-point, giving rise to a plasma with a single helical magnetic axis. Experimentally the transition to these states has been detected observing the behaviour of the electron temperature profile measued along a diameter of the vacuum chamber cross-section. A clear transition has been observed from a profile with a localized hot structure, corresponding to the magnetic island, to one with a large hot region, extending on the two sides of the chamber geometric axis.

In this paper we describe a technique for suppression of the m=1/n=1 kink mode which was previously shown to be the cause of the efficiency loss at high power in magneto-plasma-dynamic (MPD) thrusters. The suppression is obtained through the insertion in the inter-electrode region of a dielectric plate, which interrupts the helical currents of the kink mode. The result is a complete suppression of the mode, and a recovery of the thruster efficiency, and this opens new possibilities for the use of this kind of thruster in real missions. The technique has been also patented.

This paper gives an explanation for the efficiency loss observed at high power in magnetoi-plasma-dynamic (MPD) thrusters for space applications. It is shown that the efficiency loss is associated to the onset of a MHD instability corresponding to the m=1/n=1 kink mode observed in toroidal magnetic confinement experiments, which occurs when the Kruskal-Shafranov criterion is violated.

Studying the statistical proprerties of the turbulence measured in the outer region of the RFX plasma, it is shown that the Self Organized Criticality (SOC) paradigm is not appropriate for explaining turbulent transport in fusion plasmas.

In this paper it is shown, through the use of Fokker-Planck simulations which reconstruct the shape of the electron velocity distribution function, that the superthermal electron tail observed in the outer region of RFP plasmas can be explained through the effect of the dynamo electric field resulting from the combination of velocity and magnetic field perturbations due to MHD modes present into the plasma. It is therefore not necessary to assume additional mechanisms, such as a radial transport of energetic electrons from the center to the edge due to magnetic chaos.

It is shown that the turbulence measured in the outer plasma region of the RFX experiment is not self-similar, but rather displays a probability distribution function with stronger and stronger tails when moving from large to small scales. This is a manifestation of intermittency, according to the definition commonly used in the study of neutral fluid turbulence.

In this work the radial plasma potential profiles, and therefore radial electric field profiles, were measured for the first time in the outer region of a plasma confined in RFP configuration. It is shown that the radial electric field gives rise to a double shear layer of toroidal velocity.