M. Zuin

Magnetic order and confinement improvement in high-current regimes of RFX-mod with MHD feedback control

The RFX-mod machine (Sonato et al 2003 Fusion Eng. Des. 66 161) recently achieved, for the first time in a reversed-field pinch, high plasma current up to 1.6 MA with good confinement. Magnetic feedback control of magnetohydrodynamic instabilities was essential to reach the goal. As the current is raised, the plasma spontaneously accesses a new helical state, starting from turbulent multi-helical conditions. Together with this raise, the ratio between the dominant and the secondary mode amplitudes increases in a continuous way. This brings a significant improvement in the magnetic field topology, with the formation of helical flux surfaces in the core. As a consequence, strong helical transport barriers with maximum electron temperature around 1 keV develop in this region. The energy confinement time increases by a factor of 4 with respect to the lower-current, multi-helical conditions. The properties of the new helical state scale favourably with the current, thus opening promising perspectives for the higher current experiments planned for the near future.

Topology and transport in the edge region of RFX-mod helical regimes

New edge diagnostics and detailed analysis of magnetic topology have significantly improved the comprehension of the processes developing at the boundary of a reversed-field pinch (RFP) plasma in RFX-mod (a = 0.46 m, R = 2 m).An upper critical density nC ≈ 0.4 nG (nG Greenwald density) is found to limit the operational space for the improved quasi-single helical (QSH) regime: magnetic topology reconstructions and diagnostic observations suggest that this limit is due to a helical plasma–wall interaction which determines toroidally and poloidally localized edge density accumulation and cooling.The experimental evidence is provided by a variety of diagnostics: the magnetic boundary as reconstructed from equilibrium codes reveals a helical deformation, which is well correlated with the modulation of edge pressure profile as reconstructed from the thermal helium beam diagnostic. Correlations with the helical deformation are also observed on the space- and time-resolved patterns of the floating potential measured at the wall, and with the edge plasma flow, obtained from different diagnostics. The relevance of these findings is that understanding the mechanisms that limit the operational space of QSH is decisive in achieving the goal of high-density stationary helical RFP equilibrium.

High current regimes in RFX-mod

Optimization of machine operation, including plasma position control, density control and especially feedback control on multiple magnetohydrodynamic modes, has led RFX-mod to operate reliably at 1.5?MA, the highest current ever achieved on a reversed field pinch (RFP). At high current and low density the magnetic topology spontaneously self-organizes in an Ohmical helical symmetry, with the new magnetic axis helically twisting around the geometrical axis of the torus. The separatrix of the island disappears leaving a wide and symmetric thermal structure with large gradients in the electron temperature profile. The new topology still displays an intermittent nature but its overall presence has reached 85% of the current flat-top period. The large gradients in the electron temperature profile appear to be marginal for the destabilization of ion temperature gradient modes on the assumption that ions and electrons have the same gradients. There are indications that higher currents could provide the conditions under which to prove the existence of a true helical equilibrium as the standard RFP configuration.

3D effects on the RFX-mod boundary

In present fusion research a strong effort is devoted to the studies of non-axisymmetric magnetic perturbations and consequent field ergodization on the external region of the plasma. On this topic interesting results can be drawn from the helical configuration observed in high-current regimes in reversed field pinches (RFPs) where the small edge helical ripple is sufficient to modulate the plasma?wall interaction and the plasma kinetic properties. This paper presents the most recent experimental results and physical interpretation of the phenomena observed in the edge region of the RFX-mod RFP device. Experimental observations indicate that plasma pressure and floating potential are spatially modulated according to the helical deformation. Helical flow is observed at the edge as a consequence of an ambipolar electric field. Emphasis will be devoted to the determination of the actual phase relation between magnetic perturbation and velocity perturbation. Evidence of the influence of the helical ripple on turbulence properties at the edge is also reported.

Analysis and modelling of the magnetic and plasma profiles during PPCD experiments in RFX

In this paper, we analyse the main features of the pulsed poloidal current drive (PPCD) technique, used in the reversed field pinch configuration to achieve improved confinement conditions. In the RFX experiment, PPCD corresponds to a decrease of the magnetic fluctuations, to a peaking of the temperature profile, and to a reduced transport and plasma–wall interaction. A three-dimensional MHD nonlinear code and one-dimensional time-dependent transport models have been applied to study the effect of PPCD on the magnetic and plasma profiles. The three-dimensional MHD simulations show that the external inductive drive pinches and peaks the current profile driving the configuration through a transient phase, where the spontaneous turbulent dynamo action is quenched. The one-dimensional transport codes indicate that the experimental profile modifications associated with PPCD are consistent with a reduction of the stochastic transport.

RFX-mod: A multi-configuration fusion facility for three-dimensional physics studiesa)

RFX-mod [Sonato et al., Fusion Eng. Des. 66, 161 (2003)] exploits its 192 active coils in both reversed-field pinch (RFP) and tokamak configurations with varying degrees of 3D shaping, providing also a test bed for validating stellarator codes and 3D nonlinear magnetohydrodynamic codes. This makes RFX-mod a unique and flexible facility for comparative studies on 3D shaping and control. The paper discusses how 3D fields allow access to RFP and tokamak advanced regimes. 3D fields are used to feedback control Single Helicity (SH) RFP equilibria with 1/7 helicity up to ∼2 MA. They also allow accessing SH regimes with higher density (Greenwald fraction up to 0.5), presently inaccessible in spontaneous SH regimes. Feedback on the 2/1 resistive-wall mode in RFX-mod tokamak plasmas allows for safe operation at q(a)<2, an almost unexplored promising regime. Forcing the 2/1 mode to saturate at finite but small level, a helical tokamak equilibrium with significant n = 1 modulation is produced and a new way to tailor...

Drift-Alfvén vortex structures in the edge region of a fusion relevant plasma

Edge turbulent structures are commonly observed in fusion devices and are generally believed to be responsible for confinement degradation. Among their origin drift-Alfven turbulence is one of the most commonly suggested. The drift-Alfven paradigm allows the existence of localized vortex-like structures observed also in various systems. Here we present the evidence of the presence of drift-Alfven vortices in the edge region of RFX-mod reversed field pinch device, showing how these structures are responsible for electromagnetic turbulence at the edge and its intermittent nature.

Electrostatic properties and active magnetic topology modification in the RFX-mod edge plasma

A better understanding of the edge phenomena regulating plasma transport and turbulence is of primary importance for the whole fusion community. In particular, in reversed field pinches (RFPs) the edge properties are found to have a strong relation with the magnetic topology. The flexibility of the RFX-mod RFP experiment (Sonato et al 2003 Fusion Eng. Des. 66–68 161–8) enables exploring different topological regimes, and the insertion of a probe equipped with a two-dimensional array of electrostatic sensors allows the characterization of the outermost plasma regions (up to 10% of the minor radius). The electric field radial profiles and its dynamics are found to be highly influenced by the presence of edge magnetic islands modelled with a Field Line Tracing code (FLiT). When a helical perturbation is applied through the saddle coil feedback system a strong modification of the edge electrostatic properties is observed. The E × B velocity field results in a convective cell-like structure according to the externally imposed toroidal periodicity. Finally, an analysis of the long-range correlations in the floating potential fluctuations (measured by two toroidally separated probes) is presented and discussed along with the electric field measurements and the edge topology.

Current sheets during spontaneous reconnection in a current-carrying fusion plasma

An analysis of plasma dynamics during impulsive magnetic reconnection events in the RFX-mod reversed field pinch (RFP) is performed by means of a large set of magnetic in-vessel coils and of an insertable edge probe, equipped with a matrix of electrostatic (Langmuir) and magnetic probes. It is observed that reconnection of field lines, which leads to a global reconfiguration of the magnetic topology (relaxation), is associated with the rapid formation of a strongly localized magnetic perturbation characterized by a main m = 0 periodicity, due to enhanced dynamo modes activity. Soon after its formation, the m = 0 perturbation is observed to move in the toroidal direction and is shown to correspond to a poloidal current sheet, whose existence was predicted by three-dimensional MHD numerical simulations on RFP sustainment through magnetic reconnection processes. A reconstruction of the current density structure associated with the rotating magnetic perturbation is performed by means of the insertable probe, along with an investigation of the large induced modification of electron temperature, density and plasma velocity shear at the edge.

MHD instabilities in magneto-plasma-dynamic thrusters

Magneto-plasma-dynamic thrusters (MPDTs) act as electromagnetic plasma accelerators and represent a high power, electric propulsion option for primary space missions. One of the major problems facing MPDT operation is the onset of a critical regime, which is found when the power is increased beyond a threshold value, depending mainly on the thruster geometry, the type and mass flow rate of the propellant and the intensity of the magnetic field applied. In this regime, large fluctuations in the electrode voltage signals, damage to the anode and decreased efficiency are observed. Since 2000, several test campaigns have been carried out to investigate the electrostatic and magnetic properties of plasma fluctuations using electromagnetic and optic probes and ultraviolet tomography. Results obtained have shown a strong relation between the onset phenomena and the growth of a large scale magnetohydrodynamic (MHD) instability, with helical kink mode features. On the basis of the experimental observations, a passive method to suppress instability is proposed and has been partially tested, with encouraging results.