L. Carraro

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 density physics in reversed field pinches: comparison with tokamaks and stellarators

Reversed field pinches (RFPs) share with tokamaks and stellarators the experimental evidence of an upper limit for the maximum value of the electron density at which they can operate. Above a certain density level, well described by the Greenwald law for tokamaks and RFPs, a radiative collapse with strong plasma cooling is observed, predominantly due to processes occurring at the plasma boundary. In the RFX-mod RFP close to the density limit a radiating belt, poloidally symmetric and toroidally localized, develops in the region where the plasma is shrunk as an effect of the m = 0 tearing modes. The phenomenology recalls that of MARFES or plasma detachment, though, unlike tokamaks, the appearance of the radiating belt is associated with a soft landing of the plasma discharge.The paper reports the experimental pattern of the RFX-mod plasmas close to the density limit, including density and radiation profiles, plasma flow and turbulence. Particles are toroidally conveyed towards the region of maximum shrinking of the plasma column where they accumulate. The interpretation is related to the topology of MHD m = 0 and m = 1 modes: the reconstruction of the magnetic topology shows that the highly radiating region corresponds to the presence of peripheral m = 0 magnetic islands well detached from the wall. The emerging indication is that in RFPs a reduction of the m = 0 activity could be a way to overcome the density limit.

Toroidal and poloidal plasma rotation in the reversed field pinch RFX

In this paper, the toroidal and poloidal rotation velocities measured in the reversed field pinch device RFX from the Doppler shift of impurity ion lines are presented and their relation with the plasma velocity is discussed. It is found that the toroidal velocity at the edge has the opposite direction with respect to the core toroidal velocity, indicating a shear of the plasma rotation velocity of about . Starting from the rotation measurements and the momentum equation, a one-dimensional impurity diffusion model for the core plasma and a Monte Carlo code for the edge region are used to estimate the radial electric field, that is found to be around outward directed in the plasma core, and about inward directed at the edge, consistent with that measured at the edge by Langmuir probes.

Locked modes induced plasma-wall interactions in RFX

Abstract In the RFX reversed field pinch ( R = 2 m , a = 0.45 m ) the plasma-wall interaction has been characterised in all of the discharges by the presence of the locking both in phase and in the laboratory frame of unstable MHD modes. These locked modes cause a helical deformation of the magnetic surfaces driving large power fluxes onto the wall. The plasma edge is severely perturbed: a power density loading as high as 100 MW m −2 may locally be exceeded, the surface of the plasma facing components reach the sublimation temperature, the impurity release and the power radiated locally increase by a factor of ∼ 100, large fluctuations in the electron density develop and halo currents flow in the inconel vacuum vessel. It is estimated that the locked modes are responsible for losses that may amount to up to about 30% of the total power input.

Impurity transport studies in RFX-mod multiple helicity and enhanced confinement QSH regimes

The transport of intrinsic and artificially introduced impurities is investigated in the multiple helicity (MH) and quasi-single helicity (QSH) plasma regimes of the RFX-mod reversed-field pinch. Impurities are introduced through Ni laser blow-off and Ne gas puffing in both magnetic confinement scenarios. Simulation of experimental spectroscopic measurements, including line and soft x-ray emission, electron density and radiated power is carried out with a one-dimensional collisional–radiative model. For both QSH and MH an outwardly directed pinch velocity with a large barrier in the edge and no reversal is determined, accompanied by a diffusion coefficient that is an order of magnitude greater in the core than at the edge. The established transport parameters are similar for the Ni and Ne injected impurities with the transition region and barrier less external and stronger in the QSH regime.

High density limit in Reversed Field Pinches

The fusion triple product depends on density, which is therefore a key parameter for the future fusion reactor. In this paper the high density limit is studied in the reversed field experiment (RFX-mod) device in Padova, Italy. A rather complete experimental picture of the high density regimes is provided, showing a series of features, such as, plasma flow inversion in the edge, density accumulation, radiation condensation (poloidally symmetric and toroidal asymmetric) which resemble the MARFE phenomenon characteristic of tokamak discharges. However, in RFX-mod high density does not cause a disruption, as often observed in tokamaks, but a soft landing of the plasma current. According to a new 1D transport/radiative code applied to analyze the high density discharges, the current decay is due to an increased need for dynamo in these highly resistive, edge-cooled discharges. The relation between the radiative pattern of RFX-mod high density plasmas, the magnetic topology, and edge radial electric field is discussed.

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.

Experimental particle transport studies by pellet injection in helical equilibria

Helical equilibria have been experimentally found in the RFX-mod reversed field pinch both as spontaneous and induced states. These states are associated with a dominant helicity in the spectrum of magnetic field fluctuations producing a helical deformation of magnetic surfaces in a large fraction of the plasma core. In particular, they are also characterized by an improvement in global plasma performances in terms of electron temperature and energy confinement time with respect to the standard axisymmetric configuration with multiple helicities in the magnetic spectrum. These observations suggest that one should also expect an improvement in terms of particle confinement although this has never been experimentally observed due to the lack of a particle source in the core of RFX-mod plasmas. To address this point perturbative experiments were done in RFX-mod by injecting pellets (a known particle source) both inside and outside the helical magnetic structure present during such states. These experiments show that plasma density asymmetries and variations in the ablation rate of pellets are correlated with the internal magnetic field structure obtained by means of a line tracing code based on an equilibrium reconstructed in toroidal geometry using external measurements. In particular, they prove that particle transport is significantly reduced in the helical states with respect to axisymmetric configurations. An increase by a factor 2–3 was determined for the global electron particle confinement time calculated with a zero-dimensional model taking into account the helical geometry.

Recent progress in reversed field pinch research in the RFX experiment

The article presents an overview of recent experimental results obtained on the RFX device. The authors obtained and studied a reversed field pinch plasma with a plasma current of up to 1 MA, negligible radiation losses and low effective charge. The local power and particle balance shows that in standard operation the plasma core is dominated by magnetic turbulence and that the global confinement is mainly provided by the edge region, where a strongly sheared radial electric field is present. With poloidal current drive the amplitude of magnetic fluctuations and the thermal conductivity of the plasma core are reduced, leading to improved confinement. Reduced heat transport is also observed when the width of the n spectrum of magnetic fluctuations is reduced.