[show abstract][hide abstract] ABSTRACT: This paper summarizes the main achievements of the RFX fusion science program in the period between the 2008 and 2010 IAEA Fusion Energy Conferences. RFX-mod is the largest reversed field pinch in the world, equipped with a system of 192 coils for active control of MHD stability. The discovery and understanding of helical states with electron internal transport barriers and core electron temperature >1.5 keV significantly advances the perspectives of the configuration. Optimized experiments with plasma current up to 1.8 MA have been realized, confirming positive scaling. The first evidence of edge transport barriers is presented. Progress has been made also in the control of first-wall properties and of density profiles, with initial first-wall lithization experiments. Micro-turbulence mechanisms such as ion temperature gradient and micro-tearing are discussed in the framework of understanding gradient-driven transport in low magnetic chaos helical regimes. Both tearing mode and resistive wall mode active control have been optimized and experimental data have been used to benchmark numerical codes. The RFX programme also provides important results for the fusion community and in particular for tokamaks and stellarators on feedback control of MHD stability and on three-dimensional physics. On the latter topic, the result of the application of stellarator codes to describe three-dimensional reversed field pinch physics will be presented.
[show abstract][hide abstract] ABSTRACT: An interesting result of magnetic chaos reduction in RFX-mod high current discharges is the development of strong electron transport barriers. An internal heat and particle transport barrier is formed when a bifurcation process changes the magnetic configuration into a helical equilibrium and chaos reduction follows, together with the formation of a null in the q shear. Strong temperature gradients develop, corresponding to a decreased thermal and particle transport. Turbulence analysis shows that the large electron temperature gradients are limited by the onset of micro-tearing modes, in addition to residual magnetic chaos. A new type of electron transport barrier with strong temperature gradients develops more externally (r/a = 0.8) accompanied by a 30% improvement of the global confinement time. The mechanism responsible for the formation of such a barrier is still unknown but it is likely associated with a local reduction of magnetic chaos. These external barriers develop primarily in situations of well-conditioned walls so that they might be regarded as attempts towards an L–H transition. Both types of barriers occur in high-current low-collisionality regimes. Analogies with tokamak and stellarators are discussed.
[show abstract][hide abstract] ABSTRACT: The effect of a non-uniform effective ion charge distribution on the dynamo contribution to the plasma resisitivity of a reversed field pinch is studied, based on a stationary energy or helicity balance. The radial distribution of the impurity is obtained from the ETA-BETA II line emission profiles. It is found that the main impurity ions are concentrated in the outer region of the plasma and, consequently, the effective ion charge, Zeff, is close to one everywhere, except in a small slab close to the plasma border, where it can be as high as three to four. The loop voltage required to sustain the RFP configuration with such 'hollow' Zeff profiles has to balance a higher dissipation in the outer region; hence, the apparent resistivity on axis is considerably greater than in the case of uniform Zeff. This effect could also be present in other RFP devices and might very well bridge the gap that was observed previously between the average effective ion charge and the resistivity anomaly factor, particularly for the low plasma density operation.
[show abstract][hide abstract] ABSTRACT: A series of measurements has been carried out on ETA-BETA II at various plasma currents between 100 and 180 kA to study the time evolution of the electron temperature Te and density ne. It is for the first time that such a study has been performed under the quasi-steady conditions of the current flat-top phase. The temperatures were found to rise from below 100 eV at current peak to more than 200 eV towards the end of the discharge with all currents. The increase of Te is closely related with the decrease of ne and hence with the increase of the ratio I/N. Linear scaling of Te with current is found only when the evaluation is performed at constant I/N. The poloidal beta varies little over a wide range of plasma parameters and shows a negative dependence on I/N. The resistance anomaly factor is found to rise from below 3 soon after current peak to as high as 15 in some cases at the end of the discharge. A universal and almost linear dependence of the increase of on I/N is apparent in the range of currents investigated.
[show abstract][hide abstract] ABSTRACT: The full three-dimensional (3D) approach is now becoming an important issue for all magnetic confinement configurations. It is a necessary condition for the stellarator but also the tokamak and the reversed field pinch (RFP) now cannot be completely described in an axisymmetric framework. For the RFP the observation of self-sustained helical configurations with improved plasma performances require a better description in order to assess a new view on this configuration. In this new framework plasma configuration studies for RFX-mod have been considered both with tools developed for the RFP as well as considering codes originally developed for the stellarator and adapted to the RFP. These helical states are reached through a transition to a very low/reversed shear configuration leading to internal electron transport barriers. These states are interrupted by MHD reconnection events and the large Te gradients at the barriers indicate that both current and pressure driven modes are to be considered. Furthermore the typically flat Te profiles in the helical core have raised the issue of the role of electrostatic and electromagnetic turbulence in these reduced chaos regions, so that a stability analysis in the correct 3D geometry is required to address an optimization of the plasma setup. In this view the VMEC code proved to be an effective way to obtain helical equilibria to be studied in terms of stability and transport with a suite of well tested codes. In this work, the equilibrium reconstruction technique as well as the experimental evidence of 3D effects and their first interpretation in terms of stability and transport are presented using both RFP and stellarator tools.
[show abstract][hide abstract] ABSTRACT: The reversed field pinch configuration is characterized by the presence of magnetic structures both in the core and at the edge: in the core, at high plasma current the spontaneous development of a helical structure is accompanied by the appearance of internal electron transport barriers; at the edge strong pressure gradients, identifying an edge transport barrier, are observed too, related to the position of the field reversal surface.The aim of this paper is the experimental characterization of both the internal and edge transport barriers in relation to the magnetic topology, discussing possible analogies and differences with other confinement schemes.
[show abstract][hide abstract] ABSTRACT: In the quest for new energy sources, the research on controlled thermonuclear fusion1 has been boosted by the start of the construction phase of the International Thermonuclear Experimental Reactor2 (ITER). ITER is based on the tokamak magnetic configuration3, which is the best performing one in terms of energy confinement. Alternative concepts are however actively researched, which in the long term could be considered for a second generation of reactors. Here, we show results concerning one of these configurations, the reversed-field pinch4, 5 (RFP). By increasing the plasma current, a spontaneous transition to a helical equilibrium occurs, with a change of magnetic topology. Partially conserved magnetic flux surfaces emerge within residual magnetic chaos, resulting in the onset of a transport barrier. This is a structural change and sheds new light on the potential of the RFP as the basis for a low-magnetic-field ohmic fusion reactor.
[show abstract][hide abstract] ABSTRACT: 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.
Physics of Plasmas 05/2009; 16(5):056109-056106. · 2.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: With the exploration of the MA plasma current regime in up to 0.5 s long discharges, RFX-mod has opened new and very promising perspectives for the reversed field pinch (RFP) magnetic configuration, and has made significant progress in understanding and improving confinement and in controlling plasma stability. A big leap with respect to previous knowledge and expectations on RFP physics and performance has been made by RFX-mod since the last 2006 IAEA Fusion Energy Conference. A new self-organized helical equilibrium has been experimentally achieved (the Single Helical Axis---SHAx---state), which is the preferred state at high current. Strong core electron transport barriers characterize this regime, with electron temperature gradients comparable to those achieved in tokamaks, and by a factor of 4 improvement in confinement time with respect to the standard RFP. RFX-mod is also providing leading edge results on real-time feedback control of MHD instabilities, of general interest for the fusion community.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: RFX-mod is a reversed field pinch (RFP) experiment equipped with a system that actively controls the magnetic boundary. In this paper we describe the results of a new control algorithm, the clean mode control (CMC), in which the aliasing of the sideband harmonics generated by the discrete saddle coils is corrected in real time. CMC operation leads to a smoother (i.e. more axisymmetric) boundary. Tearing modes rotate (up to 100 Hz) and partially unlock. Plasma–wall interaction diminishes due to a decrease of the non-axisymmetric shift of the plasma column. With the ameliorated boundary control, plasma current has been successfully increased to 1.5 MA, the highest for an RFP. In such regimes, the magnetic dynamics is dominated by the innermost resonant mode, the internal magnetic field gets close to a pure helix and confinement improves.
[show abstract][hide abstract] ABSTRACT: The particle and energy transport in reversed field pinch experiments is affected by the locking in phase of the tearing modes, also dubbed dynamo modes, that sustain the magnetic configuration. In standard RFP pulses many m = 1 and m = 0 resonant modes have a relatively large amplitude (a spectrum dubbed MH for multiple helicity). The locking in phase of m = 1 tearing modes produces a helical deformation (locked mode (LM)) of the magnetic surfaces in a region of approximately 40 toroidal degrees. The region of the LM is characterized by a strong plasma–wall interaction and by high losses of energy and particles that account for a significant fraction of the input power and of the total particle outflux. The locking in phase of m = 0 modes modifies the plasma radius, shrinking and enlarging the plasma cross section in two wide toroidal regions of about 100°. The purpose of this paper is to investigate to what extent the locking in phase of m = 0 modes introduces toroidal asymmetries in the transport properties of the plasma. This study has been carried out investigating the shape of the density profile in the RFX-mod experiment. The analyses show that the profile exhibits a dependence on the toroidal angle, which is related to the deformation of the plasma column due to the locking in phase of m = 0 modes: the least steep density gradients at the edge are found in the region where the plasma column is shrunk, entailing that in this region the particle transport is enhanced. An analogous asymmetry also characterizes the density and magnetic fluctuations at the edge, which are enhanced in the same toroidal region where the particle transport also is enhanced. This result can be considered the first experimental evidence of an instability localized where the plasma column is shrunk.
[show abstract][hide abstract] ABSTRACT: The modified RFX is a very flexible device used for a variety of control schemes for MHD instabilities and for advanced reverse field pinch scenarios. Relative to the previous machine, RFX-mod has a thin Cu shell with vertical field penetration time τS, lowered from 450 to 50 ms and shell/plasma proximity from b/a = 1.24–1.1. Toroidal equilibrium is feedback-controlled and new power supplies provide better B control. Newly designed graphite tiles protect the vessel from localized power deposition. A mesh of 192 external saddle coils, supervised by a digital feedback system, controls radial fields due to field errors and MHD modes. The paper presents an overview of the very encouraging results obtained using both new and 'standard' advanced operational modes in the current range 0.3–1 MA. A dramatic improvement of plasma performance was obtained by using the saddle coils to cancel all the radial field components, an operation mode dubbed virtual shell (VS). The toroidal voltage was lowered by more than 25% and the pulse length was tripled, up to 7 times the τS. Steady-state RFP pulses are now limited only by the applied volt-seconds. The improved magnetic boundary also has an effect on the tearing modes underlying the sustainment of the RFP, whose core amplitude is more than halved. The VS combined with new schemes for the active rotation of the MHD dynamo modes has allowed us to obtain reliable and well-controlled long RFP pulses in the MA current range. This results in a 100% increase in the particle and energy confinement time relative to the previous experiment and opens the possibility of exploring the machine performance in the 2 MA design range.
[show abstract][hide abstract] ABSTRACT: Pellet injection is considered the most promising technique to fuel ITER plasmas. However, standard low field side (LFS) pellet injection seems inadequate for an efficient fuelling of the plasma core because it is too demanding in terms of injection velocity. Alternative configurations, as injection from the high field side (HFS) or vertically from the top of the machine (VHFS) are technologically less demanding due to a drift of the ablated material down the magnetic field gradient, which helps its penetration. In the past year LFS and VHFS pellet injection experiments have been performed on the Frascati Tokamak Upgrade (FTU: BT = 8 T, Ip = 1.5 MA, R = 93.5 cm, a = 30 cm), a circular cross-section device with a molybdenum toroidal limiter located on the HFS. The rationale of these experiments was to study the advantages of VHFS injection in a high toroidal field machine, where the value of BT can be considered as representative of that in ITER. The results demonstrate that the main advantage of VHFS injection (with respect to LFS) is that it allows one to obtain the same post-pellet density profile with an injection speed 2–3 times lower than that required in the case of LFS injection. However no significant displacement of the ablated material was observed, possibly due to the small value of the ∇ B-induced drift in these ohmic, low β plasmas. The experimental measurements were compared to the predictions of the drift model (Pégourié B. et al 2007 Nucl. Fusion 47 44) showing a reasonable agreement in both the magnitude of the drift and the main features of the line integrated density during pellet ablution.
[show abstract][hide abstract] ABSTRACT: Stable operation with control on magnetohydrodynamic modes has been obtained in the modified reversed field experiment employing a set of 192 feedback controlled saddle coils. Improvements of plasma temperature, confinement (twofold), and pulse length (threefold) and, as a consequence of the magnetic fluctuation reduction, strong mitigation of plasma-wall interaction and mode locking are reported.
[show abstract][hide abstract] ABSTRACT: In the Reversed Field Pinch RFX the upper boundary of the density operating space matches fairly well the Greenwald limit. Typically reaching the ceiling procures a soft termination; hard terminations may however occur at high currents (>0.9 MA) in presence of particularly large error fields. Radiation losses are always a relatively small fracti on of the ohmic input power but the injection of highly radiating impurities shrinks the density opera ting space. The contribution of localized plasma wal l interactions (PWI) to the total radiation is of diffi cult determination so that a detailed power balance cannot be performed. The limit is never exceeded eve n in the region of privileged PWI, where the density can be higher than elsewhere, suggesting that in the RFP the limit is valid locally. In He plasmas the limit can be exceeded and the electron density source profile estimated by Monte Carlo modelling with experimental edge temperature and densities is su bstantially similar to the hydrogen case. Preliminary self-consistent simulations of high dens ity hydrogen discharges with the RITM transport code illustrate that realistic combinations of electr on temperature and particle diffusivity can lead to a saturation of the density build up as the influx is inc reased.
[show abstract][hide abstract] ABSTRACT: The results of an extensive investigation of electrostatic and magnetic turbulence in the edge region of two European Reversed Field Pinch (RFP) experiments EXTRAP-T2R and RFX are reported. In both experiments particle transport is driven by turbulence and almost 50% of the particle losses is due to coherent structures emerging from the fluctuation background. It has been found that the collision of these structures results in a diffusion coefficient comparable to that due to the background turbulence. A spontaneous highly sheared E×B flow is observed in the edge region of both experiments. The flow shear is found to result mainly from the balance between the Reynolds Stress and the anomalous viscous losses. The results indicate that a turbulence self-regulation process is in action in the edge region.