V. Antoni

Turbulent transport reduction by E×B velocity shear during edge plasma biasing: recent experimental results

Experiments in the tokamaks TEXTOR, CASTOR, T-10 and ISTTOK, as well as in the reversed field pinch RFX have provided new and complementary evidence on the physics of the universal mechanism of E×B velocity shear stabilization of turbulence, concomitant transport barrier formation and radial conductivity by using various edge biasing techniques. In TEXTOR the causality between transport reduction and induced electric fields in the edge has been for the first time clearly demonstrated. The high electric field gradients have been identified as the cause for the quenching of turbulent cells. A quantitative analysis of the measured transport reduction is in good agreement with theoretical predictions. The scaling of plasma turbulence suppression with velocity shear has been established, revealing the density-potential cross-phase as a key element. Reduction in poloidal electric field, temperature, and density fluctuations across the shear layer lead to a reduction of the anomalous conducted and convected heat fluxes resulting in an energy transport barrier that is measured directly. In CASTOR the biasing electrode is placed at the separatrix in a non-intrusive configuration which has demonstrated strongly sheared electric fields and consequent improvement of the global particle confinement, as predicted by theory. The impact of sheared E×B flow on edge turbulent structures has been measured directly using a comprehensive set of electrostatic probe arrays as well as emissive probes. Measurements with a full poloidal Langmuir probe array have revealed quasi-coherent electrostatic waves in the SOL with a dominant mode number equal to the edge safety factor. In T-10 edge biasing is clearly improving the global performance of ECR heated discharges. Reflectometry and heavy ion beam probe measurements show the existence of a narrow plasma layer with strong suppression of turbulence. On ISTTOK, the influence of alternating positive and negative electrode and (non-intrusive) limiter biasing has been compared. Electrode biasing is found to be more efficient in modifying the radial electric field Er and confinement, limiter biasing acting mainly on the SOL. In the RFX reversed field pinch it has been demonstrated that also in RFPs biasing can increase the local E×B velocity shear in the edge region, and hence substantially reduce the local turbulence driven particle flux mainly due to a change in the relative phase between potential and density fluctuations.

Intermittency in plasma turbulence

Abstract Intermittency in fluid turbulence can be evidentiated through the analysis of Probability Distribution Functions (PDF) for velocity fluctuations, which display a strong non-Gaussian behaviour at small scales. In this paper, we investigate the occurrence of intermittency in plasma turbulence by studying the departure from the Gaussian distribution of PDF for both velocity and magnetic fluctuations. We use data coming from two different experiments, namely in situ satellite observations of the inner solar wind and turbulent fluctuations in a magnetically confined fusion plasma. Moreover, we investigate also time intermittency observed in a simplified shell model which mimics 3D MHD equations. We found that the departure from a Gaussian distribution is the main characteristic of all cases. The scaling behaviour of PDFs are then investigated by using two different models built up in the past years, in order to capture the essence of intermittency in turbulence.

MHD stability analysis of force-free reversed field pinch configurations

The MHD current driven instabilities of Reversed Field Pinch (RFP) configurations are analysed for a plasma in contact with a perfectly conducting wall. The equilibrium distributions are obtained on the basis of a model which allows a parametric variation of the value of the safety factor on the axis, q(0), and of the current density distribution. ? The RFP is found to be stable in a wide range of parameters. However, when the toroidal field reversal becomes too deep, unstable modes, resonant outside the reversal surface, are found in analogy to the stability limit at ? = 1.56 for the Taylors theoretical, fully relaxed states. Nevertheless, it is shown that these modes are not very significant in that they arise in a region of the parameters space which is only rarely approached in experiments. On the other hand, for peaked current density distributions, new unstable tearing and kink modes are found when q(0) drops below the limit q(0) ? 2a/(3R). These modes are resonant inside the reversal surface and may include the mode resonant on the axis. ? The results of the MHD stability for the internal modes and in particular the limit on the value of q on axis that has been found are discussed in connection with experimental observations on mean field profiles and related oscillations.

Physics and engineering design of the accelerator and electron dump for SPIDER

The ITER Neutral Beam Test Facility (PRIMA) is planned to be built at Consorzio RFX (Padova, Italy). PRIMA includes two experimental devices: a full size ion source with low voltage extraction called SPIDER and a full size neutral beam injector at full beam power called MITICA. SPIDER is the first experimental device to be built and operated, aiming at testing the extraction of a negative ion beam (made of H− and in a later stage D− ions) from an ITER size ion source. The main requirements of this experiment are a H−/D− extracted current density larger than 355/285 A m−2, an energy of 100 keV and a pulse duration of up to 3600 s.Several analytical and numerical codes have been used for the design optimization process, some of which are commercial codes, while some others were developed ad hoc. The codes are used to simulate the electrical fields (SLACCAD, BYPO, OPERA), the magnetic fields (OPERA, ANSYS, COMSOL, PERMAG), the beam aiming (OPERA, IRES), the pressure inside the accelerator (CONDUCT, STRIP), the stripping reactions and transmitted/dumped power (EAMCC), the operating temperature, stress and deformations (ALIGN, ANSYS) and the heat loads on the electron dump (ED) (EDAC, BACKSCAT).An integrated approach, taking into consideration at the same time physics and engineering aspects, has been adopted all along the design process. Particular care has been taken in investigating the many interactions between physics and engineering aspects of the experiment. According to the robust design philosophy, a comprehensive set of sensitivity analyses was performed, in order to investigate the influence of the design choices on the most relevant operating parameters.The design of the SPIDER accelerator, here described, has been developed in order to satisfy with reasonable margin all the requirements given by ITER, from the physics and engineering points of view. In particular, a new approach to the compensation of unwanted beam deflections inside the accelerator and a new concept for the ED have been introduced.

Intermittency and self-similarity in plasma edge fluctuations

The intermittency of the floating potential fluctuations as measured at the edge of a plasma of interest for controlled thermonuclear fusion research is investigated. The probability distribution functions of fluctuations are not scale invariant, that is the wings of these functions are more important at the smallest scales, a classical signature of intermittency. Self-similarity is recovered at scales greater than about 20 μs.

Overview of RFX-mod results

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.

Electrostatic fluctuations in a direct current magnetron sputtering plasma

Electrostatic fluctuations (up to 500 kHz) have been studied in a planar dc magnetron device using Langmuir probes measuring the floating potential and the ion saturation current. Fluctuation levels as high as 30% have been found inside the magnetic trap. A two-point spectral analysis has shown that the fluctuations are due to coherent modes with a low azimuthal mode number. The modes are present only when the discharge power and the neutral gas pressure are above a threshold. Their frequency spacing decreases when the neutral gas pressure is raised, so that increasing the pressure leads to a more turbulent state. The modes have been interpreted as unstable E×B/density gradient modes.

Magnetic self organization, MHD active control and confinement in RFX-mod

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 100u2009Hz) 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 1.5u2009MA, 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.

Active MHD control at high currents in RFX-mod

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 50u2009ms 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 0.3–1u2009MA. 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 2u2009MA design range.

Mean magnetic field profiles and their dynamics in reversed field pinches

Magnetic field profiles have been measured in the ETA-BETA II experiment by means of insertable probes. The main properties of the mean magnetic field profiles and the ?-profiles at various values of the pinch parameter ? are discussed as well as experimental evidence of the existence of constraints on the radial distribution of ?. The dynamics of the magnetic distributions is investigated and given in terms of the cyclic current redistribution associated with the sustainment of the configuration.