Characterization of signals for a Divertor Tokamak Test facility interferometer/polarimeter system.

In magnetically confined fusion experiments, laser interferometer/polarimeter systems allow one to determine plasma density, give valuable information on the internal magnetic fields, and contribute to the evaluation of the plasma magnetic equilibrium and to the real-time estimation of the q profile to allow feedback configuration control. This work presents an analysis of the interferometric and polarimetric signals of a multi-chord far-infrared interferometer/polarimeter for the divertor tokamak test facility, the new tokamak device currently under construction in Italy. The polarimetric signals are calculated both with approximate formulas and by solving the equation describing the evolution of the laser beam polarization inside the plasma using the Mueller formalism. The latter method correctly accounts for crosstalk between Faraday rotation and the Cotton-Mouton effect. The impact of the plasma birefringence on the interferometric phase shift is also studied, and it is found that a perturbation of the interferometric phase shift is present also in the case of an initial fixed linear polarization of the probe laser beam.

Bifurcation to 3D helical magnetic equilibrium in an axisymmetric toroidal device.

We report the first direct measurement of the internal magnetic field structure associated with a 3D helical equilibrium generated spontaneously in the core of an axisymmetric toroidal plasma containment device. Magnetohydrodynamic equilibrium bifurcation occurs in a reversed-field pinch when the innermost resonant magnetic perturbation grows to a large amplitude, reaching up to 8% of the mean field strength. Magnetic topology evolution is determined by measuring the Faraday effect, revealing that, as the perturbation grows, toroidal symmetry is broken and a helical equilibrium is established.

Single-helical-axis states in reversed-field-pinch plasmas.

The transition to a new magnetic topology, characterized by a quasi-single-helicity state with a single helical magnetic axis has been experimentally observed for the first time in a reversed-field-pinch plasma. The occurrence of the new state, which has been dubbed a single-helical-axis state, was found to provide magnetic chaos healing and enhanced thermal content of the plasma. The helical structure extends on both sides of the vessel geometric axis, and is related to exceeding a threshold in the ratio between the amplitude of the dominant MHD mode and the amplitude of the secondary ones.

Enhanced confinement and quasi-single-helicity regimes induced by poloidal current drive.

Enhanced confinement regimes with quasihelical symmetry are reproducibly obtained in the modified reversed field pinch eXperiment (RFX)-mod by reducing the level of internal chaos through the combination of a smooth magnetic boundary (through a virtual shell scheme) and the oscillating poloidal current drive technique. The plasma moves from a chaotic multiple helicity towards a quasi-single-helicity regime with a magnetic island showing high temperature and soft x-ray emissivity. The chaos reduction involves also the plasma outside the island allowing for a global enhanced confinement with an improvement up to 50%.

Two-color medium-infrared scanning interferometer for the Frascati tokamak upgrade fusion test device.

A scanning beam interferometer installed on the Frascati tokamak upgrade (FTU) experiment is presented. The scanning beam scheme combined with the small dimensions of the beams produces a system with very high spatial resolution: more than 30 adjacent (nonoverlapping) chords sample most of the plasma cross section. A good time resolution is achieved by the use of a proper scanning device, with a scanning frequency >or=8 kHz. Very fast events are measured by three additional fixed lines of sight providing a time resolution >or=100 kHz. The instrument is a two-color medium-infrared-compensated-type interferometer; two wavelengths (colors) are used to measure both the density and the mechanical vibrations of optical components. A CO2 laser (lambda=10.6 microm) is the main light source, and a CO laser (lambda=5.4 microm) is the compensation one. The optical scheme is a double pass Mach-Zehnder type. All the retroreflector mirrors are mounted directly on the FTU mechanical structure thanks to the compensation system that allows for large vibration amplitudes of optical components. Heterodyne detection at 30 and 40 MHz is obtained by frequency shifting the reference beams with two acousto-optic modulators (Bragg cells). Many features are implemented to achieve high measurement accuracy and reliability. A real-time system computes the integral density measured on one of the fixed lines of sight and provides an analog signal for density feedback control. The interferometer was used to measure density profiles both in medium-density discharges (n(e) approximately 10(20) m(-3)) and in high-density pellet injected discharges (n(e) approximately 7-8 x 10(20) m(-3)). The measurement error is approximately 2 x 10(18) m(-2) under optimal conditions but can be higher in some cases, mainly because of the large tilt of the retroreflector mirrors.

Active-feedback control of the magnetic boundary for magnetohydrodynamic stabilization of a fusion plasma.

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.

Investigation of electron-distribution function and dynamo mechanisms in a reversed-field pinch by analysis of hydrogen-pellet deflection

In reversed-field pinches, two different mechanisms have been proposed to explain the dynamo process which drives the poloidal current needed to sustain the magnetic configuration: the kinetic dynamo theory and the magnetohydrodynamic (MHD) dynamo theory. Experimentally, they can be distinguished by the radial behavior of the electron distribution function. In this Letter the trajectory deflection of frozen hydrogen pellets has been used as a diagnostic of suprathermal electrons in the plasma. The classical Spitzer-Harm distortion of the electron distribution function consistent with the MHD dynamo electric field is found to give a better modeling of the pellet trajectory.

[Clinical tolerance of oral loading with elevated doses of amiodarone in a group of patients with heart failure]. / Tolleranza clinica del carico orale, con dosi elevate di amiodarone, in un gruppo di pazienti con scompenso cardiaco.

Amiodarone is a choice drug for the treatment of patients suffering from life-threatening hyperkinetic ventricular arrhythmias and depressed ventricular function. The drug is characterized by a remarkably long halflife and a delayed initial activity after oral administration of the usual loading levels. The aim of this study was to establish: -the clinical tolerance to elevated doses in patients with heart failure presenting complex, hyperkinetic ventricular arrhythmias; -the possibility to shorten hospitalization as a result of the oral loading; -whether this administration route would take less time to be efficacious. For this purpose, 30 patients with heart failure and complex, hyperkinetic ventricular arrhythmias were treated with 0.50 mg/kg body weight of amiodarone for three days and with 0.30 mg/kg on the 4th and 5th days, followed by a maintenance period of treatment with 200-400 mg daily. All patients underwent a 24-h Holter test before and after administration and an echocardiographic examination showing an average ejection fraction of approximately 30%. Amiodarone was clinically well tolerated; only 2 patients required discontinuation of therapy whereas, among the the remaining 28 patients, 2 cases of transient hypotension and 3 cases of gastroenteric disorders were observed. It was concluded that elevated doses of amiodarone were well tolerated, which allowed to reduce the loading period and, therefore, hospitalization.