C. Mazzotta

FTU results with a liquid lithium limiter

Since the end of 2005 most of the plasma?wall interaction experiments on FTU have been focused on the possible use of liquid lithium as the plasma facing material. Liquid lithium limiter is an active method to deposit, during the plasma discharge, a lithium film on the walls with prolonged beneficial effects. Reliable operation with very clean plasmas, very low wall particle recycling, spontaneous peaking of the density profile for line-averaged density values 1.0\times 10^{20}\,{\rm m}^{-3} SRC=http://ej.iop.org/images/0029-5515/51/7/073006/nf381122in001.gif/> have been obtained. These results have allowed us to extend the density limit to the highest value so far obtained ( at Ip = 0.7?MA and BT = 7.1?T, qa = 5.0, by gas puffing only) and to increase the energy confinement time by almost 50% with respect to the average value of 50?ms of the old ohmic FTU database. An accurate analysis of these plasmas has been carried out by means of a gyrokinetic code to establish the role of collisionality and density gradients on the observed phenomenology.

Improved Confinement in JET hybrid discharges

A new technique has been developed to produce plasmas with improved confinement relative to the H-98,H-y2 scaling law (ITER Physics Expert Groups on Confinement and Transport and Confinement Modelling and Database ITER Physics Basics Editors and ITER EDA 1999 Nucl. Fusion 39 2175) on the JET tokamak. In the mid-size tokamaks ASDEX upgrade and DIII-D heating during the current formation is used to produce a flat q-profile with a minimum close to 1. On JET this technique leads to q-profiles with similar minimum q but opposite to the other tokamaks not to an improved confinement state. By changing the method utilizing a faster current ramp with temporary higher current than in the flattop (current overshoot) plasmas with improved confinement (H-98,H-y2 = 1.35) and good stability (beta(N) approximate to 3) have been produced and extended to many confinement times only limited by technical constraints. The increase in H-98,H-y2-factor is stronger with more heating power as can be seen in a power scan. The q-profile development during the high power phase in JET is reproduced by current diffusion calculated by TRANSP and CRONOS. Therefore the modifications produced by the current overshoot disappear quickly from the edge but the confinement improvement lasts longer, in some cases up to the end of the heating phase.

Modelling of polarimetry measurements at JET

This paper presents a study aimed at validating the ability of the presently available models to predict the Cotton–Mouton (C–M) effect. The Faraday rotation and the C–M phase shift angle can be calculated by means of a rigorous numerical solution of Stokes equations. Numerical and approximated solutions are presented and compared with experimental data. A detailed comparison is done with the time traces of measurements, inside a limited dataset representative of JET regimes. A statistical analysis is then carried out on a dataset including data from 300 discharges. In general the C–M measurements are in agreement with the numerical model, and the line integral of plasma density deduced by the C–M measurements is in agreement with that measured by LIDAR Thomson scattering (well inside the experimental error, which is close to two fringes for the polarimetry measurements, 1 fringe = 1.14 × 1019 m−2).

Analysis of Faraday rotation in JET polarimetric measurements

ThepaperdealswithJETpolarimetermeasurementsandinparticularitpresents a study of the Faraday rotation angle, which is used as a constraint in equilibrium codes. This angle can be calculated by means of the rigorous numerical solution of Stokes equations. A detailed comparison of calculations is carried outwiththetimetracesofmeasurements, insidealimiteddatasetrepresentative of JET discharges: in general, it is found that the Faraday rotation angle and Cotton‐Mouton phase shift measurements can be represented by the numerical solution to Stokes equations. To obtain this agreement in particular for Faraday rotation, a shift of the magnetic surfaces must be included. This results in an improvement of the position of the magnetic surfaces as calculated by the EFIT equilibrium code. The approximated linear models normally used can be applied only at low density and current. The Cotton‐Mouton is calculated at high plasma density including the contribution by the Faraday rotation angle. For highplasmacurrentthenon-lineartermsinthepropagationequationscanbeimportant. These conclusions have some impact on the mathematical form of the polarimetric constraints (Faraday and Cotton‐Mouton) in equilibrium codes. (Some figures in this article are in colour only in the electronic version)

Parametric dependence of turbulent particle transport in high density electron heated FTU plasmas

In this paper we present the result of a study carried out at the Frascati Tokamak Upgrade (FTU), on a set of full non-inductive current driven, electron heated, L-mode discharges aimed at investigating the parametric dependence of the electron density profile on the electron temperature and safety factor gradients as predicted by quasi-linear drift-turbulence transport theory. Experiments in FTU allow the extension of similar studies carried out on other tokamaks to plasmas with higher density and higher magnetic field. Magnetic shear and electron temperature gradients are found to drive opposite turbulent particle flows in the gradient region (0.3 < r/a ? 0.5), while inward thermo-diffusion alone is found in the plasma core (r/a ? 0.3). Density profiles at midradius appear to be controlled by a convective term proportional to the density and independent of the gradient of temperature and magnetic shear. A linear increase in density peaking versus effective collisionality is found, differing from the scaling observed in other FTU plasma regimes.

Mutual interaction of Faraday rotation and cotton-mouton phase shift in jet polarimetric measurements

The paper presents a study of Faraday rotation (FR) angle and Cotton–Mouton (CM) phase shift measurements to determine their mutual interaction and the validity of the linear models presently used in equilibrium codes. Comparison between time traces of measurements and model calculations leads to the result that only an exact numerical solution of Stokes equations can reproduce in all the experimental data. As a consequence, approximated linear models can be applied only in a limited range of plasma parameters. In general, the nonlinear coupling between FR and CM is important for the evaluation of polarimetry parameters.

Linear microstability analysis of a low-Z impurity doped tokamak plasma

Improved electron and deuterium energy and particle confinement in the presence of low-Z impurities have been observed in many tokamaks under various experimental conditions. Peaked electron density profiles have been obtained in the Frascati Tokamak Upgrade (FTU) ohmic plasmas where a high concentration of lithium has been detected following the installation of a Liquid Lithium Limiter (LLL). This paper presents the results of a gyrokinetic study on the effects of lithium and other low-Z impurities on the linear stability of deuterium and electron temperature driven modes and their associated fluxes for plasma parameters such as those found in the core of LLL-FTU plasmas.Simulations show that a lithium concentration in excess of nLi/ne = 15%, as estimated in the initial phase of a reference FTU discharge, is found to have a strong stabilizing effect on the TEM and high-frequency ETG modes. A significant stabilization of the electron driven modes can still be observed when the lithium concentration is reduced to 3%.In the presence of a significant impurity concentration (nLi/ne = 3–15%) the long wavelength ITG modes drive an inward electron and deuterium flux and outward lithium flux. This process may lead eventually to an increased electron and deuterium density peaking and a reduced Zeff (lithium density below nLi/ne = 1%).

Molybdenum transport in high density FTU plasmas with strong radio frequency electron heating

Effects of radio frequency heating on particle confinement have been observed in various tokamaks, where it was found that impurities can be effectively pumped out of the plasma core when sufficient central ion cyclotron (ICH) and/or electron cyclotron heating (ECH) power is applied. It has been proposed that radio frequency heating prevents impurity accumulation by modifying the gradients of temperature and current and hence the turbulence regime responsible for particle anomalous flow. In this paper we present a study of the behavior of molybdenum injected in full lower-hybrid current drive plasmas in the Frascati Tokamak Upgrade with strong ECH. Three similar discharges with increasing level of ECH power are discussed in terms of plasma and power deposition profiles. Strong accumulation of the impurity is observed during off axis heating, while for central ECH deposition the concentration of molybdenum in the core of the discharges decreases at the highest level of ECH power (PECH = 1.2 MW). Microturbulence stability analysis has been carried out for the three plasmas and the relative importance of the off diagonal terms in the transport matrix for the pinch velocity of molybdenum has been studied with reference to FTU results on electron transport and in the framework of recent theoretical results.

ITB formation with counter ECCD and LHCD and Suprathermal ECCD experiments on FTU in ITER relevant conditions

In this paper the results of the experiments of combined injection of LH waves and EC waves performed in the FTU tokamak are reported. Such experiments were mainly devoted to study and to control the access to the advanced tokamak scenarios in plasma conditions close to ITER parameters (Bt≈5T or higher, ne,line≈10 20 m -3 ). Two different absorption mechanisms were used for the EC waves. In the first one the cold resonance absorption of EC waves launched with a toroidal angle was used to induce small modification of the current profile mainly maintained by LH waves. In the second one the absorption through Doppler shift due to the fast electron tails generated by LHCD was used.

Confinement and turbulence study in the Frascati Tokamak Upgrade high field and high density plasmas

The Frascati Tokamak Upgrade (FTU) is a high field and high density device which allows one to study confinement and transport at plasma parameters generally not accessible to other tokamaks and is relevant to next generation experiments (ITER). In this paper we study the effect of different density, temperature and q profiles on confinement and transport in various types of plasmas including ohmic, externally heated and pellet-fuelled discharges. Peaking of the electron density profile in pellet-fuelled discharges has been found to enhance the energy confinement time on FTU to values as high as 120 ms at densities of the order of those expected in ITER. Experiments designed to further improve confinement by injecting pellets at higher densities show the existence of a second threshold for saturation of confinement when the local electron heat conductivity corresponds to the same order of the local ion neoclassical conductivity in the region of maximum temperature gradient. Reflectometry measurements show turbulence suppression in pellet-injected discharges (as predicted by microstability analysis) and give insights to the change in turbulence in electron internal transport barrier plasmas. Finally, particle transport has been studied in experiments with full LH current drive.