F. Crisanti

HIGH DENSITY OPERATION ON FRASCATI TOKAMAK UPGRADE

Plasma behaviour in the high density regime has been investigated on the Frascati Tokamak Upgrade (FTU). The items particularly addressed are density limit, MARFE characteristics and fuelling efficiency. With gas puffing, a maximum line average density of 3.2*1020 m-3 has been obtained in the ohmic regime at q=5.1, corresponding to 1.7 times the Greenwald limit; while with pellet injection, a value of 3.7*1020 m-3 has been reached at q=3.5. The density limit appears to be connected with the impurity content and edge parameters, so the best results are obtained with very clean plasmas and peaked electron density profiles. The MARFE phenomenon always appears beyond a critical density that depends on the total input power and the effective charge: emissivities in the range of 2 to 12 MW/m3 have been measured in this highly radiating region. The fuelling efficiency, starting from a value of about 50% at low density, progressively deteriorates, falling to 10% near the density limit: this behaviour is interpreted as being due to a decrease of the scrape-off layer transparency to incoming neutrals

EAST alternative magnetic configurations: modelling and first experiments

Heat and particle loads on the plasma facing components are among the most challenging issues to be solved for a reactor design. Alternative magnetic configurations may enable tokamak operation with a lower peak heat load than a standard single null (SN) divertor. This papers reports on the creation and control of one of such alternatives: a two-null nearby divertor configuration. An important element of this study is that this two-null divertor was produced on a large superconducting tokamak as an experimental advanced superconducting tokamak. A preliminary experiment with the second null forming a configuration with significant distance between the two nulls and a contracting geometry near the target plates was performed in 2014. These configurations have been designed using the FIXFREE code and optimized with CREATE-NL tools and are discussed in the paper. Predictive edge simulations using the TECXY code are also presented by comparing the advanced divertor and SN configuration. Finally, the experimental results of ohmic and low confinement (L-mode) two-null divertor and SN discharges and interpretative two-dimensional edge simulations are discussed. Future experiments will be devoted to varying the distance between the two nulls in high confinement (H-mode) discharges.

Characterisation of the scrape-off layer plasma in the FTU tokamak

Abstract The average values of the scrape-off layer (SOL) plasma quantities of FTU, density, temperature, e-folding lengths are studied as a function of the main plasma parameters and satisfactorily compared with 2D model predictions. The cause of their poloidal asymmetry is investigated and attributed to the variation of the magnetic line connection length. A self consistent model of the FTU plasma, 0D in the core and 1D in the SOL, can reproduce the main features of the impurity production and power radiation in FTU.

A MIMO architecture for integrated control of plasma shape and flux expansion for the EAST tokamak

This paper deals with the magnetic control system of the EAST tokamak, i.e. with the system in charge of controlling the position and the shape of the plasma column into the vacuum vessel, as well as the current that flows into it. A multi-input multi-output architecture is proposed in order to achieve integrated control of both plasma boundary and flux expansion in the scrape-off layer. The proposed architecture is based on an XSC-like control approach [1]. In order to show the effectiveness of the proposed approach, a set of Matlab/Simulink® tools that allow to reproduce in simulation the EAST experiments is used. This simulation environment is validated against the experiments, and is then exploited to design and validate the proposed MIMO controller.

Particle transport in the Frascati Tokamak upgrade

Three methods of density modulation have been used in FTU discharges (gas puffing, pellet injection and current rampup). The analysis of particle balance confirms that the neoclassical theory is not adequate and the experimental results obtained by the three methods are used to verify the validity of an empirical expression for the radial particle flux. Such an expression is relevant for the design of future experiments and constitutes a constraint for any satisfactory theory.

Radial particle flow, electrical conductivity and thermal energy content in the Frascati Tokamak Upgrade

Experimental determinations of the radial plasma flow velocity, of Zeff and of the thermal energy content, made on the Frascati Tokamak Upgrade (FTU), have been compared with the predictions of the conventional neoclassical theory, of the extended neoclassical theory and of the pseudoneoclassical theory. In the comparison only the momentum balance and continuity equations were used. The best overall agreement is found for the second of these theories, thus confirming the result of previous analyses

Effect of the magnetic topology of a tokamak divertor on the power exhaust properties

The peculiarities of various advanced divertor magnetic configurations that could be adopted for a tokamak reactor are investigated with the 2D edge code TECXY applied to the different divertor options of the projected tokamak DTT (Divertor Test Tokamak). The analysis highlights very interesting features for those configurations that realize a wide region with significantly depressed poloidal field in between the main X point and the target. Here, the energy cross-field diffusion can become so fast to extend up tou2009≈10 times the width of the power flow channel, in terms of the poloidal flux coordinates. This can spread the power over a long length and then drop the peak heat load below the technologically safe value, even with no help from impurities. Furthermore, the strongly enlarged effective divertor volume can favour the dissipative processes and lead to plasma detachment from the associated target. The driving mechanism appears to rest on the strongly increased connection lengths. This reduces the pa...