Particle transport in the Frascati Tokamak upgrade

Abstract

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

Peaked density profiles and MHD activity on FTU in lithium dominated discharges

“…It can be seen that the diffusivity remains almost unchanged during and after the good confinement phase, so that the lost of central density can be ascribed to the reduction of the particle inward pinch. The low value of diffusion coefficient is not a peculiar value of this discharge but it is well aligned with usual values found with the same analysis in FTU discharges [7,20]. …”

“…Evaluating the density variation from density profile measurements we get a direct measurement of the particles flux through the surface. Assuming that the flux is given by a diffusion term and an inward pinch ( = − Un e + D ∂ n e /∂ r) we can get the transport coefficients (D and U) by plotting the normalized particle flux ( /n e ) versus the logarithm derivative of the density (1/n e ∂n e /∂r), and fitting the data with a straight line [20]. Fig.…”

Peaked density profiles due to neon injection on FTU

“…An estimate of the particle diffusion coefficient D and of the pinch velocity U has been performed using the model suggested in [15]. A detailed evolution of the density profiles is necessary for the determination of the particle flux.…”

Overview of the FTU results

“…A qualitative estimate from UV spectroscopy measurements indicates that the density behaviour cannot be attributed simply to the stripped electrons from the puffed neon, but a modification of particle transport should be invoked in order to explain the spontaneous rise of the density and the higher density peaking. The observed density peaking was analysed in terms of electron diffusion coefficient and pinch velocity, in the framework of a simple particle transport model [25], confirming the presence of a higher inward pinch (with respect to the reference case) inside half radius of the plasma, where the assumption of negligible sources is valid and the method described is more reliable. A micro-stability analysis will be performed in the near future to investigate the role of the ion and electron gradient driven modes on particle transport.…”