FTU results with a liquid lithium limiter

TLDR

FTU experiments since 2005 have focused on using liquid lithium as a plasma‑facing material. The liquid lithium limiter actively deposits a lithium film on the walls during discharge, and gyrokinetic analysis was used to study collisionality and density gradient effects. The limiter achieved reliable operation with clean plasmas, low wall recycling, and spontaneous density profile peaking, enabling the highest density limit to date and a ~50 % increase in energy confinement time.

Abstract

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 have been obtained. These results have allowed us to extend the density limit to the highest value so far obtained ( at I p = 0.7 MA and B T = 7.1 T, q a = 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.