Effects of wall boron coating on FTU, an all metallic and carbon free medium size tokamak

Abstract

Boronization of FTU tokamak strongly affects its gas recycling, impurity concentration and its temporal evolution. Two main phases are distinguished. In the first one, lasting about 60 discharges, with the machine fully boronized, the radiation losses decrease from 80% to 40% and the impurity content drops from Z eff = 6 to Z eff ≈ 1.5 at n e ≈ 0.35×10 20 m −3 in ohmic conditions. In the second phase, lasting >500 discharges, with the B film eroded from the TZM toroidal limiter surface but not from the SS walls, the reduction of radiated power (≈60%) and impurities (Z eff ≈ 2.0) though smaller, is still important with respect to pre-boronization, allowing for optimization of plasma characteristics up to the maximum RF power so far injected (2.6 MW ≈ 1.6 MW m −3). The recycling properties of the B film are fully stabilized, making the plasma operations much more reliable, while the gettering properties are preserved and they maintain the oxygen below n O 0.5%.

First experiments with lithium limiter on FTU

“…The lowest value for the ratio N p /N g has been 7.0 · 10 À3 , a factor 4 below that obtained after a fresh boronization, as reported in Ref. [7] which well describes the recycling characteristics of boronized and metallic walls. The saturation effects arising with a fresh boronization after high density pulses is not observed at all and the pre-programmed density is easily obtained.…”

“…Plasma phenomenology after lithization, with LLL extracted from the vacuum chamber, is very similar to that observed after boronization [7] but with better results as shown in Fig. 2 for two discharges at I p = 0.5 MA, B T = 6 T after lithization (#28544) and after boronization (#28819).…”

“…The radial diffusion and the recycling coefficient are not different from those typically used to simulate FTU data (D = 0.75 m 2 /s, R = 0.75) [7]. For the shot after lithization, instead, two important assumptions are needed to reproduce the experimental results: a strongly reduced recycling (R = 0.02) related to the large pumping effect of lithium on the walls and a reduced contribution of Mo to the total radiation losses (the density of Mo at the LCMS must be set a factor three lower than before lithization).…”

A Parametric Analysis of Nonlinear Lower Hybrid Effects

“…In figure 2 we plot the ratio NL NLo , ρ ρ where NLo ρ is the nonlinearity parameter for a reference Deuterium plasma, versus the ion mass (left side) and versus the electron temperature (right side), for different values of the ion-electron temperature ratio , τ ionization charge i Z and density gradient. The These parameters correspond to typical SOL profiles in FTU [12,13]. However, in order to show the effect of a strong density gradient, we increased the typical value observed in the experiments (with characteristic e-folding of the order of 1.5-2.0 cm) by a factor 2-4.…”

Operation of Alcator C-Mod with high-Z plasma facing components and implications

“…Alcator C-Mod, along with several limiter tokamaks ͓Frascati Tokamak Upgrade ͑FTU͒ and Tokamak Research Institute for Applied Mechanics ͑TRIAM͔͒, have molybdenum PFCs. [9][10][11] Molybdenum is similar to tungsten in that it has a high temperature limit and low tritium retention. 12 Other tokamak programs have been experimenting with tungsten PFCs, 13 most notably ASDEX-Upgrade where the carbon PFC tiles are gradually being replaced with tungsten-coated carbon tiles.…”