Fast Camera Images of Surviving Gallium Droplets During Plasma–Liquid Gallium Jet Interaction Experiments in ISTTOK

Abstract

Liquid metals have received a renewed interest in fusion research due to the advantages that these materials could offer to overcome the erosion problem of plasma facing components presently used in large-size fusion devices. In ISTTOK Tokamak, the plasma-liquid gallium jet interaction is currently being studied. One of the main technical problems faced during the implementation of the experimental setup was to minimize the amount of gallium that remains in the vacuum chamber. Despite the large effort put in fulfilling this task, liquid-metal droplets could not be completely eradicated from the plasma. Here, pictures of one of those remaining droplets traveling through the plasma are presented and discussed. Index Terms-Gallium, limiter, liquid metals, liquid-metal jet, plasma-surface interaction.M ATERIALS currently used in large-size fusion devices are submitted to very high power loads. A frequent replacement of the plasma facing components is to be expected in the present approach of the fusion research program as a result of the high erosion levels and thermal stress produced by such power loads, and one way to overcome that issue is linked to the use of liquid metals as plasma facing surfaces. The possibility to perform a permanent renewal of liquid surfaces has been pointed out as one adequate solution for the protection of solid walls providing, at the same time, an efficient process to exhaust power from fusion plasmas. Among a set of several liquid metals, lithium has shown the best compatibility with fusion plasmas, which is mainly due to its low Z [1]. Another candidate material that has better thermal properties, a wider liquid state range, and a lower vapor pressure is gallium. The interaction of a liquid gallium jet with plasmas is being studied in ISTTOK, which is a tokamak with R = 0.46 m, a = 0.085 m, B T = 0.45 T, n e (0) = 5 × 10 18 m −3 , T e (0) = 150 eV, I P ∼ 6 kA, and V loop ∼4 V. The jets, which have a diameter of 2.3 mm and a flow velocity of 2.5 m/s, are generated by hydrostatic pressure and received in a lower collector after interacting with the plasma. A detailed description of the experimental setup is presented in [2].Due to its high density, gallium flows have typically high Weber number (in our specific case, We∼140 [2]). One of the consequences of this effect is the splashing of gallium droplets Manuscript

Liquid gallium jet–plasma interaction studies in ISTTOK tokamak

“…The good reproducibility observed from discharge to discharge does not seem to indicate a significant increase in the plasma impurity content or a contamination of the ISTTOK chamber. No evidences of disruption induced by liquid metal had been noticed during the experiments even in the presence of macroscopic size ($1 mm radius) gallium droplet in the discharge [9].…”