Figure : Sealed thin-wall quartz capillary with FLiNaK salt inside a graphite insert. The quartz capillary is of ⇠ 3 mm diameter and of 0.01 mm wall thickness. The insert is a 3mm-diameter HOPG (highly-oriented pyrolytic graphite) graphite rod, further thinned to fit in the capillary and machined with a 2mm hollow filled with ⇠ 0.1 g salt. The quartz wool at the bottom of the capillary provides a space for the thermal expansion of graphite and protection of the capillary during assembly.