A method to produce lithium pellets for fueling and ELM pacing in NSTX-U

A device has been developed that produces spherical lithium pellets for the controlled excitation, or pacing, of ELMs and replenishing lithium coating on PFCs during a plasma shot. The device is based on a dripper design, where the lithium is forced through a small orifice with gas pressure. It is envisaged to use the dripper in two ways: first it is used in conjunction with the lithium granular injector developed at PPPL for ELM pacing and the second for replenishing lithium PFC coatings during a plasma discharge by “dripping” liquid lithium drops into the plasma edge and vaporization will redistribute the lithium to recoat the PFC during the shot. A theory has been developed for the drop formation and frequency using high-pressure gas. Experiments have been performed initially with Wood's metal and subsequently lithium. Using Wood's metal at a backing pressure of ΔP > 600 torr, frequencies up to f = 1200 Hz have been achieved with droplet diameters d > 600 μm. These agree well with theory. Measurements using lithium also show that the frequency does not quite match the theory with ΔP = 450 torr, a frequency of f = 2 kHz and but the diameters are in good agreement with d = 0.8 mm.