Importance of gas dissolution on droplet ejection from liquid gallium irradiated with inductively coupled plasma

We observed the ejection of droplets from liquid Gallium interacting with an inductively coupled plasma. The droplet election was observed at the timing of the bubble rupture on the liquid gallium surface. We examined the frequency of droplet ejection by varying the ion flux ((0.2 - 7.1) × 10¹⁴ cm^-2 s^-1), ion energy (15 - 215 eV), and the liquid gallium temperature (370 - 500 K). In the case of hydrogen plasma, the frequency of the droplet ejection increased with the ion flux and the liquid gallium temperature. On the other hand, it decreased with the ion energy. When irradiating argon, helium, and nitrogen plasmas, the frequency of the droplet ejection was lower (0 - 0.004 s^-1) compared with that in hydrogen plasma (0.01 - 0.035 s^-1). We estimated the amount of dissolved hydrogen by measuring the partial pressure of molecular hydrogen desorbed from the liquid gallium using a mass spectrometer. The amount of molecular hydrogen increased with the ion flux, and it decreased with the liquid gallium temperature. In addition, a spiky increase in the hydrogen partial pressure was observed at the timing of the bubble rupture or the droplet ejection. This result indicates the importance of dissolved hydrogen in the liquid gallium. The amount of hydrogen in the liquid gallium can reach the supersaturated state by the ion irradiation. The supersaturation results in the formation of a bubble, and droplets are ejected at the bubble rupture.