Tracer-encapsulated solid pellet (TESPEL) was injected from the outboard midplane into Wendelstein 7-X (W7-X) plasmas. Viewing from behind the flight path, the distribution of the light emissions from the TESPEL cloud particles (H, C and C2+) was recorded using an ultrafast visible camera with a temporal resolution up to 2 µs, which allows resolving both the pellet cloud evolution and the detachment of the drifting cloud. It was observed that both the B┴ and the B║ aligned dimensions of the pellet cloud fluctuate quasi-periodically, this being associated with a vertical cloud movement and an eruption/ejection of part of the cloud. It is found that, first, the pellet cloud expands parallel to the magnetic field lines for about 10 µs. Then, when the cloud reaches a certain B║ size the ionized part of the cloud (plasmoid) moves vertically (typically upwards) and detaches itself from the pellet within 10 µs. One possible explanation for the upward movement of the pellet cloud is the presence of a vertical component of grad(B) pointing downward resulting in an upward cloud drift.
Ionization–recombination processes and ablation cloud structure for a carbon pellet