TJ-II: Multipellet injection in NBI plasmas: Difference between revisions

Pellet induced Enhanced Confinement (PiEC) has been achieved with single pellet injection into NBI plasmas in TJ-II. In the 2021 autumn experimental campaign, PiEC plasmas were achieved with a single large pellet but further enhancement were achieved when additional pellets were injected (up to two additional pellets). This has demonstrated that the use of multi-pellets opens new NBI operating scenarios to achieve high confinement in TJ-II plasmas.

It has been found that the injection of more than one pellet has a clear impact on the quality of the PiEC mode. The critical pellet parameters seem to be: the time separation between pellets, the plasma radius where pellet particles are deposited after initial plasmoid drifting and the pellet size. These multi-pellet experiments were carried out with Co-counter, counter and balanced NB injection configurations and the impact of configuration on confinement was found to depend to some extent on heating scenario. Unfortunally, the series needed to investigate these effects was incompleted due to a lack of dedicated discharges.

High plasma performance due to pellet injections is described in many devices although the underlying physics is still  unclear. In TJ-II, during PiEC, the HIBPs show changes in Er (for rho<0.7) and a strong reduction in turbulence after multi-pellets injections (data analysis is still ongoing).

We propose series of multi-PIs into plasmas with different NBI heating scenarios (Co-counter, counter, balanced, max. power in both injectors) in order to optimize PiEC mode while varying pellet settings (size, pellet flight path, and time delays between them). We will also investigate the evolution of plasma potential and Er profiles (HIBPs and DR) as well as of the plasma density and its fluctations (HIBPs and DR) for these plasma conditions.