TJ-II: Multipellet injection in NBI plasmas
Multipellet injection in NBI plasmas. Characterization of PiEC mode
Name and affiliation of proponent
Laboratorio Nacional de Fusión, CIEMAT
Details of contact person at LNF
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Description of the activity
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 was achieved when additional pellets were injected (up to two additional pellets). It was demonstrated that the synergetic effect of multi-pellets, injected one shortly after another one, opens new NBI operating scenarios to achieve high confinement in TJ-II plasmas – Multi-Pellet induced Enhanced Confinement (MPiEC) mode.
It has been found that the injection of more than one pellet has a clear impact on the quality of the MPiEC 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 and balanced NB injection configurations and the impact of magnentic 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 and MPiEC, 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, balanced, max. power in both injectors) in order to optimize MPiEC mode while varying pellet settings (size, pellet flight path, and time delays between them). We plan to increase the number of pellets up to four. We will also investigate the evolution of plasma potential and Er profiles (HIBPs and DR) as well as of plasma potential, Epol, and density fluctuations and coherence between them – all ingredients of turbulent particle flux (HIBP). These data will be supported by independent measurements of the plasma density profiles (HIBPs and DR) and its fluctuations for these plasma conditions. In addition, the evolution of poloidal turbulence rotation for plasma potential and density perturbation will be investigated during PiEC and MPiEC modes (HIBP). Long-Range Correlation (HIBP1-HIBP2A) plasma potential (LRC) and Medium-Range Correlation (HIBP2A-HIBP2B) plasma potential (LRC) will be also investigated during PIEC and MPIEC modes.
International or National funding project or entity
Include funding here (grants, national plans) Studies of fuelling and impurity control in the stellarators TJ-II and W7-X through the use of cryogenic and TESPEl pellets (Estudios de abastacimiento y control de impurezas en los stellarators mediante el uso de perdigones criogenicos y TESPEL", Ref: PID2020-116599RB-I00
Description of required resources
- Number of plasma discharges or days of operation: 3 days (scan in NBI scenarios: Co, CNTR and balanced) and 2 days for scan in magnetic configuration.
- Essential diagnostic systems: TS, HIBP, Doppler Reflectometer, He-Beam, Pellet injection
- Type of plasmas (heating configuration): ECRH, NBI
- Specific requirements on wall conditioning if any: Good control of NBI heated plasmas. Recent lithium and boron wall coatings
- External users: need a local computer account for data access: no
- Any external equipment to be integrated? No:
Preferred dates and degree of flexibility
Preferred dates: (March when NBI is well conditioned)