Plasma Fuelling using the Pellet Injector
Name and affiliation of proponent
K.J. McCarthy, N. Panadero, J. L. Velasco, J. Hernández, E. de la Cal. Laboratorio Nacional de Fusión
Details of contact person at LNF (if applicable)
Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)
Since the start of pellet injector operation, 0.66, 0.76 and 1 mm (the latter for NBI only) diameter pellets (containing ≤8x10^18, ≤1.6x10^19, and ≤4x10^19 H particles, respectively) have been successfully injected into ECRH, ECRH & NBI, and NBI-only heated plasmas created using the 100_44_64 configuration. Although the result has been significantly increased electron densities, it is found that fuelling efficiency (ratio of deposited particles to injected particles) varies from ~30% in ECRH plasmas to ~80% for high-density NBI-heated plasmas.
In all cases, pellet ablation is monitored across the plasma radius using photodiodes, and a fast camera. Furthermore, using multiple Thomson Scattering profiles, it has been possible to study particle diffusion, deposition and confinement. However, fast radial drift that leads to large particle losses, and hence low efficiency, remains to be evaluated. In addition, using the Doppler Reflectometer, which is sensitive to a local density perturbation, it may be possible to study the plasmoid that extends toroidally out from the neutral cloud surrounding an ablated pellet along magnetic field lines. if successfull this will be important for studying pellet particle deposition in stellarator devices.
In the first instance, it is intended to broaden the current pellet fuelling database by performing injections into a broad range of magnetic configurations in order to determine efficiencies, as well as pellet penetration, ablation processes, particle drift and diffusion, plus the possible role of magnetic islands and magnetic well/hill.
In parallel, is intended to determine if it is possible to increase fuelling efficiency by firstly injecting a small pellet (or using a gas puff) to pre-cool the outer plasma core just prior to injecting a fuelling pellet (here Δt ≥ 1 ms for sequential pellet injections). The TJ-II pellet injector is unique for undertaking this study as up to 4 pellets can be injected simultaneously.This will allow us to perform the first experimental comparison between pre-cooling by pellet injection and by gas puff in a magnetically confined plasma device. In parallel, it is intended to determine if the Doppler Reflectrometer measurements can provide insight into plasmoid elongation and location.
If applicable, International or National funding project or entity
MEC ENE2013-48679-R, plus EUROFUSION: WP17.S1.A4.T.1 and WP17.S1.A4.T.2
Description of required resources
- Number of plasma discharges or days of operation: 5 days
- Essential diagnostic systems: Pellet Injector, fast camera, Thomson scattering, microwave interferometer, HIBPs, Doppler Reflectrometer.
- Type of plasmas (heating configuration): ECRH and NBI, 100_24_58 to 100_54_66
- Specific requirements on wall conditioning if any: Good plasma control
- External users: need a local computer account for data access: yes/no
- Any external equipment to be integrated? Provide description and integration needs:
Preferred dates and degree of flexibility
Preferred dates: (format dd-mm-yyyy) Impracticable dates: (14-02-2017 to 16-02-2017), los jueves (02-03-2017 a 30-03-2017), los martes (07-03-2017 a 21-03-17)