TJ-II:Improving fuelling efficiency in TJ-II ECRH plasmas

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Experimental campaign

2018 Spring

Proposal title

Improving fuelling efficiency in TJ-II ECRH plasmas

Name and affiliation of proponent

M. Calvo (Universidad Politécnica de Madrid, Spain), K. J. McCarthy, N. Panadero, J. L. Velasco, J. Hernández, E. de la Cal (Laboratorio Nacional de Fusión, CIEMAT, Spain)

Details of contact person at LNF (if applicable)

Kieran J McCarthy

Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)

Since starting pellet injection operation in the TJ-II, 0.42, 0.66, 0.76 and 1 mm (the latter for NBI only) diameter pellets have been successfully injected into ECRH, ECRH & NBI, and NBI-only heated plasmas. Although injection results in increased electron densities, fuelling efficiency (number of deposited particles/number of injected particles) is found to be less than ~40% for ECRH plasmas whilst it is from 40% to ~80% in NBI-heated plasmas.[1] During injections, pellet penetration is monitored using photodiodes, whereas pellet particle diffusion, deposition and confinement is studied using a fast-frame camera, while fuelling efficiency is determined using Thomson Scattering profiles from reproducible plasmas.[2] In ECRH plasmas, it is found that fast outwards radial drifting leads to large particle losses.[3] As a pellet is ablated, the self regulating partially ionized cloud (plasmoid) that forms around the pellet occasionally detaches. Thereafter, this palsmoid drifts outwards due to a ExB acceleration. Since plasmoid drifting is most important for particles ablated close to the plasma outer edge, it is considered that deeper penetration is needed to improve efficiency. Moreover, since pellet ablation is most sensitive to electron temperature, one means of achieving deeper penetration is to first inject a small 0.42 mm pellet to cool the outer plasma and then, immediately afterwards, inject a large fuelling pellet.[4] However, penetration depth is critical as pellet ablation rate falls quickly once a pellet has penetrated beyond the plasma axis. A second means of cooling the plasma edge is by gas puffing, albeit particle accounting become more difficult.

Such a study is possible with the TJ-II pellet injector system as it permits a series of pellets to be discharged with millisecond time intervals between pellets. It is intended to inject pairs of pellets into TJ-II, the first pellet being the smaller. The study will consist in changing the time interval between pellets and determining the fuelling efficiency achieved. This will be done for a fixed magnetic configuration in the first instance. Similar injections will be attempted with gas puffing as the cooling mechanism.

If applicable, International or National funding project or entity

FIS2017-89326-R and WP18.S1.A4

Description of required resources

Required resources:

  • Number of plasma discharges or days of operation: 2 days
  • Essential diagnostic systems:Fast-frame camera with fibre-optic bundle. TS, microwave interferometer, ECE, and plasma current measurements.
  • Type of plasmas (heating configuration): ECRH with Ne > 4e18 m^-3
  • Specific requirements on wall conditioning if any:
  • External users: need a local computer account for data access: no
  • Any external equipment to be integrated? Provide description and integration needs:

Preferred dates and degree of flexibility

Preferred dates: Not available 12-04-2018, from 24-04-2018 to 26-04-2018.

References

  1. K. J. McCarthy, et al., Nucl. Fusion 57 (2017) 056039
  2. K. J. McCarthy, et al, Proc of Science (ECPD2015) 134
  3. N. Panadero, et al, Nucl. Fusion 58 (2018) 026025
  4. L. R. Baylor, et al, Nucl. Fusion, 37 (1997) 445

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