TJ-II: Influence of edge radial electric fields on impurity transport

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Experimental campaign

Spring 2022

Proposal title

TJ-II:Influence of edge radial electric fields on impurity transport

Name and affiliation of proponent

Dominique Escande, Marseille University

Eduardo de la Cal, Igor Voldiner, Marian Ochando, Carlos Hidalgo, Ciemat

Details of contact person at LNF

Eduardo de la Cal, Ciemat

Description of the activity

Rational. It has been recently argued that the existence of a time delay in the feedback loop relating radiation and impurity production on divertor plates can affect the density limit [1]. Furthermore, recent experiments in the TJ-II stellarator have shown the presence and amplification of low frequency Zonal Flows in the vicinity of this limit [2]. Whether the threshold radiation value for the density limit would be partially affected by the amplitude of (fluctuating and DC) zonal flows as well as the role of radiation to reduce the free energy available to the turbulence and the ZF-drive are open questions.

Goal. It is proposed to investigate the role of edge radial electric fields and low frequency zonal flows on impurity (He) inward transport in the plasma boundary region in the TJ-II stellarator.

Scenario. Experiments will be carried out in the proximity of the elctron-ion root transition and in the proximity to de NBI density limit.

The ambipolarity condition (i.e. the equality of ion and electron fluxes) determining the radial neoclassical electric field has two stable roots in stellarators: the ion root with typically negative Er, usually achieved in high density plasmas, and the electron root with positive Er, that is typically realized when electrons are subject to strong heating. In addition the neoclassical viscosity vanishes as electron – ion root transition [J.L. Velasco et al., PRL-2012]. This allows large deviations of Er from NC ambipolarity including the amplification of zonal flows [M.A. Pedrosa et al., PRL-2008]. The electron – ion root transition allows to modify DC radial electric fields in a continuos and controlled manner.

Diagnostics. Recently TJ-II diagnostic capabilities have been expanded to include a fast camera system with a gas puffing injection that allows 3 simultaneous filtered frames to apply the He I ratio technique.

International or National funding project or entity

Include funding here (grants, national plans)

Description of required resources

Required resources:

  • Number of plasma discharges or days of operation: 2
  • Essential diagnostic systems: Fast caremas with triple boundle
  • Type of plasmas (heating configuration): ECRH (electron - ion root) and NBI (density limit)
  • 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: (format dd-mm-yyyy)

References

  1. D.F. Escande et al 2022 Nucl. Fusion 62 026001
  2. D. Fernández-Ruiz et al 2021 Nucl. Fusion 61 126038

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