TJ-II: Pellet induced Enhanced Confinement: the role of Er and turbulence

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Experimental campaign

Autumn 2021

Proposal title

Pellet induced Enhanced Confinement regime in NBI plasmas: the role of Er and turbulence in PiEC phase

Name and affiliation of proponent

I. García-Cortés, K. McCarthy, Nerea Panadero, Macarena Liniers, Teresa Estrada, Daniel Carralero and HIBP group

Laboratorio Nacional de Fusión, CIEMAT

Details of contact person at LNF

If applicable, enter contact person here or write N/A

Description of the activity

A Pellet-induced Enhanced Confinement (PiEC) regime has been recently reported [1] in discharges made in the TJ-II after a cryogenic fuel pellet is injected into its neutral beam injection (NBI) heated phase. In addition to increased particle density in the plasma core after pellet injection (PI), these plasmas show diamagnetic energy content up to 40% higher to in reference shots without PI. Moreover, the energy confinement time, as determined using a diamagnetic loop, is enhanced when compared to predictions obtained using the International Stellarator Scaling law [2]. In general, the PiEC in TJ-II is characterized by increased density gradients, by enhanced negative radial electric fields that extend from the plasma edge region to the core, as observed by HIBP and Doppler reflectometer systems, as well as by reductions in density, plasma potential and magnetic fluctuations in the plasma core.

However, the underlying physics that can induce this regime is still unclear in TJ-II. Taking into account that Er and turbulence seems to play a key role in similar phenomena observed in the W7-X (enhanced confinement after pellet injection)[3], we propose series of PIs into plasmas with controlled density and different NBI heating powers (from low power, with 1 NBI working, to high power, with both NBI working) in order to study simultaneously the evolution of plasma potential and Er profiles (HIBPs and DR) as well as of the plasma density and its fluctations (HIBPs and DR).

International or National funding project or entity

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

Required resources:

  • Number of plasma discharges or days of operation: 2 days (scan in NBI power and 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
  • 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: 26-10-2021 to 30-11-2021 when HIBPs available


References

  1. I. García-Cortés et al. submitted for publication in Nucl. Fusion
  2. H. Yamada et al., Nucl. Fusion 45 (2005) 1684
  3. T. Estrada et al., Nucl. Fusion 61 (2021) 046008

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