TJ-II:Understanding an often observed transient rise in core electron temperature during pellet injection into TJ-II plasmas

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Experimental campaign

2018 Spring

Proposal title

Understanding an often observed transient rise in core electron temperature during pellet injection into TJ-II plasmas

Name and affiliation of proponent

K.J. McCarthy, J.L. Velasco, N. Panadero, J. Hernández, E. de la Cal, Laboratorio Nacional de Fusión, CIEMAT, Spain, N. Tamura, National Institute for Fusion Science, Toki, Japan, M. Calvo, Universidad Politécnica de Madrid, Madrid, 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)

Pellet injection is a widely used tool in magnetically confined plasma devices for core fueling and for studying impurity transport, as well as other for other objectives.[1] While much of the physics of pellet ablation and of the subsequent particle deposition and transport processes is understood, significant research is still needed to achieve complete knowledge. For instance, phenomena such as cold waves that travel ahead of ablating pellets have been detected in some devices or increased core electron temperatures following pellet injections are reported in the literature.[2] However, satisfactory explanations for such occurrences are still required. Moreover, it is known that while pellet penetration into a plasma is dependent on plasma electron density, Ne, as well as on pellet mass and velocity, it is most sensitive to plasma electron temperature, Te.[3] Hence, given that particle deposition is dependent on the pellet ablation profile and penetration depth, it is imperative to evaluate and comprehend any phenomenon that may modify these as well as to understand the plasma response to injections.[4]

A short-lived (≤200 μs) transient rise of core electron temperature has often been observed before an injected pellet is completely ablated by the TJ-II plasma.[5] It is detected by both the Thomson Scattering and Electron Cyclotron Emission diagnostic systems and when observed, this core temperature rise begins within ~100 μs after a pellet enters the plasma through the last-closed magnetic flux surface, as it is approaches the plasma core. Such behaviour occurs in plasmas maintained by either on- or off-axis electron cyclotron resonance heating. It is postulated that a steepening of the radial temperature gradient leads to a more positive radial electric field in the core so that the plasma moves deeper in Core Electron Root Confinement. The resultant improved confinement of injected heating power then leads to the raised core temperature. Conversely, it is observed that, when a pellet is injected into plasma with a peaked core electron temperature profile, the recovery time for core temperatures is significantly longer than for edge temperatures. Given the possible implications for pellet penetration and particle deposition, it is intended to make a systematic study by injecting pellets in plasmas in which such a core temperature rise is observed and using the TS diagnostic to understand the influence of the temperature gradient.

If applicable, International or National funding project or entity

FIS2017-89326-R

Description of required resources

Required resources:

  • Number of plasma discharges or days of operation: 2 days
  • Essential diagnostic systems:Thomson Scattering, microwave interferometer, ECE, soft x-rays, and plasma current measurements. Fast-frame camera with fibre-optic bundle.
  • Type of plasmas (heating configuration): ECRH
  • 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, 24-04-2018 to 26-04-2018)

References

  1. KJ McCarthy et al, Nucl Fusion 57 (2017) 056039
  2. P. Mantica, et al. Phys. Rev. Lett., 82 (1999) 5048
  3. L. R. Baylor, et al, Nucl. Fusion, 37 (1997) 445
  4. K. J. McCarthy, et al,Europhys lett 120 (2017) 25001
  5. K. J. McCarthy, et al, Proc. 43rd EPS Conference, Leuven, Belgica (2016)

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