TJ-II: Impurity injection with TESPEL in TJ-II stellarator

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Experimental campaign

2019 Autumn

Proposal title

'TJ-II: impurity injection with TESPEL in TJ-II stellarator

Name and affiliation of proponent

N. Tamura, National Institute for Fusion Science, Toki, Japan

Details of contact person at LNF

Kieran Mc Carthy

Description of the activity

In the TJ-II heliac, in-surface electrostatic potential asymmetries have been found in the edge region with Langmuir probes,[1] and very recently in the core region with heavy ion beam probes. These differences of electrostatic potential could have possibilities of giving rise to substantial changes of radial impurity flux, and then this variation may be a key component of understanding a HDH mode in the W7-AS stellarator and an impurity hole in the LHD heliotron. Therefore, the effect of such in-surface electrostatic potential variations on the impurity transport must be investigated experimentally. To this end, it is intended to continue the tracer-encapsulated solid pellet (TESPEL) injections performed in TJ-II during 2015 and 2016,[2] when 300 μm TESPELs were injected into ECRH & NBI plasmas. As we already know, the TESPEL can deposit locally the tracer impurity particles, with a known amount, inside the plasma. [3] Owing to these advantages, we could investigate more clearly its effect on the in-surface impurity density variations. In the proposed experiment, a plasma density scan will be a basic action. In this line, we can also investigate a parametric window of tracer impurity accumulation in TJ-II. This investigation could shed light on the nature of the impurity accumulation in helical plasmas. If time permitted, we would like to check the ECRH effect of the tracer impurity accumulation in TJ-II.

The scheme of the experiment proposed here is as follows: • Tracer impurity will be injected by a tracer-encapsulated solid pellet (TESPEL). As a secondary study, VUV emission lines of TESPEl impurities are of high interest for plasma impurity studies. • Plasma density will be scanned from low to high. The experiment, with the same condition as which the HIBP and/or Langmuir probes have confirmed the electrostatic potential asymmetries previously, will be repeated.

• [If time permits] ECRH effect will be investigated by 1. ECRH power scan


International or National funding project or entity

If applicable, enter funding here or write N/A

Description of required resources

Required resources:

  • Number of plasma discharges or days of operation:
  • Essential diagnostic systems: VUV spectrometer, bolometers and SXR, ECE, HIBP, Doppler reflectomety and Langmuir probes
  • Type of plasmas (heating configuration): ECRH & NBI
  • 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: TESPEL injector

Preferred dates and degree of flexibility

Preferred dates:(February or March 2020

References

  1. M.A. Pedrosa et al., Nucl. Fusion 55 (2015) 052001
  2. N. Tamura et al., Rev. Sci. Instrum. 87 (2016) 11D6191
  3. N. Tamura et al., Plasma Fusion Res. 10 (2015) 1402056

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