TJ-II: On the physics of the density limit

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Experimental campaign

Autumn 2021

Proposal title

TJ-II: On the physics of the density limit: edge biasing

Name and affiliation of proponent

P. Diamond, University of California San Diego, USA

Details of contact person at LNF

Carlos Hidalgo

Description of the activity

Motivation. Recent experiments in TJ-II have addressed the relation between the density limit (DL) and the formation of low frequency, coherent electrostatic fluctuations in NBI heated plasmas. Once the density limit is achieved, highly coherent low frequency (f < 10 kHz) fluctuations in plasma potential with a near-zero phase difference between distant probe signals were detected. The increase in the amplitude of low frequency Zonal Flows like structures near the DL is qualitatively correlated to the increase in the level turbulence and the negative values in the radial electric field [1].

From the theoretical perspective, it has been argued that the collapse of Zonal Flows could be linked to an increase of radial transport that can lead to edge cooling limiting the plasma density [2].

Goals. The goal of the present research proposal is twofold: 1. Explore edge turbulence spreading and SOL width as approaching the density limit 2. Explore whether edge biasing can affect the density limit by driving external edge sheared flows.

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: edge probes, fast camera with triple bundle, HIBP, Doppler and edge biasing
  • Type of plasmas (heating configuration): NBI where the density limit will be surpassed during density ramps
  • Specific requirements on wall conditioning if any:
  • External users: need a local computer account for data access: yes/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. Fernandez- Ruiz et al., Master Thesis 2021
  2. Singh & Diamond Nuclear Fusion 2021 (in press)

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