TJ-II:Turbulence properties near a rational surface

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Experimental campaign

Spring 2022

Proposal title

Turbulence properties near a rational surface

Name and affiliation of proponent

B.P. van Milligen, I. Voldiner, B.A. Carreras, C. Hidalgo

Details of contact person at LNF


Description of the activity

In previous work, we have established that the intermittence parameter C(1) varies in a systematic way near rational surfaces[1]. This was found to be the case both in a numerical model of resistive MHD turbulence, and confirmed using data from the W7-X stellarator. Hence, the intermittence parameter provides an indirect diagnostic of the magnetic configuration.

In more recent work, the iota scan experiments of 2013[2] were revisited, and a remarkably detailed confirmation of this phenomenon was obtained[3]. The latter paper also suggested that a radial electric field (i.e., poloidal rotation) may affect the intermittence parameter significantly. In the present experiment, we therefore plan to repeat the iota scan experiments while applying a radial electric field, induced via probe biasing.

The plan is to scan iota between configurations 100_40 and 100_44, while the B and D Langmuir probes are located at $ \rho \simeq 0.85-0.90 $ (the reason being that this scan will move the important 8/5 rational surface across the Langmuir probe location, based on the results from the latest paper cited). The applied voltage to the biasing probe will be [-300, -150, 0, 150, 300], making 5 discharges at each voltage to verify reproducibility[4]. Apart from intermittence, we will quantify all other turbulence parameters that can be measured by the probes: $ V_f, I_{\rm sat}, E_r, E_\theta, v_\theta, \Gamma $, the RMS of various quantities, long-range correlations between probes B and D, cross phase (between $ E_\theta, I_{\rm sat} $), etc.[5]

International or National funding project or entity


Description of required resources

Required resources:

  • Dynamic iota scan between configurations 100_40 and 100_44
  • OH current control
  • Biasing at fixed voltage [-300, -150, 0, 150, 300] V
  • Number of plasma discharges or days of operation: 30 discharges, 1 day
  • Essential diagnostic systems: Langmuir probes
  • Desirable diagnostic systems: Doppler reflectometer, HIBP (at similar radial positions as the Langmuir probes)
  • Type of plasmas (heating configuration): ECRH heated, density constant, below critical density ($ n_e^{\rm crit} \simeq 0.6\cdot 10^{19} {\mathrm m}^{-3} $)

Preferred dates and degree of flexibility

Preferred dates: N/A


  1. B. Carreras, L. García, J. Nicolau, B. van Milligen, U. Hoefel, M. Hirsch, and the TJ-II and W7-X Teams. Intermittence and turbulence in fusion devices. Plasma Phys. Control. Fusion, 62:025011, 2020.
  2. B.Ph. van Milligen et al., Parallel and perpendicular turbulence correlation length in the TJ-II stellarator. Nucl. Fusion, 53:093025 (2013)
  3. B. P. van Milligen, B. Carreras, L. García, and C. Hidalgo. The localization of low order rational surfaces based on the intermittence parameter in the TJ-II stellarator. Nucl. Fusion, 60:056010, 2020.
  4. B.Ph. van Milligen, B.A. Carreras, L. García, G. Grenfell, I. Voldiner, C. Hidalgo, and the TJ-II Team. The impact of radial electric fields and plasma rotation on intermittence in TJ-II. Plasma Phys. Control. Fusion, 2021.
  5. B. van Milligen, B. Carreras, I. Voldiner, U. Losada, C. Hidalgo, and the TJ-II Team. Causality, intermittence and crossphase evolution during confinement transitions in the TJ-II stellarator. Phys. Plasmas, 28:092302, 2021.

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