# TJ-II:Turbulence properties near a rational surface

Spring 2022

## Proposal title

Turbulence properties near a rational surface

N/A

## 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]

N/A

## 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

## References

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.