TJ-II:Turbulence properties near a rational surface: Difference between revisions

In previous work, we have established that the intermittence parameter C(1) varies in a systematic way near rational surfaces<ref>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.</ref>. 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<ref>B.Ph. van Milligen et al., ''Parallel and perpendicular turbulence correlation length in the TJ-II stellarator''. Nucl. Fusion, 53:093025 (2013)</ref> were revisited, and a remarkably detailed confirmation of this phenomenon was obtained<ref>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.</ref>. The latter paper also suggested that a radial electric field (i.e., poloidal rotation) may affect the intermittence parameter significantly.

The plan is to scan iota between configurations 100_40 (or 100_38) and 100_44, while the B and D Langmuir probes are located at <math>\rho \simeq 0.85-0.90</math> (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.

Apart from intermittence, we will quantify all other turbulence parameters that can be measured by the probes: <math>V_f, I_{\rm sat}, E_r, E_\theta, v_\theta, \Gamma</math>, the RMS of various quantities, long-range correlations between probes B and D, cross phase (between <math>E_\theta, I_{\rm sat}</math>), etc.<ref>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.</ref>