Turbulence radial correlation length measurements using Doppler reflectometry in TJ-II

Abstract

First measurements carried out using radial correlation Doppler reflectometry in the TJ-II stellarator are presented. Broad scans in the probing beam angle are performed to study the behaviour of the radial correlation length measured by Doppler reflectometry. A comparison with the simulation results obtained using a two-dimensional full-wave code (Blanco and Estrada 2013 Plasma Phys. Control. Fusion 55 125006) indicates that both linear and non-linear regimes are found. In linear regimes, the procedure to calculate the best estimation for the turbulence radial correlation length found in the simulations is applied providing sensible results. These measurements, viewed as a validation of the simulation results, show that Doppler reflectometry provides a good estimation of the turbulence radial correlation length in linear regimes if the probing beam angle is scanned.

Plasma flow, turbulence and magnetic islands in TJ-II

“…on the probing beam frequency and angle. This dependence has been also confirmed in previous TJ-II experiments [17]. In any case, the results obtained with the Doppler reflectometer synthetic diagnostic suggest that a Doppler reflectometry diagnostic should be able to capture the signature of a magnetic island.…”

Core turbulence behavior moving from ion-temperature-gradient regime towards trapped-electron-mode regime in the ASDEX Upgrade tokamak and comparison with gyrokinetic simulation

“…The Doppler reflectometry measurement is scale-selective, i.e. the measurement depends on the turbulent structure scale k ⊥ , such that the turbulence response to a change in plasma parameters can be monitored for a variety of different structure sizes, such that measurements of wavenumber spectra become possible [41][42][43][44][45][46]. In the experiments reported here, the diagnostic scale selectivity has been varied to measure turbulent structures from about 0.3 mm to 1.5 cm in a radial range of ρ pol = 0.5 -1.0, where ρ pol is the normalized poloidal flux radius, such that about half of the AUG minor radius has been monitored.…”

A theoretical investigation of the turbulent structures tilting measurements with radial correlation Doppler reflectometry

“…To obtain the signal corresponding to linear scattering it is enough to substitute expression for the unperturbed electric field (given by the solution of equation ( 1) when perturbation δk is neglected) into the expression (2). Considering the background density profile to be linear, the solution of equation ( 1) without the density fluctuations can be expressed in terms of an integral over Airy functions corresponding to poloidal harmonics of the probing beam:…”

“…Since full-wave calculations are demanding and results of the paper were obtained within the Born approximation, a faster approach, similar to the one described in [21] was used. Namely, the solution (3) of the unperturbed Helmholtz equation combined with turbulence spectrum (11) was used to calculate the CCF according to (2) and (8) without the need to average over random fluctuations or to calculate perturbed electric field.…”