Validation of gyrokinetic simulations with measurements of electron temperature fluctuations and density-temperature phase angles on ASDEX Upgrade

Abstract

Measurements of turbulent electron temperature fluctuation amplitudes, δT e⊥ /Te, frequency spectra and radial correlation lengths, Lr(T e⊥ ), have been performed at ASDEX Upgrade using a newly upgraded Correlation ECE diagnostic in the range of scales k ⊥ < 1.4cm −1 , kr < 3.5cm(k ⊥ ρs < 0.28 and krρs < 0.7). The phase angle between turbulent temperature and density fluctuations, αnT , has also been measured by using an ECE radiometer coupled to a reflectometer along the same line of sight. These quantities are used simultaneously to constrain a set of ion-scale nonlinear gyrokinetic turbulence simulations of the outer core (ρtor = 0.75) of a low density, electron heated L-mode plasma, performed using the gyrokinetic simulation code, GENE. The ion and electron temperature gradients were scanned within uncertainties. It is found that GK simulations are able to match simultaneously the electron and ion heat flux at this radius within the experimental uncertainties. The simulations were performed based on a reference discharge for which δT e⊥ /Te measurements were available, and Lr(T e⊥ ) and αnT were then predicted using synthetic diagnostics prior to measurements in a repeat discharge. While temperature fluctuation amplitudes are overestimated by > 50% for all simulations within the sensitivity scans performed, good quantitative agreement is found for Lr(T e⊥ ) and αnT . A validation metric is used to quantify the level of agreement of individual simulations with experimental measurements, and the best agreement is found close to the experimental gradient values.

Validation study of GENE on ASDEX Upgrade using perturbative thermal diffusivity measured with partial sawtooth heat pulses

“…The period of the time average only begins after initial transients have died off in the simulation, and the effect of E × B shear has been fully realized. Figure 4 shows the time trace from the heat flux ‡ Note that the uncertainty in Pe quoted in this study (18 %) is larger than that in Reference [34] (14 %). This is due to additional uncertainty discovered in the routines that calculate the radiated power profiles since the publication of that study.…”

“…Note that the validation study presented in this work is for the same plasma discharge used in the validation study in Reference [34]. The work here, however, focuses on a different radial location and utilizes the perturbative thermal diffusivity as an additional validation constraint (for the first time on ASDEX Upgrade), instead of fluctuation measurements.…”

“…Please see Reference [34] for the full profiles in this discharge. These diagnostics have the following uncertainties, calculated by combining variations in time with the instrumental uncertainty quoted in each diagnostic reference: 15 % in T e , 20 % in T i , 10 % in n e , and 50 % in v rot .…”

“…To put the strong agreement between experiment and simulation with electron and ion heat fluxes and perturbative diffusivity into context, consider the validation metric, χ, originally developed in Reference [45] and used in Reference [34] (not to be confused with the perturbative diffusivity χ pert e ). χ is to some extent a weighted sum of the differences between the experimental and simulated quantities.…”

Advances in turbulence measurements using new Correlation ECE and nT-phase diagnostics at ASDEX Upgrade

“…We find that the CECE channel comb significantly improves the reliability of the radial alignment between radiometer and reflectometer. All quantities have been measured simultaneously at the same radius in dominantly electron heated L-mode plasma and have been compared directly to nonlinear ion scale GK simulations [7]. It is found that both electron and ion heat flux can be matched, along with α nT and L r (T e⊥ ), however δT e⊥ /T e is higher in the simulations than measured.…”