(TH/1-3) Identification of TEM Turbulence through Direct Comparison of Nonlinear Gyrokinetic Simulations with Phase Contrast Imaging Density Fluctuation Measurements

Abstract.   Nonlinear gyrokinetic simulations of Trapped Electron Mode (TEM) turbulence have reproduced measured particle fluxes and thermal energy fluxes, within experimental uncertainty, in Alcator C-Mod. This has provided a model for internal transport barrier control with on-axis ICRH in Alcator C-Mod, without adjustable model parameters. The onset of TEM turbulent transport limits the density gradient, preventing radiative collapse. Here we move beyond comparisons of simulated and measured fluxes to a more fundamental and direct comparison with density fluctuation spectra. Using a new synthetic diagnostic, excellent agreement is obtained between wavelength spectra from nonlinear GS2 simulations, and spectra measured by Phase Contrast Imaging. The density fluctuations are associated with the steep density gradient in the C-Mod ITB, which provides spatial localization for the chordal PCI measurement. Gyrokinetic stability analysis shows that Trapped Electron Modes are strongly destabilized inside the ITB foot by the addition of on-axis ICRH. Nonlinear GS2 simulations reproduce the relative increase in fluctuation level when on-axis heating is applied. Further, we have extended the GS2 Lorentz collision operator to include classical diffusion associated with the ion finite Larmor radius, and have implemented collisional energy diffusion, together with particle, momentum, and energy conservation terms. Classical diffusion is shown to strongly stabilize shorter wavelength trapped electron modes for realistic C-Mod collisionalities. A series of detailed nonlinear gyrokinetic simulations show the nonlinear upshift in the TEM critical density gradient increases favorably with collisionality.

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