Direct Comparison of Gyrokinetic Turbulence Simulations with Phase Contrast Imaging Fluctuation Measurements

We have developed a synthetic diagnostic$^1$ for the GS2 gyrokinetic code for direct comparisons with phase contrast imaging (PCI) measurements of density fluctuations in Alcator C-Mod. The gyrokinetic simulation is carried out in a local, field line following flux-tube, while PCI measures density fluctuations along 32 chords passing vertically through the plasma cross-section.$^2$ Transforming from Clebsch to cartesian coordinates, and integrating appropriately over portions of the flux tube viewed by the diagnostic, yields a density fluctuation spectrum versus wavenumber $k_R$ in the major radius direction. To achieve vertical localization, we examine an ITB case in which the spectrum is dominated by a strong trapped electron mode, localized near the half-radius. The wavelength spectrum from the simulations, using the synthetic diagnostic, closely reproduces the PCI spectrum. Contributions from $k_{\psi}$, where $\mathbf{B}=\nabla \alpha \times \nabla \psi $, downshift the GS2 $k_{\alpha}$ spectrum to improve upon our previous raw comparison with the PCI $k_R$ spectrum.$^3$ \\ $^1$A. Long, D. R. Ernst \textit{et al.,} Bull. Am. Phys. Soc. \textbf{50}(8) p. 153, GP1.48, also \urllink{p. 235, LP1.37} {http://www.psfc.mit.edu/research/alcator/pubs/APS/APS2005/ernst.pdf}. $^2$N. P. Basse \textit{et al.,} Phys. Plasmas \textbf{12}, 052512 (2005). $^3$D. R. Ernst \textit{et al.,} 2004 IAEA Fusion Energy Conference, \urllink{IAEA-CN116/TH/4-1} {http://www-naweb.iaea.org/napc/physics/fec/fec2004/datasets/TH4-1.html}, see also Phys. Plasmas \textbf{11} (2004) 2637.