New measurements of H-mode core density fluctuation wavenumber spectra and tests of quasilinear turbulence modeling*

Measurements of the density fluctuation wavenumber spectrum, δn_e(k), obtained with Doppler backscattering (DBS) in ECH-heated H-mode DIII-D plasmas, are reported and used to test predictions from the TGLF code. Remarkable agreement is found between DBS measurements and our novel synthetic DBS diagnostic using measured profiles. The back-scattered power spectrum, P_s(k), was directly measured with DBS over a broad wavenumber range, 0.5 ≤ k ≤ 16 cm^-1 in electron-heated H-mode plasmas possessing low collisionality (ν^*_e < 1), T_e/T_i > 1, and zero injected torque – a regime expected to be relevant for future devices. Measurements reveal a nonuniform spectrum with weak decay (k^-0.6) at low wavenumbers increasing to rapid decay (k^-9.4) at high-k. Starting with the SCOTTY beam tracing code, a novel synthetic DBS diagnostic was developed that allows us to calculate the back-scattered power, P_s, using the TGLF model δn_e(k). TGLF predicts that R/L_Te-driven modes (TEM/ETG) dominate the transport spectra in this plasma regime. Parameter scans with TGLF predict the P_s(k) spectrum is sensitive to small changes in R/L_Te at low and intermediate-k. Interestingly, +10% R/L_Te destabilizes electron modes near k_θρ_s = 1.0, nonlinearly increasing electron thermal and particle fluxes. With +10% R/L_Te, the synthetic DBS diagnostic predicts the formation of a peak near k_θρ_s = 1.0 in the P_s(k) spectrum – which was not observed experimentally. These TGLF predictions, combined with DBS measurements, suggest the mid-radius of this plasma is in a state of mixed ion-electron turbulence. Our results, fluctuation wavenumber spectrum measurements and a novel synthetic diagnostic, allow for significantly improved tests of both reduced turbulence/transport models and nonlinear gyrokinetic simulations (currently underway).