Density wavenumber spectrum measurements, synthetic diagnostic development, and tests of quasilinear turbulence modeling in the core of electron-heated DIII-D H-mode plasmas

Measurements of the turbulent density wavenumber spectrum, delta n(e)(k(perpendicular to)), using the Doppler Back-Scattering (DBS) diagnostic are reported from DIII-D H-mode plasmas with electron cyclotron heating (ECH) as the only auxiliary heating method. These electron-heated plasmas have low collisionality, nu*(e) < 1, T-e/T-i > 1, and zero injected torque - a regime expected to be relevant for future fusion devices. We probe density fluctuations in the core (rho approximate to 0.7) over a broad wavenumber range, 0.5 <= k(perpendicular to) <= 16 cm(-1) (0.1 <= k(perpendicular to)rho(s) <= 5) to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, P-s(k(perpendicular to)) - which is directly measured by DBS - to the underlying electron density fluctuation spectrum, delta n(k(perpendicular to)). The synthetic DBS P-s(k(perpendicular to)) spectrum is calculated by combining the SCOTTY beam-tracing code with a model delta n(k(perpendicular to)) predicted either analytically or numerically. In this work we use the quasi-linear code TGLF to approximate the delta n(k(perpendicular to)) spectrum. We find that TGLF, using the experimental profiles, is capable of closely reproducing the DBS measurements. Both the DBS measurements and the TGLF-DBS synthetic diagnostic show a wavenumber spectrum with variable decay. The measurements show weak decay (k(-0.6)) for k < 3.5 cm(-1), with k(-2.6) at intermediate-k (3.5 <= k <= 8.5 cm(-1)), and rapid decay (k(-9.4)) for k > 8.5 cm(-1). Scans of physics parameters using TGLF suggest that the normalized del T-e scale-length, R/LTe, is an important factor for distinguishing microturbulence regimes in these plasmas. A combination of DBS observations and TGLF simulations indicate that fluctuations remain peaked at ITG-scales (low k) while R/LTe-driven TEM/ETG-type modes (intermediate/high k) are marginally sub-dominant.