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, δ n ˆ e (k ⊥) , using the Doppler Back-Scattering (DBS) diagnostic are reported from DIII-D H-mode plasmas with electron cyclotron heating as the only auxiliary heating method. These electron-heated plasmas have low collisionality, ν 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 (ρ ≈ 0.7) over a broad wavenumber range, 0.5 ⩽ k ⊥ ⩽ 16 cm⁻¹ ( 0.1 ⩽ k ⊥ ρ 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 ⊥) —which is directly measured by DBS—to the underlying electron density fluctuation spectrum, δ n ˆ e (k ⊥). The synthetic DBS P s (k ⊥) spectrum is calculated by combining the SCOTTY beam-tracing code with a model δ n ˆ e (k ⊥) predicted either analytically or numerically. In this work we use the quasi-linear code Trapped Gyro-Landau Fluid (TGLF) to approximate the δ n ˆ e (k ⊥) 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 ∇ T e scale-length, R / L T e , 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 / L T e -driven TEM/ETG-type modes (intermediate/high k) are marginally sub-dominant. [ABSTRACT FROM AUTHOR]