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

Abstract

Measurements of the turbulent density wavenumber spectrum, δn̂e(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, νe∗ < 1, Te/Ti > 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 ≼ kρs ≼ 5), to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, Ps(k)—which is directly measured by DBS—to the underlying electron density fluctuation spectrum, δn̂e(k). The synthetic DBS Ps(k) spectrum is calculated by combining the SCOTTY beam-tracing code with a model δn̂e(k) predicted either analytically or numerically. In this work we use the quasi-linear code Trapped Gyro-Landau Fluid (TGLF) to approximate the δn̂e(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 ∇Te 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.