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, , 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, , , 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, cm−1 (), to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, —which is directly measured by DBS—to the underlying electron density fluctuation spectrum, . The synthetic DBS spectrum is calculated by combining the SCOTTY beam-tracing code with a model predicted either analytically or numerically. In this work we use the quasi-linear code Trapped Gyro-Landau Fluid (TGLF) to approximate the 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 ( cm−1), and rapid decay (k −9.4) for k > 8.5 cm−1. Scans of physics parameters using TGLF suggest that the normalized scale-length, , 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 -driven TEM/ETG-type modes (intermediate/high k) are marginally sub-dominant.