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, $\delta \hat{n}_e(k_\perp)$ , 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, $\nu^*_e \lt 1$ , $T_e/T_i \gt 1$ , 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, $0.5 \unicode{x2A7D} k_\perp \unicode{x2A7D} 16$ cm ^−1 ( $0.1 \unicode{x2A7D} k_\perp\rho_s \unicode{x2A7D} 5$ ), to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, $P_s(k_\perp)$ —which is directly measured by DBS—to the underlying electron density fluctuation spectrum, $\delta \hat{n}_e(k_\perp)$ . The synthetic DBS $P_s(k_\perp)$ spectrum is calculated by combining the SCOTTY beam-tracing code with a model $\delta \hat{n}_e(k_\perp)$ predicted either analytically or numerically. In this work we use the quasi-linear code Trapped Gyro-Landau Fluid (TGLF) to approximate the $\delta \hat{n}_e(k_\perp)$ 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 8.5 cm ^−1 . Scans of physics parameters using TGLF suggest that the normalized $\nabla T_e$ 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.

Keywords

• density fluctuation wavenumber spectrum
• Doppler back-scattering
• ECH H-mode
• synthetic diagnostics