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Measurements of the turbulent density wavenumber spectrum, δne(k⊥), 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, ν*e < 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 ≤ k⊥ρs ≤ 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, δne(k⊥). The synthetic DBS Ps(k⊥) spectrum is calculated by combining the SCOTTY beam-tracing code with a model δne(k⊥) predicted either analytically or numerically. In this work we use the quasi-linear code TGLF to approximate the δne(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.

Measurements of the turbulent density wavenumber spectrum, δnˆe(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⩽k⊥ρs⩽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, δnˆe(k⊥) . The synthetic DBS Ps(k⊥) spectrum is calculated by combining the SCOTTY beam-tracing code with a model δnˆe(k⊥) predicted either analytically or numerically. In this work we use the quasi-linear code Trapped Gyro-Landau Fluid (TGLF) to approximate the δnˆe(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.