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

Journal Article · Mon Nov 13 23:00:00 EST 2023 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/ad0906· OSTI ID:2208880

Pratt, Q.; Hall-Chen, V.; Neiser, T. F.; Hong, R.; Damba, J.; Rhodes, T. L.; Thome, K. E.; Yang, J.; Haskey, S. R.; Cote, T.; Carter, T.

Abstract

Measurements of the turbulent density wavenumber spectrum, $\delta {\hat{n}}_e\left(k_{\perp }\right)$ , 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, ${\nu }_e^{\ast }<1$ , $T_e/T_i>1$ , 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, $0.5\text{⩽}{k}_{\perp }\text{⩽}16$ cm ⁻¹ ( $0.1\text{⩽}{k}_{\perp }{\rho }_s\text{⩽}5$ ), to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, $P_s\left(k_{\perp }\right)$ —which is directly measured by DBS—to the underlying electron density fluctuation spectrum, $\delta {\hat{n}}_e\left(k_{\perp }\right)$ . The synthetic DBS $P_s\left(k_{\perp }\right)$ spectrum is calculated by combining the SCOTTY beam-tracing code with a model $\delta {\hat{n}}_e\left(k_{\perp }\right)$ predicted either analytically or numerically. In this work we use the quasi-linear code Trapped Gyro-Landau Fluid (TGLF) to approximate the $\delta {\hat{n}}_e\left(k_{\perp }\right)$ 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 ( $3.5\text{⩽}k\text{⩽}8.5$ cm ⁻¹ ), and rapid decay ( k ^−9.4 ) for k  > 8.5 cm ⁻¹ . Scans of physics parameters using TGLF suggest that the normalized $\mathrm{\nabla }{T}_e$ 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.

• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: FC02-04ER54698
• OSTI ID: 2208880
• Alternate ID(s): OSTI ID: 2204547; OSTI ID: 2205231; OSTI ID: 2248052
• Journal Information: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 1 Vol. 64; ISSN 0029-5515
• Publisher: IOP Publishing
• Country of Publication: IAEA
• Language: English