Turbulence link to L-mode, I-mode, and H-mode confinement in the DIII-D tokamak

Journal Article · 08 July 2024 · Nuclear Fusion

DOI: https://doi.org/10.1088/1741-4326/ad5aae · OSTI ID:2396372

Chen, J. ; Brower, D. L.; McClenaghan, J. ; Yan, Z. ; Hubbard, A. E.; Groebner, R.

Abstract Understanding the physics of low-confinement (L-), improved-confinement (I-), and high-confinement (H-) modes is critical for fusion reactors. The finding herein reports observations of two types of turbulence coexisting near the L-mode edge, one magnetohydrodynamic (MHD)-like and another micro-tearing mode (MTM)-like, linked to the H-mode and I-mode confinement in the DIII-D tokamak. Ion-scale magnetic and density turbulence is measured using a Faraday-effect radial-interferometer-polarimeter and beam-emission-spectroscopy (BES). Broadband turbulence spectra of up to ∼600 kHz are observed in two discharges where transitions between L-mode, I-mode, and H-mode occurs. Turbulence is found to be inversely correlated with confinement, meaning lower turbulence power at higher confinement. Distinctively, the high-frequency (HF, >∼100 kHz) magnetic turbulence power changes by the most (55%) during transitions primarily involving energy confinement change, whereas the low-frequency (LF, <∼100 kHz) magnetic and density turbulence power changes by the most (80%) during transitions primarily involving particle confinement change. The LF turbulence amplitude oscillates with and leads to deuterium-alpha emission oscillations before an H-mode. These results imply that HF turbulence mainly affects energy confinement whereas LF turbulence can affect particle confinement. The magnetic and density turbulence exhibits coherence up to 0.6 and cross-phase magnitude close to $\pi /2$ in most cases, suggesting they have a common origin in both the LF and HF ranges. BES suggests that LF turbulence resides at the edge ( $\rho =0.95$ ) and HF turbulence can be at the outer core ( $\rho =0.8$ ) or edge ( $\rho =0.95$ ). Comparisons of measurements, theory, and gyrokinetic simulations suggest that HF turbulence is MTM-like in all cases, whereas LF turbulence is more consistent with MHD-like modes and the exact instability might change during transitions—except that a drift-wave origin is possible in a low collisionality H-mode. These results suggest that the H-mode involves suppressed MHD-like turbulence, whereas the I-mode mitigates MTM-like turbulence along with largely unchanged MHD-like turbulence.

• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: FC02-04ER54698
• OSTI ID: 2396372
• Alternate ID(s): OSTI ID: 2376247; OSTI ID: 2397332
• Journal Information: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 8 Vol. 64; ISSN 0029-5515
• Publisher: IOP PublishingCopyright Statement
• Country of Publication: IAEA
• Language: English