A seminar by Bill Heidbrink
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Alfvén eigenmode (AE) induced zonal modes cause dramatic reductions in ion temperature gradient (ITG) turbulence and concomitant increases in both ion and electron temperature Ti and Te in beam-heated, L-mode, DIII-D discharges. Beam emission spectroscopy (BES) data simultaneously measure AE and ITG amplitudes [1,2]. Reductions in turbulence are anti-correlated with AE amplitude on short (~1 ms) and long (~100 ms) timescales in many different discharge conditions but, in some discharges, nearly complete suppression occurs for as long as 375 ms. This strong suppression regime only occurs when the fast-ion AE drive is above a threshold [3]. Onset to this regime occurs when one or more AEs acquire an electrostatic component to their polarization [2]. (The mode polarization is inferred from the difference between electron temperature and density fluctuations and [4,5].) The change in polarization from purely electromagnetic to partially electrostatic is strong evidence that AEs drive the zonal flows (ZF), since (in theory) AEs cannot nonlinearly drive an electrostatic ZF potential otherwise [6]. Once entered, the AE activity gradually evolves, characterized by more modes of smaller radial extent [2,7]. During this transition, a narrow shear flow layer forms, driven by an enhanced Reynolds stress force, with a shearing rate that exceeds the local turbulence decorrelation rate, leading to turbulence suppression [2]. Although evolution into the strongly suppressed state is gradual, the back transition is sudden. Theoretically [6], when AEs excite zonal flows, they also excite zonal currents (ZC). Comparison of motional Stark effect (MSE) data from suppressed discharges with carefully matched unsuppressed discharges show that currents are indeed excited, causing a fractional change in q of ~5% [7]. In suppressed plasmas, Ti and Te rise in both the narrow suppression region and throughout the plasma [2]. These observations fuel optimism that reactor regimes may exist where AEs improve overall performance despite their adverse impact on fast-ion confinement.
*Supported by DOE DE-SC0020337, DE-FC02-04ER54698, DE-SC0019352, DE- SC0020287, and DE-FG02-08ER54999.
[1] “Wave-number based classification of Alfvén eigenmodes and drift-wave turbulence,” K.J. Callahan, X.D. Du et al, J. Plasma Phys. (2026) submitted.
[2] “Microturbulence suppression by Alfvén eigenmodes in the DIII-D tokamak,” X.D. Du, W.W. Heidbrink, Z. Yan et al., Phys. Rev. Lett. 135 (2025) 265101.
[3] “Suppression of ion temperature gradient modes by Alfvén activity above a drive threshold in DIII-D,” X.D. Du, W.W. Heidbrink, Z. Yan et al., Phys. Pl. (2026) submitted.
[4] “First measurement of drift-Alfvén wave polarization in magnetically confined fusion plasmas,” X.D. Du, Liu Chen, W.W. Heidbrink et al., Phys. Rev. Lett. 132 (2024) 215101.
[5] “Measurements of the polarization of several instabilities in the DIII-D tokamak,” W.W. Heidbrink, X.D. Du, Liu Chen et al., Nucl. Fusion 65 (2025) 112002.
[6] “The effects of zonal fields on energetic particle excitations of reversed-shear Alfvén eigenmode: simulation and theory,” Liu Chen et al., Nucl. Fusion 65 (2025) 016018.
[7] “First observation of fine-scale currents induced by Alfvén eigenmodes,” W.W. Heidbrink, X.D. Du et al., Phys. Rev. Lett. (2026) submitted.
Professor Heidbrink earned his B.A. degree from the University of California, San Diego in 1977. For the next two years he performed industrial research in pulsed power at Maxwell Laboratories. In 1984, he received his Ph.D. from Princeton University. After working as a staff member on the TFTR tokamak (Princeton) and the DIII-D tokamak (General Atomics), he joined the UCI Physics Department in 1988. He was the 1995 recipient of the Lauds & Laurels award for Distinguished Teaching and was named a Fellow of the American Physical Society in 1996. In 2004, he received the Excellence in Plasma Physics Research award from the American Physical Society (APS). He won the UCI Academic Senate's Distinguished Faculty Award for Teaching in 2017-2018 and has won departmental teaching awards three times. In 2025, the APS awarded him the Maxwell Prize for lifetime achievement in plasma physics, with the citation "For studies of resonant and non-resonant energetic particle transport in magnetized plasmas, innovative diagnostic methods, and the experimental discovery of detrimental fast-ion driven instabilities."
