George R. McKee

A first-of-its-kind experiment was conducted in the DIII-D tokamak under WEST similarity constraints on plasma shape and core parameters. This work presents a detailed transport study comparing a reference regime dominated by intrinsic carbon radiation and a high-radiation regime resulting from controlled tungsten (W) injection using the laser blow-off system, with a core tungsten concentration n W / n e ∼ 3 × 10 − 4 and a radiated-power fraction f rad > 0.5 . The W-induced radiative cooling lowered the electron temperature, thereby decreasing T e / T i and stabilizing trapped-electron-mode (TEM) turbulence. This transition in turbulence regime reduced momentum and ion thermal diffusivities, yielding ion temperature peaking and a factor-of-two increase in toroidal rotation. At the outer plasma region, enhanced E × B shear and increased collisionality further suppressed ion-scale turbulence, causing a sharp drop in ion heat flux. Consequently, impurity transport, predominantly turbulent in the low-radiation regime, acquired a strong neoclassical inward W convection during radiative cooling, bootstrapping the cooling cycle. Despite f rad > 0.5 , radiative collapse was not observed, likely owing to collisional ion-to-electron energy exchange acting as an electron-energy reservoir, together with 1 / 1 MHD activity modulating the radiated power through core impurity neoclassical T i -screening. These results support preparation for a tungsten wall change in DIII-D by elucidating tungsten-induced turbulence stabilization. They also provide key insights for interpreting plasma performance in WEST and are relevant to future reactors expected to operate with radiating tungsten-walled plasmas.

This Department of Energy-Office of Fusion Energy Science-sponsored project at the University of Wisconsin-Madison (UW) has determined the characteristics, behavior and dynamics of turbulence and related instabilities in magnetically confined fusion plasmas at the DIII-D National Fusion Facility, operated by General Atomics in San Diego, CA. Our UW team has designed, developed, implemented and operated a suite of multifield, high-performance optical diagnostics at the DIII-D tokamak facility to remotely measure fluctuation characteristics in a wide range of tokamak plasmas. Major diagnostics in this suite include Beam Emission Spectroscopy (BES), Ultra-Fast Charge Exchange Recombination Spectroscopy (UF-CHERS), Charge eXchange Imaging (CXI), and the Beam Emission Manifold Fast Spectrometer (JrS), and a visible survey spectrometer. Measurements from these instruments have been used to study the L-H transition process, H-mode pedestal and ELM dynamics and ELM-suppression mechanisms, core turbulence properties, near-Scrape-Off-Layer turbulence, Alfven Eigenmodes, and Magnetohydrodynamic instabilities. Measurements have been obtained in a wide variety of operational scenarios, ranging from standing ELM’ing H-modes to hybrid scenarios, RMP-ELM suppressed H-modes, RI-modes, Negative Triangularity plasmas, ITER Baseline scenarios, and high-beta-poloidal plasmas, as several examples. A central element of this research has been comparison with and validation of gyrokinetic simulations and transport modes. Many UW graduate students have performed their PhD thesis research under this project on topics ranging from diagnostic development to plasma turbulence research. Our UW team has presented many Invited talks, contributed talks, and poster presentations at prestigious national and international conferences, such as the American Physical Society-Division of Plasma Physics and IAEA-Fusion Energy Conference, as well as specialized workshops. Our Team has also participated in and lead many plasma fusion committees and organizations. This UW-provided advanced fluctuation diagnostic capability has been available to and utilized by the full DIII-D National program, including many collaborators from other universities, multiple national laboratories, General Atomics, and other company collaborators.

A recent study demonstrates that the suppression of ion temperature gradient (ITG) modes can occur during the nonlinear evolution of toroidicity-induced Alfvén eigenmodes (TAEs) in the DIII-D tokamak [Du et al., Phys. Rev. Lett. 135, 265101 (2025)]. In that work, ITG was suppressed by the formation of a narrow, TAE-induced shear flow layer, whose shearing rate exceeded the ITG decorrelation rate. The shear flow arises from an imbalance between Reynolds and Maxwell stress forces, as the TAE departs from the conventional shear Alfvén wave polarization. This follow-up paper systematically identifies the plasma conditions required for robust ITG suppression through a series of comparative experiments. The results show that TAEs routinely suppress ITG turbulence in plasmas with higher local safety factor (q), elevated fast-ion beta, and larger populations of fast ions on passing orbits, i.e., conditions consistent with the substantial TAE drive. Database analysis further reveals the existence of a threshold in TAE drive for ITG suppression. That is, once the drive exceeds a critical value within a favorable q window, the system undergoes the nonlinear bifurcation process, characterized by a sharp increase in TAE saturated amplitudes at a nearly fixed fast ion drive, suppression of ITG turbulence, and the formation of an internal thermal transport barrier at the localized radii.

Microtearing modes (MTMs) ( ω ∼ ω d e , k θ ρ s ∼ 0.1 , k r ρ s ∼ 0.3 − 1.0 ) are predicted to cause substantial electron thermal transport in the H-mode pedestal, limiting fusion performance on high-power devices. Localized 2D measurements of low-to-intermediate wavenumber density fluctuations in the edge of DIII-D H-mode plasmas suggest the presence of temporally intermittent or ‘bursting’ MTMs during the inter-ELM cycle in the steep gradient region. These measurements are obtained using a new intermediate radial resolution multichannel Charge eXchange Imaging (CXI) diagnostic that analyzes the intensity of the CVI emission at 529 nm. The mode is peaked near ρ = 0.97 with a radial full-width half maximum of 1.5–2.0 cm and a normalized density fluctuation amplitude peak of d n C n C ∼   5 % , peaking near 60 kHz in the lab frame. The mode additionally features a broad spectral width from 30 to 180 kHz and propagates in the electron diamagnetic direction in the plasma frame. CXI measurements taken during discharges with high density and current ramps provide evidence that MTMs become unstable at finite collisionality and remain at nearly the same location in ρ -space despite scanning of rational surface position. This indicates that the location of the peak in the electron diamagnetic frequency is a stronger driving force than individual rational surfaces.

A low-noise, high-speed optical detector module is characterized and successfully commissioned for the measurement of high-frequency, low-intensity beam emission on Wendelstein 7-X (W7-X). An ultra-narrow bandpass optical filter is employed to selectively transmit the desired emission line while suppressing broadband plasma background emissions. Carbon density fluctuations are investigated by observing the carbon C-VI emission line (n = 8 → 7, λ ∼ 529 nm), arising from charge exchange (CX) between the neutral beam atoms and the intrinsic carbon population. We present the characterization of the optical detector module and experimental measurements of carbon density fluctuations using available fibers on W7-X. The initial performance of the detector is presented in both active and passive CX measurements of intrinsic carbon density fluctuations. The low frequency dynamics of fluctuation is observed in response to the neutral beam and the pellet injection, demonstrating that the optical detector module is capable of providing a sufficient signal level with an adequate signal-to-noise ratio. In the upcoming OP2.4 campaign, this optical detector module will be adapted for use in a beam emission spectroscopy system by replacing the optical bandpass filter with one centered at 654 nm (90% transmission: 653–655.3 nm), which facilitates two-dimensional measurements of ion gyro-scale turbulence on W7-X.

Mitigation and suppression of low-k turbulence are observed during the nonlinear evolution of toroidicity-induced Alfvén eigenmodes (TAEs) in DIII-D experiments. Turbulence mitigation begins when the dominant TAE starts to depart from the typical shear Alfvén wave polarization. TAE activity then evolves into a new state, characterized by more discrete coherent modes, increased total amplitude, and more localized radial structures. 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 a full turbulence suppression. These observations indicate the imbalance between Reynolds and Maxwell stress forces during the nonlinear evolution of the TAE and their important roles in shear flow generation and turbulence reduction.

The density limit is investigated in the DIII-D negative triangularity plasmas which lack a standard H-mode edge. We find the limit may not be a singular disruptive boundary but a multifaceted density saturation phenomenon governed by distinct core and edge transport mechanisms. Sustained, non-disruptive operation is achieved at densities up to 1.8 times the Greenwald limit ( n G ) until the termination of auxiliary heating. Systematic power scans show distinct power scalings for the core ( n e ∝ P SOL 0.27 ± 0.03 ) and edge ( n e ∝ P SOL 0.42 ± 0.04 ) density limits. The edge density saturation is triggered by the onset of a non-disruptive, high-field side radiation front and the associated cooling, which clamps the edge density below n G . In contrast, the core density continues to rise until it saturates, a state characterized by enhanced core turbulence. Core transport evolves from a diffusive to an intermittent, avalanche-like state, as indicated by heavy-tailed probability density functions (kurtosis ≈ 6), increased Hurst exponents, and a 1 / f -type power spectrum. These findings suggest that the density limit in the low-confinement regime is determined by a combination of edge radiative cooling and core turbulent transport. This distinction provides separate targets for control strategies aimed at extending the operational space of future fusion devices.

Recently, a method to infer the polarization of modes with frequencies much less than the ion cyclotron frequency was published [X.D. Du et al., Phys. Rev. Lett. 132 (2024) 215101]. The method uses measurements of electron temperature and density fluctuations δTe and δne at the same spatial position to infer the local ratio of “acoustic polarization,” |δϕ∥|/(|δϕ∥| + |δψ|), where δϕ∥ is the effective parallel potential and δψ is related to the parallel magnetic vector potential A∥. This paper summarizes key formulas, with emphasis on their range of validity, and elaborates on the workflow required to infer the acoustic polarization from experimental data. The drift-acoustic polarization of ellipticity-induced, toroidicity-induced, and reversed shear Alfv´en eigenmodes is nearly zero, as expected for modes with predominately shear-Alfv´enic polarization. The polarization of beta-induced Alfv´en eigenmodes contains an acoustic component that increases with poloidal wave number. “Low frequency modes,” (instabilities that appear transiently when the minimum of the safety factor qmin passes through rational values) have large and highly variable acoustic polarization. In both experiment and simulation, fishbones have non-zero acoustic polarization that increases as the mode chirps down in frequency.