Colin Chrystal

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.

Rapid and accurate machine learning (ML) based analysis of the Doppler broadened and shifted Balmer-α spectra from the main ion charge exchange recombination spectroscopy (MICER) diagnostic on the DIII-D tokamak has been developed to provide raw measurements of the main ion (typically D+) temperature, toroidal rotation, density, and beam emission based measurements directly from the active Balmer-α spectra. The temperatures and toroidal rotations are typically within 150 eV and 10 km/s of those obtained from the standard least squares fitting approach. The raw measurements are then corrected for systematic errors using a second set of ML models that were trained as inverse surrogates for the FIDASIM three dimensional collisional radiative model. FIDASIM provides a comprehensive forward model of the raw measurements, including atomic physics and spatial smearing effects. At the pedestal top, corrections to the ion temperature are typically 150 eV but can be up to 300 eV. The standard deviation of the difference between the surrogate and the full model based correction is less than 50 eV, with some of this difference due to Monte Carlo noise from the FIDASIM model. 20 000 FIDASIM simulations spanning the relevant DIII-D parameter space were used for the training and validation of the ML model, incorporating variations in ion and electron profiles, as well as variations in the neutral beam injector parameters and magnetic equilibrium mapping. The complexity of the MICER spectra and associated corrections has historically limited analysis to high priority discharges, despite the wealth of information contained in the spectra. These advances have dramatically reduced the analysis time for single discharges from several hours to seconds. The speed-up now allows bulk analysis of thousands of historical discharges acquired by the diagnostic during its 12+ years of operation on DIII-D, enabling a variety of studies, such as those involving H-mode main ion pedestal profiles.

At the DIII-D tokamak, the Charge Exchange Recombination (CER) spectroscopy diagnostic routinely measures impurity temperature, rotation, and density. The ability to make these measurements in real time within DIII-D’s Plasma Control System (PCS) has recently been expanded and improved. High-speed network connections send raw data from 24 tangential CER views to the PCS, so spectral analysis can be performed. These views make measurements on the outboard midplane of DIII-D from the magnetic axis to the scrape off layer. Spectral analysis is completed quickly by a simplified least-squares fitting routine. Results are typically available within a millisecond of when the raw data become available and new raw data for all 24 views are typically sent to the PCS every 5 ms (for the most common CER sampling frequency). Fully ionized carbon is the most common impurity used for analysis, and analysis can also be performed for other highly charged impurities with relatively simple spectra such as boron, neon, and argon. The large majority of the real-time temperature and rotation measurements are within 20% of the most accurate automatic analysis available. As DIII-D discharges are typically separated by 10–15 min, the more rapid availability of the real-time analysis can be a major aid for control room analysis during an experiment. The real-time analysis can also be used in the PCS for control purposes (e.g., plasma profile feedback).

Similarity experiments were performed on the DIII-D and TCV tokamaks to explore the scaling of energy confinement in negative triangularity (NT) plasmas using non-dimensional variables. Near up-down symmetric plasmas with large top-bottom averaged NT were created in a lower single null configuration, with the shape of the separatrix being closely matched between the two devices. The normalized energy confinement is found to weakly improve at increasing collisionality and, between the two devices, shows a machine size scaling behavior between Bohm and gyro-Bohm. Engineering scaling on a large DIII-D dataset is in agreement with the non-dimensional experiment.

Experiments on DIII-D have demonstrated a density-confinement synergy that enables sustainment of high performance in a previously unattained parameter regime of simultaneous very high energy confinement quality (H _98y2 ⩾ 1.5), very high line-average density Greenwald fraction ( ƒ _Gr = πa ² < n >/ I _P ⩾ 1.4), and high toroidal beta ( β T ⩾ 3%). Tokamak operation in this regime is essential for a compact steady-state FPP, as well as for Q = 10 with 500 MW of fusion power in ITER at I _P ≪ 15 MA. These experiments leveraged the knowledge that, in the high-poloidal-beta ( β P ) regime, density gradients can enhance turbulence stabilization caused by high α MHD ( α MHD ∼ d β P / d r ). This was described by theoretical predictions and gyrokinetic transport simulations (Kotschenreuther et al 2024 Nucl. Fusion 64 076033), and later confirmed by experiments on DIII-D (Ding et al 2024 Nature 629 555). To increase both β P and β T , the new experiments increased the ideal-wall stability β N -limit by using a smaller plasma-outer wall distance and higher triangularity in the plasma cross section (top/bottom average δ ∼ 0.9), enabled by the recent ‘shape & volume rise’ (SVR) modification to the DIII-D divertor. The higher triangularity also contributed to achieving higher ƒ _Gr by enabling higher pedestal density. At high density, the pedestal is ballooning limited and exhibits small and frequent ELMs, while the divertor is near detachment even without any impurity seeding. Highplasma performance was attained and sustained reproducibly, with the eventual terminations brought about by an MHD mode destabilized as the current profile slowly continued to evolve. A path to stationary fully noninductive operation might include ECH injection to reduce both core impurity accumulation and the electron collisionality, thus increasing the bootstrap current. These experiments provide the first experimental demonstration of the ƒ _Gr , H _{98 y 2} , and β T values required simultaneously for ITER Q = 10 at I _P < 10 MA, pointing to practical ways to improve the energy confinement in a fusion reactor.

Comparative modelling of ion temperature gradient (ITG)/trapped electron mode (TEM) turbulence in negative (NT) and positive (PT) triangularity plasma shapes was done using the nonlinear global full-f gyrokinetic particle code JOREK-GK in the realistic X -point tokamak geometry including the Scrape Off Layer (SOL) for TCV and DIII-D parameters. A comparison of JOREK-GK code with the gyrokinetic codes GS2 and GENE was done using NT/PT triangularity TCV L-modes parameters showing good agreement between codes in linear growth rates and clear beneficial effect of NT as compared to PT. Global non-linear modelling of the ITG/TEM saturated turbulence for realistic DIII-D NT pulses was done and compared with numerically constructed PT equilibrium with the same plasma profiles. Existence of longer correlation length of density fluctuations in PT compared to NT was demonstrated. Stronger and more sheared zonal flows are generated via Reynolds stress in NT compared to PT. These factors are stabilizing for TEM/ITG turbulence in NT and lead to smaller heat fluxes and heat conductivities in NT compared to PT configuration. Weak dependence of plasma confinement on collisionality and plasma rotation was found in modelling of DIII-D NT shots similar to the experiment. The confinement scaling with normalized ion gyro-radius ρ * was estimated both for NT and PT. Bohm-like scaling was obtained in both configurations, however with better confinement for NT compared to PT which could be favourable factor for reactor size machines.

New spectroscopic measurements of deuterium Balmer- α emission are used to infer the spatial distribution of neutral particles near the X -point in a diverted high confinement mode plasma. The charge exchange neutral spectroscopy (CENS) diagnostic—recently installed on the DIII-D tokamak—uses 15 lines of sight extending from the edge of the confined region ( ρ ≈ 0.7) to the X -point in lower single-null plasmas. Each CENS line of sight is spectrally resolved around the deuterium Balmer- α wavelength (6561 Å) to measure the Doppler shift, line broadening, and intensity of D _α emission. Thermal line broadening is used to identify emission from neutrals undergoing charge-exchange (CX) with high temperature ions in the confined plasma. This spectral information allows a more accurate determination of the neutral density deeper inside the confined plasma compared to traditional filter-based diagnostics. We present two methods of analyzing CENS measurements: (1) fitting the spectrum from each view-chord separately, and (2) a tomographic method for inverting the neutral density over a 2D region of space using all CENS views collectively. The neutral density profile is found to decay exponentially in the radial direction with two scale-lengths. In the pedestal region the neutral density decays at a rate approximately equal to the local mean-free-path for CX collisions, L n D 0 ≈ λ CX . Further inside the plasma the neutral density decays at a rate equal to the mean-free-path for ionization, L n D 0 ≈ λ inz. . The separatrix n D 0 value is found to be approximately 2 × 10 15 m ⁻³ . Based on results from the 2D inversion, the density of neutrals is found to increase along the separatrix approaching the X -point by a factor of ≈ 5 × over the region covered by the CENS diagnostic.

Three-dimensional magnetic sensors are employed to identify the amplitude and toroidal phase of error fields (EFs) by analyzing the torque balance for magnetic islands entrained by rotating resonant magnetic perturbations (RMPs) in DIII-D H-mode plasmas. This technique of torque balance allows for efficient identification of EFs, offering a valuable tool for scenario-specific and optimized EF compensation (EFC), and requires only magnetic diagnostics. The torque balance used in this work includes the contribution from electromagnetic torque due to EFs, wall response, RMP fields and viscous torque. Results show that viscous torque plays a crucial role, particularly during locked modes and H-mode plasmas, ensuring accurate data fits with lower residuals. The torque balance technique reveals that L- and H-mode plasmas have distinct EF configurations, consistent with a model-based EF analysis including the ideal magnetohydrodynamic plasma response in IPEC and the SURFMN EF simulation. This technique shows great robustness in measuring the intrinsic EF amplitude regardless of its amplitude or toroidal phase. Repeated discharges with EFC disparities exhibit consistent results of intrinsic EF within a reasonable range near the ‘standard’ EFC. Additionally, the use of a rotating n = 1 RMP offers the advantage of reducing the risk of disruption by entraining saturated magnetic islands. These findings are instrumental for optimizing EF correction in fusion devices, thereby enhancing tearing mode suppression and overall plasma stability.

To simplify and routinize particle transport analysis in fusion devices, a novel closed-form linear inversion of the 1D convection–diffusion equation to estimate diffusion and convection profiles D(r→), v(r→), and source distribution s(r→) of a single species from measured data is derived and demonstrated on synthetic data. Formulas for profile estimates of D(r→), v(r→), s(r→), and their uncertainties are given as a matrix expression constructed directly from the incoming density data of the transported species in space and time, as well as physics assumptions such as particle conservation and experimental geometry. The derived matrix expression can be applied to a pumped or nonpumped recycling species, or a non-recycling species that is effectively pumped by plasma-facing surfaces.

Reduced radial transport, short midplane-to-target parallel connection lengths, and a strong effect of cross-field drifts were responsible for the high densities required for detachment in strong negative triangularity (NT) configurations in DIII-D. Dissipative divertor conditions were achieved in NT discharges at different triangularity, injected power, plasma current, and toroidal field direction. Differences between negative and positive triangularity (PT) discharges are analyzed in this paper to understand the requirements for access to detached divertor conditions: power balance, geometry, radial transport and effect of cross-field particle drifts. Parametric dependencies of access to detachment on plasma current and power flowing into the scrape-off layer remained similar in negative and PT and impurity seeding was observed to reduce the density needed to detach by up to 30% at the expense of core impurity dilution. The impact of triangularity on core-edge integration was tested varying bottom triangularity at fixed top triangularity. The high density needed to detach was not intrinsic to the NT edge as shapes with positive lower triangularity and negative upper triangularity were able to detach at lower upstream densities while maintaining an ELM-free NT edge. Confinement degradation at deeper detachment levels was however observed in all NT shapes, often associated with radiation instabilities.