Negative Triangularity

Negative triangularity (NT) is a potentially transformative scenario for fusion energy with its high-performance core, L-mode-like edge, and low-field-side divertors that could readily scale to an integrated reactor solution free of a deleterious edge instability (edge localized modes (ELMs)). The DIII-D NT campaign, conducted January-February 2023 and including approximately 22 plasma run days, explored the value of NT as a reactor concept, providing promising results warranting further investigation.

Learn more about the unique plasma geometry of NT and the special engineering required for this campaign at DIII-D in the video below:

This campaign was made possible by the flexibility of the DIII-D tokamak and expertise of the DIII-D team, which allowed rapid install and removal of the necessary armor tiles. During this special campaign, ~647 NT plasmas were created, with 533 having a pulse length longer than 2 seconds. Thirteen plasma experiments on a variety of topics were completed during this period, and many supplemental physics experiments were performed in this relatively unexplored tokamak regime. 

Most experiments were conducted utilizing plasmas with strong NT in the diverted configuration, and NT was able to access unique operating space not typically achieved with standard positive triangularity regimes. Good performance was exhibited over a wide range of operational space. Plasmas were robustly ELM-free as long as the NT maintained was sufficiently strong; additionally, NT plasmas could access detached divertor configurations. This research is just the beginning of the exploration of NT regimes, with the results indicating that the NT approach has notable potential for fusion power plant design. Although historically thought to be impractical, this DIII-D special campaign suggests that NT may be an important approach for stable, economic operation. 

Learn more about the NT campaign: Flipping Plasma Geometry at DIII-D Demonstrates Stable Pathway to Practical Fusion Energy and DIII-D National Fusion Facility Completes Highest-Powered Negative Triangularity Experiments in History of U.S. Fusion Research Program

Topical Areas

Below, the three 2023 Physics Groups are broken down by topical area. The subsections describe some of the major highlights for the listed topical areas during the FY2023 run campaign at DIII-D. 

Divertor Science and Innovation (Edge and Boundary Physics) 

• A series of experiments reached the highest levels of injected power and current, achieving the highest parallel heat flux reach to date in DIII-D. This work sets the stage for even higher predicted current work to be achieved with the Shape and Volume Rise (SVR) divertor being installed as an upgrade for the FY2024 campaign. 
• The capability to use extrinsic nitrogen (rather than released carbon) to seed plasmas so that that they become dominantly radiating was demonstrated, with simultaneous absolutely calibrated measurement of multiple impurity species and charge states via both visible and EUV/VUV diagnostics. 

• A multiyear coordinated campaign comprising experimental and modeling work on the Small Angle Slot (SAS) divertor in DIII-D was completed. The results showed that in regard to the onset of detachment, the V-shaped divertor acts similarly to a flat-bottom slot for both directions of ion BxgradB drift. This work provides critical data for the future design of slotted divertors with baffling for enhanced dissipation and neutral trapping.

Plasma-Material Interactions (Edge and Boundary Physics) 

• Integrated testing of new tungsten (W)-based materials was performed to inform future development of reactor-relevant plasma-facing components (PFCs). Samples of innovative W-based materials (button samples, 6 mm in diameter; 34-mm-diameter W disc reinforced with W fibers) were exposed in the lower divertor of DIII-D to several plasma discharges with intense transients using the Divertor Evaluation Material System (DiMES) manipulator, and post-mortem analysis of the disc revealed that the fibers inhibited thermally-induced crack propagation.   
• Commissioning of a new ultraviolet spectroscopy system successfully demonstrated quantification of W gross erosion and re-deposition, allowing estimation of these processes in the DIII-D divertor during plasma discharges. These new measurement capabilities improve the ability to predict material lifetime due to the significant degree of re-deposition expected in the divertors of future fusion devices. 

Pedestal and QH Mode (Edge and Boundary Physics) 

• Fueling physics, including evaluation of particle transport coefficients with measured sources, physical explanation of in/out asymmetries of a particle source, comparison of neutral and electron density profiles for hydrogen versus deuterium discharges, and quantitative evaluation of fueling from thermal neutrals, were investigated.  
• FY2022 experiments, with a particular focus on using the LLAMA diagnostic to study pedestal fueling and the density profile, were analyzed. 

3D and Stability Physics (Integrated Plasma Scenarios) 

• Experiments investigated indirect electron cyclotron current drive (ECCD) stabilization of m,n=2,1 neoclassical tearing modes (NTMs) in the ITER Baseline Scenario (IBS) by removing magnetic stochasticity from higher mode number islands. This work explored new paths toward a stable ITER baseline, setting a foundation for more electron cyclotron heating-dominated IBS studies as more gyrotrons become available at DIII-D.  
• A number of important disruption-free error field identification experiments prioritized by the ITER organization were performed. Together with those of related experiments performed at JET and ASDEX-Upgrade, the results of these experiments will guide the early start-up phases of ITER. 

Plasma Control (Integrated Plasma Scenarios) 

• Adaptive resonant magnetic perturbation (RMP) achieved ELM suppression at a greatly reduced perturbation coil current, allowing recovery of high core performance, in a collaborative KSTAR/DIII-D upgrade. 

Isotope Physics (Burning Plasma Physics) 

• Experiments that provided insight into key isotopic dependencies of using hydrogen and deuterium plasma comparisons were performed.  
• A 35% reduction in the L-H power threshold was achieved in ITER-similar-shape (ISS) hydrogen plasmas via applied n=3 non-RMPs. MARS-F plasma response calculations demonstrated that n=3 or n=4 non-RMPs create localized edge counter-current rotation and hold great promise for L-H power threshold reduction in non-nuclear ITER hydrogen plasmas.
• Positive power scaling of the H-mode density limit (HDL) was found in ISS deuterium and hydrogen plasmas (n_HDL ~ P^0.3). Significant power hysteresis between the L-H transition power threshold and the required H-mode holding power (P_LH/P_HL ≤ 0.85) was observed in hydrogen and deuterium plasmas below the Greenwald density limit. 

Turbulence and Transport (Burning Plasma Physics) 

• The physical mechanism by which turbulence drives intrinsic rotation in toroidal plasmas was investigated. 
• The interaction of fast ion transport with Alfvén eigenmodes and large-scale instabilities, such as sawteeth and 3D fields, was evaluated. 

Thrusts and Taskforces

Disruption Mitigation (Integrated Plasma Scenarios) 

• Particle assimilation and runaway electron (RE) generation properties were characterized during ITER-like shattered pellet injection shutdowns. 
• Equilibrium requirements for RE mitigation based on dispersal by internal magnetohydrodynamic (MHD) activity were assessed. 

High-Performance Non-ELMing Regimes (Edge and Boundary Physics) 

• A new understanding of impurity sourcing and transport, including SOLPS-ITER simulations with drifts, was achieved in Wide Pedestal Quiescent H-Mode (WPQH-Mode) plasmas. This was exploited to reduce WPQH-Mode Zeff by more than a factor of two (Zeff=2.1); QH/WPQH-Mode was attained in hydrogen plasmas for the first time, which reduced carbon sputtering. Radiative nitrogen injection for divertor cooling also reduced the concentrations of both carbon and tungsten in WPQH-Mode.  
• In conjunction with the 2022 U.S. DOE Joint Research Target, Extended magnetohydrodynamics, gyrokinetic turbulence and neoclassical simulations were used to determine the mechanisms underlying pedestal transport WPQH-Mode.  Neoclassical impurity transport was found to play an increasingly important role as impurity charge increases, and multiple instabilities spanning MHD to electron scales were identified. 
• A key result for turbulence-limited pedestals was observed for the first time: The exhaust layer width was doubled relative to empirical scaling by driving increased turbulent transport in the plasma edge, without coherent MHD activity or ELM transients. The results were quantitatively reproduced in comprehensive electromagnetic XGC gyrokinetic particle-in-cell simulations, and simulations with BOUT++ suggest turbulence spreading may play a role. 
• The operating spaces of WPQH-Mode and I-Mode were significantly extended. WPQH-mode operation at reactor-relevant low injected torque was expanded to a lower q95 of 4.2 and sustained in an ITER similar shape and in both ion magnetic drift directions. I-Mode operation in DIII-D was extended to 11.9 MW using AUG-style β Feedback Control to maintain high power just below the H-Mode threshold. 

Heating and Current Drive System Readiness (Burning Plasma Physics) 

• The Lower Hybrid Coupler was produced for the High-Field-Side Lower Hybrid Current Drive system by a novel additive manufacturing process. 
• Commissioning of the Helicon heating and current drive system at 850 kW klystron power and execution of power deposition experiments with this system in both L- and H-modes were performed. 

Negative Triangularity (Burning Plasma Physics) 

• The physics behavior and performance benefits of NT-shaped plasmas were investigated. The areas of study included evaluating L-H confinement transition behavior (or lack thereof) as a function of NT, exploring the operational space that NT can access, determining the confinement time and MHD pressure limits, assessing confinement behavior and turbulence dynamics, and evaluating boundary parameters, detachment physics, and impurity radiation tolerance. 

Joint DIII-D/EAST Task Force 

• Good integration of high-performance high-poloidal-beta core and full divertor detachment was achieved in DIII-D SAS-V divertor plasmas with an ITER-like shape and impurity seeding.