arXiv subjects

A. Hyatt

Publications and source records attributed to A. Hyatt.

6 recordsLinked to original sources

arXiv2026

Overview of Enhanced D-alpha and Quasi Continuous Exhaust H-modes at DIII-D

We review the Enhanced D-alpha (EDA) and Quasi-continuous Exhaust (QCE) H-modes that have recently been achieved in the DIII-D tokamak. These scenarios are free of large type-I edge-localized modes (ELMs) and achieved at high fueling compatible with divertor detachment. This makes them promising operational regimes as ELMs are a major risk for fusion pilot plants (FPPs). Access to EDA H-mode and the associated quasi-coherent mode (QCM) was observed at low heating power barely above the L-H power threshold, and increased density enabled access to EDA H-mode even at higher heating powers. The QCE H-mode was achieved at simultaneously high density and heating power, characterized also by a QCM and small ELMs with transient peaks of the total power into the divertor below about 3MW, in line with observations at ASDEX Upgrade. During the QCE H-mode, the turbulence parameter alpha_t at the separatrix is about 1 and does not separate ELMy or EDA from QCE H-modes well. Peeling ballooning stability calculations suggest that the EDA H-mode pedestal is limited by transport far away from ideal magnetohydrodynamic instability, and the QCE H-mode pedestal operates close to the ballooning branch of the stability boundary. BOUT++ simulations qualitatively reproduce the pressure collapse characteristics of the different regimes and suggest an important role of n ~ 50 resistive ballooning modes localized near the separatrix at the X-point during EDA and QCE H-modes. This work provides a foundation and guidance for further research on the access conditions and physics mechanisms of EDA and QCE H-modes necessary for the extrapolation of these regimes towards an FPP.

physics.plasm-ph↗

arXiv2026

The Negative Triangularity Tokamak Path for Fusion Pilot Plants: Experimental Progress and Future Prospects

This paper reviews the experimental progress of negative triangularity (NT), a tokamak configuration where the poloidal cross-section is a reversed-D shape compared to the conventional positive triangularity (PT) shape. NT is a promising reactor scenario that addresses the fundamental tension between performance, exhaust, and robustness. NT studies have accelerated globally across these three pillars over the past several years. While tokamak pilot plants are typically designed for the standard PT H-mode regime, this approach faces significant challenges in balancing high core performance with manageable heat and particle exhaust as well as reliable robustness. In contrast, NT plasmas have achieved H-mode-level confinement while remaining robustly free of the deleterious edge localized mode (ELM) instability. Regarding exhaust, NT offers a larger divertor wetted area on the outboard side and demonstrates compatibility with detachment and operation at high core radiation fraction without the constraints of the L-H power threshold, while also exhibiting low core impurity retention. NT operates with high reproducibility over a wide operating space, demonstrated by robust discharge-to-discharge consistency, and has access to plasmas with very high Greenwald fractions and/or low edge safety factors compared to PT H-mode plasmas. Further research is required to answer outstanding questions related to reactor confinement extrapolation, the optimal triangularity for a reactor, and core-edge integration. NT studies in existing and planned tokamaks are increasing, as is interest in possible reactor concepts. The unique physics and engineering advantages of NT offer a robust and simplified foundation for a viable fusion power plant.

physics.plasm-ph↗

arXiv2024

Designing a Validation Experiment for Radio Frequency Condensation

Theoretical studies have suggested that nonlinear effects can lead to "radio frequency condensation", which coalesces RF power deposition and driven current near the center of a magnetic island. It is predicted that an initially broad current profile can coalesce in islands when they reach sufficient width, providing automatic stabilization. Experimental validation of the theory has thus far been lacking. This paper proposes experiments on DIII-D for testing and refining the theory of the nonlinear effects.

physics.plasm-ph↗

arXiv2024

Characterization of the ELM-free Negative Triangularity Edge on DIII-D

Tokamak plasmas with strong negative triangularity (NT) shaping typically exhibit fundamentally different edge behavior than conventional L-mode or H-mode plasmas. Over the entire DIII-D database, plasmas with sufficiently negative triangularity are found to be inherently free of edge localized modes (ELMs), even at injected powers well above the predicted L-H power threshold. A critical triangularly ($δ_\mathrm{crit}\simeq-0.15$), consistent with inherently ELM-free operation is identified, beyond which access to the second stability region for infinite-$n$ ballooning modes closes on DIII-D. It is also possible to close access to this region, and thereby prevent an H-mode transition, at weaker average triangularities ($δ\lesssimδ_\mathrm{crit}$) provided that at least one of the two x-points is still sufficiently negative. Enhanced low field side magnetic fluctuations during ELM-free operation are consistent with additional turbulence limiting the NT edge gradient. Despite the reduced upper limit on the pressure gradient imposed by ballooning stability, NT plasmas are able to support small pedestals and are typically characterized by an enhancement of edge pressure gradients beyond those found in traditional L-mode plasmas. Further, the pressure gradient inside of this small pedestal is unusually steep, allowing access to high core performance that is competitive with other ELM-free regimes previously achieved on DIII-D. Since ELM-free operation in NT is linked directly to the magnetic geometry, NT fusion pilot plants are predicted to maintain advantageous edge conditions even in burning plasma regimes, potentially eliminating reactor core-integration issues caused by ELMs.

physics.plasm-ph↗

arXiv2023

Robust avoidance of edge-localized modes alongside gradient formation in the negative triangularity tokamak edge

In a series of high performance diverted discharges on DIII-D, we demonstrate that strong negative triangularity (NT) shaping robustly suppresses all edge-localized mode (ELM) activity over a wide range of plasma conditions: $\langle n\rangle=0.1-1.5\times10^{20}$m$^{-3}$, $P_\mathrm{aux}=0-15$MW and $|B_\mathrm{t}|=1-2.2$T, corresponding to $P_\mathrm{loss}/P_\mathrm{LH08}\sim8$. The full dataset is consistent with the theoretical prediction that magnetic shear in the NT edge inhibits access to ELMing H-mode regimes; all experimental pressure profiles are found to be at or below the infinite-$n$ ballooning stability limit. Importantly, we also report enhanced edge pressure gradients at strong NT that are significantly steeper than in traditional ELM-free L-mode plasmas and provide significant promise for NT reactor integration.

physics.plasm-ph↗

arXiv2015

Avoiding Tokamak disruptions by applying static magnetic fields that align locked modes with stabilizing wave-driven currents

Non-rotating (`locked') magnetic islands often lead to complete losses of confinement in tokamak plasmas, called major disruptions. Here locked islands were suppressed for the first time, by a combination of applied three-dimensional magnetic fields and injected millimetre waves. The applied fields were used to control the phase of locking and so align the island O-point with the region where the injected waves generated non-inductive currents. This resulted in stabilization of the locked island, disruption avoidance, recovery of high confinement and high pressure, in accordance with the expected dependencies upon wave power and relative phase between O-point and driven current.

physics.plasm-ph↗