Disentangling core and edge mechanisms of the density limit in DIII-D negative triangularity plasmas

Nuclear Fusion · 2025-11-06

Abstract 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) until the termination of auxiliary heating. Systematic power scans show distinct power scalings for the core ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mi>e</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>SOL</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.27</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) and edge ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mi>e</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mi>SOL</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.42</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">G</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> </mml:math> -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.

Influence of <i>n</i> = 1 resonant magnetic perturbation on flow and turbulence towards L-H transition

Nuclear Fusion · 2025-05-02 · 2 citations

Abstract The application of resonant magnetic perturbation (RMP) with toroidal mode number n = 1 on HL-3 tokamak inhibits the L-H transition at specific heating power. Following RMP application, the electron density increases in the outer plasma region ( ρ &gt; 0.85, where ρ is the normalized toroidal flux), while the electron/ion temperature decreases. Notably, the equilibrium flow shear in the edge region is substantially reduced. This reduction, combined with enhanced micro-instabilities driven by increased profile gradients, leads to enhanced turbulence levels. Consequently, the diminished flow shear becomes less effective in suppressing turbulence, providing a comprehensive explanation for the inhibited access to H-mode. Through a modified one-dimensional predator–prey model that incorporates the effects of RMP-induced radial magnetic perturbations, we have conducted a quantitative analysis of the turbulence and flow dynamics during the L-H transition process. Our results indicate that as the strength of magnetic perturbation increases, the turbulence intensity increases and edge flow shear decreases, in agreement with experimental observations. Additionally, we found that the L-H transition power threshold increases almost linearly with the square of the radial magnetic perturbation intensity. These results enhance our understanding of RMP-induced changes in edge plasma transport, providing valuable insights for optimizing the operation of future tokamaks and improving the performance of fusion reactors.

Multi-scale Interaction Mechanism for Edge-Localized-Mode Suppression in the Tokamak Edge

Nature Communications · 2025-11-21 · 1 citations

A central challenge in fusion energy is reconciling the high-confinement mode required for reactor performance with the intense intermittent relaxation events it produces, known as edge-localized modes. These instabilities arise in the steep pressure pedestal at the plasma edge when magnetohydrodynamic thresholds are crossed, inflicting damaging heat loads on reactor components. Here, we show that multiscale interactions between microscopic turbulence and macroscopic magnetohydrodynamic modes provide encouraging prospects for self-organized edge-localized modes regulation. Using direct quantitative measurements of multiscale modes, eddy dynamics, and turbulent flux, we show that small-scale electron drift wave turbulence actively scatters the large-scale peeling-ballooning modes. This scattering decorrelates the pressure and velocity fields of the instability, so arresting its growth. Our modeling and theoretical analysis confirm this suppression mechanism is effective even when conventional linear stability thresholds are exceeded. This work establishes a nonlinear principle for edge-localized modes stability, revealing how ambient micro-turbulence can be leveraged to maintain a macro-stable, high-performance pedestal for future fusion reactors.

Layered patterns of active scalar fields in a two-dimensional magnetohydrodynamic system

Physical review. E · 2025-05-21 · 2 citations

We observe the formation of staircase patterns in the magnetic potential (A) in a weakly magnetized two-dimensional magnetohydrodynamic system driven by a forced, fluctuating vortex array. Layering occurs due to inhomogeneous mixing of A by vortex cells. Magnetic Reynolds number (R_{m})-dependent quenching of the turbulent diffusion of A by weak magnetic fields increases the disparity between the (short) cell circulation time and the (long) time for intercell transport of magnetic potential. Thus, magnetic fields strengthen transport barriers between cells and reinforce the staircase, relative to its passive scalar counterpart. The analysis reveals a feedback mechanism, which promotes staircase formation. Magnetic staircases persist in both the flux expulsion (R_{m}v_{A}^{2}/U_{0}^{2}<1) and vortex disruption (R_{m}v_{A}^{2}/U_{0}^{2}≥1) limits. In the latter case, residual vortex cells homogenize A. Global layering morphology is shown to be well characterized by staircase curvature. Stochastic forcing of magnetic potential can support magnetic staircases against resistive decay.

Metabolic dysfunction over a life course key to healthy ageing inequality

Aging Clinical and Experimental Research · 2025-06-23

The UK is experiencing a decline in healthy life expectancy, now at 62.4 years for men and 60.9 years for women. Socioeconomic deprivation plays a significant role in health disparities, affecting individuals across the life arc. Girls born in the most deprived areas may live 19 fewer years in good health compared to those in wealthier areas. Health inequalities are particularly severe for ethnic minorities, with Black and Asian individuals reporting poorer health at a younger age. Health inequalities correlate with socioeconomic status. In old age, 2.1 million older adults live in poverty, with Black and Asian communities again disproportionately affected. While ageing increases the risk of morbidities, poor health is not inevitable; however, disadvantaged populations face early-life risk factors, such as low birth weight, linked to future conditions like diabetes and cardiovascular disease. CELLO, an interdisciplinary network, focuses on cellular metabolism throughout life in disadvantaged populations, examining how both genetic and environmental factors shape metabolic dysfunction and contribute to social health inequalities. This review stems from discussions within the network, aiming to advance understanding of healthy ageing across the life course.

Erratum: “Potential Vorticity Mixing in a Tangled Magnetic Field” (2020, ApJ, 892, 24)

The Astrophysical Journal · 2025-09-10

Operation above the Greenwald density limit in high performance DIII-D negative triangularity discharges

Plasma Physics and Controlled Fusion · 2025-06-05 · 4 citations

Abstract The density limit in strongly-shaped negative triangularity (NT) discharges is studied experimentally in the DIII-D tokamak. Record-high Greenwald fractions <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mtext>G</mml:mtext> </mml:msub> </mml:mrow> </mml:math> are obtained, using gas puff injection only, with values up to near 2, where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mtext>G</mml:mtext> </mml:msub> </mml:mrow> </mml:math> is defined as the ratio of the line-averaged density over <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mtext>G</mml:mtext> </mml:msub> <mml:mo>=</mml:mo> <mml:msub> <mml:mi>I</mml:mi> <mml:mi>p</mml:mi> </mml:msub> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>π</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>a</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> , with I p [MA] the plasma current and a [m] the plasma minor radius. A clear higher operational limit with higher auxiliary power is also demonstrated, with the ohmic density limit about two times lower than with additional neutral beam injection heating. The evolution of the electron density, temperature and pressure profiles are analyzed as well. The core density can be up to twice the Greenwald density and keeps increasing, while the value at the separatrix remains essentially constant and slightly below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mtext>G</mml:mtext> </mml:msub> </mml:mrow> </mml:math> . The edge temperature gradient collapses to near zero and NT plasmas are shown to be resilient to such profiles in terms of disruptivity. We also present the time evolution of the inverse electron pressure scale length with the value at the last closed flux surface (LCFS) decreasing below the value at the normalized radius 0.9 near the density limit, demonstrating the clear drop of confinement starting from the edge. This inverse scale length ‘collapse’ at the LCFS also defines well the characteristic behavior of the kinetic profiles approaching a density limit.

Comparative studies of cross-phase dynamics in turbulent momentum flux and particle flux at the tokamak edge

Reviews of Modern Plasma Physics · 2025-02-10 · 3 citations

Abstract Turbulent transport events, including turbulent transport flux of momentum (i.e., turbulent momentum flux or Reynolds stress) and turbulent transport flux of particle (i.e., turbulent particle flux), have important effects on the confinement performance of magnetic confinement fusion devices. Poloidal Reynolds stress is the ensemble average of the product of radial velocity fluctuations and poloidal velocity fluctuations, i.e., $$\langle {\widetilde{v}}_{r}{\widetilde{v}}_{\theta }\rangle$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>⟨</mml:mo> <mml:msub> <mml:mover> <mml:mi>v</mml:mi> <mml:mo>~</mml:mo> </mml:mover> <mml:mi>r</mml:mi> </mml:msub> <mml:msub> <mml:mover> <mml:mi>v</mml:mi> <mml:mo>~</mml:mo> </mml:mover> <mml:mi>θ</mml:mi> </mml:msub> <mml:mo>⟩</mml:mo> </mml:mrow> </mml:math> . Turbulent particle flux is the ensemble average of the product of radial velocity fluctuations and density fluctuations, i.e., $$\langle \widetilde{n}{\widetilde{v}}_{r}\rangle$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>⟨</mml:mo> <mml:mover> <mml:mi>n</mml:mi> <mml:mo>~</mml:mo> </mml:mover> <mml:msub> <mml:mover> <mml:mi>v</mml:mi> <mml:mo>~</mml:mo> </mml:mover> <mml:mi>r</mml:mi> </mml:msub> <mml:mo>⟩</mml:mo> </mml:mrow> </mml:math> . Changes in either amplitude of fluctuations or cross phase between fluctuations can cause changes in turbulent transport. In this paper, cross-phase dynamics in the Reynolds stress and turbulent particle flux at the tokamak edge are studied in detail. Reynolds stress and turbulent particle flux are, respectively, written as the product of fluctuation amplitudes and an average cross-phase factor. The mathematical expressions of the average cross-phase factors are derived. The average cross-phase factors and the power spectra of cross phase are obtained using experimental measurement data. It is found that the cross-phase dynamics in Reynolds stress and particle flux are very different. Reynolds stress is found to be more sensitive to cross phase than particle flux is. In the strong $$E\times B$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>E</mml:mi> <mml:mo>×</mml:mo> <mml:mi>B</mml:mi> </mml:mrow> </mml:math> shear layer, spatial slips of cross phase lead to the obvious radial gradient of Reynolds stress. In the no/weak $$E\times B$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>E</mml:mi> <mml:mo>×</mml:mo> <mml:mi>B</mml:mi> </mml:mrow> </mml:math> shear region, the cross phase in Reynolds stress tends to lock. Here, phase locking refers to that the power spectra of phase tend to distribute around a fixed phase which does not change with radial position, while phase slip means that the power spectra of cross phase tend to distribute around a phase that varies with radial position. Phase slip or locking mainly describes the central phase weighted by the power spectra, while the phase scattering mainly describes the dispersion of the power spectrum distribution of the phase. The increased scattering of cross phase, which indicates the power spectra distribution of the phase is more dispersed, contributes to the decreased Reynolds stress for higher collisionality. The cross phase in particle flux tends to lock in both strong and no/weak shear regions. The degree of scattering of cross phase in the particle flux does not change obviously as collisionality increases. For higher collisionality, it is the increased density fluctuation amplitude rather than cross-phase dynamics that leads to the increased particle flux. The underlying physical mechanism that causes Reynolds stress and particle flux to exhibit different phase dynamics is discussed.

Zonal Flow and Self-regulating Mechanism in a Hydrodynamic Disk

The Astrophysical Journal · 2025-03-24

Abstract This study addresses key aspects of momentum transport in hydrodynamic disks, which is critical for understanding zonal flow generation and turbulence in compressible hydrodynamic disks. We find that nonlinear momentum/density transport leads to the formation of zonal flows from the Rossby wave instability in disks. We analytically derive the generation and location of zonal flows and describe a modified Taylor identity applicable to compressible disk flows. We further present a self-regulation model, revealing a dynamic interplay between zonal flow and fluctuations driven by Rossby wave instability that regulates the nonlinear saturation state. This theoretical framework contributes insights into the dynamics of disks such as protoplanetary disks, shedding light on the intricate processes governing momentum/density transport and the emergence of zonal flows in the saturation of protoplanetary disks.

On the Orbital Evolution of Multiple Wide Super-Jupiters: How Disk Migration and Dispersal Shape the Stability of the PDS 70 System

The Astrophysical Journal · 2025-12-16

Abstract Direct imaging has revealed exoplanet systems hosting multiple wide-orbit Super-Jupiters, where planet–planet interactions can shape their long-term dynamical evolution. These strong perturbations may lead to orbital instability, raising questions about the long-term survival of such systems. Shortly after formation, planet–disk interactions can shepherd planets into mean-motion resonances, which may promote long-term stability, as seen in HR 8799. However, early-stage processes such as photoevaporation and disk viscosity can influence these outcomes. The ∼5 Myr old PDS 70 system offers a unique laboratory to investigate these processes: its two massive (&gt;4 M Jup ), wide-orbit (&gt;20 au) giants are still embedded in their natal disk. We perform 2D hydrodynamic simulations of the system, allowing the disk to disperse via photoevaporation. Once the disk dissipates, we continue to track the planets’ orbital evolution over gigayear timescales using N -body simulations. We find that the system is likely to remain stable for &gt;1 Gyr. To assess the importance of disk-driven evolution, we compare these results with disk-free N -body simulations using orbital parameters constrained by orbit fits that include recent relative astrometry and radial velocities from the literature. In this case, we find that only ≲4% of posterior is stable for 100 Myr, highlighting the importance of considering disk-driven evolution for long-term dynamics stability of exoplanetary systems. We also simulate two three-planet configurations including the proposed inner candidate “PDS 70 d,” finding that a higher photoevaporation leads the system to become unstable in &lt;10 Myr.