arXiv subjects

David Dickinson

Publications and source records attributed to David Dickinson.

6 recordsLinked to original sources

arXiv2026

Global Gyrokinetic Simulations of Electromagnetic Turbulence in STEP

This paper presents gradient-driven global electromagnetic gyrokinetic simulations for a conceptual burning flat-top operating point of STEP [1], STEP-EC-HD, and investigates how non-local effects influence the nonlinear saturation and transport of the electromagnetic turbulence at finite ${\beta}$. Local gyrokinetic simulations have shown that including ${\delta}B_{\parallel}$ is essential for the dominant hybrid kinetic ballooning modes, or hKBMs, to be unstable in STEP [2]. Using the long-wavelength ${\delta}B_{\parallel}$ solver [3] implemented in GENE [4], this work demonstrates that the linear mode spectrum can be accurately captured in global geometry, which results in good agreement with an ensemble of local flux-tube simulations. The global framework reproduces the hKBMs identified in [5], while microtearing modes remain challenging to resolve due to their shorter radial scales. Nonlinear simulations reveal clear evidence of an electromagnetic transition to states with extremely large heat fluxes, consistent with local simulations and with the predicted loss of zonal-flow regulation for this proposed operating point [6]. These findings establish the capability of global gyrokinetics to capture finite-${\beta}$ dynamics in STEP-like plasmas and motivate future work to identify the conditions governing this transition.

physics.plasm-ph↗

arXiv2024

Omnigenous stellarator equilibria with enhanced stability

To build an economically viable stellarator, it is essential to find a configuration that satisfies a set of favorable properties to achieve efficient steady-state nuclear fusion. One such property is omnigenity, which ensures confinement of trapped particles. After creating an omnigenous equilibrium, one must also ensure reduced transport resulting from kinetic and magnetohydrodynamic (MHD) instabilities. This study introduces and leverages the GPU-accelerated DESC optimization suite, which is used to design stable high-$\beta$ omnigenous equilibria, achieving Mercier, ideal ballooning, and enhanced kinetic ballooning stability. We explain the link between ideal and kinetic ballooning modes and discover stellarators with second stability, a regime of large pressure gradient where an equilibria becomes ideal ballooning stable.

physics.plasm-ph↗

arXiv2023

Photometry of Type II Supernova SN 2023ixf with a Worldwide Citizen Science Network

We present highly sampled photometry of the supernova (SN) 2023ixf, a Type II SN in M101, beginning 2 days before its first known detection. To gather these data, we enlisted the global Unistellar Network of citizen scientists. These 252 observations from 115 telescopes show the SN's rising brightness associated with shock emergence followed by gradual decay. We measure a peak $M_{V}$ = -18.18 $\pm$ 0.09 mag at 2023-05-25 21:37 UTC in agreement with previously published analyses.

astro-ph.HE↗

arXiv2023

On electromagnetic turbulence and transport in STEP

In this work, we present first-of-their-kind nonlinear local gyrokinetic simulations of electromagnetic turbulence at mid-radius in the burning plasma phase of the conceptual high-$\beta$, reactor-scale, tight-aspect-ratio tokamak STEP (Spherical Tokamak for Energy Production). A prior linear analysis in D. Kennedy et al. 2023 Nucl. Fusion 63 126061 reveals the presence of unstable hybrid kinetic ballooning modes, where inclusion of the compressional magnetic field fluctuation, $\delta B_{\parallel}$, is crucial, and subdominant microtearing modes are found at binormal scales approaching the ion-Larmor radius. Local nonlinear gyrokinetic simulations on the selected surface in the central core region suggest that hybrid kinetic ballooning modes can drive large turbulent transport, and that there is negligible turbulent transport from subdominant microtearing modes when hybrid kinetic ballooning modes are artificially suppressed (through the omission of $\delta B_{\parallel}$). Nonlinear simulations that include perpendicular equilibrium flow shear can saturate at lower fluxes that are more consistent with the available sources in STEP. This analysis suggests that hybrid kinetic ballooning modes could play an important role in setting the turbulent transport in STEP, and possible mechanisms to mitigate turbulent transport are discussed. Increasing the safety factor or the pressure gradient strongly reduces turbulent transport from hybrid kinetic ballooning modes in the cases considered here. Challenges of simulating electromagnetic turbulence in this high-$\beta$ regime are highlighted. In particular the observation of radially extended turbulent structures in the absence of equilibrium flow shear motivates future advanced global gyrokinetic simulations that include $\delta B_\parallel$.

physics.plasm-ph↗

arXiv2023

Electromagnetic gyrokinetic instabilities in STEP

We present herein the results of a linear gyrokinetic analysis of electromagnetic microinstabilites in the conceptual high-$\beta$, reactor-scale, tight-aspect-ratio tokamak STEP (Spherical Tokamak for Energy Production, https://step.ukaea.uk). We examine a range of flux surfaces between the deep core and the pedestal top for two candidate flat-top operating points of the prototype device. Local linear gyrokinetic analysis is performed to determine the type of microinstabilities that arise under these reactor-relevant conditions. We find that the equilibria are dominated at ion binormal scales by a hybrid version of the Kinetic Ballooning Mode (KBM) instability that has significant linear drive contributions from the ion temperature gradient and from trapped electrons, while collisional Microtearing Modes (MTMs) are sub-dominantly also unstable at similar binormal scales. The hybrid-KBM and MTM exhibit very different radial scales. We study the sensitivity of these instabilities to physics parameters, and discuss potential mechanisms for mitigating them. The results of this investigation are compared to a small set of similar conceptual reactor designs in the literature. A detailed benchmark of the linear results is performed using three gyrokinetic codes; alongside extensive resolution testing and sensitivity to numerical parameters providing confidence in the results of our calculations, and paving the way for detailed nonlinear studies in a companion article.

physics.plasm-ph↗

arXiv2022

Microstability of $\beta \sim 1$ tokamak equilibria

High-power-density tokamaks offer a potential solution to design cost-effective fusion devices. One way to achieve high power density is to operate at a high $\beta$ value (the ratio of thermal to magnetic pressure), i.e., $\beta \sim 1$. However, a $\beta \sim 1$ state may be unstable to various pressure- and current-driven instabilities or have unfavorable microstability properties. To explore these possibilities, we generate $\beta \sim 1$ equilibria and investigate their stability. Initially, we study an analytical technique that was used in the past to generate $\beta \sim 1$ equilibria and outline its limitations. Hence, we demonstrate the generation of high-$\beta$ equilibria with the computer code $\texttt{VMEC}$. We then analyze these equilibria to determine their stability against the infinite-$n$ ideal ballooning mode. We follow that by engaging in a detailed microstability study, beginning with assessments of electrostatic ITG and TEM instabilities. We observe interesting behavior for the high-$\beta$ equilibria -- stabilization of these modes through two distinct mechanisms. Finally, we perform electromagnetic gyrokinetic simulations and again observe stabilizing trends in the equilibria at high $\beta$. These trends are different from their lower $\beta$ counterparts and offer an alternative, potentially favorable regime of tokamak operation.

physics.plasm-ph↗