## Samenvatting

Available energy (Æ), which quantifies the maximum amount of thermal energy that may be liberated and converted into instabilities and turbulence, has shown to be a useful metric for predicting saturated energy fluxes in trapped-electron-mode-driven turbulence. Here, we calculate and investigate the Æ in the analytical tokamak equilibria introduced by Miller et al. (Phys. Plasmas, vol. 5, issue, 4, 1998, pp. 973-978). The Æ of trapped electrons reproduces various trends also observed in experiments; negative shear, increasing Shafranov shift, vertical elongation and negative triangularity can all be stabilising, as indicated by a reduction in Æ, although it is strongly dependent on the chosen equilibrium. Comparing Æ with saturated energy flux estimates from the TGLF (trapped gyro-Landau fluid) model, we find fairly good correspondence, showcasing that Æ can be useful to predict trends. We go on to investigate Æ and find that negative triangularity is especially beneficial in vertically elongated configurations with positive shear or low gradients. Furthermore, we extract a gradient-threshold-like quantity from Æ and find that it behaves similarly to gyrokinetic gradient thresholds: it tends to increase linearly with magnetic shear, and negative triangularity leads to an especially high threshold. We next optimise the device geometry for minimal Æ and find that the optimum is strongly dependent on equilibrium parameters, for example, magnetic shear or pressure gradient. Investigating the competing effects of increasing the density gradient, the pressure gradient, and decreasing the shear, we find regimes that have steep gradients yet low Æ, and that such a regime is inaccessible in negative-triangularity tokamaks.

Originele taal-2 | Engels |
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Artikelnummer | 905890522 |

Aantal pagina's | 34 |

Tijdschrift | Journal of Plasma Physics |

Volume | 89 |

Nummer van het tijdschrift | 5 |

DOI's | |

Status | Gepubliceerd - okt. 2023 |

### Financiering

We wish to thank J. Ball, J.M. Duff, R. Wolf, A. Goodman, P. Mulholland, P. Costello, M.J. Pueschel, F. Jenko, M. Barnes and E. Rodriguez for insightful discussions. This work was partly supported by a grant from the Simons Foundation (560651, P.H.), and this publication is part of the project ‘Shaping turbulence – building a framework for turbulence optimisation of fusion reactors’, with project no. OCENW.KLEIN.013 of the research program ‘NWO Open Competition Domain Science’ which is financed by the Dutch Research Council (NWO). This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Program (grant agreement no. 101052200 – EUROfusion). Views and opinions expressed are, however, those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

Financiers | Financiernummer |
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Simons Foundation | 560651, OCENW.KLEIN.013 |

European Commission | 101052200 |

Nederlandse Organisatie voor Wetenschappelijk Onderzoek |