arXiv subjects

Hannes Bergström

Publications and source records attributed to Hannes Bergström.

4 recordsLinked to original sources

arXiv2026

Implementation and verification of the avalanche source in a 3D full-f particle-in-cell model of relativistic electrons for studies of tokamak disruptions

Tokamak disruptions may lead to the acceleration of some electrons to relativistic energies. These so-called runaway electrons (REs) can multiply exponentially via knock-on collisions with thermal electrons. As the resulting RE avalanche is exponentially sensitive to the pre-disruption plasma current, multi-MA RE beams may form in large future devices, risking severe localized wall damage. In this work, an energy and momentum conserving knock-on collision operator is implemented in the 3D nonlinear MHD code JOREK for the full-f relativistic hybrid fluid-kinetic model that describes the REs using the particle-in-cell (PiC) approach both for full-orbit and drift-kinetic markers, which will enable accurate modeling of the RE phase-space dynamics in realistic 3D electromagnetic fields. Such a self-consistent treatment of the RE avalanche and competing losses in the stochastic fields of MHD-active plasmas is required to further the understanding of RE transport and phase-space dynamics in self-consistent interaction with the 3D plasma evolution, which is needed for developing reliable predictions as well as reliable mitigation methods. To make such novel high-fidelity simulations computationally viable, a resampling technique was also implemented to restrict the number of markers. The avalanche model is verified using analytical expressions from literature and applied to a JET-like termination scenario, demonstrating its applicability to realistic 3D MHD active scenarios. Future work on porting to accelerated high-performance computing systems will be needed to cross the long time scales involved, e.g., in periodic termination and re-avalanching that could occur in large devices like ITER.

physics.plasm-ph↗

arXiv2026

FIREWALL: A surrogate model for the rapid assessment of tokamak wall loading and melting by runaway electrons

Runaway electron (RE) beams generated during tokamak disruptions can deposit highly localized heat loads on plasma-facing components, posing a serious risk of melting and damage. Monte Carlo particle transport simulations coupled with three-dimensional thermomechanical response modeling can quantify this damage but are too computationally demanding for device-scale assessments and extensive scenario scans. We present FIREWALL (Fast Integrated Runaway Electron WALL loads), a surrogate model that combines a database of \textsc{Geant4} volumetric energy-deposition profiles with a one-dimensional heat-diffusion solver for each wall element. FIREWALL retains the energy and incident angle distributions of impacting REs and predicts the spatiotemporal temperature evolution of detailed three-dimensional wall geometries up to the melting threshold. FIREWALL thus provides a fast physics-based framework for translating global RE simulations into global wall melting predictions, enabling large-scale screening of disruption scenarios while directing high-fidelity costly workflows to the limited wall regions where they are actually required.

physics.plasm-ph↗

arXiv2026

A new model for runaway electron transport based on chaotic Hamiltonian systems

The transport of runaway electrons (RE) in ergodic magnetic geometries is an area of active study. Computing the transport from the direct simulation of particle trajectories is computationally expensive. Instead, diffusion models, such as the one by Rechester and Rosenbluth, are often employed to incorporate transport effects into reduced simulations. However, the comparison of diffusion-based to direct simulations reveals that the transport is typically not purely diffusive. In this paper, we introduce a simple transport model, based on chaos theory, which goes beyond the Rechester-Rosenbluth approximation. Besides chaotic diffusion, our model takes into account the effect of so-called sticky regions, a trapping layer around magnetic islands, where particle escape slows down to a power-law decay rather than an exponential decay. We demonstrate the applicability of the model both in the Ullmann-Caldas map with parameters corresponding to the TBR-1 tokamak, and in a JOREK simulation of a JET disruption scenario, with remarkably good fits achieved in both cases.

physics.plasm-ph↗

arXiv2023

Bayesian optimization of massive material injection for disruption mitigation in tokamaks

A Bayesian optimization framework is used to investigate scenarios for disruptions mitigated with combined deuterium and neon injection in ITER. The optimization cost function takes into account limits on the maximum runaway current, the transported fraction of the heat loss and the current quench time. The aim is to explore the dependence of the cost function on injected densities, and provide insights into the behaviour of the disruption dynamics for representative scenarios. The simulations are conducted using the numerical framework DREAM (Disruption Runaway Electron Analysis Model). We show that irrespective of the quantities of the material deposition, multi-megaampere runaway currents will be produced in the deuterium-tritium phase of operations, even in the optimal scenarios. However, the severity of the outcome can be influenced by tailoring the radial profile of the injected material; in particular if the injected neon is deposited at the edge region it leads to a significant reduction of both the final runaway current and the transported heat losses. The Bayesian approach allows us to map the parameter space efficiently, with more accuracy in favorable parameter regions, thereby providing us information about the robustness of the optima.

physics.plasm-ph↗