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Modelling runaway electron generation in tokamaks
KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetics and Plasma Physics.ORCID iD: 0009-0000-6127-9787
2026 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Tokamak disruptions can convert a large fraction of the plasma current into a beam of relativistic runaway electrons. In a reactor-scale device such as ITER,a runaway electron beam could carry several megaamperes and, if left uncontrolled, could cause severe damage to plasma-facing components. Predicting whether a given disruption scenario leads to a dangerous runaway beam, and designing injection schemes that prevent it, requires models that capture the interplay between material injection, rapid plasma cooling, electric field evolution, and the various mechanisms by which runaway electrons are born,multiply, and are lost. This thesis addresses runaway electron physics from seed formation to disruption mitigation through numerical modelling.

A synthetic electron cyclotron emission (ECE) framework is developed and applied to vertical ECE measurements on the TCV tokamak, combining Fokker-Planck calculations of the electron distribution function with ray tracing and radiative transfer. The analysis demonstrates that vertical ECE can resolve the energy-dependent dynamics of suprathermal electrons in the 20–100 keV range, providing constraints on the nascent runaway seed that are difficult to obtain with conventional diagnostics.

The disruption simulation framework Dream is then extended with several physics models relevant to ITER: runaway electron losses from vertical plasma displacement, cross-field drift of pellet ablation material, stochasticity driven current-profile relaxation, and an updated Compton scattering source for the ITER first wall. These are applied to a systematic study of shattered pellet injection scenarios in ITER showing that avoiding a multi-megaampere runaway beam depends sensitively on the thermal quench timescale, the injected material composition, and the competition between runaway multiplication and scrape-off losses. Finally, a viable theoretical pathway that limits the runaway current to tolerable levels even in the presence of nuclear runaway sources is identified.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2026. , p. ix, 71
Series
TRITA-EECS-AVL ; 2026:59
Keywords [en]
Nuclear fusion, Tokamak, Runaway electrons, Disruptions
National Category
Fusion, Plasma and Space Physics
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-382158ISBN: 978-91-8106-637-1 (print)OAI: oai:DiVA.org:kth-382158DiVA, id: diva2:2061895
Presentation
2026-06-12, H1, Teknikringen 33, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20260525

Available from: 2026-05-25 Created: 2026-05-22 Last updated: 2026-05-29Bibliographically approved
List of papers
1. Experimental and numerical investigation of suprathermal electron dynamics using vertical electron cyclotron emission
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2026 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 68, no 1, p. 015029-015029Article in journal (Refereed) Published
Abstract [en]

The Tokamak à configuration variable (TCV) is equipped with an advanced set of diagnostics for studying suprathermal electron dynamics. Among these, the vertical electron cyclotron emission (VECE) diagnostic offers valuable insights into the electron energy distribution by measuring electron cyclotron emission (ECE) along a vertical line-of-sight. However, reconstructing the electron distribution from ECE measurements is inherently challenging due to harmonic overlap and thermal radiation noise. A more practical approach leverages forward modelling of ECE based on kinetic simulations. To this end, we introduce Yoda, a novel synthetic ECE diagnostic framework that simulates emission and (re)absorption of electron cyclotron (EC) radiation for arbitrary electron distributions and antenna geometries. The framework is validated against the well-established synthetic ECE code Spece, using an ohmic TCV discharge as a reference case. In this study, the 3D bounce-averaged Fokker–Planck code Luke is used to model electron distributions in two EC current drive experiments. The synthetic spectra generated using the combined Luke-Yoda framework successfully reproduce the main features of the experimental VECE measurements in both simulated discharges. The combination of kinetic and synthetic ECE simulations allow the identification of the features in the electron distribution function which give rise to certain signatures in the VECE signal.

Place, publisher, year, edition, pages
IOP Publishing, 2026
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-382173 (URN)10.1088/1361-6587/ae3344 (DOI)001668077700001 ()2-s2.0-105033450576 (Scopus ID)
Funder
Swedish Research Council, 2024-04879
Note

QC 20260525

Available from: 2026-05-25 Created: 2026-05-25 Last updated: 2026-05-25Bibliographically approved
2. Runaway electron generation in ITER mitigated disruptions with improved physics models
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(English)Manuscript (preprint) (Other academic)
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-382030 (URN)
Note

Submitted to Nuclear Fusion

QC 20260521

Available from: 2026-05-21 Created: 2026-05-21 Last updated: 2026-05-25Bibliographically approved
3. Reduced modelling of scrape-off losses of runaway electrons during tokamak disruptions
Open this publication in new window or tab >>Reduced modelling of scrape-off losses of runaway electrons during tokamak disruptions
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2025 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 91, no 3, article id E78Article in journal (Refereed) Published
Abstract [en]

Accurate modelling of runaway electron generation and losses during tokamak disruptions is crucial for the development of reactor-scale tokamak devices. In this paper, we present a reduced model for runaway electron losses due to flux surface scrape-off caused by the vertical motion of the plasma. The model is made compatible with computationally inexpensive one-dimensional models averaging over a fixed flux-surface geometry, by formulating it as a loss term outside an estimated time-varying minor radius of the last closed flux surface. We then implement this model in the disruption modelling tool DREAM and demonstrate its impact on selected scenarios relevant for ITER. Our results indicate that scrape-off losses may be crucial for making complete runaway avoidance possible even in a 15 MA DT H-mode ITER scenario. The results are however sensitive to the details of the runaway electron generation and phenomena affecting the current density profile, such as the current profile relaxation at the beginning of the disruption.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2025
Keywords
fusion plasma, plasma dynamics, Runaway electrons
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-364018 (URN)10.1017/S0022377825000327 (DOI)001489654900001 ()2-s2.0-105005485847 (Scopus ID)
Note

QC 20250603

Available from: 2025-06-02 Created: 2025-06-02 Last updated: 2026-05-25Bibliographically approved
4. Simulation of shattered pellet injections with plasmoid drifts in ASDEX Upgrade and ITER
Open this publication in new window or tab >>Simulation of shattered pellet injections with plasmoid drifts in ASDEX Upgrade and ITER
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2025 (English)In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 67, no 10, article id 105034Article in journal (Refereed) Published
Abstract [en]

Pellet injection is an important means to fuel and control discharges and mitigate disruptions in reactor-scale fusion devices. To accurately assess the efficiency of these applications, it is necessary to account for the drift of the ablated material towards the low-field side. In this study, we have implemented a semi-analytical model for ablation cloud drifts in the numerical disruption modelling tool DREAM. We show that this model is capable of reproducing the density evolution in shattered pellet injection (SPI) experiments in ASDEX Upgrade, for model parameters within the expected range. The model is then used to investigate the prospects for disruption mitigation by staggered SPIs in 15MA DT H-mode ITER scenarios. We find that the drifts may decrease the assimilation of pure deuterium SPIs by about an order of magnitude, which may be important to consider when designing the disruption mitigation scheme in ITER. The ITER scenarios studied here generally result in similar multi-MA runaway electron (RE) currents, regardless of the drift assumptions, but the effect of the drift is larger in situations with a fast and early thermal quench. The RE current may also be more strongly affected by the drift losses when accounting for RE losses caused by the vertical plasma motion.

Place, publisher, year, edition, pages
IOP Publishing, 2025
Keywords
disruption mitigation, shattered pellet injection, plasmoid drift, plasma simulation, ASDEX Upgrade, ITER
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-375526 (URN)10.1088/1361-6587/ae140f (DOI)001607541200001 ()2-s2.0-105034123515 (Scopus ID)
Note

QC 20260416

Available from: 2026-01-27 Created: 2026-01-27 Last updated: 2026-05-25Bibliographically approved

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