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Variability of MHD instabilities in benign termination of high-current runaway electron beams in the JET and DIII-D tokamaks.

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Title: Variability of MHD instabilities in benign termination of high-current runaway electron beams in the JET and DIII-D tokamaks.
Authors: Zimmermann, C.F.B.1 (AUTHOR) benedikt.zimmermann@columbia.edu, Paz-Soldan, C.1 (AUTHOR), Su, G.1 (AUTHOR), Reux, C.2 (AUTHOR), Battey, A.F.1 (AUTHOR), Ficker, O.3 (AUTHOR), Gerasimov, S.N.4 (AUTHOR), Hansen, C.J.1 (AUTHOR), Jachmich, S.5 (AUTHOR), Lvovskiy, A.6 (AUTHOR), Puchmayr, J.7 (AUTHOR), Schoonheere, N.2 (AUTHOR), Sheikh, U.8 (AUTHOR), Stewart, I.G.1 (AUTHOR), Szepesi, G.4 (AUTHOR)
Source: Nuclear Fusion. 2026, Vol. 66 Issue 5, p1-15. 15p.
Subjects: Magnetohydrodynamic instabilities, JET Project (Joint European Torus), Plasma currents, Electron beams, Fusion reactors, Tokamaks, Magnetohydrodynamics
Abstract: Benign termination, in which magnetohydrodynamic (MHD) instabilities deconfine runaway electrons (REs) following hydrogenic injections, is a promising strategy for mitigating dangerous RE loads after disruptions. Recent experiments on the Joint European Torus (JET) have explored this scenario at higher pre-disruptive plasma currents than are achievable on other devices, revealing challenges in obtaining benign terminations at I p ⩾ 2.5 MA. This work analyzes the evolution of these high-current RE beams and their terminating MHD events using fast magnetic sensor measurements and EFIT equilibrium reconstructions for approximately 40 JET and 20 DIII-D tokamak discharges. On JET, unsuccessful non-benign terminations occur at low edge safety factor ( q edge ≈ 2 ), and are preceded by intermittent, non-terminating MHD events at higher rational q edge. Trends in the internal inductance l i indicate more peaked RE current profiles in the high- I p non-benign population, which may hinder successful recombination through re-ionization of the companion plasma. In contrast, benign terminations on JET typically occur at higher q edge ⩾ 3 and exhibit less peaked RE current profiles. DIII-D displays a broader range of terminating edge safety factors, again correlated with the measured l i values. Across both tokamaks, the RE current peaking is therefore found to determine which MHD instability boundary is encountered, a result confirmed by linear resistive MHD modeling with the CASTOR3D code. Measured growth rates are similar for benign and non-benign cases, indicating that ideal MHD timescales at low density after hydrogenic injection do not alone explain efficient RE deconfinement. Instead, non-benign cases are most readily characterized by their comparably lower overall MHD perturbation amplitudes δ B. These observations suggest that the interplay between ideal and resistive dynamics governs the termination process, with implications for extrapolating benign RE termination to high- I p reactor scenarios. [ABSTRACT FROM AUTHOR]
Copyright of Nuclear Fusion is the property of IOP Publishing and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Variability of MHD instabilities in benign termination of high-current runaway electron beams in the JET and DIII-D tokamaks.
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  Data: <searchLink fieldCode="AR" term="%22Zimmermann%2C+C%2EF%2EB%2E%22">Zimmermann, C.F.B.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> benedikt.zimmermann@columbia.edu</i><br /><searchLink fieldCode="AR" term="%22Paz-Soldan%2C+C%2E%22">Paz-Soldan, C.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Su%2C+G%2E%22">Su, G.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Reux%2C+C%2E%22">Reux, C.</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Battey%2C+A%2EF%2E%22">Battey, A.F.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Ficker%2C+O%2E%22">Ficker, O.</searchLink><relatesTo>3</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Gerasimov%2C+S%2EN%2E%22">Gerasimov, S.N.</searchLink><relatesTo>4</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Hansen%2C+C%2EJ%2E%22">Hansen, C.J.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Jachmich%2C+S%2E%22">Jachmich, S.</searchLink><relatesTo>5</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Lvovskiy%2C+A%2E%22">Lvovskiy, A.</searchLink><relatesTo>6</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Puchmayr%2C+J%2E%22">Puchmayr, J.</searchLink><relatesTo>7</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Schoonheere%2C+N%2E%22">Schoonheere, N.</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Sheikh%2C+U%2E%22">Sheikh, U.</searchLink><relatesTo>8</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Stewart%2C+I%2EG%2E%22">Stewart, I.G.</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Szepesi%2C+G%2E%22">Szepesi, G.</searchLink><relatesTo>4</relatesTo> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Nuclear+Fusion%22">Nuclear Fusion</searchLink>. 2026, Vol. 66 Issue 5, p1-15. 15p.
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  Data: Benign termination, in which magnetohydrodynamic (MHD) instabilities deconfine runaway electrons (REs) following hydrogenic injections, is a promising strategy for mitigating dangerous RE loads after disruptions. Recent experiments on the Joint European Torus (JET) have explored this scenario at higher pre-disruptive plasma currents than are achievable on other devices, revealing challenges in obtaining benign terminations at I p ⩾ 2.5 MA. This work analyzes the evolution of these high-current RE beams and their terminating MHD events using fast magnetic sensor measurements and EFIT equilibrium reconstructions for approximately 40 JET and 20 DIII-D tokamak discharges. On JET, unsuccessful non-benign terminations occur at low edge safety factor ( q edge ≈ 2 ), and are preceded by intermittent, non-terminating MHD events at higher rational q edge. Trends in the internal inductance l i indicate more peaked RE current profiles in the high- I p non-benign population, which may hinder successful recombination through re-ionization of the companion plasma. In contrast, benign terminations on JET typically occur at higher q edge ⩾ 3 and exhibit less peaked RE current profiles. DIII-D displays a broader range of terminating edge safety factors, again correlated with the measured l i values. Across both tokamaks, the RE current peaking is therefore found to determine which MHD instability boundary is encountered, a result confirmed by linear resistive MHD modeling with the CASTOR3D code. Measured growth rates are similar for benign and non-benign cases, indicating that ideal MHD timescales at low density after hydrogenic injection do not alone explain efficient RE deconfinement. Instead, non-benign cases are most readily characterized by their comparably lower overall MHD perturbation amplitudes δ B. These observations suggest that the interplay between ideal and resistive dynamics governs the termination process, with implications for extrapolating benign RE termination to high- I p reactor scenarios. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
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  Data: <i>Copyright of Nuclear Fusion is the property of IOP Publishing and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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      – SubjectFull: Tokamaks
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      – SubjectFull: Magnetohydrodynamics
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