Showing 1–20 of 20 results for author: Sheikh, U

AXUV synthetic diagnostic for ASDEX Upgrade and its application for SPI simulations

Authors: Ferenc Lengyel, Weikang Tang, Matthias Hölzl, Matthias Bernert, Matěj Tomeš, Peter Halldestam, Paul Heinrich, Gergely Papp, Stefan Jachmich, Umar Sheikh, Mathias Dibon, Pascal de Marné, Jörg Hobirk, Thomas Eberl, Gergő I. Pokol, for the ASDEX Upgrade Team, the EUROfusion Tokamak Exploitation Team

Abstract: We introduce an Absolute eXtended UltraViolet (AXUV) diode-based camera forward-modelling tool to support the validation of mitigated disruption simulations and the interpretation of experimental phenomena, with applications to the ASDEX Upgrade (AUG) tokamak. AXUV diodes measure electromagnetic radiation across a wide spectral range with a significantly higher time resolution (~microseconds) than… ▽ More We introduce an Absolute eXtended UltraViolet (AXUV) diode-based camera forward-modelling tool to support the validation of mitigated disruption simulations and the interpretation of experimental phenomena, with applications to the ASDEX Upgrade (AUG) tokamak. AXUV diodes measure electromagnetic radiation across a wide spectral range with a significantly higher time resolution (~microseconds) than foil bolometers (~milliseconds), albeit with a non-uniform spectral responsivity. AXUV is suitable for examining fast phenomena, such as shattered pellet injection (SPI), where the radiation localisation and radiated power provide information on the deposition of pellet material. Due to the characteristics and degradation of AXUV diodes, absolute power measurements are subject to large systematic uncertainties, especially when the spectra are time-varying, as in e.g. mixed Ne/D2 SPI experiments. These challenges motivated the development of a synthetic diagnostic within the Cherab-Raysect optical modelling framework, which is applied here to four AXUV cameras in two poloidal cross-sections of AUG. The synthetic diagnostic provides a means to understand how the diodes measure radiation under SPI conditions and to connect first-principles plasma simulations with experimental measurements. The details of the synthetic diagnostic are presented, and the capabilities are illustrated with applications to AUG SPI simulations performed in JOREK. The synthetic signals generated from these simulations are compared with experimental measurements from the 2022 SPI campaign and show qualitatively similar features in many respects. Particularly good agreement was found in the time evolution of the studied high Ne-content (10%) pellet, whereas a different, low Ne-content (0.17%) case exhibited more pronounced differences, likely due to the absence of background impurities in the underlying SPI simulations. △ Less

Submitted 21 August, 2026; originally announced August 2026.

Comments: 16 figures (some multi-panel), 2 supplementary movies. Submitted to Plasma Physics and Controlled Fusion

Synthetic Diagnostic Modeling for Plasma Tomography: Geometry Matrix Computation Methods and Impact of Model Accuracy

Authors: D. Hamm, G. Partesotti, C. Theiler, U. Sheikh

Abstract: Tomographic emissivity reconstruction from plasma diagnostics data relies on a synthetic model mapping the plasma emissivity to the measured signals. The model, referred to as a geometry matrix in the plasma imaging community, is often built using the line-of-sight (LoS) approximation. This approximation neglects the finite width of the detector viewing beams and can therefore introduce systematic… ▽ More Tomographic emissivity reconstruction from plasma diagnostics data relies on a synthetic model mapping the plasma emissivity to the measured signals. The model, referred to as a geometry matrix in the plasma imaging community, is often built using the line-of-sight (LoS) approximation. This approximation neglects the finite width of the detector viewing beams and can therefore introduce systematic errors. Physically correct volume-of-sight (VoS) models remove this inaccuracy by accounting for the full 3D extent of the viewing beams. Their adoption, however, is sometimes hindered by the difficulty of independently validating them. We present an intuitive and easily inspectable voxel-to-detector (V2D) approach for computing physically accurate VoS geometry matrices, based on discretizing the tokamak vessel into voxels and estimating the contribution of each voxel to the measurements of each detector. We apply the V2D approach to the soft X-ray (SXR) and bolometry systems of the TCV tokamak. Through phantom-based studies on physically realistic emissivity profiles, we quantify the improvement in reconstruction quality obtained by using VoS rather than LoS models. We find that the VoS model yields overall better accuracy and precision; however, interestingly, the simpler LoS model does not introduce a significant systematic bias in the estimated total, core, divertor and main chamber radiated powers. We further compare the V2D geometry matrix with an independent ray-tracing implementation, finding excellent agreement that validates both approaches for routine use at TCV. All routines developed in this work are made openly available. △ Less

Submitted 4 August, 2026; originally announced August 2026.

Evolution of SPI-induced disruptions in ASDEX Upgrade

Authors: P. Heinrich, G. Papp, S. Jachmich, J. Artola, M. Bernert, P. de Marné, M. Dibon, R. Dux, T. Eberl, O. Ficker, P. Halldestam, J. Hobirk, M. Hoelzl, F. Klossek, M. Lehnen, T. Lunt, M. Maraschek, A. Patel, T. Peherstorfer, N. Schwarz, U. Sheikh, B. Sieglin, J. Svoboda, W. Tang, the ASDEX Upgrade Team , et al. (1 additional authors not shown)

Abstract: Disruptions are a major concern for future fusion reactors based on the tokamak principle. To ensure machine protection, the thermal loads and vessel forces that arise during disruptions have to be mitigated reliably. For the ITER disruption mitigation system (DMS), the shattered pellet injection (SPI) technology has been selected. It can provide a prompt delivery of the injection material into th… ▽ More Disruptions are a major concern for future fusion reactors based on the tokamak principle. To ensure machine protection, the thermal loads and vessel forces that arise during disruptions have to be mitigated reliably. For the ITER disruption mitigation system (DMS), the shattered pellet injection (SPI) technology has been selected. It can provide a prompt delivery of the injection material into the plasma core, with the mitigation efficiency depending on fragment size and velocity. A highly flexible SPI system was built and installed at the tokamak ASDEX Upgrade (AUG) to aid the finalization process of the ITER DMS and provide crucial input for modeling. The SPI-induced disruptions in the 2022 AUG experiments follow a typical chain of events, which are discussed in this paper: The first light, main fragment arrival, plasma movement event, MARFE, thermal quench/plasma current spike, current quench, and vertical displacement event phase. Depending on the injection parameters, these phases may vary significantly or some might not be present at all. In this paper, we will focus on the characterization of these disruption phases and figures of merit for the mitigation efficiency, depending on the SPI configuration. With increasing amount of assimilated neon in the plasma - primarily influenced by the neon content in the pellet but also the shattering parameters - the disruptions exhibit different behaviors. This disruption evolution seems to be a continuous process, with the most prominent feature being the changing disruption time scales and plasma current time trace shape during the CQ from convex (poorly or unmitigated) $\rightarrow$ concave (well mitigated/radiation dominated). Depending on the injection, pre-TQ durations between 15 - 0.5 ms and early CQ durations ($Δ\textrm{t}_\textrm{CQ}^{100 \rightarrow 80}$) between 13.3 - 8.2 ms had been achieved at AUG. △ Less

Submitted 8 July, 2026; v1 submitted 7 April, 2026; originally announced April 2026.

Comments: 23 pages, 14 figures

Non-dimensional confinement scaling in similar negative triangularity plasmas on the DIII-D and TCV tokamaks

Authors: A. Marinoni, C. Chrystal, S. Coda, R. Coosemans, C. Marini, M. Podesta, O. Sauter, M. Agostini, M. E. Austin, E. Belli, J. Candy, M. Gorelenkova, D. Hamm, A. W. Hyatt, M. Knolker, M. La Matina, P. Lunia, S. Mordijck, A. O. Nelson, T. H. Osborne, C. Paz-Soldan, L. Porte, U. Sheikh, F. Scotti, K. E. Thome , et al. (3 additional authors not shown)

Abstract: Similarity experiments were performed on the DIII-D and TCV tokamaks to explore the scaling of energy confinement in negative triangularity plasmas using non-dimensional variables. Near up-down symmetric plasmas with large top-bottom averaged negative triangularity were created in a lower single null configuration, with the shape of the separatrix being closely matched between the two devices. The… ▽ More Similarity experiments were performed on the DIII-D and TCV tokamaks to explore the scaling of energy confinement in negative triangularity plasmas using non-dimensional variables. Near up-down symmetric plasmas with large top-bottom averaged negative triangularity were created in a lower single null configuration, with the shape of the separatrix being closely matched between the two devices. The normalized energy confinement is found to weakly improve at increasing collisionality and, between the two devices, shows a machine size scaling behavior between Bohm and gyro-Bohm. Engineering scaling on a large DIII-D dataset is in agreement with the non-dimensional experiment. △ Less

Submitted 12 March, 2026; originally announced March 2026.

Infrared Thermography in the Tokamak à Configuration Variable

Authors: M. Zurita, H. Reimerdes, C. Colandrea, H. Elaian, M. Pedrini, Y. Andrebe, F. Crisinel, S. Koncewiez, J. -D. Landis, D. Mykytchuk, U. Sheikh, the TCV team

Abstract: In the Tokamak à Configuration Variable (TCV), infrared thermography (IR) is currently composed of the horizontal, vertical, and tangential infrared systems (HIR, VIR, TIR), which all use Equus 81k M cameras. The IR diagnostics obtain the surface temperature of TCV's graphite tiles for post-discharge analysis. Target heat flux profiles are inferred from the tile temperature with the THEODOR (Therm… ▽ More In the Tokamak à Configuration Variable (TCV), infrared thermography (IR) is currently composed of the horizontal, vertical, and tangential infrared systems (HIR, VIR, TIR), which all use Equus 81k M cameras. The IR diagnostics obtain the surface temperature of TCV's graphite tiles for post-discharge analysis. Target heat flux profiles are inferred from the tile temperature with the THEODOR (Thermal Energy Onto Divertor) code. Fast transient analysis is possible in reduced frame mode, with acquisition frequencies above 10kHz. The main views are the lower inner wall for HIR, the floor for VIR, and the lower outer wall for TIR. The HIR camera can also be moved to view the midplane inner wall, while TIR can be moved to see the midplane inner wall and the upper outer wall, mainly to measure synchrotron radiation and heat deposition due to runaway electrons. Recent developments in TCV's IR systems include (i) tile diffusivity and conductivity measurements to assure the precision of heat flux estimates; (ii) the addition of one new VIR heated valley tile and two rooftop TIR tiles, for measurements of fast heat flux transients; (iii) the implementation of long-pass wavelength filter of 4095 nm, to diminish the measurement of plasma parasitic infrared light, mainly from deuterium 5-4 emission at 4051 nm. Despite these developments, the main sources of uncertainty for IR in TCV are still parasitic infrared light and the determination of the surface layer heat transmission factor, both of which mainly affect the VIR system. △ Less

Submitted 1 April, 2026; v1 submitted 12 March, 2026; originally announced March 2026.

Comments: 29 pages, 12 figures

Real-time Tomography-based Bayesian Inference from TCV Bolometry Data

Authors: D. Hamm, C. Theiler, L. Simons, B. P. Duval, U. Sheikh, the TCV team

Abstract: Radiated power information is crucial to diagnose and optimize the performance of fusion plasmas. Traditionally, at the TCV tokamak, radiated power analysis has only ever been possible following plasma discharge termination. However, recently, TCV bolometer data have become available in real-time. This offers the opportunity of integrating the radiated power information into the TCV plasma control… ▽ More Radiated power information is crucial to diagnose and optimize the performance of fusion plasmas. Traditionally, at the TCV tokamak, radiated power analysis has only ever been possible following plasma discharge termination. However, recently, TCV bolometer data have become available in real-time. This offers the opportunity of integrating the radiated power information into the TCV plasma control system. In this work, we propose a novel real-time tomography-based Bayesian technique allowing estimation of the power radiated from user-defined regions of interest in the plasma. The real-time estimates are obtained as computationally cheap linear combinations of bolometer measurements, using pre-computed coefficients that are optimized for the specific discharge planned. This method is not, thus, trained on a set of synthetic or tomographically reconstructed emissivity profiles. We detail the derivation of the technique and show its equivalence to traditional tomographic estimates under suitable conditions. We then demonstrate that this technique enables accurate real-time estimation of the total, core, divertor and main chamber radiated power, by its application to a representative and heterogeneous set of TCV discharges. Finally, we discuss the robustness of the technique to faulty detectors, showing that simple precautions allow safe handling of many common issues. The computational routines implementing the described technique are provided as open-source code. △ Less

Submitted 12 March, 2026; originally announced March 2026.

Quantitative 3D non-linear simulations of shattered pellet injection in ASDEX Upgrade using JOREK

Authors: W. Tang, M. Hoelzl, P. Heinrich, D. Hu, F. J. Artola, P. de Marne, M. Dibon, M. Dunne, O. Ficker, P. Halldestam, S. Jachmich, M. Lehnen, E. Nardon, G. Papp, A. Patel, U. Sheikh, the ASDEX Upgrade Team, the EUROfusion Tokamak Exploitation Team, the JOREK Team

Abstract: Shattered pellet injection (SPI) as primary mitigation method for major disruptions in ITER has a large parameter space available for optimization including the total amount of injected material, the size of the individual pellet fragments, the material composition, and the timing of multiple injections. This flexibility needs to be exploited to simultaneously minimize thermal heat loads, electrom… ▽ More Shattered pellet injection (SPI) as primary mitigation method for major disruptions in ITER has a large parameter space available for optimization including the total amount of injected material, the size of the individual pellet fragments, the material composition, and the timing of multiple injections. This flexibility needs to be exploited to simultaneously minimize thermal heat loads, electromagnetic vessel forces, and formation of relativistic electrons and their impacts on plasma facing components. In this article, we apply 3D non-linear magnetohydrodynamic modelling to SPI experiments in the ASDEX Upgrade tokamak, going beyond our previous work [Tang et al Nucl. Fusion 65 116003 (2025)] by resolving some discrepancies between simulations and experiment and thus opening the path to quantitative model validation and experiment interpretation. The key element that enables the transition from merely qualitative comparisons to quantitatively reliable predictions of the thermal quench duration and the radiation fraction is the incorporation of a simplified treatment of parallel heat-flux limiting. The work increases the confidence of matching the key processes of disruption mitigation with this high fidelity modelling in view of predictive studies for ITER. △ Less

Submitted 13 February, 2026; originally announced February 2026.

Variability of MHD Instabilities in Benign Termination of High-Current Runaway Electron Beams in the JET and DIII-D Tokamaks

Authors: C. F. B. Zimmermann, C. Paz-Soldan, G. Su, C. Reux, A. F. Battey, O. Ficker, S. N. Gerasimov, C. J. Hansen, S. Jachmich, A. Lvovskiy, J. Puchmayr, N. Schoonheere, U. Sheikh, I. G. Stewart, G. Szepesi, JET Contributors, the EUROfusion Tokamak Exploitation Team

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 obt… ▽ More 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 $I_p \geq 2.5$ 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 ($q_{\text{edge}} \approx 2$), and are preceded by intermittent, non-terminating MHD events at higher rational $q_{\text{edge}}$. 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. In contrast, benign terminations on JET typically occur at higher $q_{\text{edge}} \geq 3$ and exhibit less peaked RE current profiles. DIII-D displays a range of terminating edge safety factors, 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, 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 characterized by their lower MHD perturbation amplitudes $δB$. These observations suggest that the interplay between ideal and resistive dynamics governs the termination process. △ Less

Submitted 12 February, 2026; v1 submitted 5 January, 2026; originally announced January 2026.

Journal ref: Nucl. Fusion 66 (2026) 056004

Runaway electron-induced plasma facing component damage in tokamaks

Authors: S. Ratynskaia, M. Hoelzl, E. Nardon, P. Aleynikov, F. J. Artola, V. Bandaru, M. Beidler, B. Breizman, D. del-Castillo-Negrete, M. De Angeli, V. Dimitriou, R. Ding, J. Eriksson, O. Ficker, R. S. Granetz, E. Hollmann, M. Hoppe, M. Houry, I. Jepu, H. R. Koslowski, C. Liu, J. R. Martin-Solis, G. Pautasso, Y. Peneliau, R. A. Pitts , et al. (9 additional authors not shown)

Abstract: This Roadmap article addresses the critical and multifaceted challenge of plasma-facing component (PFC) damage caused by runaway electrons (REs) in tokamaks, a phenomenon that poses a significant threat to the viability and longevity of future fusion reactors such as ITER and DEMO. The dramatically increased RE production expected in future high-current tokamaks makes it difficult to avoid or miti… ▽ More This Roadmap article addresses the critical and multifaceted challenge of plasma-facing component (PFC) damage caused by runaway electrons (REs) in tokamaks, a phenomenon that poses a significant threat to the viability and longevity of future fusion reactors such as ITER and DEMO. The dramatically increased RE production expected in future high-current tokamaks makes it difficult to avoid or mitigate REs when a plasma discharge terminates abnormally. Preventing damage from the intense localised heat loads REs can cause requires a holistic approach that considers plasma, REs and PFC damage. Despite decades of progress in understanding the physics of REs and the thermomechanical response of PFCs, their complex interplay remains poorly understood. This document aims to initiate a coordinated, interdisciplinary approach to bridge this gap by reviewing experimental evidence, advancing diagnostic capabilities, and improving modelling tools across different scales, dimensionalities and fidelities. Key topics include RE beam formation and transport, damage mechanisms in brittle and metallic PFCs, and observations in major facilities such as JET, DIII-D, WEST and EAST. The Roadmap emphasises the urgency of predictive, high-fidelity modelling validated against well-diagnosed controlled experiments, particularly in the light of recent changes in ITER's wall material strategy and the growing importance of private sector initiatives. Each section of the article is written to provide a concise overview of one area of this multidisciplinary subject, with an assessment of the status, a look at current and future challenges, and a brief summary. The ultimate goal of this initiative is to guide future mitigation strategies and design resilient components that can withstand the loads imposed by REs, thus ensuring the safe and sustainable operation of the next generation of fusion power plants. △ Less

Submitted 12 June, 2025; originally announced June 2025.

Comments: Submitted for publication in the journal Plasma Physics and Controlled Fusion

Broadband Absorption in Cadmium Telluride Thin-Film Solar Cells via Composite Light Trapping Techniques

Authors: Asif Al Suny, Tazrian Noor, Md. Hasibul Hossain, A. F. M. Afnan Uzzaman Sheikh, Mustafa Habib Chowdhury

Abstract: Composite light-trapping structures offer a promising approach to achieving broadband absorption and high efficiency in thin-film solar cells (TFSCs) in order to accelerate sustainable energy solutions. As the leading material in thin-film solar technology, cadmium telluride (CdTe) faces challenges from surface reflective losses across the solar spectrum and weak absorption in the near-infrared (N… ▽ More Composite light-trapping structures offer a promising approach to achieving broadband absorption and high efficiency in thin-film solar cells (TFSCs) in order to accelerate sustainable energy solutions. As the leading material in thin-film solar technology, cadmium telluride (CdTe) faces challenges from surface reflective losses across the solar spectrum and weak absorption in the near-infrared (NIR) range. This computational study addresses these limitations by employing a dual light trapping technique: the top surfaces of both the CdS and CdTe layers are tapered as nanocones (NCs), while germanium (Ge) spherical nanoparticles (NPs) are embedded within the CdTe absorber layer to enhance broadband absorption. Numerical simulations using Finite-Difference Time Domain (FDTD) and other methods are used to optimize the parameters and configurations of both nanostructures, aiming to achieve peak optoelectronic performance. The results show that a short-circuit current density ($J_{sc}$) of 35.38 mA/$cm^2$ and a power conversion efficiency (PCE) of 27.76% can be achieved with optimal nanocone (NC) texturing and spherical Ge nanoparticle (NP) configurations, a 45.45% and 80.72% increase compared to baseline structure in $J_{sc}$ and PCE respectively. To understand the enhancement mechanisms, the study includes analyses using diffraction grating theory and Mie theory. Fabricability of these structures is also evaluated. Furthermore, an additional study on the effects of incident angle variation and polarization change demonstrates that the optimal structure is robust under practical conditions, maintaining consistent performance. △ Less

Submitted 27 February, 2025; originally announced February 2025.

Comments: 18 pages of main paper and 5 pages of supplementary material. 16 figures and 5 tables in total

An upper pressure limit for low-Z benign termination of runaway electron beams in TCV

Authors: M Hoppe, J Decker, U Sheikh, S Coda, C Colandrea, B Duval, O Ficker, P Halldestam, S Jachmich, M Lehnen, H Reimerdes, C Paz-Soldan, M Pedrini, C Reux, L Simons, B Vincent, T Wijkamp, M Zurita, the TCV team, the EUROfusion Tokamak Exploitation Team

Abstract: We present a model for the particle balance in the post-disruption runaway electron plateau phase of a tokamak discharge. The model is constructed with the help of, and applied to, experimental data from TCV discharges investigating the so-called ``low-Z benign termination'' runaway electron mitigation scheme. In the benign termination scheme, the free electron density is first reduced in order fo… ▽ More We present a model for the particle balance in the post-disruption runaway electron plateau phase of a tokamak discharge. The model is constructed with the help of, and applied to, experimental data from TCV discharges investigating the so-called ``low-Z benign termination'' runaway electron mitigation scheme. In the benign termination scheme, the free electron density is first reduced in order for a subsequently induced MHD instability to grow rapidly and spread the runaway electrons widely across the wall. We show that the observed non-monotonic dependence of the free electron density with the measured neutral pressure is due to plasma re-ionization induced by runaway electron impact ionization. At higher neutral pressures, more target particles are present in the plasma for runaway electrons to collide with and ionize. Parameter scans are conducted to clarify the role of the runaway electron density and energy on the free electron density, and it is found that only the runaway electron density has a noticeable impact. While the free electron density is shown to be related to the spread of heat fluxes at termination, the exact cause for the upper neutral pressure limit remains undetermined and an object for further study. △ Less

Submitted 15 June, 2025; v1 submitted 19 December, 2024; originally announced December 2024.

Comments: 11 pages, 8 figures, special issue paper after the Joint Varenna-Lausanne International Workshop

Radiated energy fraction of SPI-induced disruptions at ASDEX Upgrade

Authors: Paul Heinrich, Gergely Papp, Stefan Jachmich, Javier Artola, Matthias Bernert, Pascal de Marné, Mathias Dibon, Ralph Dux, Thomas Eberl, Jörg Hobirk, Michael Lehnen, Tobias Peherstorfer, Nina Schwarz, Umar Sheikh, Bernhard Sieglin, Jakub Svoboda, the ASDEX Upgrade Team, the EUROfusion Tokamak Exploitation Team

Abstract: Future large tokamaks will operate at high plasma currents and high stored plasma energies. To ensure machine protection in case of a sudden loss of plasma confinement (major disruption), a large fraction of the magnetic and thermal energy must be radiated to reduce thermal loads. The disruption mitigation system for ITER is based on massive material injection in the form of shattered pellet injec… ▽ More Future large tokamaks will operate at high plasma currents and high stored plasma energies. To ensure machine protection in case of a sudden loss of plasma confinement (major disruption), a large fraction of the magnetic and thermal energy must be radiated to reduce thermal loads. The disruption mitigation system for ITER is based on massive material injection in the form of shattered pellet injection (SPI). To support ITER, a versatile SPI system was installed at the tokamak ASDEX Upgrade (AUG). The AUG SPI features three independent pellet generation cells and guide tubes, and each was equipped with different shatter heads for the 2022 experimental campaign. We dedicated over 200 plasma discharges to the study of SPI plasma termination, and in this manuscript report on the results of bolometry (total radiation) analysis. The amount of neon inside the pellets is the dominant factor determining the radiated energy fraction ($f_{rad}$). Large and fast fragments, produced by the 12.5° rectangular shatter head, lead to somewhat higher values of frad compared to the 25° circular or rectangular heads. This effect is strongest for neon content of $< 3\times10^{20}$ neon atoms ($f_\textrm{neon} \lesssim 1.25\%$ neon) injected, where a lower normal velocity component (larger fragments) seems slightly beneficial. While full-sized, 8 mm diameter, 100% deuterium ($D_2$) pellets lead to a disruption, the 4 mm or shortened 8 mm pellets of 100% $D_2$ did not. The disruption threshold for 100% $D_2$ is found to be around $1\times10^{22}$ $D_2$ molecules inside the pellet. While the radiated energy fraction of non-disruptive SPI is below 20%, this is increased to 40% during the TQ and VDE phase of the disruptive injections. For ($D_2$-Ne-mix pellets, frad values of $< 90$% are observed, and the curve saturates around 80% for 10% neon mixed into the 8 mm pellets ($2\times10^{21}$ neon atoms). △ Less

Submitted 16 April, 2025; v1 submitted 1 October, 2024; originally announced October 2024.

Comments: 17 pages, 10 figures

Expulsion of runaway electrons using ECRH in the TCV tokamak

Authors: J. Decker, M. Hoppe, U. Sheikh, B. P. Duval, G. Papp, L. Simons, T. Wijkamp, J. Cazabonne, S. Coda, E. Devlaminck, O. Ficker, R. Hellinga, U. Kumar, Y. Savoye-Peysson, L. Porte, C. Reux, C. Sommariva, A. Tema Biwolé, B. Vincent, L. Votta, the TCV Team, the EUROfusion Tokamak Exploitation Team

Abstract: Runaway electrons (REs) are a concern for tokamak fusion reactors from discharge startup to termination. A sudden localized loss of a multi-megaampere RE beam can inflict severe damage to the first wall. Should a disruption occur, the existence of a RE seed may play a significant role in the formation of a RE beam and the magnitude of its current. The application of central electron cyclotron reso… ▽ More Runaway electrons (REs) are a concern for tokamak fusion reactors from discharge startup to termination. A sudden localized loss of a multi-megaampere RE beam can inflict severe damage to the first wall. Should a disruption occur, the existence of a RE seed may play a significant role in the formation of a RE beam and the magnitude of its current. The application of central electron cyclotron resonance heating (ECRH) in the Tokamak à Configuration Variable (TCV) reduces an existing RE seed population by up to three orders of magnitude within only a few hundred milliseconds. Applying ECRH before a disruption can also prevent the formation of a post-disruption RE beam in TCV where it would otherwise be expected. The RE expulsion rate and consequent RE current reduction are found to increase with applied ECRH power. Whereas central ECRH is effective in expelling REs, off-axis ECRH has a comparatively limited effect. A simple 0-D model for the evolution of the RE population is presented that explains the effective ECRH-induced RE expulsion results from the combined effects of increased electron temperature and enhanced RE transport. △ Less

Submitted 22 July, 2024; v1 submitted 15 April, 2024; originally announced April 2024.

Power exhaust and core-divertor compatibility of the baffled snowflake divertor in TCV

Authors: Sophie Gorno, Claudia Colandrea, Olivier Février, Holger Reimerdes, Christian Theiler, Basil P Duval, Tilmann Lunt, Harshita Raj, Umar A Sheikh, Luke Simons, Andrew Thornton

Abstract: A baffled Snowflake Minus Low-Field Side (SF-LFS) is geometrically-optimised in TCV, increasing divertor neutral pressure, to evaluate the roles of divertor closure (comparing with an unbaffled SF-LFS) and magnetic geometry (comparing with a baffled Single Null, SN) in power exhaust and core-divertor compatibility. Ohmically-heated L-mode discharges in deuterium, with a line-averaged core density… ▽ More A baffled Snowflake Minus Low-Field Side (SF-LFS) is geometrically-optimised in TCV, increasing divertor neutral pressure, to evaluate the roles of divertor closure (comparing with an unbaffled SF-LFS) and magnetic geometry (comparing with a baffled Single Null, SN) in power exhaust and core-divertor compatibility. Ohmically-heated L-mode discharges in deuterium, with a line-averaged core density of approximately 4x10(19) m-3, are seeded with nitrogen to approach detached conditions. Baffles in the SF-LFS configuration are found to reduce the peak outer target heat flux by up to 23%, without significantly affecting the location of the inter-null radiation region or the core-divertor compatibility. When compared to the baffled SN, the baffled SF-LFS exhibits a reduction in outer target heat flux by up to 66% and the ability to balance the strike-point distribution of heat flux. These benefits are less significant with N2 seeding, with similar peak target quantities (such as heat flux, electron temperature and ion flux) and divertor radiated power. Despite a radiating region located farther from the confined plasma for the SF-LFS than the baffled SN, no change in core confinement is observed. Core effective charge even indicates an increase in core impurity penetration for the SF-LFS. These experiments constitute a good reference for detailed model validations and extrapolations, exploring important physics such as core impurity shielding and the dependence of divertor cross-field transport on magnetic geometry. △ Less

Submitted 23 November, 2022; v1 submitted 7 September, 2022; originally announced September 2022.

Improved Heat and Particle Flux Mitigation in High Core Confinement, Baffled, Alternate Divertor Configurations in the TCV tokamak

Authors: Harshita Raj, C. Theiler, A. Thornton, O. Fevrier, S. Gorno, F. Bagnato, P. Blanchard, C. Colandrea, H. de Oliveira, B. P. Duval, B. Labit, A. Perek, H. Reimerdes, U. Sheikh, M. Vallar, B. Vincent

Abstract: Nitrogen seeded detachment has been achieved in the Tokamak a Configuration Variable (TCV) in advanced divertor configurations (ADCs), namely X-divertor and X-point target, with and without baffles in H-mode plasmas with high core confinement. Both ADCs show a remarkable reduction in the inter-ELM particle and heat fluxes to the target compared to the standard divertor configuration. 95-98% of the… ▽ More Nitrogen seeded detachment has been achieved in the Tokamak a Configuration Variable (TCV) in advanced divertor configurations (ADCs), namely X-divertor and X-point target, with and without baffles in H-mode plasmas with high core confinement. Both ADCs show a remarkable reduction in the inter-ELM particle and heat fluxes to the target compared to the standard divertor configuration. 95-98% of the peak heat flux to the target is mitigated as a synergetic effect of ADCs, baffling, and nitrogen seeded detachment. The effect of divertor geometry and baffles on core-divertor compatibility is investigated in detail. The power balance in these experiments is also investigated to explore the physics behind the observed reduction in heat fluxes in the ADCs. △ Less

Submitted 27 July, 2022; originally announced July 2022.

Tomographic reconstruction of the runaway distribution function in TCV using multispectral synchrotron images

Authors: T. A. Wijkamp, A. Perek, J. Decker, B. Duval, M. Hoppe, G. Papp, U. A. Sheikh, I. G. J. Classen, R. J. E. Jaspers, the TCV team, the EUROfusion MST1 team

Abstract: Synchrotron radiation observed in a quiescent TCV runaway discharge is studied using filtered camera images targeting three distinct wavelength intervals. Through the tomographic SART procedure the high momentum, high pitch angle part of the spatial and momentum distribution of these relativistic particles is reconstructed. Experimental estimates of the distribution are important for verification… ▽ More Synchrotron radiation observed in a quiescent TCV runaway discharge is studied using filtered camera images targeting three distinct wavelength intervals. Through the tomographic SART procedure the high momentum, high pitch angle part of the spatial and momentum distribution of these relativistic particles is reconstructed. Experimental estimates of the distribution are important for verification and refinement of formation-, decay- and transport-models underlying runaway avoidance and mitigation strategy design. Using a test distribution it is demonstrated that the inversion procedure provides estimates accurate to within a few tens of percent in the region of phase-space contributing most to the synchrotron image. We find that combining images filtered around different parts of the emission spectrum widens the probed part of momentum-space and reduces reconstruction errors. Next, the SART algorithm is used to obtain information on the spatiotemporal runaway momentum distribution in a selected TCV discharge. The momentum distribution is found to relax towards an avalanche-like exponentially decaying profile. Anomalously high pitch angles and a radial profile increasing towards the edge are found for the most strongly emitting particles in the distribution. Pitch angle scattering by toroidal magnetic field ripple is consistent with this picture. An alternative explanation is the presence of high frequency instabilities in combination with the formation of a runaway shell at the edge of the plasma. △ Less

Submitted 5 February, 2021; v1 submitted 30 October, 2020; originally announced November 2020.

Comments: 18 pages, 13 figures, submitted to Nuclear Fusion

Runaway electron synchrotron radiation in a vertically translated plasma

Authors: M. Hoppe, G. Papp, T. Wikjamp, A. Perek, J. Decker, B. Duval, O. Embreus, T. Fülöp, U. A. Sheikh

Abstract: Synchrotron radiation observed from runaway electrons (REs) in tokamaks depends upon the position and size of the RE beam, the RE energy and pitch distributions, as well as the location of the observer. We show that experimental synchrotron images of a vertically moving runaway electron beam sweeping past the detector in the TCV tokamak agree well with predictions from the synthetic synchrotron di… ▽ More Synchrotron radiation observed from runaway electrons (REs) in tokamaks depends upon the position and size of the RE beam, the RE energy and pitch distributions, as well as the location of the observer. We show that experimental synchrotron images of a vertically moving runaway electron beam sweeping past the detector in the TCV tokamak agree well with predictions from the synthetic synchrotron diagnostic Soft. This experimental validation lends confidence to the theory underlying the synthetic diagnostics which are used for benchmarking theoretical models of and probing runaway dynamics. We present a comparison of synchrotron measurements in TCV with predictions of kinetic theory for runaway dynamics in uniform magnetic fields. We find that to explain the detected synchrotron emission, significant non-collisional pitch angle scattering as well as radial transport of REs would be needed. Such effects could be caused by the presence of magnetic perturbations, which should be further investigated in future TCV experiments. △ Less

Submitted 18 August, 2020; v1 submitted 23 March, 2020; originally announced March 2020.

Comments: 7 pages, 4 figures. Accepted for publication in Nuclear Fusion

Journal ref: Nucl. Fusion 60 094002 (2020)

An improved understanding of the roles of atomic processes and power balance in divertor target ion current loss during detachment

Authors: Kevin Verhaegh, Bruce Lipschultz, Basil Duval, Olivier Février, Alexandre Fil, Christian Theiler, Mirko Wensing, Christopher Bowman, Daljeet Gahle, James Harrison, Benoit Labit, Claudio Marini, Roberto Maurizio, Hugo de Oliveira, Holger Reimerdes, Umar Sheikh, Cedric Tsui, Nicola Vianello, Wouter Vijvers

Abstract: The process of divertor detachment, whereby heat and particle fluxes to divertor surfaces are strongly diminished, is required to reduce heat loading and erosion in a magnetic fusion reactor to acceptable levels. In this paper the physics leading to the decrease of the total divertor ion current (It), or 'roll-over', is experimentally explored on the TCV tokamak through characterization of the loc… ▽ More The process of divertor detachment, whereby heat and particle fluxes to divertor surfaces are strongly diminished, is required to reduce heat loading and erosion in a magnetic fusion reactor to acceptable levels. In this paper the physics leading to the decrease of the total divertor ion current (It), or 'roll-over', is experimentally explored on the TCV tokamak through characterization of the location, magnitude and role of the various divertor ion sinks and sources including a complete analysis of particle and power balance. These first measurements of the profiles of divertor ionisation and hydrogenic radiation along the divertor leg are enabled through novel spectroscopic techniques. Over a range in TCV plasma conditions (plasma current and electron density, with/without impurity-seeding) the $I_t$ roll-over is ascribed to a drop in the divertor ion source; recombination remains small or negligible farther into the detachment process. The ion source reduction is driven by both a reduction in the power available for ionization, Precl, and concurrent increase in the energy required per ionisation, $E_{ion}$: often described as 'power starvation' (or 'power limitation'). The detachment threshold is found experimentally (in agreement with analytic model predictions) to be $\sim P_{recl}/I_t {E_{ion}} \sim 2$, corresponding to a target electron temperature, $T_t \sim E_{ion}/γ$ where $γ$ is the sheath transmission coefficient. The target pressure reduction, required to reduce the target ion current, is driven both by volumetric momentum loss as well as upstream pressure loss. The measured evolution through detachment of the divertor profile of various ion sources/sinks as well as power losses are quantitatively reproduced through full 2D SOLPS modelling through the detachment process as the core density is varied. △ Less

Submitted 20 August, 2019; v1 submitted 11 October, 2018; originally announced October 2018.

Comments: The analysis part of this paper is now presented arxiv:1903.08157

Journal ref: 2019 Nucl. Fusion 59 126038

Impurity seeding for suppression of the near Scrape-Off Layer heat flux feature in tokamak limited plasmas

Authors: F. Nespoli, B. Labit, I. Furno, C. Theiler, U. Sheikh, C. K. Tsui, J. A. Boedo, the TCV team

Abstract: In inboard-limited plasmas, foreseen to be used in future fusion reactors start-up and ramp down phases, the Scrape-Off Layer (SOL) exhibits two regions: the "near" and "far" SOL. The steep radial gradient of the parallel heat flux associated with the near SOL can result in excessive thermal loads onto the solid surfaces, damaging them and/or limiting the operational space of a fusion reactor. In… ▽ More In inboard-limited plasmas, foreseen to be used in future fusion reactors start-up and ramp down phases, the Scrape-Off Layer (SOL) exhibits two regions: the "near" and "far" SOL. The steep radial gradient of the parallel heat flux associated with the near SOL can result in excessive thermal loads onto the solid surfaces, damaging them and/or limiting the operational space of a fusion reactor. In this article, leveraging the results presented in [F. Nespoli et al., Nuclear Fusion 2017], we propose a technique for the mitigation and suppression of the near SOL heat flux feature by impurity seeding. First successful experimental results from the TCV tokamak are presented and discussed. △ Less

Submitted 4 December, 2017; originally announced December 2017.

Spectroscopic investigations of divertor detachment in TCV

Authors: K. Verhaegh, B. Lipschultz, B. P. Duval, J. R. Harrison, H. Reimerdes, C. Theiler, B. Labit, R. Maurizio, C. Marini, F. Nespoli, U. Sheikh, C. K. Tsui, N. Vianello, W. A. J. Vijvers, TCV team, MST1 team

Abstract: The aim of this work is to provide an understanding of detachment at TCV with emphasis on analysis of the Balmer line emission. A new Divertor Spectroscopy System has been developed for this purpose. Further development of Balmer line analysis techniques has allowed detailed information to be extracted from the three-body recombination contribution to the n=7 Balmer line intensity. During densit… ▽ More The aim of this work is to provide an understanding of detachment at TCV with emphasis on analysis of the Balmer line emission. A new Divertor Spectroscopy System has been developed for this purpose. Further development of Balmer line analysis techniques has allowed detailed information to be extracted from the three-body recombination contribution to the n=7 Balmer line intensity. During density ramps, the plasma at the target detaches as inferred from a drop in ion current to the target. At the same time the Balmer $6\rightarrow2$ and $7\rightarrow2$ line emission near the target is dominated by recombination. As the core density increases further, the density and recombination rate are rising all along the outer leg to the x-point while remaining highest at the target. Even at the highest core densities accessed (Greenwald fraction 0.7) the peaks in recombination and density may have moved not more than a few cm poloidally away from the target which is different to other, higher density tokamaks, where both the peak in recombination and density continue to move towards the x-point as the core density is increased. The inferred magnitude of recombination is small compared to the target ion current at the time detachment (particle flux drop) starts at the target. However, recombination may be having more localized effects (to a flux tube) which we cannot discern at this time. Later, at the highest densities achieved, the total recombination does reach levels similar to the particle flux. △ Less

Submitted 16 January, 2017; v1 submitted 15 July, 2016; originally announced July 2016.

Comments: Article accepted for publication in Journal of Nuclear Materials and Energy

Journal ref: Nuclear Materials and Energy Volume 12, August 2017, Pages 1112-1117