Recent Publications

Experimental Identification of the n=2 Error Field and its Interaction with MHD Activity and Plasma Rotation in MAST-U

Lidia Piron, David Anthony Ryan, Andrew Kirk, Alessandra Tonel, Paolo Zanca, Geoffrey Cunningham, Matteo Baruzzo, Sam Blackmore, Christopher J Ham, Scott Alan Silburn, et al.

Spurious magnetic field perturbations, known as error fields (EFs), with toroidal mode number n = 2 can have deleterious effects similar to those associated with n = 1 error fields. An n = 2 error field source is expected in MAST-U because the poloidal field coils P4 and P5, retained from the previous device MAST, exhibit n = 1 and n = 2 deformations due to coil manufacturing imperfections. This work presents the n = 2 error field identification studies carried out in MAST-U. The n = 2 compass scan indicates that, for 750 kA plasma current, double-null divertor H-mode plasmas, the n = 2 error field is relatively small when assessed using locked mode onset and rotation braking as metrics, suggesting that the n = 2 EF is effectively screened by the plasma in this scenario. Furthermore, during the n = 2 EF identification studies, an interesting interplay between the onset times of n = 1 and n = 2 rotating modes and plasma rotation was observed. This observation enables the identification of control strategies aimed at delaying the onset of n = 1 and n = 2 MHD modes while sustaining plasma rotation.

Mitigation of sawtooth transient heat flux in the MAST-U Super-X divertor with deuterium and nitrogen

Rory Scannell, Jack Flanagan, Zhouji Huang, James R Harrison, Kevin Verhaegh, Scott Alan Silburn, Peter Ryan, Stuart Scott Henderson, Nicola Lonigro, Sidney Leigh, et al.

The MAST Upgrade Super-X divertor protects plasma-facing components from heat fluxes during transient events. This paper examines heat loads from sawtooth crashes with energies ΔWsawtooth ≈ 2–9kJ and investigates the impact of deuterium and nitrogen gas pressure on their mitigation. In a D2 gas scan for sawteeth with energies up to 6kJ, the peak heat flux (q⊥,max) scales with the transient energy and inversely with the neutral pressure, with variations in ΔW/Pgas accounting for 70% of the observed variation in q⊥,max. Larger sawtooth transients (≈ 6–9 kJ) fall outside this trend and exhibit much higher heat fluxes. With D2 puffing in the divertor, the Super-X configuration exhibits substantially lower peak q⊥,max for comparable transient energy loads than observed in conventional divertor configurations. However, this reduction is broadly consistent with geometric expectations especially at higher pressures. Nitrogen gas seeding was applied to transients with energies below ≈ 6 kJ. Increasing the nitrogen pressure reduces q⊥,max, by 4.5MW/m2/Pa in Super-X while nitrogen seeding is found to have much less impact on transients in conventional divertor configuration. In the case of N2 seeding, the Super-X divertor outperforms conventional divertor configuration beyond geometric expectations. Direct Super-X divertor Te and ne measurements during transients were obtained from Thomson scattering. These indicate quiescent inter-transient temperatures of < 1 eV, corresponding to strong detachment. The temperatures rise to 6–8 eV during typical sawtooth transients and 10 eV where divertor heat loads exceed q⊥,max > 2MWm−2. The experimental data are compared to modelling results using a 1D exhaust code in the ReMKiT1D framework. The modelling highlights the importance of recycling on understanding the buffering of transients.

Identifying non-performing or Dud Plasmas for Burning Plasma Control: Insights from JET and TFTR Deuterium-Tritium Campaigns

Lidia Piron, Nicolò Ferron, Eric Fredrickson, Morten Lennholm, Alessandro Pau, Timo Ravensbergen, Olivier Sauter, Fulvio Auriemma, Matteo Baruzzo, Krassimir K. K Kirov, et al.

Among the burning plasma controllers for research fusion reactors, the dud detector will be of primary importance as it determines whether the plasma is performing well or if it is a dud. In the latter case, the discharge needs to be terminated to remain within tritium and neutron activation limits. To this scope, monitors which track the plasma performance will be integrated in the plasma control system. In this work, we present a novel dud detector that has been empirically identified based on Deuterium-Tritium campaigns carried out at JET and TFTR. This controller estimates a proxy of the neutron rate using a combination of the diamagnetic energy and the density peakedness. If the predicted neutron rate deviates from the real-time measurement, then the dud detector will trigger an alarm leading to a safe plasma termination if plasma recovery is not expected or, to actuator requests when adjustment is possible. This monitoring function can also be coupled with equilibrium solver and control-oriented models, such as RAPDENS, as proposed in the 15 MA plasma current, 5.3 T toroidal magnetic field baseline Deuterium-Tritium ITER scenario.

The physics of ballooning-limited ELM-free regimes in EUROfusion tokamaks

Mike G Dunne, Michael Faitsch, Olivier Sauter, Eleonora Viezzer, Benoit Labit, Athina Kappatou, David Keeling, Branka Vanovac, Itziar Balboa, Petra Bilkova, et al.

The development of operational scenarios without large Type-I ELMs is of utmost importance for the stable operation and longevity of future tokamaks. The EUROfusion tokamak exploitation program has therefore made the understanding of ELM-free regimes a major topic of exploration across all its contributing devices (ASDEX Upgrade, JET, MAST-Upgrade, TCV, and WEST). An integrated program to investigate a range of Type-I ELM-free regimes has been developed covering the enhanced D-alpha (EDA), magnetic perturbations (MP), negative triangularity (NT), quasi-continuous exhaust (QCE), quiescent H-mode (QH), the baseline small ELMs (SE), I-mode, and X-point radiator (XPR) regimes. This contribution focuses on the development and understanding of the NT and QCE regimes on ASDEX Upgrade, JET, and TCV. The importance of transport via ballooning modes in both regimes is highlighted, as well as the progress in developing access models based on ideal-MHD. In the case of the QCE, this can also be expressed as a minimum separatrix density, which corresponds well to experimentally measured separatrix densities. Particular focus is paid to the performance of the QCE in terms of the achieved pedestal top values, which, when appropriately normalised, do not differ significantly from ELMy H-mode plasmas. This, combined with the predicted minimum separatrix density for the 15~MA ITER baseline plasma, highlight the relevance of the QCE as a potential operational scenario for both ITER and future reactors.

Development of a high-current, high fusion performance scenario on JET: physics insights and operational challenges

Luca Garzotti, Domenico Frigione, Peter J Lomas, Fernanda Rimini, Dirk Van Eester, Vito Konrad Zotta, Spyridon Aleiferis, Edoardo Alessi, Fulvio Auriemma, Rennan Bianchetti Morales, et al.

Since the installation of a beryllium-tungsten(Be/W) first wall on JET an intense scenario development programme has taken place to realize a high-current (3.5-4 MA) scenario, denominated JET baseline scenario, with $q_{95} \sim 3$, $\beta_N \sim 1.8$ and capable of delivering high fusion performance ($P_{fusion} > 10$ MW) for 5 s in the D-T campaigns conducted in 2021 and 2023. The JET baseline scenario was realized in D-T at 3.5 MA in 2021 (DTE2 campaign), but, despite achieving transitorily $P_{fus} \sim 8$ MW, it could not be sustained for more than 3 s. In this paper we will revisit and expand the analysis of the baseline scenario at 3.5 MA and present new results obtained in D-T at 3.0 MA in DTE3, where we were able to sustain the scenario for 5 s. We will also present recent modelling results showing how available semi-empirical and first-principles transport models can explain a variety of phenomena ranging from fusion performance, impurity transport and control of the fuelling mix. Many of these points are potentially relevant for bigger machines such as ITER. Finally, we will discuss open questions and improvements to our present modelling capabilities required to extrapolate the results to power plant devices.

Validation of Integrated Modelling of KSTAR Plasmas with Carbon Divertor using JINTRAC and Prediction for Tungsten Divertor

Beomsu Kim, Sun Hee Kim, Florian Koechl, Francis J Casson, Romain Futtersack, Chan-Young Lee, SeulChan Hong, JAEGON LEE, Boseong Kim, Yong-Su Na

The JINTRAC integrated modelling suite has been adapted for the KSTAR tokamak, with its primary goal being the establishment of a core-edge integrated modelling framework for predictive analysis of KSTAR plasmas. This framework was validated by benchmarking JINTRAC's physics modules against both established codes and experimental data from a KSTAR carbon (C) divertor discharge (#25458), demonstrating high fidelity. Based on the validation, predictive core-edge coupled simulations were performed to investigate the effects of changing the divertor material from C to tungsten (W). The simulations predicted a significant increase in core radiation and a corresponding decrease in plasma temperatures, consistent with the behavior of high-Z materials. Specifically, the high tungsten influx causes radiative cooling in the pedestal and core regions, leading to a reduction in edge bootstrap current and a compensatory increase in core inductive current. This redistribution of current density drops the central safety factor (q 0 ) below unity, triggering sawtooth instability which serves to suppress core impurity accumulation. Sensitivity analysis indicated that while toroidal rotation exhibits a non-monotonic correlation with tungsten profile peaking, the total tungsten inventory and radiated power (P rad ) increased with higher rotation. Additionally, increasing the separatrix electron density effectively suppressed the tungsten source at the divertor target, yet the reduction in core tungsten inventory was limited compared to the extent of source reduction. These findings imply that achieving high-performance operation requires a comprehensive control strategy that minimizes the absolute impurity influx across the separatrix through combined source suppression and transport control.

Detailed modelling of alpha transport due to ELM control fields in ITER: implications for PFCs and diagnostic design

Fabio Camilo de Souza, Kenneth G McClements, Alexander Philip Kofi Prokopyszyn, Antti Snicker, Alex Reyner-Vinolas, Javier Gonzalez Martin, Lucia Sanchis, Rafael Marqués Gómez

This paper presents simulations of alpha-particle transport in ITER driven by static 3D magnetic field perturbations in the high-performance 15 MA Q = 10 baseline scenario, specifically resonant magnetic perturbations (RMPs) arising from edge-localized mode (ELM) control coils, combined with toroidal field ripple (TFR) and effects from ferromagnetic materials. We employ the Lorentz-Orbit Code for Use in Stellarators and Tokamaks (LOCUST), which tracks fast-ion orbits under the Lorentz force and Monte Carlo collisions with the bulk plasma, taking into account the detailed geometry of ITER plasma-facing components (PFCs). LOCUST uses GPU cards to enable the high-resolution modelling required to accurately resolve power fluxes across surfaces with complex morphologies, including unprotected cooling pipes beneath the dome divertor, and the generation of reliable synthetic diagnostics for the ITER Fast Ion Loss Detector (FILD) to support its design. The simulations include a range of ELM control coil current profiles with toroidal mode number n=3. The results indicate that the total alpha-particle energy loss has a negligible impact on plasma performance, remaining below 1% of the alpha energy produced in D–T reactions. Furthermore, the power flux density on the divertor structures and the first wall remains well below design limits and is comparable to thermal and radiative loads. The simulated alpha flux on the FILD scintillator plate is well above the noise threshold and can be significantly higher than conservative estimates.

Initial observations in X-point target divertor discharges on MAST-U

Nicola Lonigro, Kevin Verhaegh, James R Harrison, Bruce Lipschultz, Christopher Bowman, Fabio Federici, Jack Flanagan, Daniel Greenhouse, David Moulton, Peter Ryan, et al.

The first high-power (≥ 3 MW) H-mode experiments using a double-null X-point-target (XPT) divertor configuration have been performed on MAST-U. The XPT geometry is obtained by combining a large strike point radius, similar to the Super-X divertor (SXD), with an additional X-point near the separatrix in the baffled outer divertor chambers and leads to additional exhaust benefits over the SXD. The broader electron density profile near the secondary X-point leads to additional plasma-neutral interactions, evidenced by a broader hydrogenic emission profile, and resulting in larger power and ion sinks. The increase in plasma-neutral interactions also leads to lower target electron temperatures and heat fluxes. These benefits appear to extend to transients, and preliminary evidence of improved ELM buffering in the XPT is presented. These results showcase how multiple alternative divertor configuration strategies can be combined to improve momentum, power, and particle losses, which may be required for the challenging exhaust conditions of future reactors.

First wall particle fluxes and Be erosion in high-performance JET-ITER baseline plasmas

Eduardo de la Cal, Carine Giroud, Henri Aaron Kumpulainen, Juri Romazanov, Itziar Balboa, Scott Alan Silburn, Pedro Carvalho, Juuso Karhunen, Beth Thomas, Alex Tookey, et al.

In next step large fusion devices with a tungsten (W) first wall such as ITER, impurity control will be a challenge to avoid strong core radiation from highly ionized metals. Here we analyse the deuterium (D) fluxes impacting on the beryllium (Be) first wall limiters and the resulting surface erosion in high-performance neon (Ne) seeded ITER-baseline plasmas with up to 35 MW heating power in JET tokamak. Visible cameras are used to quantify the fluxes using the spectroscopic S/XB method at the regions of strongest plasma-wall interaction: the Outer Midplane (OMP) and the Upper Dump Plates (UDP). We first describe how Ne seeding, which significantly improves core plasma performance and power exhaust control, modifies the plasma flux dynamics and ELM properties at the walls. It is then shown, that the global Be erosion by sputtering is not significantly affected by Ne seeding. This is because the time-averaged fluxes are dominated by the inter-ELM phase, where the main erosion precursor is deduced to be D+. Furthermore, we observe a beneficial strong decrease of the fluxes at the OMP when slightly increasing the Separatrix-limiter clearance, indicating short far scrape-off layer (SOL) decay lengths. We also describe, how the fluxes change with average plasma density and toroidal magnetic field and plasma current. Regarding the fluxes at the UDP, we show the critical effect of the magnetic topology at the top of the chamber. The formation of a secondary Separatrix in this region enhances the parallel plasma fluxes to the UDP, which eventually becomes the main plasma-wall interaction area. Finally, we compare the experimentally estimated Be effective sputtering yields at the OMP for the inter- and intra-ELM phases with the values calculated using the SDTrim code to infer information on the mean ion impinging energies Ei and to show that the main global erosion precursor is D+. The obtained Ei values are in the range or at least compatible with those expected for the plasmas analysed: approximately 30 eV for the inter- and 1 keV for the intra-ELM periods respectively.

Measurements and analysis of short-term activation of ITER samples exposed in irradiation-end during DT operations at JET

Ewa Łaszyńska, Jakub Piotr Włodarczyk, Sabahattin Akbas, Barbara Bieńkowska, Zamir Ghani, Callum Grove, Xavier L Litaudon, Lee William Packer, Chantal Shand, Rosaria Villari, et al.

Short-lived activation products in fusion materials are critical for assessing radiological safety and maintenance planning in future fusion reactors like ITER. These products influence shutdown dose rates, impact maintenance procedures, and provide essential validation for nuclear data and activation models in the short term. Understanding their formation and decay behavior under deuterium-tritium (DT) plasma conditions allows for more accurate predictions of material performance and safety constraints. This study analyzes the short-term activation of ITER materials exposed at the KN2 6U irradiation end in the JET (Joint European Torus) tokamak during the DTE2 and DTE3 campaigns. CuCrZr, IWS A286, and SS316L(N) samples were activated during DTE2, while tungsten monoblock, a key ITER divertor material, was irradiated during DTE3. Activated samples were promptly transported via a pneumatic post system for gamma spectrometry measurements using UKAEA’s 190% relative efficiency HPGe (High-Purity Germanium) detector. The efficiency calibration was performed with the MCNP 6.1 code, employing an MCNP detector model validated by a certified multi-gamma calibration source. The experimental data were compared with FISPACT-II activation calculations, enabling the determination of calculation-to-experiment (C/E) ratios. This study provides key validation data for nuclear activation models and improves the accuracy of computational predictions for material behavior in fusion reactors.

Real-time observation of toroidal current redistributions induced by three-dimensional MHD phenomena triggering vertical displacement events in tokamak plasmas

Matthew Tobin, Steve A Sabbagh, Veronika Zamkovska, Guillermo Bustos Ramirez, Hankyu Lee, Joseph R Jepson, Juan Riquezes, Frederick C Sheehan, Grant A Tillinghast, Keith Erickson, et al.

Three-dimensional MHD instabilities, including edge-localized modes (ELMs) and internal reconnection events (IREs), have been observed to precipitate loss of vertical stability in tokamak plasmas, resulting in vertical displacement events (VDEs). This vertical destabilization can occur due to toroidal current redistributions and/or shape changes resulting from these phenomena. Using a recently introduced method for rapidly reconstructing the two-dimensional toroidal plasma current density profile in real-time, results are presented that demonstrate the specific current distribution changes that occur during ELMs (on KSTAR) and IREs (on MAST-U) that lead to loss of vertical control. The method most efficiently reconstructs the toroidal current density profile by doing so on a basis of principal components of historical profiles. These principal components isolate dominant current profile dynamics, improving interpretability, increasing speed, and reducing dimensionality of the profile computation. On KSTAR, this computation is executed in the real-time plasma control system at a rate of 10 kHz (limited by available CPU cycle times), allowing the current profile evolution to be assessed at several times over the course of each ELM event. Further, by incorporating the reconstructions into a novel vertical stability metric, the contribution of specific current profile dynamics to the loss of vertical stability can be assessed in real-time for VDE avoidance and improved understanding of the causal relationship between three-dimensional MHD phenomena and VDEs. The success of this method in approximating toroidal current density profiles from kinetic equilibrium reconstructions is also presented ($R^2=0.990$), along with its capability to produce other equilibrium quantities of interest in real-time at high time resolution.

First observations of ion cyclotron emission in the TCV tokamak

A. Jansen van Vuuren, M.B. Dreval, R. Ochoukov, S.E. Sharapov, B.P. Duval, H. Elaian, L. Simons, C. Paraskevopoulos, A.N. Karpushov

The first observations of ion cyclotron emission (ICE) in the Tokamak à Configuration Variable (TCV) tokamak are reported. The measured ICE frequency closely follows the deuterium ion cyclotron frequency at the magnetic axis, with temporal variations consistent with equilibrium evolution and associated shifts of the magnetic axis. Core ICE is observed primarily during phases with combined electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI). In most discharges the second harmonic dominates, while in others higher harmonics (third or fourth) are observed. Fine spectral structure, including branch splitting and frequency chirping, is frequently detected, together with additional modes consistent with emission at cyclotron harmonics from regions closer to the plasma edge. Core ICE is observed during both co-current and counter-current NBI. The frequency offset of core ICE relative to the ion cyclotron harmonic at the magnetic axis reverses sign between co- and counter-current injection, and this behavior is confirmed in discharges with reversed plasma current. The observed frequency shifts are qualitatively consistent with Doppler-shifted cyclotron resonance arguments.

Numerical investigation of the impact of conductivity modelling in fluid simulations of RF drivers for giant negative ion sources

Iacopo Regoli, Roman Zagorski, Isabella Mario, Antonio Pimazzoni, Emanuele Sartori, Alastair Shepherd, Valeria Candeloro, Edgard Zuin, Gianluigi Serianni