Recent Publications

Estimating global energy fluxes during COMPASS disruptions via magnetic measurements

Nicola Isernia, Ondřej Ficker, Bojana Stefanoska, Daniil Kabirov, Victor Fernández-Pacheco Albandea, Gabriele Ferrari, Vadim Yanovskiy, Fabio Villone, Martin Imrisek, Petr Vondracek, et al.

During tokamak disruptions, a significant amount of poloidal magnetic energy is converted into other energy forms within the plasma, contributing to the thermal load on the first wall and the surrounding plasma facing components. The amount of magnetic energy converted to plasma thermal losses depends on the duration of the current quench as compared to the electromagnetic time of surrounding conductors. We implement a first-principle method to estimate plasma thermal losses during disruptions for the tokamak COMPASS. The method assumes the plasma evolves through MHD equilibrium states during the disruption, but it does not require any equilibrium reconstruction, elaborating exclusively raw magnetic measurements. The robustness of the method is validated against numerical simulations and different diagnostic measurements. An extensive analysis of energy fluxes during COMPASS disruptions is considered, showing how the poloidal magnetic energy influx contributes to losses and the role of halo currents in the task.

Kilojoule-scale laser acceleration enabling efficient generation of electron-positron and muon beams

R. Babjak, M. Pouyez, C. Badiali, T. Grismayer, M. Vranic

Despite many experimental attempts, laboratory pair-beam sources have not yet reached the charge, collimation, and density needed to observe kinetic pair-plasma instabilities. Here we show that an optimised direct-laser-acceleration (DLA) regime in a gas target can close this gap in the regime of kilojoule-class petawatt lasers. Quasi-3D PIC and Geant4 simulations show that DLA-accelerated electron bunches carry tens of nC at GeV-scale energies with low divergence. Converted in a high-Z target, they generate up to 3x10^12 positrons with tunable energy spectra and transverse sizes of several skin depths in the beam rest frame. A proof-of-principle PIC simulation of the pair beam propagating through background gas shows the growth of a current-filamentation instability. This provides direct numerical evidence that kinetic pair dynamics are within reach of near-term facilities such as ELI-L4. The same electron driver also generates 6.5x10^5 Bethe-Heitler muons per shot, at up to 580 muons/J. We derive a scaling law for the muon yield and validate it against Geant4 simulation. Above a modest driver-energy threshold, the yield depends on the total energy delivered to the electron beam rather than on its peak energy - a design principle for future laser-based muon sources. These results establish DLA-driven secondary sources as a practical, near-term pathway to laboratory pair plasmas, high-yield muon beams, and lepton-accelerator injectors.

A real-time data acquisition system for the magnetic diagnostic of COMPASS-U

João Oliveira, André Torres, António Joaquim Batista, Bernardo B Carvalho, Ivan Ďuran, Aleš Havránek, Tomas Markovic, Marie Vanakova, P Sladek, Jorge Sousa, et al.

A bespoke real-time data acquisition system for the magnetic diagnostic of COMPASS-U was developed. The system targets a high density of galvanicly isolated channels, each with a dedicated 2 MSPS 18-bit converter. Based on the ATCA standard, it supports up to 576 channels in a single sub-rack. The system connects the inductive magnetic sensors to the real-time tokamak control system, performing the digitization, pre-processing, and numerical integration of the magnetic signals. The modular architecture of the system is described and its advantages in handling different types of magnetic sensors are discussed. The front-end modules sit on a data acquisition main-board which reads the digital data from the converters and does the numerical integration of the signals. Since the system operates in a real-time environment where latency is one of the primary concerns, an FPGA serves as its main processing element. The FPGA is part of a System on Module (SoM) which also contains a CPU running a Linux operating system, enabling advanced remote management and supervision capabilities. A prototype of the data acquisition system was built for the purposes of validating its architecture, performance and electronic design. We have successfully demonstrated the operation of the data acquisition system and confirmed its performance meets the requirements for COMPASS-U operation.

Accuracy of COMPASS Upgrade equilibrium reconstruction

Marie Vanakova, David Fridrich, Ondrej Kovanda, Tomas Markovic, Stefano Marchioni, Lukas Kripner, Martin Imrisek, Pavel Bělina, Viktor Veselovsky, Petr Vondracek, et al.

We carry out an analysis of the reconstruction accuracy of plasma equilibrium during the plasma current flat-top phase of tokamak COMPASS Upgrade, utilizing the codes EFIT and LIUQE. We do so for the nominal set of input synthetic magnetic diagnostics signals, as well as those with varying degrees of introduced measurement errors (normal distribution with 2 % standard deviation) within 75 Monte Carlo data sets inputs. The analysis was performed for three COMPASS Upgrade baseline scenarios using the measurements of standard and backup magnetic diagnostics set, and for a case study of missing magnetics sensors on low field side. We show that both in the absence of the measurement errors as well as for conservative estimates of 2 % error the reconstruction accuracy of LIUQE code is within the 1 cm limit, albeit approaching it on the top and bottom of the plasma column. EFIT also performs well in the absence of the input measurement errors, but with errors included the inaccuracy of the reconstruction reaches slightly above the error limits on several positions. We conclude that the utilization of LIUQE for tokamak COMPASS Upgrade plasma control will be viable, albeit real-time implementation of the reconstruction algorithms will have to be subject of our future work.

Transient behavior analysis of the intermediate-circuit-balanced HCPB fusion power plant

Jan Loskot, Jan Štěpánek, Slavomír Entler, Pavel Zácha

Direct comparison of 3D non-linear JOREK simulations of shattered pellet injection with ASDEX Upgrade experiments

Weikang Tang, Matthias Hoelzl, Paul Heinrich, Di Hu, F J Artola, Pascal de Marne, Mathias Dibon, Mike G Dunne, Ondřej Ficker, Peter Halldestam, et al.

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 carrying out direct qualitative and quantitative comparisons to experimental measurements. The key element that enables the transition is the incorporation of the parallel heat-flux limit, which is done here in a simplified form. The work increases the confidence of reproducing key processes of disruption mitigation in direct 3D non-linear simulations in view of future predictive studies for ITER.

TPC magnetic field sensors for fusion reactors: Instrumented neutron irradiation test

Pavel Turjanica, Jiri Svarny, Vaclav Smidl, Ondřej Bareš, Jaroslav Soltes, Jan Reboun, Ivan Ďuran, Ladislav Viererbl, Lukas Sobotka, Slavomir Entler

Future thermonuclear fusion reactors will impose extreme demands on diagnostic components, particularly regarding radiation hardness, high-temperature tolerance, and reliability, with limited or no opportunities for maintenance or replacement. To meet these challenges, innovative, thoroughly tested, and qualified solutions are essential. The exploitation of Thick Printed Copper (TPC) technology has recently enabled the development of compact and efficient inductive and steady-state magnetic field sensors. TPC coils, with an optimized winding arrangement designed to minimize cross-field sensitivity and reduce temperature gradient-induced spurious voltages, have been extensively tested, including exposure to high neutron fluence during instrumented neutron irradiation test up to 0.5 dpa at the LVR-15 fission reactor. This contribution will describe these new design solutions and present the results of comprehensive testing. Data analysis of resistance measurements including temperature model and irradiation model are presented.

Validation of the GBS code against COMPASS experimental data

P. Macha, J. Seidl, D. Oliveira, D. Galassi, K. Lim, D. Tskhakaya, J. Horacek, J. Adamek, J. Cavalier, P. Bilkova, et al.

This work presents the first full-size turbulence simulation of the COMPASS tokamak edge with the three-dimensional, self-consistent, flux-driven, two-fluid Global Braginskii Solver (GBS) code. The simulation is validated against experimental data from a standard L-mode COMPASS discharge, covering the scrape-off layer (SOL) from the outer midplane to the divertor region. The dataset combines four diagnostic systems and provides 43 observable profiles, including electron temperature and plasma potential measurements that enable the first direct comparison of their fluctuations and higher-order statistics in COMPASS. The validation results show good agreement between simulation and experiment in profile shape, magnitude, and fluctuation levels for most outer midplane observables. The parallel heat-flux fall-off length is also reproduced. In the divertor, the mean profiles agree reasonably well in magnitude, but the simulated fluctuation levels remain significantly underestimated. Advanced blob analysis shows that both simulation and experiment lie mainly in the sheath-connected regime, with partial presence in the connected ideal-interchange regime. The remaining discrepancies in the COMPASS simulation are probably a consequence of simplified parameters and boundary conditions, reflecting limitations that are common to fluid turbulence models. Overall, the validation demonstrates that GBS captures the dominant upstream SOL turbulence dynamics, while identifying divertor physics as the main target for further improvement.

Generalisation of two foil method for joint estimation of electron temperature and impurity content

Jakub Svoboda, Jakub Seidl, Matěj Tomeš, Martin Imríšek

This paper presents a probabilistic formulation and a generalised version of the two foil method for estimating electron temperature from soft x-ray radiation, motivated by its application to the COMPASS-U tokamak. Using synthetic data representative of the expected diagnostic configuration, we benchmark the performance of the method and investigate the impact of tungsten line radiation on its accuracy. The results show that neglecting tungsten line radiation can lead to a substantial underestimation of the electron temperature, highlighting a key limitation of the standard approach. To address this issue, we propose a generalised two foil method that explicitly incorporates line radiation from plasma impurities into the probabilistic model. The generalised formulation relates the ratio of emissivities measured by two detectors with different filters to the electron temperature, electron density, effective charge, and tungsten density. The method is demonstrated using simulations of COMPASS-U plasma scenarios with a synthetic model of a planned soft x-ray diagnostic system consisting of two detector sets with different filters capable of tomographic inversions. Benchmarking is performed under different a priori assumptions on plasma parameters, considering both the standalone method and cases where additional constraints from other diagnostics are included. We identify three distinct regimes in plasma parameter space in which the diagnostic sensitivity is dominated by the electron temperature, by the impurity content, or coupled dependence on both parameters.

Modelling and inversion of bremsstrahlung from runaway electrons on the GOLEM tokamak

M Tunkl, L Lobko, S Malec, O Ficker, J Cerovsky, V Svoboda, G I Pokol

Runaway electron (RE) losses are usually diagnosed with a small number of hard x-ray (HXR) detectors, which provide limited spatial information. Although multi-chord HXR spectrometers on large devices allow reconstruction of RE parameters in the plasma volume, equivalent methods for localized RE losses at plasma-facing components are scarce. We investigated the feasibility of inferring the RE strike-point position and in principle, pitch from bremsstrahlung generated at the poloidal limiter of the GOLEM tokamak. Geant4 simulations were used to construct a forward model, and synthetic detector signals were inverted with a tomographic algorithm. Benchmark tests indicated that strike-point localization should be possible with modest detector coverage, but pitch inference requires substantially more. With only five detectors available, we were able to demonstrate strike-point localization but no pitch estimation in the experiment. The results demonstrate the potential of thick-target bremsstrahlung as a diagnostic of RE losses.

Thermal performance of deposited W and W/Mo coatings on graphite for plasma-facing components

Alžběta Endrychová, Slavomír Entler, Vojtěch Smolík, Tomáš Polcar, Jitka Kabátová, David Rada