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The COMPASS-U tokamak (R = 0.894 m, a = 0.27 m, Bt = 5 T, Ip = 2 MA) is a new medium-size device with fully metallic plasma facing components, currently under construction at the Institute of Plasma Physics of the Czech Academy of Sciences in Prague. It features a unique combination of parameters, such as a high temperature of the tokamak walls up to 500 ◦C allowing a high recycling regime, a high magnetic field connected with a high plasma density above 1020 m -3 and with a high heat flux (perpendicular to divertor targets) density at the outer strikepoint up to 90 MW/m2 in attached conditions. These parameters of the device pose strict constraints and requirements on the design of individual diagnostic systems. Strategy and present status of the development of the diagnostic systems for COMPASS-U are provided. Plans for a diagnostic set for the first plasma are reviewed. The review of the diagnostics systems involves the high-temperature compatible slow (up to 20 kHz) and fast (up to several MHz) inductive and non-inductive magnetic sensors (including Thick Printed Copper coils and Hall sensors), the sub-millimetre interferometer with an unambiguous channel, Electron Cyclotron Emission, the interlock and overview cameras, high resolution Thomson scattering, radiation diagnostics (neutron diagnostics, soft and hard X-ray diagnostics, bolometers, impurity monitors, effective ion charge), probe diagnostics (including rail probes) and manipulators.

COMPASS upgrade (COMPASS-U) is a high-magnetic field, medium-sized tokamak with high-temperature (< 500 °C) operation. The scientific program is aimed to address the topics of plasma exhaust, liquid metals, enhanced confinement modes, and edge plasma physics. The plasma current is up to 2 MA and the toroidal magnetic field is up to 5 T. Therefore, plasma disruptions can produce large electromagnetic forces on the vacuum vessel (VV) and other conducting structures. This article presents the study of different VV design concepts, which were considered during the COMPASS-U conceptual design phase. This article describes the electromagnetic forces exerted during plasma disruption, high-temperature operation requirements, and other design constraints. INCONEL 625 is selected as a reference material for VVs. FE simulations of the 45° sector of the vacuum vessel were carried out for various load combinations. This article summarizes the wall thickness optimization that limits the vessel deformation and minimizes the stress in the shell. The results are broken down into different categories of stress according to the ASME code and compared with material limits.

The Thomson scattering (TS) diagnostic, one of the key diagnostics used on the tokamaks around the world, is planned for the COMPASS-U tokamak, which is recently under design and construction in the Institute of Plasma Physics in Prague, Czech Republic. This tokamak is supposed to be a world-unique, high magnetic field device with hot walls, allowing for the study of the plasma exhaust in advanced operational scenarios and testing cutting-edge technologies relevant to future fusion reactors, e.g., use of liquid metals. The core and edge TS systems are planned to be designed and operational, with a limited performance, already in the early stage of the tokamak operation. In this contribution, requirements and the most important constraints defining the TS system design are presented. The impact of both the possible collection lens location and spatial resolution on the plasma pedestal observation is simulated. Design considerations also take into account the high-resolution TS core and edge systems available from the COMPASS tokamak, which will be reused. The collection lenses will be newly built. Extension of the detection system will complete the plasma radius coverage in the future. The divertor TS is considered for later periods.

COMPASS-U, a high magnetic field tokamak with hot walls, will be designed and built at IPP Prague. Unique features of this new device bring noticeable constraints and requirements on plasma diagnostics, which make their development highly demanding. In this paper, the main expected constraints influencing the conceptual design of diagnostic tools for COMPASS-U (high temperature of the vacuum vessel, high plasma density, high heat flux density, strong auxiliary plasma heating, spatial constraints, liquid metals in the divertor) are reviewed and possible solutions are indicated.

A new fast divertor infra-red thermography system was put into operation on COMPASS. It provides full radial coverage of the bottom open divertor with pixel resolution ∼0.6–1.1 mm/px on the target surface and temporal resolution better than 20 μs. The system consists of fast IR camera TELOPS Fast-IR 2K placed in a magnetic shielding box, a positionable holder, a 1 m long IR endoscope consisting of 14 Ge and Si lenses securing off-axis view from an upper inner vertical port and a special graphite divertor tile optimized for IR thermography. The tile is equipped with a heating system allowing tile preheating up to 250 °C. Embedded thermoresistors and a calibration target (a deep narrow hole acting as a black body radiator) allows in-situ calibration of the system including estimation of the target surface emissivity. Furthermore, a roof-top shaped structure on top of the tile increases magnetic field incidence angles above 3 degrees. Laboratory tests of the system performed during its commissioning are presented. The global transmission of the optical system was found to be τ ≈ 40–50%. Poor spatial resolution compared to the design value was observed. Too large surface error of individual lenses was identified as the main cause and re-manufacturing of the most critical lens was suggested. First experimental results obtained using the IR system are presented: divertor heat flux profiles in L-mode with the heat flux decay length λ q omp = 2.1 − 3.3 mm and average H-mode heat flux profiles in an inter-ELM period and during an ELM heat flux maximum with λ q omp ≈ 0.6 mm and λ q omp ≈ 6.7 mm, respectively.

Substantial power dissipation in the edge plasma is required for the safe operation of ITER and next-step fusion reactors, otherwise unmitigated heat fluxes at the divertor plasma-facing components (PFCs) would easily exceed their material limits. Traditionally, such heat flux mitigation is linked to the regime of detachment, which is characterised by a significant pressure gradient between upstream and downstream scrape-off layer (SOL). However, the physics phenomena responsible for power dissipation and pressure loss are distinctly different, especially when the power dissipation is achieved by impurity seeding. In principle, it is possible to achieve substantial mitigation of the heat fluxes while maintaining conservation of the pressure along the open field lines in the SOL. This regime can be accessed by injection of medium- or high-Z impurities, which mostly radiate inside the last closed flux surface. The critical question related to such an approach is the effect on confinement and perspective fusion power generation in future thermonuclear reactors. In this work, we report on experiments at COMPASS tokamak, where neon and argon impurities were injected in ohmic or NBI-heated low confinement plasmas. With appropriate seeding waveform, stable scenarios were achieved, avoiding the radiative collapse of plasmas. Significant reduction of heat fluxes at the outer target was observed, with heat flux pattern similar to the one previously achieved by nitrogen seeding. The reduction of downstream pressure was, however, accompanied by an equal reduction of upstream pressure, indicating that the power dissipation occurred inside the separatrix. Indeed, the impurity cooling is causing a significant drop of edge temperature; however, the effect in the plasma centre is much less pronounced.

COMPASS Upgrade is a new medium size, high magnetic field tokamak (R = 0.9 m, Bt = 5 T, Ip = 2 MA) currently under design in the Czech Republic. It will provide unique capabilities for addressing some of the key challenges in plasma exhaust physics, advanced confinement modes and advanced plasma configurations as well as testing new plasma facing materials and liquid metal divertor concepts. This paper contains an overview of the preliminary engineering design of the main systems of the COMPASS Upgrade tokamak (vacuum vessel, central solenoid and poloidal field coils, toroidal field coils, support structure, cryostat, cryogenic system, power supply system and machine monitoring and protection system). The description of foreseen auxiliary plasma heating systems and plasma diagnostics is also provided as well as a summary of expected plasma performance and available plasma configurations.

The fast microwave reflectometer system on the COMPASS tokamak consists of an O-mode polarized K-band (18 GHz–26 GHz), Ka-band (26 GHz–40 GHz), and a part of U-band (40 GHz–54 GHz). The plasma density profile from the edge plasma area is measured using a fast sweeping rate up of to 6 µs of the probing wave. The processing of the reflected signal is realized by the heterodyne detection configuration based on the I/Q modulator. Two different methods of dynamic calibration of the required linear sweep frequency, together with static frequency and dispersion calibration, were used. The electron density profile was reconstructed by a spectrogram-based method with four sweeps on average. The system has the capability to measure the mid-plane low-field side electron density profile in the density range from 4 × 1018 m−3 to 3.6 × 1019 m−3. Experimental results obtained on COMPASS discharges are presented to demonstrate the performance of the diagnostics.

The COMPASS tokamak (R = 0.56 m, a = 0.2 m, BT = 1.3 T, Ip ~ 300 kA, pulse duration 0.4 s) operates in ITER-like plasma shape in H-mode with Type-I ELMs. In 2019, we plan to install into the divertor a test target based on capillary porous system filled with liquid lithium/tin. This single target will be inclined toroidally in order to be exposed to ITER-relevant surface heat flux (20 MW/m2). Based on precisely measured actual heat fluxes, our simulations predict (for 45° inclination, without accounting for the lithium vapor shielding) the surface temperature rises up to 700°C within 120 ms of the standard ELMy H-mode heat flux with ELM filaments reaching hundreds MW/m2. Significant lithium vaporization is expected. The target surface will be observed by spectroscopy, fast visible and infrared cameras. The scientific program will be focused on operational issues (redeposition of the evaporated metal, ejection of droplets, if any) as well as on the effect on the plasma physics (improvement of plasma confinement, L–H power threshold, Zeff, etc.). After 2024, a closed liquid divertor may be installed into the planned COMPASS Upgrade tokamak (R = 0.84 m, a = 0.3 m, BT = 5 T, Ip = 2 MA, Pin = 8 MW, pulse duration ~2 s) with ITER-relevant heat fluxes loading the entire toroidal divertor.

The microwave reflectometry system on COMPASS tokamak uses the frequency modulated continuous wave (FM-CW) in the K and Ka bands. The fast swept synthesizer together with the homodyne detection provides the complex beat frequency spectrum for the density profile reconstruction. The homodyne detection limits the use of reflectometry in the other applications like correlation or Doppler reflectometry. Our effort was to implement a heterodyne detection based on the existing system. We have decided to use a single-sideband modulator (SSBM) scheme to keep the compatibility with the fast sweeping. Lack of suitable SSBMs on the market leads us to use I/Q modulator, which is modulated by the in-quadrature signals.

A new fast infrared camera was purchased on the COMPASS tokamak recently. It is equipped with a medium wavelength infrared (3–5 mm) InSb detector and is capable of reaching framerate up to 1.9 kHz in full frame acquisition mode (320 × 256 px.) and up to 90 kHz in sub-windowed mode (64 × 4 px.). First experimental measurements of plasma heat flux to inner and outer wall limiters of the COMPASS tokamak using the new camera are presented. Time evolution of parallel heat flux during sawtooth instability is studied showing strong modulation of the heat flux absolute value whilst keeping constant its radial decay length. Fast modulation of outboard midplane limiter heat loading due to runaway electrons is also presented. The camera will be part of a new fast divertor thermographic system with exceptional spatial resolution (∼0.6–1.3 mm/px. on the target plane, 0.04–0.14 mm/px. mapped to the outer midplane) with a possibility of measurements of radial profiles on the divertor with 320 × 4 px. with temporal resolution better than 20 μs. A design of the foreseen optical divertor system is described together with a design of a special divertor graphite tile used for the IR thermography, that will allow in-situ surface emissivity calibration.

The COMPASS-U is a new tokamak in the design phase at Institute of Plasma Physics, Prague, Czech Republic. It is a medium-sized tokamak with high magnetic field (Bt = 5 T) and high temperature vessel ( This paper outlines the latest status on the design activities of the vacuum vessel concept and its mechanical verification against the electromagnetic forces. The vessel design concept is developed considering the spatial constraints, fabrication simplicity, minimum cost and the design is verified for minimum deformation and stresses in the vessel. It also includes the design evolution of PSP, PSP support and VV support. The mechanical analyses are carried out for load combinations of plasma transient events.

In this article the forward model and feasibility study of the CXRS (Charge Exchange Recombination Spectroscopy) diagnostic for the COMPASS Upgrade tokamak [1] based on the Raysect and Cherab framework [2] , [3] , [4] is presented. The neutral beam model available in Cherab is expanded and based on individual acceleration grid beamlets. The spatial density distribution calculation takes into account 3D geometry of the port structure. Beam density profile calculated with the expanded model is used to estimate realistic radial resolution of measurement and to produce forward models of the spectra. Effects of background plasma and beam halo are not included. The spectra are used to reconstruct the plasma profiles and are compared with the simulation input to estimate measurement errors with respect to positions of collecting optics.

The COMPASS-U tokamak is an advanced and more complicated successor of the COMPASS tokamak. Almost all main technological parts will be different. Both the higher performance of COMPASS-U and the increased number of parts and systems lead to a higher risk of failure. Therefore, new control systems must be developed regarding handling dangerous situations and integrating new systems. The COMPASS-U also opens the opportunity to make significant changes in the current systems based on the experience gained from COMPASS operation and to upgrade the systems to nowadays technologies. This article focuses on the conceptual design of the COMPASS-U control systems. It shows all CODAC (Control, Data Access, and Communication) systems their purpose, functionality, operation principle, interaction, and main used technologies. The COMPASS-U will use EPICS (Experimental Physics and Industrial Control System) for Slow control and supervision system. The real-time plasma control system will utilize MARTe2 (Multi-threaded Application Real-Time executor). The base control loop period of the Real-time plasma control system will be 50 μs as of COMPASS.

First, it is shown that during electromagnetic transients in COMPASS-U the poloidal field coils must drain sizeably the current from the vessel and, therefore, reduce disruption forces and their duration. Next, the role of poloidal eddy current (which is absent in some approaches) in the dynamics of vertical and radial forces is found to be essential. Finally, to verify the CarMa0NL modelling for COMPASS-U, the numerical results are cross-validated with general analytical predictions (Pustovitov 2015 Nucl. Fusion 55 113032): the computed vertical force on the tokamak wall is found to be almost zero during rapid ( jump-like) transients, as it should be because of strong skin-effect. This test proves the credibility of the simulation model and computational realization 2021 IAEA, Vienna.

The COMPASS-U tokamak is designed at IPP CAS Prague. It will be a compact device that should achieve high toroidal magnetic field and high wall temperatures. In order to observe fast phenomena, photodiode arrays combined with beryllium filters and a pinhole are planned to be used as Soft X-Ray (SXR) detectors. Operational temperatures of photodiodes are typically below 100 °C and planned first wall temperatures as high as 500 °C for the later phases of operation. Therefore, an actively cooled support structure housing the detector has to be designed. Moreover, in order to obtain good inversion results the layout of tomographic diagnostics systems have to be optimised to provide the best possible coverage of the plasma. The proposed SXR system consists of two sets of detectors with different filter thickness enabling the use of the two filter method for electron temperature estimation. Both sets will share the same line of sight layout for better comparability. Optimisation of this layout is adapted for limited options of detector placement and extensive testing of tomography capabilities is performed. Results of these tests are shown for one of considered layouts.

The COMPASS-Upgrade tokamak, being designed as a medium-sized tokamak, operating with a hot first wall, allows for the study of DEMO-relevant plasma exhaust physics, crucial for future reactors. Bolometry diagnostics for COMPASS-U, consisting of metal foil bolometers and AXUV diodes, are proposed to measure spatially- and time-resolved radiation losses. Metal foil bolometers supply the absolute value of radiation power, whereas the AXUV diodes can observe fast phenomena such as MHD activity. Coverage of the whole poloidal cross-section by bolometers’ cameras allows tomography reconstruction of the local plasma emissivity. The metallic foil bolometry system will be based on sensors with a gold absorber on a silicon nitride substrate with a platinum resistor. Special modifications, e.g., channel separation, will be applied to the detectors to fulfil the requirements. Due to the high temperature, effective thermal shielding and cooling are essential to reduce the risk of damage to the detectors as well as reducing noise in the measured signal. The pin-hole cameras spatial configuration was optimized to provide the best performance under the given engineering constraints. The position of the cameras strongly depends on the space available behind the plasma-facing components (PFC). The proposed layout allows for an efficient observation of the confined plasma as well as that of the divertor region.

The reduction of the incident heat flux onto the divertor will be a necessity for the future thermonuclear reactors. Impurity seeding is recognized as an efficient way to achieve the partial detachment regime, which allows to dissipate a large fraction of power flux by radiation. This paper presents a heat flux real-time feedback system (RTFS) based on impurity seeding controlled by a combined ball-pen and Langmuir probe divertor array in the COMPASS tokamak. A number of features of the system have been studied, such as the type of impurity, seeding location, constants used in the real-time controller and the diagnostic selections. A detailed description of the designed RTFS and the results of the implementation are presented. The findings confirm the applicability of the RTFS for reduction and control of the divertor heat fluxes. Another important implication of this research is the ability of installing such systems in next-step devices.

This contribution presents a Thomson scattering module developed for the Raysect and Cherab framework. Detailed models of spectroscopic diagnostic systems can be created in the framework, which deliver synthetic data with high precision due to accurate physical treatment of ray propagation and radiation phenomena. The addition of the presented module will allow us to model Thomson scattering systems that can aid both data validation and design. Two examples of such application are given. The first example shows the application of the module on the COMPASS tokamak edge Thomson scattering diagnostic and experimental data. The second example shows the possibility to use the framework and the Thomson scattering module as a design support tool.

The effect of a minor concentration of energetic particles produced by parallel NB injection on Geodesic Acoustic Modes (GAM) spectrum is analyzed using fully kinetic equation. It is found that the GAM frequency is reduced by the effective mass renormalization due to that new energetic GAM appears with higher frequency. The electron current in combination with NB driven ion flow, modeled by shifted Maxwell distribution, may overcome the ion Landau damping thus resulting in the GAM instability when electron current velocity is larger than the effective parallel GAM phase velocity Rqω . Qualitative agreement of the theory with co/counter NB injection experiments in COMPASS tokamak is demonstrated.

The poloidal distribution of electromagnetic loads during fast transients in the vacuum vessel of COMPASS-U tokamak is calculated analytically. The estimates are then compared with CarMa0NL numerical simulations. The results show that the poloidal eddy currents in the vacuum vessel must be taken into account for the proper evaluation of disruption forces in the COMPASS-U tokamak.

Due to an increased interest in runaway electron (RE) phenomena in tokamak research, the need for diagnostics of runaway electron population in plasma has emerged. A novel diagnostic of the nonthermal electron cyclotron emission from runaway electrons can be utilized for this purpose. It was designed and installed at the COMPASS tokamak based on the available heterodyne radiometer. The vertical ECE (V-ECE) system uses a 16-channel heterodyne radiometer with a vertically placed E2-band horn antenna with a 76.5-90 GHz frequency range front-end. Simulations with the ray-tracing SPECE code have shown a measurement feasibility of the runaway electrons with energies up to 1 MeV. Due to a low optical depth of the plasma in COMPASS during RE discharges, reflected waves from the tokamak wall can be detected as well. First results show strong connection with other RE diagnostics at COMPASS. The V-ECE can obtain important information about RE population evolution and primary generation mechanism.

Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened. Peer Reviewed Article escrit per 365 autors/autores: Labit, B.; Eich, T.; Harrer, G. F.; Wolfrum, E.; Bernert, M.; Dunne, M. G.; Frassinetti, L.; Hennequin, P.; Maurizio, R.; Merle, A.; Meyer, H.; Saarelma, S.; Sheikh, U.; Adamek, J.; Agostini, M.; Aguiam, D.; Akers, R.; Albanese, R.; Albert, C.; Alessi, E.; Ambrosino, R.; Andr be, Y.; Angioni, C.; Apruzzese, G.; Aradi, M.; Arnichand, H.; Auriemma, F.; Avdeeva, G.; Ayllon-Guerola, J. M.; Bagnato, F.; Bandaru, V. K.; Barnes, M.; Barrera-Orte, L.; Bettini, P.; Bilato, R.; Biletskyi, O.; Bilkova, P.; Bin, W.; Blanchard, P.; Blanken, T.; Bobkov, V.; Bock, A.; Boeyaert, D.; Bogar, K.; Bogar, O.; Bohm, P.; Bolzonella, T.; Bombarda, F.; Boncagni, L.; Bouquey, F.; Bowman, C.; Brezinsek, S.; Brida, D.; Brunetti, D.; Bucalossi, J.; Buchanan, J.; Buermans, J.; Bufferand, H.; Buller, S.; Buratti, P.; Burckhart, A.; Calabr, G.; Calacci, L.; Camenen, Y.; Cannas, B.; Cano Megías, P.; Carnevale, D.; Carpanese, F.; Carr, M.; Carralero, D.; Carraro, L.; Casolari, A.; Cathey, A.; Causa, F.; Cavedon, M.; Cecconello, M.; Ceccuzzi, S.; Cerovsky, J.; Chapman, S.; Chmielewski, P.; Choi, D.; Cianfarani, C.; Ciraolo, G.; Coda, S.; Coelho, R.; Colas, L.; Colette, D.; Cordaro, L.; Cordella, F.; Costea, S.; Coster, D.; Cruz Zabala, D. J.; Cseh, G.; Czarnecka, A.; Cziegler, I.; D’Arcangelo, O.; Dal Molin, A.; David, P.; De Carolis, G.; De Oliveira, H.; Decker, J.; Dejarnac, R.; Delogu, R.; Den Harder, N.; Dimitrova, M.; Dolizy, F.; Domínguez-Palacios Durán, J. J.; Douai, D.; Drenik, A.; Dreval, M.; Dudson, B.; Dunai, D.; Duval, B. P.; Dux, R.; Elmore, S.; Embréus, O.; Erds, B.; Fable, E.; Faitsch, M.; Fanni, A.; Farnik, M.; Faust, I.; Faustin, J.; Fedorczak, N.; Felici, F.; Feng, S.; Feng, X.; Ferreira, J.; Ferr, G.; Février, O.; Ficker, O.; Figini, L.; Figueiredo, A.; Fil, A.; Fontana, M.; Francesco, M.; Fuchs, C.; Futatani, S.; Gabellieri, L.; Gadariya, D.; Gahle, D.; Galassi, D.; Gałązka, K.; Galdon-Quiroga, J.; Galeani, S.; Gallart, D.; Gallo, A.; Galperti, C.; Garavaglia, S.; Garcia, J.; Garcia-Lopez, J.; Garcia-Mu oz, M.; Garzotti, L.; Gath, J.; Geiger, B.; Giacomelli, L.; Giannone, L.; Gibson, S.; Gil, L.; Giovannozzi, E.; Giruzzi, G.; Gobbin, M.; Gonzalez-Martin, J.; Goodman, T. P.; Gorini, G.; Gospodarczyk, M.; Granucci, G.; Grekov, D. 1; Grenfell, G.; Griener, M.; Groth, M.; Grover, O.; Gruca, M.; Gude, A.; Guimarais, L.; Gyergyek, T.; Hacek, P.; Hakola, A.; Ham, C.; Happel, T.; Harrison, J.; Havranek, A.; Hawke, J.; Henderson, S.; Hesslow, L.; Hitzler, F.; Hnat, B.; Hobirk, J.; Hoelzl, M.; Hogeweij, D.; Hopf, C.; Hoppe, M.; Horacek, J.; Hron, M.; Huang, Z.; Iantchenko, A.; Iglesias, D.; Igochine, V.; Innocente, P.; Ionita-Schrittwieser, C.; Isliker, H.; Ivanova-Stanik, I.; Jacobsen, A.; Jakubowski, M.; Janky, F.; Jardin, A.; Jaulmes, F.; Jensen, T.; Jonsson, T.; Kallenbach, A.; Kappatou, A.; Karpushov, A.; Kasilov, S.; Kazakov, Y.; Kazantzidis, P. 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A.; Morales, J.; Moret, J.-M.; Moro, A.; Moulton, D.; Nabais, F.; Naulin, V.; Naydenkova, D.; Nem, R. D.; Nespoli, F.; Newton, S.; Nielsen, A. H.; Nielsen, S. K.; Nikolaeva, V.; Nocente, M.; Nowak, S.; Oberkofler, M.; Ochoukov, R.; Ollus, P.; Olsen, J.; Omotani, J.; Ongena, J.; Orain, F.; Orsitto, F. P.; Paccagnella, R.; Palha, A.; Panaccione, L.; Panek, R.; Panjan, M.; Papp, G.; Paradela Perez, I.; Parra, F.; Passeri, M.; Pau, A.; Pautasso, G.; Pavlichenko, R.; Perek, A.; Pericoli Radolfini, V.; Pesamosca, F.; Peterka, M.; Petrzilka, V.; Piergotti, V.; Pigatto, L.; Piovesan, P.; Piron, C.; Piron, L.; Plyusnin, V.; Pokol, G.; Poli, E.; Pölöskei, P.; Popov, T.; Popovic, Z.; Pór, G.; Porte, L.; Pucella, G.; Puiatti, M. E.; Pütterich, T.; Rabinski, M.; Juul Rasmussen, J.; Rasmussen, J.; Rattá, G. A.; Ratynskaia, S.; Ravensbergen, T.; Réfy, D.; Reich, M.; Reimerdes, H.; Reimold, F.; Reiser, D.; Reux, C.; Reznik, S.; Ricci, D.; Rispoli, N.; Rivero-Rodriguez, J. F.; Rocchi, G.; Rodriguez-Ramos, M.; Romano, A.; Rosato, J.; Rubinacci, G.; Rubino, G.; Ryan, D. A.; Salewski, M.; Salmi, A.; Samaddar, D.; Sanchis-Sanchez, L.; Santos, J.; Särkimäki, K.; Sassano, M.; Sauter, O.; Scannell, R.; Scheffer, M.; Schneider, B. S.; Schneider, P.; Schrittwieser, R.; Schubert, M.; Seidl, J.; Seliunin, E.; Sharapov, S.; Sheeba, R. R.; Sias, G.; Sieglin, B.; Silva, C.; Sipilä, S.; Smith, S.; Snicker, A.; Solano, E. R.; Hansen, S. 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T.; Walkden, N.; Wauters, T.; Weiland, M.; Weinzettl, V.; Wensing, M.; Wiesen, S.; Wiesenberger, M.; Wilkie, G.; Willensdorfer, M.; Wischmeier, M.; Wu, K.; Xiang, L.; Zagorski, R.; Zaloga, D.; Zanca, P.; Zaplotnik, R.; Zebrowski, J.; Zhang, W.; Zisis, A.; Zoletnik, S.; Zuin, M.

This paper presents two scenarios used for generation of a runaway electron (RE) beam in the COMPASS tokamak with a focus on the decay phase and control of the beam. The first scenario consists of massive gas injection of argon into the current ramp-up phase, leading to a disruption accompanied by runaway plateau generation. In the second scenario, injection of a smaller amount of gas is used in order to isolate the RE beam from high-temperature plasma. The performances of current control and radial and vertical position feedback control in the second scenario were experimentally studied and analysed. The role of RE energy in the radial position stability of the RE beam seems to be crucial. A comparison of the decay phase of the RE beam in various amounts of Ar or Ne was studied using absolute extreme ultraviolet (AXUV) tomography and hard x-ray (HXR) intensity measurement. Argon clearly leads to higher HXR fluxes for the same current decay rate than neon, while radiated power based on AXUV measurements is larger for Ne in the same set of discharges.

The role of the COMPASS tokamak in research of generation, confinement and losses of runaway electron (RE) population is presented. Recently, two major groups of experiments aimed at improved understanding and control of the REs have been pursued. First, the effects of the massive gas injection (~1021 Ar/Ne particles) and impurity seeding (~1018 particles) were studied systematically. The observed phenomena include generation of the post-disruption RE beam and current conversion from plasma to RE. Zero loop voltage control was implemented in order to study the decay in simplified conditions. A distinctive drop of background plasma temperature and electron density was observed following an additional deuterium injection into the RE beam. With the loop voltage control the parametric dependence of the current decay rate dI/dt can be studied systematically and possibly extrapolated to larger facilities. Second, recent results of experiments focused on the role of the magnetic field in physics of RE were analysed. In this contribution, special attention is given to the observed effects of the resonant magnetic perturbation on the RE population. The benefits of the RE experiments on COMPASS was reinforced by diagnostic enhancements (fast cameras, Cherenkov detector, vertical ECE etc) and modelling efforts (in particular, coupling of the METIS and LUKE codes).

The plasma diagnostic and control (D&C) system for a future tokamak demonstration fusion reactor (DEMO) will have to provide reliable operation near technical and physics limits, while its front-end components will be subject to strong adverse effects within the nuclear and high temperature plasma environment. The ongoing developments for the ITER D&C system represent an important starting point for progressing towards DEMO. Requirements for detailed exploration of physics are however pushing the ITER diagnostic design towards using sophisticated methods and aiming for large spatial coverage and high signal intensities, so that many front-end components have to be mounted in forward positions. In many cases this results in a rapid aging of diagnostic components, so that additional measures like protection shutters, plasma based mirror cleaning or modular approaches for frequent maintenance and exchange are being developed. Under the even stronger fluences of plasma particles, neutron/gamma and radiation loads on DEMO, durable and reliable signals for plasma control can only be obtained by selecting diagnostic methods with regard to their robustness, and retracting vulnerable front-end components into protected locations. Based on this approach, an initial DEMO D&C concept is presented, which covers all major control issues by signals to be derived from at least two different diagnostic methods (risk mitigation).

Power handling experiments with a special liquid metal divertor module based on the capillary porous system technology were performed in the tokamak COMPASS. The performance of two metals (Li and LiSn alloy) were tested for the first time in a divertor under ELMy H-mode conditions. No damage of the capillary mesh and a good exhaust capability were observed for both metals in two separate experiments with up to 12 MW/m(2) of deposited perpendicular, inter-ELM steady-state heat flux and with ELMs of relative energy similar to 3% and a local peak energy fluence at the module similar to 15 kJ.m(-2). No droplets were directly ejected from the mesh top surface and for the LiSn experiment, no contamination of the core and SOL plasmas by Sn was observed. The elemental depth profile analysis of 14 stainless-steel samples located around the vacuum vessel for each experiment provides information about the migration of evaporated/redeposited liquid elements.

In the COMPASS tokamak, series of experiments were performed aimed at studying the impact of nitrogen, neon, and argon impurity seeding on the electron energy distribution function (EEDF) in the divertor region. The experiments were conducted in D-shaped, L-mode, deuterium plasmas. In order to obtain the radial distribution of the floating potential, ion saturation current, electron temperatures, and densities, the current-voltage characteristics were measured by Langmuir probes embedded in the COMPASS tokamak divertor. The properties of the plasma in the divertor region were measured before and during impurity seeding. Before the N2 seeding, the EEDF was bi-Maxwellian with a low-energy electron fraction with temperatures 3.5–5 eV, and a higher-energy one with temperatures in the range of 10 eV to 23 eV. During seeding with an increasing number of molecules per second, the EEDF changed from bi-Maxwellian to Maxwellian and the electron temperature decreased. The time-evolution was studied of the change in the EEDF during N2 seeding. When the seeding was carried out by a valve in the private flux region, the duration of the transition from a bi-Maxwellian to a Maxwellian EEDF was about 10–15 ms. When the N2 seeding took place through a low-field side valve, the transition from a bi-Maxwellian to a Maxwellian EEDF took longer −25–45 ms. The temporal evolution was also analyzed of the plasma parameters’ radial profiles when neon and argon were puffed using a valve in the divertor low-field side. The application is discussed of the probe measurements’ results to calculating the parallel heat-flux densities in the divertor region of the COMPASS tokamak.

O-mode frequency-modulated continuous wave (FMCW) reflectometry provides an alternative to magnetic measurements in the determination of the plasma separatrix position for plasma position control. This type of measurement proves to be particularly attractive for the control of future fusion reactors where the harsh radiation environment may damage magnetic probes or induce non-compensable measurement drifts. Plasma position reflectometry (PPR), first demonstrated in ASDEX-Upgrade, is a control technique that is increasingly important to validate in diversified experimental devices and relevant plasma regimes. The COMPASS tokamak provides suitable conditions for such advanced demonstrations and regular PPR operation and development, thanks to its O-mode reflectometer and Multi-Threaded Application Real-Time executor (MARTe) based real-time control system. Herein we present the integration of a PPR system on COMPASS, both at hardware and software levels. Reflectometry swept measurements require signals to be acquired in bursts of data and streamed to the corresponding MARTe-PPR node through PCIe ® fibre-optic links. The data transferred in real-time is used to reconstruct the radial density profiles from which the outer separatrix position is estimated. This estimate is then delivered to the central MARTe controller node via a dedicated Xilinx ® Aurora ® link at a rate matching COMPASS's 500 μ s slow control cycle. The implemented system systematically met the required latency specifications, being able to deliver an estimation of the plasma radial position capable of successfully replacing the corresponding magnetic measurements in the plasma position feedback control loops.

Microsecond probe measurements of the electron temperature during the tokamak edge localised mode (ELM) instability show that the peak values significantly exceed those obtained by conventional techniques. The temperatures measured at the plasma facing component (divertor) are around 80% of the initial value (at the pedestal). This challenges the current understanding, where only several percent of the pedestal value are measured at the divertor. Our results imply a negligible energy transfer from the electrons to the ions during the ELM instability, and therefore no associated increase of the ion power loads on the divertor. This observation is supported by the simple analytic free-streaming model, as well as by full kinetic simulations. The energetic ELM ion loads are expected to be one of the main divertor damaging factors; therefore, the obtained results give an optimistic prediction for next generation fusion devices.

In COMPASS tokamak, the ordinary mode (O-mode) microwave reflectometry diagnostic has been equipped with an upgraded acquisition system and dedicated computational node running the Multi-Threaded Application Real-Time executor (MARTe). This upgrade aims at the deployment of a real-time (RT) plasma position reflectometry (PPR) diagnostic system for future application in closed-loop plasma position control experiments. The system is synchronized with COMPASS MARTe main controller slow cycle, running at 500 μs, and allows a maximum measurement delivery latency of 450 μs. The RT signal processing algorithms, used to reconstruct the O-mode density profiles are the most time consuming step in the control cycle. This is due to the computation of a large number of Fast Fourier Transforms (FFT) over long data arrays, produced for each reflectometry profile measurement. MARTe is a software control framework that allows the integration of external C/C++ libraries and code such as the FFTW3 C library, a highly optimized and performant implementation of the FFT algorithm into the real-time designs. The framework also provides a set of tools to handle the execution of parallel threads, which can be used to expedite the execution of computationally demanding signal processing algorithms. Herein, we present the processing architecture implemented for the multi-threaded reflectometry density profile reconstruction on MARTe framework, using the FTTW3 library. Consistent systematic results have shown that the implemented solution satisfies COMPASS slow control cycle maximum data latency requirement. Moreover, the obtained reduced latency opens the possibility to either increase the number of processed measurements, leading to a more robust plasma position estimate or even to provide multiple position measurements during each control cycle.

Dedicated experiments have been performed in the COMPASS tokamak to thoroughly study the power deposition processes occurring on poloidal and toroidal edges of castellated plasma-facing components in tokamaks during steady-state L-mode conditions. Surface temperatures measured by a high resolution infra-red camera are compared with reconstructed synthetic data from a 2D thermal model using heat flux profiles derived from both the optical approximation and 2D particle-in-cell (PIC) simulations. In the case of poloidal leading edges, when the contribution from local radiation is taken into account, the parallel heat flux deduced from unperturbed, upstream measurements is fully consistent with the observed temperature increase at the leading edges of various heights, respecting power balance assuming simple projection of the parallel flux density. Smoothing of the heat flux deposition profile due to finite ion Larmor radius predicted by the PIC simulations is found to be weak and the power deposition on misaligned poloidal edges is better described by the optical approximation. This is consistent with an electron-dominated regime associated with a non-ambipolar parallel current flow. In the case of toroidal gap edges, the different contributions of the total incoming flux along the gap have been observed experimentally for the first time. They confirm the results of recent numerical studies performed for ITER showing that in specific cases the heat deposition does not necessarily follow the optical approximation. Indeed, ions can spiral onto the magnetically shadowed toroidal edge. Particle-in-cell simulations emphasize again the role played by local non-ambipolarity in the deposition pattern.

A new control module for the MAST-Upgrade Langmuir Probe system has been developed. It is based on a Xilinx Zynq FPGA, which allows for excellent configurability and ease of retrieving data. The module is capable of arbitrary bias voltage waveform generation, and digitises current and voltage readings from 16 probes. The probes are biased and measured one at a time in a time multiplexed fashion, with the multiplexing sequence completely configurable. In addition, simultaneous digitisation of the floating potential of all unbiased probes is possible. A suite of these modules, each coupled with a high voltage amplifier, enables biasing and digitisation of 640 Langmuir Probes in the MAST-Upgrade Super-X divertor. The system has been successfully tested on the York Linear Plasma Device and on the COMPASS tokamak. It will be installed on MAST-Upgrade ready for operations in 2018.

The ITER first wall is designed for start-up and ramp-down in limiter configuration. The wall panels are toroidally shaped in order to spread the incident parallel power flux q|| uniformly, assuming a single decay length λq whose value is not known from first principles. In order to study the scaling of q|| with plasma parameters, infra-red viewing of specially-designed limiters has been used on the COMPASS tokamak in ∼100 discharges with scans in Ip, ne and for all combinations of magnetic field and Ip directions. The IR measurement clearly shows that in addition to the main SOL heat flux profile with λq > 40 mm, a steep gradient (λqnear = 4 ± 2 mm) dominates q|| near separatrix. This appears independently of limiter shaping, insertion with respect to neighbors and incident field-line angles. Good agreement is found between the measured λqnear and the prediction of a heuristic drift-based model.

The narrow scrape-off layer power component observed in COMPASS inner wall limiter circular discharges by means of IR thermography is investigated by Langmuir probes embedded in the limiter. The power flux profiles are in good agreement with IR observations and can be described by a double-exponential decay with a short decay length (

This paper describes a new filamentary probe recently introduced on the COMPASS tokamak. It allows the measurement of electrostatic and magnetic properties of the filaments and their changes in dependence on distance from the separatrix in the region between a divertor and midplane. The probe head is mounted on a manipulator moving the probe radially on a shot-to-shot basis. This configuration is suitable for the long term statistical measurement of the plasma filaments and the measurement of their evolution during their propagation from the separatrix to the wall. The basics of the filamentary probe construction, the evolution of the plasma parameters, and first conditional averages of the plasma filaments in the scrape-off layer of the COMPASS tokamak during the L-mode regime are presented.

We report the latest results on fast measurements of the electron temperature and parallel heat flux in the COMPASS tokamak scrape-off layer (SOL) and divertor region during ELMy H-mode plasmas. The system of ball-pen and Langmuir probes installed on the divertor target, the horizontal reciprocating manipulator and the fast data-acquisition system with sampling frequency rate f = 5 MSa s−1 allow us to measure the electron temperature and parallel heat flux during inter-ELM and ELM periods with high temporal resolution. The filamentary structure of the electron temperature and parallel heat flux was observed during ELMs in the SOL as well as in the divertor region. The position of the filaments within ELMs is not regular and therefore the resulting conditionally averaged ELM neglects the peak values of the electron temperature and parallel heat flux. We have found a substantial difference between the value of the radial power decay length in the inter-ELM period λ q,inter = 2.5 mm and the decay length of the peak ELM heat flux λ q,ELM = 13.1 mm. The decay length of the ELM energy density was found to be λ E,ELM = 5.4 mm.

The significant role of magnetic perturbations in mitigation and losses of runaway electrons (REs) was documented in dedicated experimental studies of RE at the COMPASS tokamak. REs in COMPASS are produced both in low density quiescent discharges and in disruptions triggered by massive gas injection (MGI). The role of the RE seed produced in the beginning of the discharge on the subsequent RE population proved significant. Modulation of the RE losses by MHD instabilities was observed at several characteristic frequencies, as well as by magnetic field oscillations related to power supplies. Magnetic islands seem to suppress the losses as the HXR signal is low and coherent with the island rotation frequency. Moreover, periods of increased losses of REs observed in the current quench (CQ) and early RE beam plateau phase of the MGI disruptions seem to be linked to the bursts of magnetic perturbation, and to the observation of filaments in the fast visible camera images.

The Thomson scattering diagnostics (TS) at the COMPASS tokamak (operated in the Institute of Plasma Physics ASCR, Prague, Czech Republic) is based on two high-power Nd:YAG lasers, which have a pulse energy of 1.5 J and repetition rate of 30 Hz each. A new timing and synchronization unit (TSU) based on a Field Programmable Gate Array (FPGA) was designed and constructed in order to assure reliable and synchronized control of flash lamps and Q-switches of both lasers. Correct delay between laser pulses is necessary to obtain the highest possible laser energy and to prevent the active medium, the Nd:YAG rod, from thermal damage. An FPGA development kit was chosen as core hardware for the TSU and dedicated input and output circuits were manufactured to adapt the kit interfaces for COMPASS TS needs. In addition, the FPGA firmware, the control software, and the graphical user interface were developed to control the TSU hardware. This paper describes the TS regimes that are addressed by the new TSU. Further, the TSU design, the operation flow, and the firmware and software development are presented. Finally, the results of commissioning are shown.

The COMPASS plasma control system is based on the MARTe real-time framework. Thanks to MARTe modularity and flexibility new algorithms have been developed for plasma diagnostic (plasma position calculation), control (shaping field control), and protection systems (central solenoid protection). Moreover, the MARTe framework itself was modified to broaden the communication capabilities via Aurora. This paper presents the recent upgrades and improvements made to the COMPASS real-time plasma control system, focusing on the issues related to precision of the real-time calculations, and discussing the improvements in terms of discharge parameters and stability. In particular, the new real-time system has given the possibility to analyze and to minimize the transport delays of each control loop.

This paper presents an overview of the Control, Data Acquisition, and Communication system (CODAC) at the COMPASS tokamak: the hardware set-up, software implementation, and communication tools are described. The diagnostics and the data acquisition are tailored for high spatial and temporal resolution required by the COMPASS physics programme, which aims namely at studies of the plasma edge, pedestal, and Scrape-off-Layer (SOL). Studies of instabilities and turbulence are also an integral part of the programme. Therefore, the data acquisition consists of more than 1000 channels, sampled at rates from 500 kS/s up to 2 GS/s. Presently, the feedback system controls the plasma position and shape, plasma current, and density and it includes 32 analogue input channels as well as 1 digital input/output channel and 8 analogue outputs. The feedback control runs within the Multi-threaded Application Real-Time executor (MARTe) framework with two threads, a 500 μs cycle to control slow systems and a 50 μs cycle to control the fast feedback power supplies for plasma position control. In this paper, special attention is paid to the links between the systems, to the hardware and software connections, and to the communication. The hardware part is described, the software framework is addressed, and the particular implementation – the dedicated software modules, communication protocols, and links to the database are described.

A new technique for fitting the full radial profiles of electron density and temperature obtained by the Thomson scattering diagnostic in H-mode discharges on the COMPASS tokamak is described. The technique combines the conventionally used modified hyperbolic tangent function for the edge transport barrier (pedestal) fitting and a modification of a Gaussian function for fitting the core plasma. Low number of parameters of this combined function and their straightforward interpretability and controllability provide a robust method for obtaining physically reasonable profile fits. Deconvolution with the diagnostic instrument function is applied on the profile fit, taking into account the dependence on the actual magnetic configuration.