P. Bohm - Academia.edu (original) (raw)

Papers by P. Bohm

FOM-Institute for Plasma Physics Rij nhuizen*, Association EURATOM-FOM, P.O. Box 1207, 3430 BE Ni... more FOM-Institute for Plasma Physics Rij nhuizen*, Association EURATOM-FOM, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands Ioffe Institute, RAS, Saint Petersburg 194021, Russia Institute for Energy Research – Plasma Physics*, Forschungszentrum Jülich GmbH, Association EURATOM-FZJ, D-52425 Jülich, Germany 4 Institute of Plasma Physics AS CR, 182 00 Prague 8, Czech Republic Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China *Partners in the Trilateral Euregio Cluster

L. Frassinetti, S. Saarelma, F. Imbeaux, G. Verdoolaege, P. Bilkova, P. Bohm, R. Fridström, E. Gi... more L. Frassinetti, S. Saarelma, F. Imbeaux, G. Verdoolaege, P. Bilkova, P. Bohm, R. Fridström, E. Giovannozzi, M. Owsiak, M. Dunne, B. Labit, R. Scannell, J.C. Hillesheim and JET contributors 1Division of Fusion Plasma Physics, KTH Royal Institute of Technology, Stockholm SE 2 CCFE, Culham Science Centre, Abingdon, Oxon OX14 3DB, UK 3CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France 4Department of Applied Physics Ghent University Sint-Pietersnieuwstraat 41 B-9000 Ghent, Belgium 5Laboratory for Plasma Physics—Royal Military Academy, Avenue de la Renaissancelaan 30, Brussels, Belgium 6 Institute of Plasma Physics of the CAS, Za Slovankou 3, 182 00 Prague 8, Czech Republic 7ENEA, Fusion and Nuclear Safety Department, Via E. Fermi 45, 00044 Frascati, Italy 8Poznan Supercomputing and Networking Center, IChB PAS, Noskowskiego 12/14, Poznan, Poland 9Max-Planck-Institut für Plasmaphysik, Garching, Germany 10Swiss Plasma Center (SPC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne,Sw...

Review of Scientific Instruments, 2016

Review of Scientific Instruments, 2021

The Thomson scattering (TS) diagnostic, one of the key diagnostics used on the tokamaks around th... more 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.

Journal of Instrumentation, 2017

The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measurements of the electro... more The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measurements of the electron temperature and density profiles which are critical to the understanding of the ITER plasma. The diagnostic must satisfy the ITER project requirements, which translate to requirements on performance as well as reliability, safety and engineering. The implications are particularly challenging for beam dump lifetime, the need for continuous active alignment of the diagnostic during operation, allowable neutron flux in the interspace and the protection of the first mirror from plasma deposition. The CPTS design has been evolving over a number of years. One recent improvement is that the collection optics have been modified to include freeform surfaces. These freeform surfaces introduce extra complexity to the manufacturing but provide greater flexibility in the design. The greater flexibility introduced allows for example to lower neutron throughput or use fewer surfaces while improving optical performance. Performance assessment has shown that scattering from a 1064 nm laser will be sufficient to meet the measurement requirements, at least for the system at the start of operations. Optical transmission at λ < 600 nm is expected to degrade over the ITER lifetime due to fibre darkening and deposition on the first mirror. For this reason, it is proposed that the diagnostic should additionally include measurements of TS 'depolarised light' and a 1319 nm laser system. These additional techniques have different spectral and polarisation dependencies compared to scattering from a 1064 nm laser and hence provide greater robustness into the inferred measurements of Te and ne in the core.

Journal of Instrumentation, 2016

Thomson scattering is a widely used diagnostic tool for local measurement of both electron temper... more Thomson scattering is a widely used diagnostic tool for local measurement of both electron temperature and electron density. It is used for both low and high temperature plasmas and it is a key diagnostic on all fusion devices. The extremely low cross-section of the reaction increases the complexity of the design. Since the early days of fusion, when a simple single point measurement was used, the design moved to a multi-point system with a large number of spatial points, LIDAR system or high repetition Thomson scattering diagnostic which are used nowadays. The initial low electron temperature approximation has been replaced by the full relativistic approach necessary for large devices as well as for ITER with expected higher plasma temperature. Along the way, the different development needs and the issues that exist need to be addressed to ensure that the technique is developed sufficiently to handle challenges of the bigger devices of the future as well as current developments needed for ITER. For large devices, the achievement of the necessary temperature range represents an important task. Both high and low temperatures can be measured, however, a large dynamic range makes the design difficult as size of detector and dynamic range are linked together. Therefore, the requirements of the new devices are extending the boundaries of these parameters. Namely, ITER presents challenges as access is also difficult but big efforts have been made to cope with this. This contribution contains a broad review of Thomson scattering diagnostics used in current devices together with comments on recent progress and speculation regarding future developments needed for future large scale devices.

1 Institute of Plasma Physics, Association EURATOM-IPP.CR, Prague, Czech Republic 2 Max-Planck-In... more 1 Institute of Plasma Physics, Association EURATOM-IPP.CR, Prague, Czech Republic 2 Max-Planck-Institut fur Plasmaphysik, EURATOM Association, Garching, Germany 3 Institute for Ion Physics and Applied Physics, University of Innsbruck, Association EURATOM/ÖAW, Austria Systematic measurements of the radial profiles of the electron temperature have been performed on the tokamaks COMPASS and ASDEX Upgrade by using ball-pen probe and Thomson scattering diagnostic. The ball-pen probe (BPP) [1, 2] offers a diagnostic technique capable of measuring the plasma potential with a robust implementation that may withstand particle and heat fluxes from the plasma at radii up to and within the separatrix. A combination of BPP and common Langmuir probe (LP) provides the value of the electron temperature by the difference between the plasma and floating potential. Therefore, the BPP/LP mounted on the midplane reciprocating manipulator on COMPASS and ASDEX Upgrade can provide the radial profiles of th...

Nuclear Fusion, 2017

A new system of probes was recently installed in the divertor of tokamak COMPASS in order to inve... more A new system of probes was recently installed in the divertor of tokamak COMPASS in order to investigate the ELM energy density with high spatial and temporal resolution. The new system consists of two arrays of rooftop-shaped Langmuir probes (LPs) used to measure the floating potential or the ion saturation current density and one array of Ball-pen probes (BPPs) used to measure the plasma potential with a spatial resolution of ~3.5 mm. The combination of floating BPPs and LPs yields the electron temperature with microsecond temporal resolution. We report on the design of the new divertor probe arrays and first results of electron temperature profile measurements in ELMy Hmode and L-mode. We also present comparative measurements of the parallel heat flux using the new probe arrays and fast infrared termography (IR) data during L-mode with excellent agreement between both techniques using a heat power transmission coefficient γ = 7. The ELM energy density  || was measured during a set of NBI assisted ELMy H-mode discharges. The peak values of  || were compared with those predicted by model and with experimental data from JET, AUG and MAST with a good agreement.

Plasma Physics and Controlled Fusion, 2021

The reduction of the incident heat flux onto the divertor will be a necessity for the future ther... more 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.

Fusion Engineering and Design, 2019

COMPASS-U, a high magnetic field tokamak with hot walls, will be designed and built at IPP Prague... more 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.

Nuclear Fusion, 2017

Axisymmetric geodesic acoustic mode (GAM) oscillations of the magnetic field, plasma potential an... more Axisymmetric geodesic acoustic mode (GAM) oscillations of the magnetic field, plasma potential and electron temperature have been identified on the COMPASS tokamak. This work brings an overview of their electromagnetic properties studied by multi-pin reciprocating probes and magnetic diagnostics. The n = 0 fluctuations form a continuous spectrum in limited plasmas but change to a single dominant peak in diverted configuration. At the edge of diverted plasmas the mode exhibits a non-local structure with a constant frequency over a radial extent of at least several centimeters. Nevertheless, the frequency still reacts on temporal changes of plasma temperature caused by an auxiliary NBI heating as well as those induced by periodic sawtooth crashes. Radial wavelength of the mode is found to be about 1-4 cm, with values larger for the plasma potential than for the electron temperature. The mode propagates radially outward and its radial structure induces oscillations of a poloidal E × B velocity, that can locally reach the level of the mean poloidal flow. Bicoherence analysis confirms a non-linear interaction of GAM with a broadband ambient turbulence. The mode exhibits strong axisymmetric magnetic oscillations that are studied both in the poloidal and radial components of the magnetic field. Their poloidal standing-wave structure was confirmed and described for the first time in diverted plasmas. In limited plasmas their amplitude scales with safety factor. Strong suppression of the magnetic GAM component, and possibly of GAM itself, is observed during co-current but not counter-current NBI.

Review of Scientific Instruments, 2016

The ball-pen probe (BPP) technique is used successfully to make profile measurements of the elect... more The ball-pen probe (BPP) technique is used successfully to make profile measurements of the electron temperature on the ASDEX Upgrade (Axially Symmetric Divertor Experiment), COMPASS (COMPact ASSembly), and ISTTOK (Instituto Superior Tecnico TOKamak) tokamak. The electron temperature is provided by a combination of the BPP potential (Φ BPP) and the floating potential (V fl) of the Langmuir probe (LP), which is compared with the Thomson scattering diagnostic on ASDEX Upgrade and COMPASS. Excellent agreement between the two diagnostics is obtained for circular and diverted plasmas and different heating mechanisms (Ohmic, NBI, ECRH) in deuterium discharges with the same formula T e = (Φ BPP − V fl)/2.2. The comparative measurements of the electron temperature using BPP/LP and triple probe (TP) techniques on the ISTTOK tokamak show good agreement of averaged values only inside the separatrix. It was also found that the TP provides the electron temperature with significantly higher standard deviation than BPP/LP. However, the resulting values of both techniques are well in the phase with the maximum of cross-correlation function being 0.8.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010

Two new Thomson scattering (TS) diagnostic systems are being built on the COMPASS tokamak to get ... more Two new Thomson scattering (TS) diagnostic systems are being built on the COMPASS tokamak to get radial profiles of both electron temperature and electron density. The core TS system will measure at 24 spatial points with E10 mm resolution across the core plasma while the edge TS system will measure at 32 spatial points with E (3-5) mm resolution across the edge plasma. Two independent Nd:YAG lasers (1.5 J, 30 Hz each) could be combined to produce a repetition rate of 60 Hz. The lasers will follow separate beam paths to the COMPASS vessel. Scattered light will be collected by two objectives: one for the core region, the second for the plasma edge. Both TS systems are based on Nd:YAG/APD (Avalanche photodiodes) technology. Scattered light is spectrally resolved in polychromators equipped with a set of spectral filters and APDs. The output signal is digitized by fast sampling (GS/s) 8 bit analog digital converters (ADCs).

Review of Scientific Instruments, 2012

COMPASS tokamak shots at low magnetic field feature overdense plasmas during the extended current... more COMPASS tokamak shots at low magnetic field feature overdense plasmas during the extended current flat-top phase. The first harmonic of the electron cyclotron emission is completely cutoff for O and X modes and so the emission caused by electron Bernstein waves (EBWs) propagating obliquely with respect to the magnetic field and undergoing so called EBW-X-O conversion process can be observed. We perform an angular scan of the EBW emission during a set of comparable shots in order to determine the optimum antenna direction. A weak dependence of the radiative temperature on the antenna angles indicates an influence of multiple reflections from the vessel wall. The low temperature at the mode conversion region is responsible for the collisional damping of EBW, which can explain several times lower measured radiative temperature than the electron temperature measured by the Thomson scattering system.

symposium on fusion technology, 2021

Abstract. The COMPASS tokamak, originally from Culham laboratory, UKAEA, has been reinstalled at ... more Abstract. The COMPASS tokamak, originally from Culham laboratory, UKAEA, has been reinstalled at Institute of Plasma Physics, Prague. Many innovations will be done and new diagnostics have to be constructed. Electron temperature Te and density ne as key parameters of fusion plasma will be measured directly by the newly designed high spatial resolution incoherent Thomson scattering system. The scientific programme of reinstalled COMPASS is aimed at an edge plasma region, where these physical quantities have large gradients. Several construction options of TS are considered. In this paper these options are discussed from a point of view of data analysis and estimation of background radiation.

Nuclear Fusion, 2021

Substantial power dissipation in the edge plasma is required for the safe operation of ITER and n... more 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 confinem...

Plasma Physics and Controlled Fusion, 2019

FOM-Institute for Plasma Physics Rij nhuizen*, Association EURATOM-FOM, P.O. Box 1207, 3430 BE Ni... more FOM-Institute for Plasma Physics Rij nhuizen*, Association EURATOM-FOM, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands Ioffe Institute, RAS, Saint Petersburg 194021, Russia Institute for Energy Research – Plasma Physics*, Forschungszentrum Jülich GmbH, Association EURATOM-FZJ, D-52425 Jülich, Germany 4 Institute of Plasma Physics AS CR, 182 00 Prague 8, Czech Republic Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China *Partners in the Trilateral Euregio Cluster

L. Frassinetti, S. Saarelma, F. Imbeaux, G. Verdoolaege, P. Bilkova, P. Bohm, R. Fridström, E. Gi... more L. Frassinetti, S. Saarelma, F. Imbeaux, G. Verdoolaege, P. Bilkova, P. Bohm, R. Fridström, E. Giovannozzi, M. Owsiak, M. Dunne, B. Labit, R. Scannell, J.C. Hillesheim and JET contributors 1Division of Fusion Plasma Physics, KTH Royal Institute of Technology, Stockholm SE 2 CCFE, Culham Science Centre, Abingdon, Oxon OX14 3DB, UK 3CEA, IRFM, F-13108 Saint-Paul-lez-Durance, France 4Department of Applied Physics Ghent University Sint-Pietersnieuwstraat 41 B-9000 Ghent, Belgium 5Laboratory for Plasma Physics—Royal Military Academy, Avenue de la Renaissancelaan 30, Brussels, Belgium 6 Institute of Plasma Physics of the CAS, Za Slovankou 3, 182 00 Prague 8, Czech Republic 7ENEA, Fusion and Nuclear Safety Department, Via E. Fermi 45, 00044 Frascati, Italy 8Poznan Supercomputing and Networking Center, IChB PAS, Noskowskiego 12/14, Poznan, Poland 9Max-Planck-Institut für Plasmaphysik, Garching, Germany 10Swiss Plasma Center (SPC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne,Sw...

Review of Scientific Instruments, 2016

Review of Scientific Instruments, 2021

The Thomson scattering (TS) diagnostic, one of the key diagnostics used on the tokamaks around th... more 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.

Journal of Instrumentation, 2017

The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measurements of the electro... more The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measurements of the electron temperature and density profiles which are critical to the understanding of the ITER plasma. The diagnostic must satisfy the ITER project requirements, which translate to requirements on performance as well as reliability, safety and engineering. The implications are particularly challenging for beam dump lifetime, the need for continuous active alignment of the diagnostic during operation, allowable neutron flux in the interspace and the protection of the first mirror from plasma deposition. The CPTS design has been evolving over a number of years. One recent improvement is that the collection optics have been modified to include freeform surfaces. These freeform surfaces introduce extra complexity to the manufacturing but provide greater flexibility in the design. The greater flexibility introduced allows for example to lower neutron throughput or use fewer surfaces while improving optical performance. Performance assessment has shown that scattering from a 1064 nm laser will be sufficient to meet the measurement requirements, at least for the system at the start of operations. Optical transmission at λ < 600 nm is expected to degrade over the ITER lifetime due to fibre darkening and deposition on the first mirror. For this reason, it is proposed that the diagnostic should additionally include measurements of TS 'depolarised light' and a 1319 nm laser system. These additional techniques have different spectral and polarisation dependencies compared to scattering from a 1064 nm laser and hence provide greater robustness into the inferred measurements of Te and ne in the core.

Journal of Instrumentation, 2016

Thomson scattering is a widely used diagnostic tool for local measurement of both electron temper... more Thomson scattering is a widely used diagnostic tool for local measurement of both electron temperature and electron density. It is used for both low and high temperature plasmas and it is a key diagnostic on all fusion devices. The extremely low cross-section of the reaction increases the complexity of the design. Since the early days of fusion, when a simple single point measurement was used, the design moved to a multi-point system with a large number of spatial points, LIDAR system or high repetition Thomson scattering diagnostic which are used nowadays. The initial low electron temperature approximation has been replaced by the full relativistic approach necessary for large devices as well as for ITER with expected higher plasma temperature. Along the way, the different development needs and the issues that exist need to be addressed to ensure that the technique is developed sufficiently to handle challenges of the bigger devices of the future as well as current developments needed for ITER. For large devices, the achievement of the necessary temperature range represents an important task. Both high and low temperatures can be measured, however, a large dynamic range makes the design difficult as size of detector and dynamic range are linked together. Therefore, the requirements of the new devices are extending the boundaries of these parameters. Namely, ITER presents challenges as access is also difficult but big efforts have been made to cope with this. This contribution contains a broad review of Thomson scattering diagnostics used in current devices together with comments on recent progress and speculation regarding future developments needed for future large scale devices.

1 Institute of Plasma Physics, Association EURATOM-IPP.CR, Prague, Czech Republic 2 Max-Planck-In... more 1 Institute of Plasma Physics, Association EURATOM-IPP.CR, Prague, Czech Republic 2 Max-Planck-Institut fur Plasmaphysik, EURATOM Association, Garching, Germany 3 Institute for Ion Physics and Applied Physics, University of Innsbruck, Association EURATOM/ÖAW, Austria Systematic measurements of the radial profiles of the electron temperature have been performed on the tokamaks COMPASS and ASDEX Upgrade by using ball-pen probe and Thomson scattering diagnostic. The ball-pen probe (BPP) [1, 2] offers a diagnostic technique capable of measuring the plasma potential with a robust implementation that may withstand particle and heat fluxes from the plasma at radii up to and within the separatrix. A combination of BPP and common Langmuir probe (LP) provides the value of the electron temperature by the difference between the plasma and floating potential. Therefore, the BPP/LP mounted on the midplane reciprocating manipulator on COMPASS and ASDEX Upgrade can provide the radial profiles of th...

Nuclear Fusion, 2017

A new system of probes was recently installed in the divertor of tokamak COMPASS in order to inve... more A new system of probes was recently installed in the divertor of tokamak COMPASS in order to investigate the ELM energy density with high spatial and temporal resolution. The new system consists of two arrays of rooftop-shaped Langmuir probes (LPs) used to measure the floating potential or the ion saturation current density and one array of Ball-pen probes (BPPs) used to measure the plasma potential with a spatial resolution of ~3.5 mm. The combination of floating BPPs and LPs yields the electron temperature with microsecond temporal resolution. We report on the design of the new divertor probe arrays and first results of electron temperature profile measurements in ELMy Hmode and L-mode. We also present comparative measurements of the parallel heat flux using the new probe arrays and fast infrared termography (IR) data during L-mode with excellent agreement between both techniques using a heat power transmission coefficient γ = 7. The ELM energy density  || was measured during a set of NBI assisted ELMy H-mode discharges. The peak values of  || were compared with those predicted by model and with experimental data from JET, AUG and MAST with a good agreement.

Plasma Physics and Controlled Fusion, 2021

The reduction of the incident heat flux onto the divertor will be a necessity for the future ther... more 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.

Fusion Engineering and Design, 2019

COMPASS-U, a high magnetic field tokamak with hot walls, will be designed and built at IPP Prague... more 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.

Nuclear Fusion, 2017

Axisymmetric geodesic acoustic mode (GAM) oscillations of the magnetic field, plasma potential an... more Axisymmetric geodesic acoustic mode (GAM) oscillations of the magnetic field, plasma potential and electron temperature have been identified on the COMPASS tokamak. This work brings an overview of their electromagnetic properties studied by multi-pin reciprocating probes and magnetic diagnostics. The n = 0 fluctuations form a continuous spectrum in limited plasmas but change to a single dominant peak in diverted configuration. At the edge of diverted plasmas the mode exhibits a non-local structure with a constant frequency over a radial extent of at least several centimeters. Nevertheless, the frequency still reacts on temporal changes of plasma temperature caused by an auxiliary NBI heating as well as those induced by periodic sawtooth crashes. Radial wavelength of the mode is found to be about 1-4 cm, with values larger for the plasma potential than for the electron temperature. The mode propagates radially outward and its radial structure induces oscillations of a poloidal E × B velocity, that can locally reach the level of the mean poloidal flow. Bicoherence analysis confirms a non-linear interaction of GAM with a broadband ambient turbulence. The mode exhibits strong axisymmetric magnetic oscillations that are studied both in the poloidal and radial components of the magnetic field. Their poloidal standing-wave structure was confirmed and described for the first time in diverted plasmas. In limited plasmas their amplitude scales with safety factor. Strong suppression of the magnetic GAM component, and possibly of GAM itself, is observed during co-current but not counter-current NBI.

Review of Scientific Instruments, 2016

The ball-pen probe (BPP) technique is used successfully to make profile measurements of the elect... more The ball-pen probe (BPP) technique is used successfully to make profile measurements of the electron temperature on the ASDEX Upgrade (Axially Symmetric Divertor Experiment), COMPASS (COMPact ASSembly), and ISTTOK (Instituto Superior Tecnico TOKamak) tokamak. The electron temperature is provided by a combination of the BPP potential (Φ BPP) and the floating potential (V fl) of the Langmuir probe (LP), which is compared with the Thomson scattering diagnostic on ASDEX Upgrade and COMPASS. Excellent agreement between the two diagnostics is obtained for circular and diverted plasmas and different heating mechanisms (Ohmic, NBI, ECRH) in deuterium discharges with the same formula T e = (Φ BPP − V fl)/2.2. The comparative measurements of the electron temperature using BPP/LP and triple probe (TP) techniques on the ISTTOK tokamak show good agreement of averaged values only inside the separatrix. It was also found that the TP provides the electron temperature with significantly higher standard deviation than BPP/LP. However, the resulting values of both techniques are well in the phase with the maximum of cross-correlation function being 0.8.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2010

Two new Thomson scattering (TS) diagnostic systems are being built on the COMPASS tokamak to get ... more Two new Thomson scattering (TS) diagnostic systems are being built on the COMPASS tokamak to get radial profiles of both electron temperature and electron density. The core TS system will measure at 24 spatial points with E10 mm resolution across the core plasma while the edge TS system will measure at 32 spatial points with E (3-5) mm resolution across the edge plasma. Two independent Nd:YAG lasers (1.5 J, 30 Hz each) could be combined to produce a repetition rate of 60 Hz. The lasers will follow separate beam paths to the COMPASS vessel. Scattered light will be collected by two objectives: one for the core region, the second for the plasma edge. Both TS systems are based on Nd:YAG/APD (Avalanche photodiodes) technology. Scattered light is spectrally resolved in polychromators equipped with a set of spectral filters and APDs. The output signal is digitized by fast sampling (GS/s) 8 bit analog digital converters (ADCs).

Review of Scientific Instruments, 2012

COMPASS tokamak shots at low magnetic field feature overdense plasmas during the extended current... more COMPASS tokamak shots at low magnetic field feature overdense plasmas during the extended current flat-top phase. The first harmonic of the electron cyclotron emission is completely cutoff for O and X modes and so the emission caused by electron Bernstein waves (EBWs) propagating obliquely with respect to the magnetic field and undergoing so called EBW-X-O conversion process can be observed. We perform an angular scan of the EBW emission during a set of comparable shots in order to determine the optimum antenna direction. A weak dependence of the radiative temperature on the antenna angles indicates an influence of multiple reflections from the vessel wall. The low temperature at the mode conversion region is responsible for the collisional damping of EBW, which can explain several times lower measured radiative temperature than the electron temperature measured by the Thomson scattering system.

symposium on fusion technology, 2021

Abstract. The COMPASS tokamak, originally from Culham laboratory, UKAEA, has been reinstalled at ... more Abstract. The COMPASS tokamak, originally from Culham laboratory, UKAEA, has been reinstalled at Institute of Plasma Physics, Prague. Many innovations will be done and new diagnostics have to be constructed. Electron temperature Te and density ne as key parameters of fusion plasma will be measured directly by the newly designed high spatial resolution incoherent Thomson scattering system. The scientific programme of reinstalled COMPASS is aimed at an edge plasma region, where these physical quantities have large gradients. Several construction options of TS are considered. In this paper these options are discussed from a point of view of data analysis and estimation of background radiation.

Nuclear Fusion, 2021

Substantial power dissipation in the edge plasma is required for the safe operation of ITER and n... more 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 confinem...

Plasma Physics and Controlled Fusion, 2019