Over the last few years, experiments have been performed on JET to study the dependence of the AE... more Over the last few years, experiments have been performed on JET to study the dependence of the AE stability limits on the main plasma parameters in different operating scenarios. The measurements are compared with theoretical models with the aim of improving the prediction capabilities for burning plasma experiments, such as ITER. An increase in the edge magnetic shear provides a significant stabilising contribution for AEs in plasmas characterised by a monotonic q-profile. Conversely, with non-monotonic q-profiles and Internal Transport Barriers, multiple weakly damped modes exist in the Alfvén frequency range even in the presence of a high edge magnetic shear, with possible negative implications for the AE stability in ITER. The dependence of the frequency and damping rate of n=1 TAEs on the bulk plasma β was also analysed using NBI heating in limiter plasmas. The mode frequency decreases for increasing β, in agreement with fluid and gyrokinetic predictions. Conversely, contrary t...
High power combined NB+ICRF (H)D heating experiments have been carried out in the JET MKIIa diver... more High power combined NB+ICRF (H)D heating experiments have been carried out in the JET MKIIa divertor configuration, both in deuterium (DD) and in deuterium-tritium (DT) plasmas. Results from a wide range of RF injected power, up to 9.5 MW, NB power up to 22 MW, plasma currents, up to 4.2 MA, and toroidal field values, up to 3.6 T, show a clear improvement in electron temperature, DD reactivity and stored energy with respect to NB only discharges. High energy Neutral Particle Analyser (NPA) data show that acceleration of the NB injected Deuterons takes place at the second harmonic deuterium resonance. This is confirmed by numerical simulations with the PION code. Experiments have also been carried out in the (He 3)D heating scenario. ICRF heating has been an essential ingredient in the DT experiments in the ELM-free Hot-Ion regime, contributing to the achievement of a record fusion power of 16.1 MW and a record stored energy of 17 MJ.
Fast protons can react with tritons in an endothermic nuclear reaction which can act as a source ... more Fast protons can react with tritons in an endothermic nuclear reaction which can act as a source of neutrons in magnetically confined fusion plasmas. We have performed an experiment to systematically study this reaction in low tritium concentration (≈ 1%) plasmas in the Joint European Torus (JET). A linear dependence is found between excess neutron rate and tritium concentration when the DT fusion rate is low. We discuss the properties of the neutron emission, including anisotropy, from the proton-triton reaction in a fusion reactor environment and derive simple models for the calculation of the neutron yield from this reaction in terms of tritium density, fast ion temperature and fast ion energy content.
... 3]. Alfvén waves, which constitute the most significant stable part of the MHD spectrum, and ... more ... 3]. Alfvén waves, which constitute the most significant stable part of the MHD spectrum, and in particular the discrete spectra of toroidal Alfvén eigenmodes (TAEs), which exist in the toroidal geometry [4], were successfully used previously [2 and 3] for the MHD spectroscopy of ...
This paper reports on the results of recent experiments performed on the JET tokamak on toroidal ... more This paper reports on the results of recent experiments performed on the JET tokamak on toroidal Alfvén eigenmodes (TAEs) with toroidal mode number (n) in the range up to |n| = 7. The stability properties of these medium-n TAEs are investigated experimentally using a set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate
Electron cyclotron resonance heating is applied to the edge of a high-confinement (H-mode) plasma... more Electron cyclotron resonance heating is applied to the edge of a high-confinement (H-mode) plasma featuring type I edge-localized modes (ELMs) in the TCV tokamak. As the deposition location is shifted gradually in a highly controlled manner towards the plasma pressure pedestal, an increase in the ELM frequency by a factor 2 and a decrease in the energy loss per ELM by the same factor are observed, even though the power absorption efficiency is reduced. This unexpected and, as yet, unexplained phenomenon, observed for the first time, runs contrary to the intrinsic type I ELM power dependence and provides a new approach for ELM mitigation.
The contents of this preprint and all other JET EFDA Preprints and Conference Papers are availabl... more The contents of this preprint and all other JET EFDA Preprints and Conference Papers are available to view online free at www.iop.org/Jet. This site has full search facilities and e-mail alert options. The diagrams contained within the PDFs on this site are hyperlinked from the year 1996 onwards. 2. operAtIon of Jet wIth the Iter-lIke wAll. 2.1 Plasma imPurity content and imPurity sources from Plasma facing comPonents From the very first JET-ILW plasmas the impurity content has been significantly reduced as compared to JET-C conditions. The carbon content is on average a factor 20 lower than in comparable JET-C plasmas as taken by the density normalised intensity of CIII (97.7nm) line (Fig.2). There is no evidence of any significant increase in residual carbon in time, indicating that no damage of the W-coatings on CFC substrate in the divertor has occurred. Oxygen levels are also lower by roughly one order of magnitude with respect to JET-C with non-optimal wall conditions (although only marginally lower than in JET C-wall plasmas with well conditioned wall and following Be evaporations). This large reduction of residual impurity levels is similar to that found earlier by AUG when going from a all-C to a boronised [4] all-W wall. The line averaged Z eff decreases from ~2 (JET-C) to 1.2 (JET-ILW) for similar values of density and heating power. The lower residual carbon and oxygen level in JET-ILW is thought to be mainly due to gettering of carbon and oxygen by beryllium [5]. Specific effort has been devoted to investigate the impact to the operational space due to W core accumulation [6] and surface melting [7]. Beryllium erosion can shorten component lifetimes, contribute to tritium retention by re-deposition [8] and cause significant sputtering of tungsten. Tungsten sputtering has been studied in Land H-mode discharges. It is found that for the present range of temperatures and impurity content at JET most of the W sputtering in L-mode can be attributed to low Z impurities, more specifically Be ions (Fig.3). Experiments in L-mode have been performed to compare the impurity content under neutral beam (NBI) and Ion Cyclotron Resoance Frequency (ICRF) heating. As shown in Fig.4, the bulk radiation, defined as power radiated from inside the separatrix, is higher with ICRF [9] although significant electron heating is obtained and the increase in plasma energy is similar to C-wall values. It was estimated that around 80% of the radiation comes from W and 20% by Nickel with sources from the divertor entrance and main chamber [10] [11]. The cause for this increased radiation remains the subject of ongoing investigation [9]. It could be due to the generation of fast Be ions in front of the antenna following the field lines to the divertor and giving rise to enhanced W sputtering. In H-mode plasmas, measurements of intra and inter ELM radiation from WI lines [12] indicate that ELM events dominate the sputtering of W (Fig.5). Transient increases have been observed in total radiated power, probably associated with small particles of medium/high Z materials, mostly W, entering the plasma. Since the start of plasma operation with the ILW, the frequency of such events has first increased, with the progressive increase of the additional heating power to the plasma, and then decreased. These sudden influxes are not necessarily fatal for the plasma which often recovers from the increased radiation [13]. 2.2. Plasma Breakdown and current rumP-uP The strong decrease of carbon content in the machine with the ILW has lead to a significant reduction
This paper presents the multiple set of requirements for the ITER magnetic diagnostic systems and... more This paper presents the multiple set of requirements for the ITER magnetic diagnostic systems and the current status of the various R&D activities performed by the EU partners.
ABSTRACT The measurement of the plasma current in ITER plays an outstanding role as it is part of... more ABSTRACT The measurement of the plasma current in ITER plays an outstanding role as it is part of the machine protection and is a safety-relevant measurement: it will be used in relation with regulatory limits to show that the operation remains within the safe envelope defined in the ITER license. The Continuous External Rogowski (CER) is an inductive sensor designed for current measurements and located in the casing of 3 Toroidal Field Coils (TFCs). After the completion of the design of the CER, 4 prototypes of the sensor were manufactured and R&D activities were performed under a Grant with the European Domestic Agency (F4E-GRT-012). The work was carried out between 2010 and 2011 by the ITERMAG consortium comprising 3 laboratories: CRPP (Switzerland) as leader, CEA (France) and RFX (Italy). The R&D campaign on CER prototypes consisted in the measurement of about 100 parameters to characterize the CER in terms of electrical, thermal, mechanical and also of vacuum compatibility. From these results, electromagnetic modeling of the CER response was performed. It is demonstrated that the CER fulfills ITER requirements. However, the vacuum compatibility of the prototype has to be improved and solutions to cope with this issue are proposed.
In this work we report the successful application of an innovative method, based on the Sparse Re... more In this work we report the successful application of an innovative method, based on the Sparse Representation of signals, to perform a real-time, unsupervised detection of the individual components in a frequency degenerate, multi-harmonic spectrum, using a small number of data unevenly sampled in the spatial domain. This method has been developed from its original applications in astronomy, and is now routinely used in the JET thermonuclear fusion experiment to obtain the decomposition of a spectrum of high-frequency (~10-500kHz range) magnetic instabilities with a sub-ms time resolution, allowing the real-time tracking of its individual components as the plasma background evolves. This work opens a path towards developing real-time control tools for electromagnetic instabilities in future fusion devices aimed at achieving a net energy gain. More generally, the speed and accuracy of this algorithm is recommended for instances of physics measurements and control engineering where an unsupervised, real-time decomposition of a degenerate signal is required from a small number of data.
A range of ITER relevant plasma stability issues have been studied in the JET tokamak; these incl... more A range of ITER relevant plasma stability issues have been studied in the JET tokamak; these include the effects of fast particles on sawteeth, control of NTMs via sawteeth seeds, scaling of (2, 1) NTM thresholds and the effect of error fields on plasma rotation and vice-...
Over the last few years, experiments have been performed on JET to study the dependence of the AE... more Over the last few years, experiments have been performed on JET to study the dependence of the AE stability limits on the main plasma parameters in different operating scenarios. The measurements are compared with theoretical models with the aim of improving the prediction capabilities for burning plasma experiments, such as ITER. An increase in the edge magnetic shear provides a significant stabilising contribution for AEs in plasmas characterised by a monotonic q-profile. Conversely, with non-monotonic q-profiles and Internal Transport Barriers, multiple weakly damped modes exist in the Alfvén frequency range even in the presence of a high edge magnetic shear, with possible negative implications for the AE stability in ITER. The dependence of the frequency and damping rate of n=1 TAEs on the bulk plasma β was also analysed using NBI heating in limiter plasmas. The mode frequency decreases for increasing β, in agreement with fluid and gyrokinetic predictions. Conversely, contrary t...
High power combined NB+ICRF (H)D heating experiments have been carried out in the JET MKIIa diver... more High power combined NB+ICRF (H)D heating experiments have been carried out in the JET MKIIa divertor configuration, both in deuterium (DD) and in deuterium-tritium (DT) plasmas. Results from a wide range of RF injected power, up to 9.5 MW, NB power up to 22 MW, plasma currents, up to 4.2 MA, and toroidal field values, up to 3.6 T, show a clear improvement in electron temperature, DD reactivity and stored energy with respect to NB only discharges. High energy Neutral Particle Analyser (NPA) data show that acceleration of the NB injected Deuterons takes place at the second harmonic deuterium resonance. This is confirmed by numerical simulations with the PION code. Experiments have also been carried out in the (He 3)D heating scenario. ICRF heating has been an essential ingredient in the DT experiments in the ELM-free Hot-Ion regime, contributing to the achievement of a record fusion power of 16.1 MW and a record stored energy of 17 MJ.
Fast protons can react with tritons in an endothermic nuclear reaction which can act as a source ... more Fast protons can react with tritons in an endothermic nuclear reaction which can act as a source of neutrons in magnetically confined fusion plasmas. We have performed an experiment to systematically study this reaction in low tritium concentration (≈ 1%) plasmas in the Joint European Torus (JET). A linear dependence is found between excess neutron rate and tritium concentration when the DT fusion rate is low. We discuss the properties of the neutron emission, including anisotropy, from the proton-triton reaction in a fusion reactor environment and derive simple models for the calculation of the neutron yield from this reaction in terms of tritium density, fast ion temperature and fast ion energy content.
... 3]. Alfvén waves, which constitute the most significant stable part of the MHD spectrum, and ... more ... 3]. Alfvén waves, which constitute the most significant stable part of the MHD spectrum, and in particular the discrete spectra of toroidal Alfvén eigenmodes (TAEs), which exist in the toroidal geometry [4], were successfully used previously [2 and 3] for the MHD spectroscopy of ...
This paper reports on the results of recent experiments performed on the JET tokamak on toroidal ... more This paper reports on the results of recent experiments performed on the JET tokamak on toroidal Alfvén eigenmodes (TAEs) with toroidal mode number (n) in the range up to |n| = 7. The stability properties of these medium-n TAEs are investigated experimentally using a set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate
Electron cyclotron resonance heating is applied to the edge of a high-confinement (H-mode) plasma... more Electron cyclotron resonance heating is applied to the edge of a high-confinement (H-mode) plasma featuring type I edge-localized modes (ELMs) in the TCV tokamak. As the deposition location is shifted gradually in a highly controlled manner towards the plasma pressure pedestal, an increase in the ELM frequency by a factor 2 and a decrease in the energy loss per ELM by the same factor are observed, even though the power absorption efficiency is reduced. This unexpected and, as yet, unexplained phenomenon, observed for the first time, runs contrary to the intrinsic type I ELM power dependence and provides a new approach for ELM mitigation.
The contents of this preprint and all other JET EFDA Preprints and Conference Papers are availabl... more The contents of this preprint and all other JET EFDA Preprints and Conference Papers are available to view online free at www.iop.org/Jet. This site has full search facilities and e-mail alert options. The diagrams contained within the PDFs on this site are hyperlinked from the year 1996 onwards. 2. operAtIon of Jet wIth the Iter-lIke wAll. 2.1 Plasma imPurity content and imPurity sources from Plasma facing comPonents From the very first JET-ILW plasmas the impurity content has been significantly reduced as compared to JET-C conditions. The carbon content is on average a factor 20 lower than in comparable JET-C plasmas as taken by the density normalised intensity of CIII (97.7nm) line (Fig.2). There is no evidence of any significant increase in residual carbon in time, indicating that no damage of the W-coatings on CFC substrate in the divertor has occurred. Oxygen levels are also lower by roughly one order of magnitude with respect to JET-C with non-optimal wall conditions (although only marginally lower than in JET C-wall plasmas with well conditioned wall and following Be evaporations). This large reduction of residual impurity levels is similar to that found earlier by AUG when going from a all-C to a boronised [4] all-W wall. The line averaged Z eff decreases from ~2 (JET-C) to 1.2 (JET-ILW) for similar values of density and heating power. The lower residual carbon and oxygen level in JET-ILW is thought to be mainly due to gettering of carbon and oxygen by beryllium [5]. Specific effort has been devoted to investigate the impact to the operational space due to W core accumulation [6] and surface melting [7]. Beryllium erosion can shorten component lifetimes, contribute to tritium retention by re-deposition [8] and cause significant sputtering of tungsten. Tungsten sputtering has been studied in Land H-mode discharges. It is found that for the present range of temperatures and impurity content at JET most of the W sputtering in L-mode can be attributed to low Z impurities, more specifically Be ions (Fig.3). Experiments in L-mode have been performed to compare the impurity content under neutral beam (NBI) and Ion Cyclotron Resoance Frequency (ICRF) heating. As shown in Fig.4, the bulk radiation, defined as power radiated from inside the separatrix, is higher with ICRF [9] although significant electron heating is obtained and the increase in plasma energy is similar to C-wall values. It was estimated that around 80% of the radiation comes from W and 20% by Nickel with sources from the divertor entrance and main chamber [10] [11]. The cause for this increased radiation remains the subject of ongoing investigation [9]. It could be due to the generation of fast Be ions in front of the antenna following the field lines to the divertor and giving rise to enhanced W sputtering. In H-mode plasmas, measurements of intra and inter ELM radiation from WI lines [12] indicate that ELM events dominate the sputtering of W (Fig.5). Transient increases have been observed in total radiated power, probably associated with small particles of medium/high Z materials, mostly W, entering the plasma. Since the start of plasma operation with the ILW, the frequency of such events has first increased, with the progressive increase of the additional heating power to the plasma, and then decreased. These sudden influxes are not necessarily fatal for the plasma which often recovers from the increased radiation [13]. 2.2. Plasma Breakdown and current rumP-uP The strong decrease of carbon content in the machine with the ILW has lead to a significant reduction
This paper presents the multiple set of requirements for the ITER magnetic diagnostic systems and... more This paper presents the multiple set of requirements for the ITER magnetic diagnostic systems and the current status of the various R&D activities performed by the EU partners.
ABSTRACT The measurement of the plasma current in ITER plays an outstanding role as it is part of... more ABSTRACT The measurement of the plasma current in ITER plays an outstanding role as it is part of the machine protection and is a safety-relevant measurement: it will be used in relation with regulatory limits to show that the operation remains within the safe envelope defined in the ITER license. The Continuous External Rogowski (CER) is an inductive sensor designed for current measurements and located in the casing of 3 Toroidal Field Coils (TFCs). After the completion of the design of the CER, 4 prototypes of the sensor were manufactured and R&D activities were performed under a Grant with the European Domestic Agency (F4E-GRT-012). The work was carried out between 2010 and 2011 by the ITERMAG consortium comprising 3 laboratories: CRPP (Switzerland) as leader, CEA (France) and RFX (Italy). The R&D campaign on CER prototypes consisted in the measurement of about 100 parameters to characterize the CER in terms of electrical, thermal, mechanical and also of vacuum compatibility. From these results, electromagnetic modeling of the CER response was performed. It is demonstrated that the CER fulfills ITER requirements. However, the vacuum compatibility of the prototype has to be improved and solutions to cope with this issue are proposed.
In this work we report the successful application of an innovative method, based on the Sparse Re... more In this work we report the successful application of an innovative method, based on the Sparse Representation of signals, to perform a real-time, unsupervised detection of the individual components in a frequency degenerate, multi-harmonic spectrum, using a small number of data unevenly sampled in the spatial domain. This method has been developed from its original applications in astronomy, and is now routinely used in the JET thermonuclear fusion experiment to obtain the decomposition of a spectrum of high-frequency (~10-500kHz range) magnetic instabilities with a sub-ms time resolution, allowing the real-time tracking of its individual components as the plasma background evolves. This work opens a path towards developing real-time control tools for electromagnetic instabilities in future fusion devices aimed at achieving a net energy gain. More generally, the speed and accuracy of this algorithm is recommended for instances of physics measurements and control engineering where an unsupervised, real-time decomposition of a degenerate signal is required from a small number of data.
A range of ITER relevant plasma stability issues have been studied in the JET tokamak; these incl... more A range of ITER relevant plasma stability issues have been studied in the JET tokamak; these include the effects of fast particles on sawteeth, control of NTMs via sawteeth seeds, scaling of (2, 1) NTM thresholds and the effect of error fields on plasma rotation and vice-...
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