Publications (71)78.28 Total impact
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Article: Simulation of ITER full-field ICWC scenario in JET: RF physics aspects
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ABSTRACT: ITER as a superconducting fusion machine needs efficient wall conditioning techniques for application in the presence of the permanent high toroidal magnetic field for (i) reducing the in-vessel impurity content, (ii) controlling the surface hydrogen isotopic ratio and (iii) mitigating the in-vessel long-term tritium inventory build-up. Encouraging results recently obtained with ion-cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have raised ICWC to the status of one of the most promising techniques available to ITER for routine inter-pulse and overnight conditioning with the ITER main ICRF heating system in the presence of the permanent high toroidal magnetic field. This paper is dedicated to a milestone experiment in ICWC research: the first simulation of ICWC operation in an equivalent ITER full-field scenario and the assessment of the wall conditioning effect on the carbon wall in the largest present-day tokamak JET. In addition, we address in this paper the following topics: (i) an analysis of the radio frequency (RF) physics of ICWC discharges, (ii) the optimization of the operation of ICRF antennas for plasma startup and (iii) an outlook for the performance of ICWC in ITER using the ICRF heating system. Important operational aspects of the conventional ICRF heating system in JET (the so-called A2 antenna system) for use in the ICWC mode are highlighted: (i) the ability of the antenna to ignite the cleaning discharge safely and reliably in different gases, (ii) the capacity of the antennas to couple a large fraction of the RF generator power (>50%) to low-density (≈1016–1018 m−3) plasmas and (iii) the ICRF absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. Successful optimization of the JET-ICWC discharge parameters (BT = 3.3 T, f = 25 MHz) resulted in a reliable operation of the JET A2 antennas and a high conditioning efficiency in a scenario imitating closely ITER full-field operation (BT = 5.3 T, f = 40 MHz) with the fundamental ion-cyclotron resonance for deuterium (ω = D+ ) located on-axis. Numerical modelling with the 3D electromagnetic code Micro Wave Studio, a 1D RF full wave code and a 0D plasma code allows extrapolating the results obtained on JET and other present-day tokamaks to ITER and provides good prospects for the use of the ITER ICRF antennas for ICWC purposes.Plasma Physics and Controlled Fusion 01/2012; 54(074014). · 2.42 Impact Factor -
Article: Vessel thermal map real-time system for the JET tokamak
Phys. Rev. ST Accel. Beams. 01/2012; 15(5):054701. -
Conference Proceeding: Exploitation of Modularity in the JET Tokamak Vertical Stabilization System
50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC'11),; 01/2011 -
Article: Inboard and outboard electron temperature profile measurements in JET using ECE diagnostics
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ABSTRACT: In this paper a novel use of the heterodyne radiometer in JET is presented. The operation of the diagnostic has been extended to allow simultaneously measurement of the pedestal temperature profile at the inboard and outboard midplane with high spatial and temporal resolution. Access to the inboard midplane region is obtained by measuring ordinary mode electron cyclotron emission (ECE) (no harmonic overlap) using an antenna located on the low-field side of the torus. To assess the potential of the ECE temperature measurements in the inboard pedestal region, a detailed analysis of ECE spectrum will be discussed. The measured electron temperature profiles are presented, including a discussion of the main uncertainties in the analysis. The comparison of inboard and outboard temperature pedestal profiles in a variety of H-mode plasmas shows a reasonable agreement in both shape and magnitude.Plasma Physics and Controlled Fusion 07/2010; 52(8):085010. · 2.42 Impact Factor -
Conference Proceeding: Real-time systems in tokamak devices. A case study: The JET tokamak
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ABSTRACT: The achievement of the required performances during the operation of large fusion experimental reactors, is strictly related to the flexibility and reliability of the realtime infrastructures. It turns out that, in tokamak reactors, the real-time infrastructure has to be designed so as to meet a number of common requirements. An overview of the realtime infrastructure currently adopted at the JET tokamak is given in this paper, focusing the attention on the solutions that have been developed for addressing these common requirements. Furthermore, three real-time systems recently deployed at JET are described as example.Real Time Conference (RT), 2010 17th IEEE-NPSS; 06/2010 -
Article: SIMULATION OF ITER ICWC SCENARIOS IN JET
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ABSTRACT: Encouraging results recently obtained with alternative ion cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have elevated ICWC to the status of one of the most promising techniques available to ITER for routine interpulse conditioning in the presence of the permanent high toroidal magnetic field. The paper presents a study of ICWC discharge performance and optimization of the conditioning output in the largest tokamak JET using the standard ICRF heating antenna A2 in a scenario envisaged at ITER full field, BT=5.3 T: on-axis location of the fundamental ICR for deuterium, ω=ωcD+. The perspective of application of the alternative technique in ITER is analyzed using the 3-D MWS electromagnetic code, 1-D RF full wave and 0-D plasma codes.PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY. 01/2010; 16(Plasma Physics-5):46-50. -
Article: Pedestal width and ELM size identity studies in JET and DIII-D; implications for ITER
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ABSTRACT: The dependence of the H-mode edge transport barrier width on normalized ion gyroradius (ρ* = ρ/a) in discharges with type I ELMs was examined in experiments combining data for the JET and DIII-D tokamaks. The plasma configuration as well as the local normalized pressure (β), collisionality (ν*), Mach number and the ratio of ion and electron temperature at the pedestal top were kept constant, while ρ* was varied by a factor of four. The width of the steep gradient region of the electron temperature (Te) and density (ne) pedestals normalized to machine size showed no or only a weak trend with ρ*. A ρ1/2 or ρ1 dependence of the pedestal width, given by some theoretical predictions, is not supported by the current experiments. This is encouraging for the pedestal scaling towards ITER as it operates at lower ρ* than existing devices. Some differences in pedestal structure and ELM behaviour were, however, found between the devices; in the DIII-D discharges, the ne and Te pedestal were aligned at high ρ* but the ne pedestal shifted outwards in radius relative to Te as ρ* decreases, while on JET the profiles remained aligned while ρ* was scanned by a factor of two. The energy loss at an ELM normalized to the pedestal energy increased from 10% to 40% as ρ* increased by a factor of two in the DIII-D discharges but no such variation was observed in the case of JET. The measured pedestal pressures and widths were found to be consistent with the predictions from modelling based on peeling–ballooning stability theory, and are used to make projections towards ITERPlasma Physics and Controlled Fusion 11/2009; 51(12):124051. · 2.42 Impact Factor -
Article: Pedestal and scrape-off layer dynamics in ELMy H-mode plasmas in JET
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ABSTRACT: Pedestal and scrape-off layer (SOL) dynamics due to edge localized modes (ELMs) have been studied on JET with improved diagnostic capability. The new high resolution Thomson scattering system enables detailed measurement of the space and time evolution of the Te and ne pedestal profiles. The pedestal and SOL dynamics for type I ELMy H-mode plasmas have been studied for a wide range of plasma conditions. During a short period of <200 µs after the ELM event radial profiles of filaments in the SOL electron density and temperature have been observed. After that period the SOL density is increased and remains high for several milliseconds. During the same period the electron temperature shows no increase compared with the pre-ELM values. This SOL dynamics has been observed for a wide range of plasma parameters and is independent of plasma pedestal collisionality. For the first time on JET the convective and conductive ELM energy losses have been quantified using the new kinetic profile measurements. The findings provide detailed confirmation of earlier observations based on different measurements and analysis. The pedestal region perturbed by the ELM is the same for both density and temperature and the ELM effect extends up to about 20% of minor radius. The convective energy losses do not vary significantly and are ~5% of the pedestal stored energy (Wped) over a large range of pedestal collisionality from below to above whereas the conductive losses strongly decrease from ~20% of Wped to 5% of Wped with increasing . The experimental observations are compared with a simple model based on losses being driven by parallel transport.Nuclear Fusion 11/2009; 49(12):125006. · 4.09 Impact Factor -
Article: Recent progress in fast ion studies on JET
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ABSTRACT: This paper presents recent results on fast ion studies on JET. A set of diagnostics for both confined and lost fast ions was employed for investigating the response of fast ions to MHD modes and for studying their behaviour in plasmas with toroidal field ripple and in shear-reversed plasmas. A dependence of the losses on MHD mode amplitude was deduced from the experimental data. A study of various plasma scenarios has shown that a significant redistribution of the fast ions happens during changes in the profile of the safety factor from shear-reversed to monotonic. Significant changes in the losses of ICRH accelerated protons were found to be associated with L–H confinement transitions in plasmas. After an L–H transition, an abrupt decrease in the ICRH proton losses was observed. In plasmas with an internal transport barrier, the loss of ICRH accelerated ions was found to increase as the barrier forms. Further results concerning fast ion losses were obtained during JET experiments in which the magnitude of the TF ripple was varied. The ripple losses of fusion products appear similar to classical losses, and are in agreement with modelling.Nuclear Fusion 05/2009; 49(6):065030. · 4.09 Impact Factor -
Article: Pedestal and ELM response to impurity seeding in JET advanced scenario plasmas
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ABSTRACT: Advanced scenario plasmas must often be run at low densities and high power, leading to hot edge temperatures and consequent power handling issues at plasma–surface interaction zones. Experiments at JET are addressing this issue by exploring the use of extrinsic impurity seeding and D2 puffing to reduce heat fluxes.The experiments presented in this paper continue the line of advanced tokamak (AT) scenario studies at high triangularity in JET by concentrating on the characterization of the edge pedestal and the ELM behaviour with deuterium and/or light impurity fuelling (neon, nitrogen). Both injection of extrinsic impurities and D2 puffing are shown to have a significant impact on the edge pedestal in typical JET AT conditions. The ELM energy loss, ΔWELM/Wdia, can be reduced to below 3% and the maximum ELM penetration depth can be limited to r/a > 0.7, thus enhancing the possibility for sustainable internal transport barriers at large plasma radius. These conditions can be achieved in two separate domains, either at a radiated power fraction (Frad) of 30% or at a fraction of >50%. At the lower Frad the ELMs are type I and a high pedestal pressure is maintained, but the occasional large ELM may still occur. At Frad > 50% the pedestal pressure is degraded by 30–50%, but the ELMs are degraded to type III. The intermediate regime at Frad ~ 40% is unattractive for ITB scenarios because large type I ELMs occur intermittently during the predominantly type III ELM phases (compound type I/III). Frad = 30% can be obtained with D2 fuelling alone, whereas neon or nitrogen seeding is needed to achieve Frad > 50%. Only a limited number of tests have been carried out with nitrogen seeding, with the preliminary conclusion that the plasma edge behaviour is similar to that with neon seeding once the radiated fraction is matched.Nuclear Fusion 08/2008; 48(9):095004. · 4.09 Impact Factor -
Conference Proceeding: Current rise studies at ASDEX Upgrade and JET in preparation for ITER
35th EPS Conference on Plasma Physics, Hersonissos; 06/2008 -
Conference Proceeding: Synchronous ELM pacing at JET using the Vertical Stabilisation controller
Proceedings of the 35th EPS Conference on Plasma Physics; 01/2008 -
Article: Development of steady-state scenarios compatible with ITER-like wall conditions
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ABSTRACT: A key issue for steady-state tokamak operation is to determine the edge conditions that are compatible both with good core confinement and with the power handling and plasma exhaust capabilities of the plasma facing components (PFCs) and divertor systems. A quantitative response to this open question will provide a robust scientific basis for reliable extrapolation of present regimes to an ITER compatible steady-state scenario. In this context, the JET programme addressing steady-state operation is focused on the development of non-inductive, high confinement plasmas with the constraints imposed by the PFCs. A new beryllium main chamber wall and tungsten divertor together with an upgrade of the heating/fuelling capability are currently in preparation at JET. Operation at higher power with this ITER-like wall will impose new constraints on non-inductive scenarios. Recent experiments have focused on the preparation for this new phase of JET operation. In this paper, progress in the development of advanced tokamak (AT) scenarios at JET is reviewed keeping this long-term objective in mind. The approach has consisted of addressing various critical issues separately during the 2006–2007 campaigns with a view to full scenario integration when the JET upgrades are complete. Regimes with internal transport barriers (ITBs) have been developed at q95 ~ 5 and high triangularity, δ (relevant to the ITER steady-state demonstration) by applying more than 30 MW of additional heating power reaching βN ~ 2 at Bo ~ 3.1 T. Operating at higher δ has allowed the edge pedestal and core densities to be increased pushing the ion temperature closer to that of the electrons. Although not yet fully integrated into a performance enhancing ITB scenario, Neon seeding has been successfully explored to increase the radiated power fraction (up to 60%), providing significant reduction of target tile power fluxes (and hence temperatures) and mitigation of edge localized mode (ELM) activity. At reduced toroidal magnetic field strength, high βN regimes have been achieved and q-profile optimization investigated for use in steady-state scenarios. Values of βN above the 'no-wall magnetohydrodynamic limit' (βN ~ 3.0) have been sustained for a resistive current diffusion time in high-δ configurations (at 1.2 MA/1.8 T). In this scenario, ELM activity has been mitigated by applying magnetic perturbations using error field correction coils to provide ergodization of the magnetic field at the plasma edge. In a highly shaped, quasi-double null X-point configuration, ITBs have been generated on the ion heat transport channel and combined with 'grassy' ELMs with ~30 MW of applied heating power (at 1.2 MA/2.7 T, q95 ~ 7). Advanced algorithms and system identification procedures have been developed with a view to developing simultaneously temperature and q-profile control in real-time. These techniques have so far been applied to the control of the q-profile evolution in JET AT scenarios.Plasma Physics and Controlled Fusion 11/2007; 49(12B):B529. · 2.42 Impact Factor -
Article: Hydrogen plasmas with ICRF inverted minority and mode conversion heating regimes in the JET tokamak
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ABSTRACT: During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimized by using hydrogen (H) plasmas where the reference ion cyclotron resonance frequency (ICRF) heating scenarios rely on minority species such as helium (3He) or deuterium (D). This paper firstly describes experiments dedicated to the study of 3He heating in H plasmas with a sequence of discharges in which 5 MW of ICRF power was reliably coupled and the 3He concentration, controlled in real-time, was varied from below 1% up to 10%. The minority heating (MH) regime was observed at low concentrations (up to 2%). Energetic tails in the 3He ion distributions were observed with effective temperatures up to 300 keV and bulk electron temperatures up to 6 keV. At around 2%, a sudden transition was reproducibly observed to the mode conversion regime, in which the ICRF fast wave couples to short wavelength modes, leading to efficient direct electron heating and bulk electron temperatures up to 8 keV. Secondly, experiments performed to study D minority ion heating in H plasmas are presented. This MH scheme proved much more difficult since modest quantities of carbon (C) impurity ions, which have the same charge to mass ratio as the D ions, led directly to the mode conversion regime. Finally, numerical simulations to interpret these two sets of experiments are under way and preliminary results are shown.Nuclear Fusion 06/2006; 46(7):S550. · 4.09 Impact Factor -
Article: Confinement transitions (H-mode) in JET inner wall limiter plasmas
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ABSTRACT: Transitions in confinement with characteristics of H-mode are observed in JET inner wall limited plasmas in experiments performed at a magnetic field of 0.8 T and at a current of 0.9 MA, using up to 7 MW of auxiliary heating power. These transitions in confinement are short lived, with durations of up to 20 ms, but the overall stored energy increases by up to 5% and edge density by up to 20%. The termination of the period of good confinement correlates with the observation of a burst of magnetic fluctuations, similar to those associated with edge localized modes (ELMs).These transitions in confinement also correlate with a significant decrease in the density fluctuations measured by the microwave reflectometer and a small decrease in the magnetic fluctuations measured using 'Mirnov' probes, accompanied by a reduction of the D(alpha) emission. The comparison of the fluctuations in the limiter H-mode in JET with the divertor H-modes shows a similar behaviour, in particular the reduction of the fluctuation levels at high frequencies (f > 10 kHz) and an increase in the fluctuation levels at lower frequencies (f < 10 kHz). The strong electrostatic nature of the plasma edge turbulence is confirmed, where a reduction of up to ~50% in the density fluctuation levels correlates with an increase of up to 50% in the confinement time during the transition to H-mode. However, the importance of instabilities with a dominant magnetic component (such as ELMs) is also highlighted, which cause significant increase in transport, including the loss of the good confinement phase in limiter H-modes.Plasma Physics and Controlled Fusion 04/2006; 48(6):757. · 2.42 Impact Factor -
Article: Expanding the operating space of ICRF on JET with a view to ITER
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ABSTRACT: This paper reports on ITER-relevant ion cyclotron resonance frequency (ICRF) physics investigated on JET in 2003 and early 2004. Minority heating of helium three in hydrogen plasmas—(3He)H—was systematically explored by varying the 3He concentration and the toroidal phasing of the antenna arrays. The best heating performance (a maximum electron temperature of 6.2 keV with 5 MW of ICRF power) was obtained with a preferential wave launch in the direction of the plasma current. A clear experimental demonstration was made of the sharp and reproducible transition to the mode conversion heating regime when the 3He concentration increased above ~2%. In the latter regime the best heating performance (a maximum electron temperature of 8 keV with 5 MW of ICRF power) was achieved with dipole array phasing, i.e. a symmetric antenna power spectrum. Minority heating of deuterium in hydrogen plasmas—(D)H—was also investigated but was found inaccessible because this scenario is too sensitive to impurity ions with Z/A = 1/2 such as C6+, small amounts of which directly lead into the mode conversion regime. Minority heating of up to 3% of tritium in deuterium plasmas was systematically investigated during the JET trace tritium experimental campaign (TTE). This required operating JET at its highest possible magnetic field (3.9 to 4 T) and the ICRF system at its lowest frequency (23 MHz). The interest of this scenario for ICRF heating at these low concentrations and its efficiency at boosting the suprathermal neutron yield were confirmed, and the measured neutron and gammay ray spectra permit interesting comparisons with advanced ICRF code simulations. Investigations of finite Larmor radius effects on the RF-induced high-energy tails during second harmonic (ω = 2ωc) heating of a hydrogen minority in D plasmas clearly demonstrated a strong decrease in the RF diffusion coefficient at proton energies ~ 1 MeV, in agreement with theoretical expectations. Fast wave heating and current drive experiments in deuterium plasmas showed effective direct electron heating with dipole phasing of the antennas, but only small changes of the central plasma current density were observed with the directive phasings, in particular at low single pass damping. New investigations of the heating efficiency of ICRF antennas confirmed its strong dependence on the parallel wavenumber spectrum. Advances in topics of a more technological nature are also summarized: ELM studies using fast RF measurements, the successful experimental demonstration of a new ELM-tolerant antenna matching scheme and technical enhancements planned on the JET ICRF system for 2006, they being equally strongly driven by the preparation for ITER.Nuclear Fusion 01/2006; 46(2):391. · 4.09 Impact Factor -
Article: Bright spots generated by lower hybrid waves on JET
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ABSTRACT: Observations of bright spots on the JET divertor aprons during lower hybrid current drive experiments are described. These bright spots are important because they can potentially cause damage to large tokamaks. The bright spots arise due to the impact of a fast particle beam. This beam originates from the front of the lower hybrid launcher, where thermal particles are accelerated according to theory by interaction with the high spatial harmonics of the lower hybrid wave. The bright spots are clearly related to the lower hybrid power as they disappear when the lower hybrid power is switched off. According to the analysis versus various parameters, the brightness of the spots clearly decreases with increasing plasma–wall distance, i.e. the distance between the last closed flux surface and the poloidal limiter. This is clearly beneficial for ITER, as it is designed to operate at a large plasma–wall distance.Plasma Physics and Controlled Fusion 06/2005; 47(7):1101. · 2.42 Impact Factor -
Article: The beta scaling of energy confinement in ELMy H-modes in JET
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ABSTRACT: The disagreement between the weak dependence of the energy confinement time on normalized pressure, β, observed in dedicated scans and the strongly negative dependence in the confinement scaling laws used for the design of next step tokamaks and future reactors, remains an outstanding problem. As such, scans of β have been undertaken in single null, low triangularity (δ ≈ 0.2) ELMy H-mode plasmas in JET with the MarkIIGB-SRP divertor. The scans varied β by a factor of 2.8 (normalized β from 0.72 to 2.04) and covered a range of magnetic fields (1.5–2.3 T), plasma currents (1.5–2.75 MA) and safety factors (q95 = 2.8 and 3.3). A weak β dependence was observed both globally (B0τE varied less than 9% across any one scan) and locally. A scan within Type I ELMy H-modes suggests that this weaker dependence is not due to ELM regimes. A statistical analysis indicates that these results are consistent with log–linear regressions performed on a wide JET database of ELMy H-modes, if correlations in this database are considered.Plasma Physics and Controlled Fusion 04/2004; 46(5A):A215. · 2.42 Impact Factor -
Article: JET divertor geometry and plasma shape effects on the L–H transition threshold
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ABSTRACT: Results from recent experiments to study the effects of divertor geometry and increased plasma shaping on the L–H transition threshold on JET are reported. Equivalent septum configurations run with the new septum replacement plate (SRP) in the MkII Gas Box divertor have shown that the presence of the septum lowers the L–H transition power threshold, Pth, by 20%. For X-point to virtual septum top distances of less than 6 cm, the SRP plasmas also demonstrate a significant decrease of the L–H Pth and pedestal electron temperature, Te with reduced X-point height. Although, the SRP plasma's Pth remains above that with the septum, there is no difference in the pedestal Te at the L–H transition. The influence of plasma shaping on the L–H transition has also been investigated for the first time on JET in a series of density scans at Ip/Bt of 2.5 MA/2.7 T. While keeping the lower triangularity, δlower, and divertor geometry constant, the upper triangularity, δupper, has been increased from 0.23 to 0.34 with no effect on Pth or the pedestal Te or Ti. In a separate edge ne scan, two configurations with different δupper/δlower values of 0.23/0.23 and 0.43/0.33 have been compared. A large difference in the transition threshold is observed at values of edge ne above 1.8 × 1019 m−3, with the higher δ plasmas characterized by Pth of up to 25% lower at the highest densities scanned. This is thought to be the result of lowered X-point and outer strike point heights with increased δ.Plasma Physics and Controlled Fusion 04/2004; 46(5A):A87. · 2.42 Impact Factor -
Article: Edge localized mode physics and operational aspects in tokamaks
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ABSTRACT: Recent progress in experimental and theoretical studies of edge localized mode (ELM) physics is reviewed for the reactor relevant plasma regimes, namely the high confinement regimes, that is, H-modes and advanced scenarios.Theoretical approaches to ELM physics, from a linear ideal magnetohydrodynamic (MHD) stability analysis to non-linear transport models with ELMs are discussed with respect to experimental observations, in particular the fast collapse of pedestal pressure profiles, magnetic measurements and scrape-off layer transport during ELMs.High confinement regimes with different types of ELMs are addressed in this paper in the context of development of operational scenarios for ITER. The key parameters that have been identified at present to reduce the energy losses in Type I ELMs are operation at high density, high edge magnetic shear and high triangularity. However, according to the present experimental scaling for the energy losses in Type I ELMs, the extrapolation of such regimes for ITER leads to unacceptably large heat loads on the divertor target plates exceeding the material limits. High confinement H-mode scenarios at high triangularity and high density with small ELMs (Type II), mixed regimes (Type II and Type I) and combined advanced regimes at high βp are discussed for present-day tokamaks. The optimum combination of high confinement and small MHD activity at the edge in Type II ELM scenarios is of interest to ITER. However, to date, these regimes have been achieved in a rather narrow operational window and far from ITER parameters in terms of collisionality, edge safety factor and βp.The compatibility of the alternative internal transport barrier (ITB) scenario with edge pedestal formation and ELMs is also addressed. Edge physics issues related to the possible combination of small benign ELMs (Type III, Type II ELMs, quiescent double barrier) and high performance ITBs are discussed for present-day experiments (JET, JT-60U, DIII-D) in terms of their relevance for ITER. Successful plasma edge control, at high triangularity (~0.5) and high density (~0.7nGR), in ITB scenarios in JET is reported.Active control of ELMs by edge current, pellet injection, impurities and external magnetic perturbations creating an ergodic zone localized at the separatrix are discussed for present-day experiments and from the perspective of future reactors.Plasma Physics and Controlled Fusion 11/2003; 45(12A):A93. · 2.42 Impact Factor
Top co-authors
Top Journals
Institutions
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2003–2012
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Culham Centre for Fusion Energy
Abingdon, ENG, United Kingdom
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2004
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General Atomics
San Diego, CA, USA
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