B. Balet

European Commission, Bruxelles, Brussels Capital Region, Belgium

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Publications (79)107.81 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The energy confinement properties of low density, high ion temperature L- and H-mode plasmas are investigated. For L-mode plasmas it is shown that, although the global confinement is independent of density, the energy confinement in the central region is significantly better at low densities than at higher densities. The improved confinement appears to be associated with the steepness of the density gradient. For the H-mode phase, although the confinement at the edge is dramatically improved, which is once again associated with the steep density gradient in the edge region, the central confinement properties are essentially the same as for the standard L-mode. The results are compared in a qualitative manner with the predictions of the ion temperature gradient instability theory and appear to be in disagreement with some aspects of this theory.
    Nuclear Fusion 01/2011; 30(10):2029. · 2.73 Impact Factor
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    ABSTRACT: The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 × 10l6 s−1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 × 1019 m−3. The corresponding fusion product nD(0)τETi(0) is between 7.0 and 8.6 × 1020 m−3skeV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.
    Nuclear Fusion 01/2011; 31(5):839. · 2.73 Impact Factor
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    ABSTRACT: The concentration of deuterium in JET plasmas, expressed as a fraction of the electron concentration, has been determined using eight different methods, four of which involve neutron detection. The results from these various methods are found to be consistent within their experimental errors. The ratio nD/ne, measured at the moment of peak neutron emission strength, is found to lie in the range from nearly unity, for discharges into which deuterium pellets are injected, down to values of 0.4 or less, for some of the highest performance discharges. This finding is based on the analysis of discharges run in 1988, when the plasma facing components within the vacuum vessel were of carbon or were carbon coated.
    Nuclear Fusion 01/2011; 30(2):307. · 2.73 Impact Factor
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    ABSTRACT: The properties of JET plasmas obtained under similar L mode conditions, but using different ion species, are compared. In hydrogenic limiter discharges with NBI heating, particle confinement and sawtooth activity display a clear isotopic dependence. However, the gain in global energy confinement from hydrogen to deuterium is less pronounced than that implied by popular L mode scaling relationships and the improvement appears to be only partly due to a reduction in the local conductive heat flux. No significant change in energy or particle confinement is observed when deuterium is replaced by 3He
    Nuclear Fusion 10/2002; 33(9):1319. · 2.73 Impact Factor
  • B. Balet, J.G. Cordey
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    ABSTRACT: The analysis of discharges with strong on- and off-axis ion cyclotron resonance heating shows that the transport of heat in JET is local and diffusive. Discharges with or without pellet injection are considered; in all cases, the electron temperature profile responds to the location of the heating. The total heat flow Q can be described by a local diffusive model of the form Q = -χ(qψ, |∇Te|) ne ∇Te, where χ is proportional to qψ1.75 and increases with |∇Te|. Several forms of the dependence of χ on |∇Te| are shown to give a reasonable fit to the data
    Nuclear Fusion 10/2002; 34(8):1175. · 2.73 Impact Factor
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    ABSTRACT: During a series of experiments with tritium (T) in deuterium (D) plasmas in the Joint European Torus (JET), the temporal evolution and the two dimensional (2-D) spatial profiles of the 2.5 and 14 MeV neutron emissivities from D-D and D-T fusion reactions were inferred from measurements with the JET neutron emission profile monitor. These experiments, involving triton production from D-D fusion, beam deposition and diffusion, D-T fusion, and tritium removed from wall tiles, were investigated in four plasma scenarios: (i) In high performance deuterium plasmas with deuterium neutral beam injection, the 14 MeV neutron emissivity due to triton burnup was observed. (ii) In discharges with 1% tritium beam injection, neutron emissivity ratios showed that approximately the same deposition profiles resulted from tritium as from deuterium beams. A thermalized tritium diffusion experiment was performed in which the T-D density ratio was found to be spatially constant across the plasma; in conjunction with similar particle source profiles, this indicates that deuterium and tritium have similar particle transport properties. (iii) In two high performance discharges for which two of the sixteen neutral beam sources operated with 100% tritium, the production rate of 14 MeV neutrons reached 6 × 1017 n.s-1 The alpha particle 2-D birth profile was directly inferred from the measured 14 MeV neutron emissivity profile. Both the axial 14 MeV neutron emissivity and the axial ion temperature saturated before the maximum global emission was reached. (iv) During the tritium cleanup phase, residual tritium entering the plasma produced a spatially constant ratio of tritium to deuterium, confirming the similarity of their particle transport properties
    Nuclear Fusion 10/2002; 33(9):1325. · 2.73 Impact Factor
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    ABSTRACT: The particle and energy transport properties of the high fusion performance JET pulses that were obtained before and during the first tritium experiments are discussed. The particle diffusion coefficient of tritium is determined by monitoring the decay of a small quantity of injected tritium in a deuterium background plasma. A good simulation of the measured 14 MeV neutron emissivity profile is obtained throughout the decay phase if the mixing of the two species is described by a model in which the tritium diffusion coefficient is similar to that of deuterium. The energy confinement of these low density, hot ion, H mode discharges is found to have both improved central and edge confinement over the conventional medium to high density H mode discharges, regardless of the presence or absence of tritium in the discharge. As the tritium concentration of these D-T discharges is small (varying from <1% to 10%), no isotopic dependence was expected and indeed none is observed. Enhancement factors of at least twice the value predicted by H mode scaling expressions are observed but only transiently. A local transport analysis is completed to try and establish the reason for the improved confinement and its transient nature. Similarities between these pulses and DIII-D VH mode discharges have been noticed, and common characteristics are discussed. In particular, the expansion of the region with access to the second stability regime certainly appears to be a possibility for the enhanced confinement. The stabilization of the ηi mode by the peaked density profile seems unlikely to be the cause of the improved confinement. Finally, for the discharge with a high concentration of tritium, it has been suggested that alpha particle driven instabilities could affect the energy confinement. A comparison is made with tile stability threshold of toroidicity induced Alfven eigenmodes (TAE), which appear to have been stable. The alpha particle statistics are also presented
    Nuclear Fusion 10/2002; 33(9):1345. · 2.73 Impact Factor
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    ABSTRACT: In nearly all hot ion H modes in JET, a magnetohydrodynamic (MHD) event is clearly observed just before the time the stored plasma energy saturates and the neutron yield starts to decline. The results of a systematic analysis of MHD observations for a large number of discharges is reported. The relationship between MHD phenomena and the onset of confinement limitation is discussed, as are aspects of the three main types of performance limiting MHD: (a) low-n modes in the outer regions of the plasma, (b) sawteeth and (c) giant edge localized modes (ELMs). Model simulations indicate how the transport is affected and allow an assessment of how much the neutron yield would be improved if the MHD activity were absent
    Nuclear Fusion 10/2002; 37(6):809. · 2.73 Impact Factor
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    ABSTRACT: The results of numerical simulation of JET L, H and VH modes are presented. It is shown that L-H transitions are accompanied by a fast and large modification of the transport coefficients not only near the plasma edge but also across a large fraction of the plasma cross-section. Both experiments and numerical simulations show that giant ELMs in JET also have a global character and their penetration length increases with the amplitude of the Dalpha signal. Transport coefficients are of the order of the L mode values during these ELMs. Analysis shows also that within experimental accuracy the transport coefficients are the same for VH and ELM-free H mode discharges. The variation in the global energy confinement is mainly due to the differences in the impurity radiation, power deposition profiles and plasma recycling and related convection
    Nuclear Fusion 10/2002; 36(3):321. · 2.73 Impact Factor
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    ABSTRACT: The MHD effects observed in the hot ion H modes in the pre-divertor configuration of JET, including those generated in the preliminary tritium experiment, are described. Some observations were found to be similar to those in high beta regimes while others are new and appear to be pertinent to high performance discharges only. The high performance phase is largely sawtooth free and dominated by fishbone activity, which increases in amplitude throughout this phase. During termination of the high performance phase, the growth of a large variety of MHD activity with low mode numbers is observed. Also, edge instabilities possibly associated with much larger mode numbers are seen in the Dalpha emission. In some cases, the unusual structure of two central m=n=1 islands was found, and resistive MHD modelling indicates that this observation is consistent with a nearly flat, non-monotonic q profile. In some discharges a sawtooth collapse immediately followed by an edge localized mode (ELM) is observed at, or shortly following, the termination of the high performance
    Nuclear Fusion 10/2002; 35(4):409. · 2.73 Impact Factor
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    ABSTRACT: A power step-down approach leads to extended, high performance operation of deuterium plasmas in the hot ion edge localized mode (ELM)-free H mode of the JET tokamak. ELM-free discharges in JET, when heated steadily at the maximum power available, are usually transient and terminate with a variety of MHD phenomena. With power stepdown, instabilities are delayed or avoided, and nearly constant plasma conditions at up to 10 MJ of stored energy are maintained by 10 MW of heating power for up to 1 s, about an energy confinement time, at 3.5 MA and 3.4 T. No large transient corrections are required for confinement calculations in this hot ion regime, and the energy confinement time scaling is found to be similar to global ELM-free H mode confinement scaling. The confinement times can increase after the transition to quasi-steady conditions, to some 10-25% above the scaling law predictions. Code simulations of the experimentally observed neutron rate and the lack of a major discontinuity in this rate before and after beam power stepdown confirm that neutron production is predominantly (>60%) from thermal fusion. The scaling of fusion rates from deuterium plasmas to deuterium-tritium (DT) mixtures yields a fusion Q approaching 1. After power stepdown, the edge pressure gradient stops increasing and helps delay the onset of instabilities. Giant ELMs still occur, associated with "outer mode" activity and continuously rising plasma density. The plasma remains in an ELM-free H mode even when the heating power after power stepdown is lower than the L-H transition power. Extended operation optimization experiments indicate that stored energy and neutron production are maximized and the density rise minimized by preferentially injecting high energy 140 keV beams after power stepdown
    Nuclear Fusion 10/2002; 37(8):1067. · 2.73 Impact Factor
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    ABSTRACT: Pellet enhanced performance (PEP) has been observed in a number of JET discharges at various plasma conditions, in both L and H modes, with the H multiplier (the confinement enhancement factor over the Goldston confinement time) covering the range from 1 to 4, and with plasma currents from 1 MA to 4.1 MA. Most of the PEP plasmas have been created by refuelling with pellets of 4 mm diameter injected at 1.2 km/s. PEPs show an improved central confinement with an effective heat conductivity reduced by factors of approximately 2-5 relative to otherwise comparable discharges. This is possibly related to the inverted shear in the plasma core due to the large local bootstrap current density. The limitations in the PEP performance seem to be set by at least two mechanisms: impurity behaviour, MHD activity or a combination of both. In certain discharges, MHD modes seem to be able to check the often observed impurity accumulation. Too much MHD mode activity, however, easily destroys the enhanced confinement of the PEP discharge. The stability of the ballooning modes has been studied and the PEP plasma core is found to be in the second stability region against ballooning modes or close to marginal stability. In a number of discharges complex high (m,n) modes have been observed with the soft X-ray cameras. The behaviour of the low (m,n) MHD modes can only be understood by considering the detailed evolution of the inverted q profile, which exists in a given discharge
    Nuclear Fusion 10/2002; 35(2):225. · 2.73 Impact Factor
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    ABSTRACT: The scaling of energy transport with dimensionless parameters has been measured in high-temperature plasmas with the goals of guiding theory and predicting energy confinement in future fusion devices. Validation of this approach requires demonstration of similarity in plasmas with identical dimensionless parameters but very different physical parameters. Within measurement uncertainties, the heat diffusivities and global energy confinement exhibit similarity in high-confinement regimes on the DIII-D and JET tokamaks and in low-confinement regimes on the DIII-D and Alcator C-Mod tokamaks.
    Nuclear Fusion 09/2002; 42(10):1193. · 2.73 Impact Factor
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    ABSTRACT: High power combined NBI + ICRF heating experiments have been carried out in the JET Mark IIa divertor configuration in the hot ion ELM-free H mode regime, both in deuterium (DD) and in deuterium-tritium (DT) plasmas. Results are presented from a wide range of additional heating power levels, ICRF up to 9.5 MW tuned to the fundamental hydrogen minority, NBI up to 22 MW, and for plasma currents up to 4.2 MA and toroidal fields up to 3.6 T. Discharges with combined NBI + ICRF heating show a clear improvement in electron temperature, DD neutron yield and stored energy with respect to NBI only discharges. High energy neutral particle analyser data show that acceleration of the NBI deuterons takes place due to absorption of ICRF power at the second harmonic deuterium resonance. This is confirmed by numerical simulations with the PION code, indicating that up to 40% of the ICRF power is absorbed by bulk and NBI ions. 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.
    Nuclear Fusion 05/2002; 39(11):1591. · 2.73 Impact Factor
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    ABSTRACT: In current large tokamaks, non-intrinsic seeded impurities have been used to produce divertor power loads which would be considered acceptable when extrapolated to ITER. Many devices have achieved the goals of high fractional radiated powers, small frequent ELMs and detachment which are characteristic of radiative H mode regimes. The influence of divertor geometry on these characteristics is described. It has been a matter of concern that the Zeff associated with the seeded impurities may exceed that allowable in ITER and also that the degradation in energy confinement may be unacceptable. Confidence can only be built in the prediction of these parameters in ITER if reliable scalings are available for impurity content and energy confinement which have a sound physics basis. Work is described at JET in this area whilst using multimachine data to characterize the size scaling and provide a context for the JET data. Predicted levels for the impurity content of seeded ITER plasmas appear to be of marginal acceptability. Discharges run in the JET Mark I, Mark IIA and Mark IIAP divertors are compared and indicate that increased divertor closure has brought relatively minor benefits in highly radiative discharges. The acceptability of the energy confinement of radiation for ITER remains unclear. Dimensionless parameter scaling experiments have been conducted in which β, q25, fractional radiated power and Zeff are held constant for a range of ρ*. The price paid for high edge radiation and small ELMs appears to be a 25% loss in total stored energy as a result of edge pedestal degradation. However, the underlying energy confinement scaling may still be consistent with gyro-Bohm scaling, which would give an adequate margin for ITER. This conclusion is, however, sensitive to the scaling of confinement with collisionality, which is difficult to determine due to the coupling between ρ* and ν* which is a consequence of radiation dominated regimes.
    Nuclear Fusion 05/2002; 39(1):19. · 2.73 Impact Factor
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    ABSTRACT: The scaling of the energy confinement in H mode plasmas with different hydrogenic isotopes (hydrogen, deuterium, DT and tritium) is investigated in JET. For ELM-free H modes the thermal energy confinement time τth is found to decrease weakly with the isotope mass (τth ~M-0.25±0.22), whilst in ELMy H modes the energy confinement time shows practically no mass dependence (τth ~M0.03±0.1). Detailed local transport analysis of the ELMy H mode plasmas reveals that the confinement in the edge region increases strongly with the isotope mass, whereas the confinement in the core region decreases with mass (τthcore M-0.16), in approximate agreement with theoretical models of the gyro-Bohm type (τgB ~M-0.2).
    Nuclear Fusion 05/2002; 39(3):301. · 2.73 Impact Factor
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    ABSTRACT: Because of its large size, single null divertor and flexible magnetic geometry, JET is capable of producing the most reactor relevant plasmas of any present generation tokamak. In recent DT experiments, the fusion performance of these plasmas was tested for the first time. Over 4 MW of fusion power was produced in a high power, steady state pulse of 5 s, limited by the duration of the heating power. The fusion QE, defined simply as the fusion energy produced divided by the input energy over this 5 s interval, was 0.18. These DT ELMy H mode discharges performed up to expectations based on DD preparation pulses and thus establish a firm basis for extrapolating to a next step machine. Operation at low q95 is possible in JET with no degradation in the confinement enhancement factor and provides an improved margin to ignition when extrapolated to ITER. Considerable uncertainties remain, nonetheless. In particular, access to high density, relative to the Greenwald limit, and operation in close proximity to the H mode threshold may both result in a degradation of the confinement in the next step machine.
    Nuclear Fusion 05/2002; 39(8):993. · 2.73 Impact Factor
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    ABSTRACT: Results are presented of experiments carried out in JET with the Mark II divertor to study ELMy H modes at high density. In these experiments the effective ion mass of the plasma (pure hydrogen, deuterium and tritium), input power, plasma current and edge triangularity were varied. The variation of density was achieved by gas fuelling. The focus is on two main issues: the variation of global energy confinement with density and the scaling of the H mode pedestal. It is found that the density dependence in the ITER97-P(y) scaling is not confirmed by experiment when the density limit is approached. For the high density regime in ELMy H modes a density independent scaling is proposed that includes the plasma triangularity in the regression variables. The core energy content of the plasma is correlated to the edge pedestal parameters, consistently with a mixed Bohm-gyroBohm transport model for H mode plasmas. It is found that with type I ELMs, the edge pedestal pressure increases with the ion mass of the plasma, the input power and the edge magnetic shear. A scaling is derived for the pressure at the top of the edge pedestal which is consistent with a model where the pedestal width is assumed to be proportional to the ion gyro-radius and the pedestal gradient is limited by ideal MHD.
    Nuclear Fusion 05/2002; 39(9):1133. · 2.73 Impact Factor
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    ABSTRACT: JET experiments using deuterium and tritium have made it possible to study sawtooth stability in plasmas approaching thermonuclear conditions. Record fusion yields were obtained in discharges where the sawtooth was delayed. In a sequence of discharges designed to study alpha heating, a new phenomenon was observed: the sawtooth period increases with tritium concentration. The internal kink stability for both high performance and alpha heating experiment plasmas was studied. Calculations for the record fusion discharge at the time of maximum fusion power (Pfus = 16 MW) showed that alpha particles make a significant stabilizing contribution to the potential energy of the m = 1 internal kink instability. The scaling of sawtooth period with tritium concentration implies a dependence on mean ion mass: possible reasons for such a dependence are considered. Calculations of the kinetic fast particle contribution to the kink potential energy indicate that the rise in sawtooth period with tritium concentration is likely to have arisen from two effects: an increase in the slowing down time of the beam ions, which in these plasmas is proportional to the mean ion mass, and an increase in the proportion of beam ions at the full injection energy.
    Nuclear Fusion 03/2002; 42(3):281. · 2.73 Impact Factor
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    ABSTRACT: The JET experimental campaign has focused on studies in support of the ITER physics basis. An overview of the results obtained is given for the reference ELMy H mode and advanced scenarios, which in JET are based on internal transport barriers. JET studies for ELMy H mode have been instrumental in the definition of ITER FEAT. Positive elongation and current scaling in the ITER scaling law have been confirmed, but the observed density scaling fits a two term (core and edge) model better. Significant progress in neoclassical tearing mode limits has been made showing that ITER operation with q(95) around 3.3 seems to be optimized. Effective helium pumping and divertor enrichment is found to be well within ITER requirements. Target asymmetries and hydrogen isotope retention are well simulated by modelling codes taking into account drift flows in the scrape-off plasmas. Striking improvements in fuelling effectiveness have been made with the new high field pellet launch facility. Good progress has been made on scenarios for achieving good confinement at high densities, both with radiation improved modes and with high field side pellets. Significant development of advanced scenarios, in view of their application to ITER, has been achieved. Progress towards integrated advanced scenarios is well developed with edge pressure control (impurity radiation). An access domain has been explored showing, in particular, that the power threshold increases with magnetic field but can be significantly reduced when lower hybrid current drive is used to produce target plasmas with negative shear. The role of ion pressure peaking on MHD has been well documented. Lack of sufficient additional heating power and interaction with the septum at high beta prevents assessment of the beta limits (steady plasmas achieved with beta (N) up to 2.6). Plasmas with a non-inductive current (I-NI/I-p = 60%), well aligned with the plasma current, high beta and good confinement have also been obtained.
    01/2001;

Publication Stats

373 Citations
107.81 Total Impact Points

Institutions

  • 2002
    • European Commission
      Bruxelles, Brussels Capital Region, Belgium
  • 1999
    • University of Toronto
      Toronto, Ontario, Canada
  • 1998
    • General Atomics
      San Diego, California, United States
  • 1991
    • University of Maryland, College Park
      Maryland, United States