Publications (35)70.43 Total impact
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Article: Modeling of energy confinement improvement in high power JET discharges with neon seeding
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ABSTRACT: In order to reduce the heat load to the wall, in particular in the presence of a metallic wall, the radiated fraction is increased by means of impurity seeding. This paper aims at investigating qualitatively the balance between a degradation of the edge confinement and a potential reduction of the core turbulent transport due to the increase of effective charge, Z eff . The pedestal degradation due to the Ne seeding is taken as input. The impact of Z eff and radiative loss on the heat transport are modeled with the quasi-linear 1D fluid code RITM. The input parameters are taken from a series of Ne seeded discharges conducted in JET. Discharges with D, D + Ne and Ne only fueling are qualitatively analyzed. The model recovers the experimentally observed fact that, while the confinement is degraded from the D to the D + Ne seeded plasma, it is improved from the D + Ne to the Ne only seeded plasma. (Some figures may appear in colour only in the online journal)Plasma Physics and Controlled Fusion 01/2012; 54(1):15004-11. · 2.42 Impact Factor -
Article: Influence of JET Divertor Geometry on the H‐Mode Power Threshold
Beiträge aus der Plasmaphysik 05/2010; 50(3‐5):356 - 361. -
Article: The influence of resonant magnetic perturbations on edge transport in limiter H-mode plasmas in TEXTOR
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ABSTRACT: a b s t r a c t In this contribution, we report on experimental results on edge transport in limiter H-mode plasmas in TEXTOR under the influence of the Dynamic Ergodic Divertor (DED). These plasmas are characterized by a pedestal structure mainly visible in the electron density, resulting in increased electron pressure gradi-ents of up to 30 kPa/m over a pedestal width of 25 mm at high pedestal collisionalities ðm e à ¼ 1 À 10Þ, and with high frequency ELMs in the range of 300–1500 Hz. Under the influence of DED the pedestal pressure is gradually reduced and completely collapses to L-mode when the laminar zone extends all the way across the pedestal width. Toroidal plasma rotation is maintained at H-mode levels by the torque introduced by DED in the stochastic region. The perturbed magnetic topology has been optimized to access conditions with a density pump-out which are strongly governed by wall pumping capabilities in TEXTOR.Journal of Nuclear Materials 06/2009; 390-391:351. · 2.05 Impact Factor -
Article: Edge transport barrier formation studies on JET with the JETTO code
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ABSTRACT: The 1.5D transport code JETTO (Cenacchi G. and Taroni A. 1988 Rapporto ENEA RT/TIB (88)5) has been applied to model the transition from the low (L) to the high confinement mode (H-mode) in the JET tokamak. Computed values of the critical power, P th , required for the L–H transition on JET are directly compared with experiment (Andrew Y. et al 2006 Plasma Phys. Control. Fusion 48 479) across line averaged density and magnetic field scans. Reasonable agreement is found between computations and experiment across all densities considered, including low density discharges, where P th increases with decreasing density. The minimum of P th (n e) dependence is explained by the enhanced contribution of the particle convection to heat losses at the edge. Higher convective losses result in lower temperature and its gradient, and therefore more power is required for the L–H transition. Computations performed for JET discharges with varied magnetic field show a rough agreement with the experiment, nonetheless both computed and experimental power thresholds are substantially higher than the inter-machine scaling predictions (ITPA H-mode Power Threshold Database Working Group Presented by Takizuka T.Nuclear Fusion 08/2008; 48(8):085006. · 4.09 Impact Factor -
Article: Modelling of pedestal transport during ELM suppression by external magnetic field perturbations
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ABSTRACT: Particle and energy transport in the edge transport barrier is analysed in the presence of magnetic field perturbations from external resonant coils successfully used recently for the mitigation of type I edge localized modes (ELMs). The modification of transport due to charged particle and heat flows along perturbed field lines in a small region near the separatrix, spanning from 2% to 4% of the total poloidal flux, where complete stochastization is provided by the overlap of the main magnetic islands, is taken into account. The observed reduction of the density in plasmas of low collisionality is explained by the generation of charged particle flows along perturbed field lines, the increase in the electron and ion temperatures in the barrier—by the reduction of the perpendicular neoclassical transport with decreasing density and non-locality of parallel heat transport. On the basis of the heat flux limit concept in a deeply collisionless regime, the parallel thermal conductivities are taken to be 17 times smaller for electrons and 7 times smaller for ions than from a standard free-streaming estimate. The model elaborated before is developed further by taking into account the radial variation of the inclination angle of stochastic field lines and convective energy losses including the acceleration of ions by the pressure gradient and ambipolar electric field. It is demonstrated that convection of parallel kinetic energy of ions gives greater losses than parallel thermal conduction in the outer 50% of the stochastic layer and its inclusion improves the agreement with experimental results. This modelling is performed by assuming in agreement with observations that the influx of recycling neutrals through the separatrix is not reduced with I-coils compared with its level between ELMs before the mitigation stage. By trying to match experimental profiles with this influx decrease, some enhanced thermal losses of another nature than that considered here are needed in order to mitigate the drop in the perpendicular thermal conductivities for the assumed density scaling. The impact of the neutral particle influx increase by gas puffing applied in order to restore the plasma density is investigated.Nuclear Fusion 02/2008; 48(2):24006-9. · 4.09 Impact Factor -
Article: Modelling of radiative power exhaust by sputtered and seeded impurities in fusion reactors with carbon and molybdenum target plates
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ABSTRACT: Argon and neon seeded ITER discharges are numerically simulated by coupling a 1D multifluid model for the plasma core with a 2D model for the SOL-divertor region. The model is fully self-consistent with respect to both the effects of impurities on the α-power level and the interaction between seeded and intrinsic impurities. This interaction leads to a significant change in the intrinsic impurity fluxes, and it is found to be essential for a correct evaluation of the average power to the target plates. Even though carbon and tungsten are the real candidates for the ITER target plates, we have compared carbon and molybdenum plates for two ITER inductive scenarios, due to the uncertainties in the tungsten atomic data. In general, the integrated edge-core scenarios with impurity seeding are more favourable in the case of carbon than of molybdenum plates. However, a high density/low confinement scenario with Ne seeding is found for which the requirements in terms of power to the plate and of power through the separatrix are also fulfilled in the case of Mo. The interplay mechanism among different impurities also holds for He ash resulting in a rather weak dependence of the power amplification factor on He confinement.Nuclear Fusion 11/2007; 47(11):1625. · 4.09 Impact Factor -
Article: A model for particle and heat losses by type I edge localized modes
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ABSTRACT: A model to estimate the particle and energy losses caused in tokamaks by type I edge localized modes (ELMs) is proposed. This model is based on the assumption that the increase in transport by ELM is due to flows along magnetic field lines perturbed by ballooning–peeling MHD modes. The model reproduces well the experimentally found variation of losses with the plasma collisionality ν * , namely, the weak dependence of the particle loss and significant reduction of the energy loss with increasing ν * . It is argued that the electron parallel heat conductivity is dominating in the energy loss at not very large ν * .Plasma Physics and Controlled Fusion 04/2007; 49(4):395-403. · 2.42 Impact Factor -
Article: Comparison of impurity transport model with measurements of He-like spectra of argon at the tokamak TEXTOR
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ABSTRACT: The measured K α spectra of He-like argon were used to determine the densities of H-, He-and Li-like argon ions in ohmically heated plasmas in the tokamak TEXTOR. The obtained ratio of the densities for the Li-and He-like ions shows a significant deviation from coronal equilibrium. In order to interpret these results numerical modelling of the impurity transport was performed by the code RITM with neoclassical and anomalous contributions to the transport coefficients. The measurements show that the transport in the plasma core is much higher than that predicted from the model and has a strong correlation with the line averaged density. This paper also demonstrates the unique ability of the He-like spectra to separate the transport properties and the charge-exchange processes in the plasma.Plasma Physics and Controlled Fusion 11/2006; 48(11):1633-1646. · 2.42 Impact Factor -
Article: Modelling and Comparison with Experiment of Radial Profiles in a Tokamak with Magnetic Field Stochastization
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ABSTRACT: A deliberate stochastization of the magnetic field in tokamaks with Ergodic Divertors (ED) modi.es essentially the transport properties at the plasma edge by permitting a radial transfer of particles and heat along stochastic field lines and affecting anomalous transport perpendicular to them. A model for the effective transport coefficients in a stochastic magnetic field, taking into account both parallel flows and perpendicular transfer of anomalous nature, has been incorporated into the 1.5D transport code RITM for self-consistent calculations of the radial profiles of plasma parameters. Computations have been performed for the tokamak TEXTOR to simulate phases in an ohmic shot without and with the Dynamic Ergodic Divertor (DED). Results show that the increase of the outward convective transport in the stochastic boundary layer leads to a drop of the line average density. The experimentally observed density variation is reproduced by adjusting the amplitude of the stochastization characteristics with their radial dependence predicted by the field line mapping. The decrease of the diamagnetic energy after DED activation is in qualitative agreement with the experimental data from the diamagnetic loop measurements. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Beiträge aus der Plasmaphysik 08/2006; 46(7‐9):704 - 710. -
Article: RITM‐Code Modelling of Plasmas with Edge Transport Barrier
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ABSTRACT: Conditions for the formation of the edge transport barrier (ETB) in tokamaks are investigated by means of onedimensional transport modeling performed for the characteristic parameter range of the TEXTOR tokamak. The computations predict the formation of the ETB at the heating power given by the multi-machine scaling if the fraction of convective heat losses from the plasma does not exceed 50%. An increase of the amount of heat lost through convection above this critical value shifts the formation of ETB to a power several times above the level given by the scaling. For given plasma parameters, the ratio of the conductive to convective heat losses at the plasma edge is determined by the penetration of neutrals. By switching from a divertor to a limiter configuration when the distance between the LCMS and neutralizing plates decreases, this ratio increases due to the higher fraction of particles ionized inside the last closed magnetic surface (LCMS). This can be the reason for the higher H-mode power threshold in limiter tokamaks. First experimental results obtained in TEXTOR demonstrate a good agreement of the power required for ETB formation with the value calculated with 1. 5D transport code RITM prior to the experiment. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Beiträge aus der Plasmaphysik 08/2006; 46(7‐9):685 - 691. -
Article: On the difference of H-mode power threshold in divertor and limiter tokamaks
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ABSTRACT: The difference in the H-mode power threshold in divertor and limiter configurations is numerically investigated by analysing the effect of boundary conditions imposed on the last closed magnetic surface (LCMS) and given by prescribed density and temperature e-folding lengths, δ n and δ T , respectively. It is demonstrated that the variation of δ n and δ T significantly affects the H-mode power threshold. This is explained by the change in the balance between conductive and convective heat losses at the edge. For the ratio δ n /δ T large enough, when the convective loss does not exceed 45% of the total power, the threshold agrees well with the experimental multi-machine scaling for divertor tokamaks. With reduction in δ n /δ T and increase in convective loss above this critical level, the power threshold significantly exceeds the scaling, in agreement with observations on different limiter tokamaks. By considering the power and particle balances in the scrape-off layer it is shown that the ratio δ n /δ T is controlled by the distance which recycling neutrals pass before entering the confined plasma and which is normally much larger in divertor machines than in the limiter ones. The calculations for the limiter tokamak TEXTOR have predicted the experimentally found conditions for the L–H transition in advance.Plasma Physics and Controlled Fusion 05/2006; 48(5A):309-317. · 2.42 Impact Factor -
Article: Influence of the boundary conditions on the H-mode power threshold
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ABSTRACT: The effect of boundary conditions at the last closed magnetic surface LCMS on the formation of the edge transport barrier ETB in tokamaks is investigated by one-dimensional transport calculations for the radial profiles of plasma parameters. For a given heating power the transition from the low confinement mode L mode to the high confinement mode H mode can be triggered by increasing the density e-folding length, n , or reducing the temperature e-folding length, T , at the LCMS. This is explained by the decrease of heat losses from the confined plasma with the convection of charged particles and changeover to the case where losses are mostly due to heat conduction. In such a case, corresponding to a divertor configuration, the computed power threshold for the L-to H-mode transition L-H transition is in a good agreement with the experimental multimachine scaling. © 2006 American Institute of Physics.Physics of Plasmas 03/2006; 13(3). · 2.15 Impact Factor -
Article: RITM-Code Modelling of Plasmas with Edge Transport Barrier partner in Trilateral Euregio Cluster
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ABSTRACT: Conditions for the formation of the edge transport barrier (ETB) in tokamaks are investigated by means of one-dimensional transport modeling performed for the characteristic parameter range of the TEXTOR tokamak. The computations predict the formation of the ETB at the heating power given by the multi-machine scaling if the fraction of convective heat losses from the plasma does not exceed 50%. An increase of the amount of heat lost through convection above this critical value shifts the formation of ETB to a power several times above the level given by the scaling. For given plasma parameters, the ratio of the conductive to convective heat losses at the plasma edge is determined by the penetration of neutrals. By switching from a divertor to a limiter configuration when the distance between the LCMS and neutralizing plates decreases, this ratio increases due to the higher fraction of particles ionized inside the last closed magnetic surface (LCMS). This can be the reason for the higher H-mode power threshold in limiter tokamaks. First experimental results obtained in TEXTOR demonstrate a good agreement of the power required for ETB formation with the value calculated with 1.5D transport code RITM prior to the experiment.Contributions to Plasma Physics 01/2006; 46(7-9):685-691. · 1.11 Impact Factor -
Article: Modelling and Comparison with Experiment of Radial Profiles in a Tokamak with Magnetic Field Stochastization
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ABSTRACT: A deliberate stochastization of the magnetic field in tokamaks with Ergodic Divertors (ED) modifies essentially the transport properties at the plasma edge by permitting a radial transfer of particles and heat along stochastic field lines and affecting anomalous transport perpendicular to them. A model for the effective transport co-efficients in a stochastic magnetic field, taking into account both parallel flows and perpendicular transfer of anomalous nature, has been incorporated into the 1.5D transport code RITM for self-consistent calculations of the radial profiles of plasma parameters. Computations have been performed for the tokamak TEXTOR to simulate phases in an ohmic shot without and with the Dynamic Ergodic Divertor (DED). Results show that the increase of the outward convective transport in the stochastic boundary layer leads to a drop of the line average density. The experimentally observed density variation is reproduced by adjusting the amplitude of the stochastization characteristics with their radial dependence predicted by the field line mapping. The decrease of the diamagnetic energy after DED activation is in qualitative agreement with the experimental data from the diamagnetic loop measurements.Contributions to Plasma Physics 01/2006; 46(7-9):704. · 1.11 Impact Factor -
Article: Local effects of gas fuelling and their impact on transport processes in the plasma edge of the tokamak TEXTOR
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ABSTRACT: Deuterium fuelling through a carbon test limiter has been applied to maximize the plasma density in the Radiative Improved Mode. The impact of the fuelling on the local plasma edge properties has been investigated, by analyzing the spectral emission of both deuterium atoms and molecules, which indicates the creation of a cold and dense plasma cloud with a local electron density at the last closed flux surface up to 8 · 10 19 m À3 , about 4 times higher than the electron density far away from the puffing location at the same plasma radius. The local electron temperature decreases to less than 10 eV. The experimental data can be reproduced by a model for the development of the cold plasma cloud and the critical fuelling rate to initiate the process based on the heat balance in the cloud. The correlation of the resulting local perturbation with the global confinement properties is discussed.Journal of Nuclear Materials 03/2005; 337-339:515-519. · 2.05 Impact Factor -
Article: Toroidal plasma rotation induced by the dynamic ergodic divertor in the TEXTOR tokamak.
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ABSTRACT: The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.Physical Review Letters 02/2005; 94(1):015003. · 7.37 Impact Factor -
Article: Predictive modelling of L and H confinement modes and edge pedestal characteristics
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ABSTRACT: The results of predictive self-consistent modelling of plasma parameters in low (L) and high (H) confinement modes by the one-dimensional transport code RITM, with particular emphasis on the properties of the edge transport barrier, are presented and discussed. The same transport model is used under both L-and H-mode conditions and includes contributions from ion temperature gradient (ITG), trapped electron, drift Alfvén (DA) and drift resistive ballooning instabilities described in the fluid approximation. The computations predict the formation of the edge transport barrier at a high enough heating power due to stabilization of ITG and DA modes, dominating the edge transport in the L-mode, through the effects of the density gradient and the pressure gradient at low collisionality, respectively. The calculated radial profiles and scalings for pedestal and confinement characteristics are compared with measurements on JET, DIII-D and JT-60U tokamaks.Nuclear Fusion 01/2005; 45:468-476. · 4.09 Impact Factor -
Article: Toroidal Plasma Rotation Induced by the Dynamic Ergodic Divertor in the TEXTOR Tokamak
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ABSTRACT: The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m=n 3=1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m=n 2=1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.Physical Review Letters 01/2005; 94:015003. · 7.37 Impact Factor -
Article: Nature of the isotope effect on transport in tokamaks.
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ABSTRACT: The reduction of energy and particle losses with the increasing mass of the hydrogen isotope is more pronounced under conditions of improved confinement when the dominant ion temperature gradient instability is suppressed and other channels of anomalous transport are of importance. In this Letter, we reconsider the dissipative trapped electron (DTE) instability by taking into account finite Larmor radius effects in the analysis of the ion response to perturbations. By applying the improved mixing length approximation in order to estimate the transport coefficients, it is demonstrated that DTE contribution is intrinsically dependent on the isotope mass and provides a plausible explanation for the isotope effect. Contrary to the common belief, it is shown that the DTE turbulence may be of importance for reactor plasmas of low collisionality.Physical Review Letters 06/2004; 92(21):215001. · 7.37 Impact Factor -
Article: Predictive modelling of the impact of argon injection on H-mode plasmas in JET with the RITM code
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ABSTRACT: Self-consistent modelling of energy and particle transport of the plasma background and impurities has been performed with the code RITM for argon seeded high density H-mode plasmas in JET. The code can reproduce both the profiles in the plasma core and the structure of the edge pedestal. The impact of argon on core transport is found to be small; in particular, no significant change in confinement is observed in both experimental and modelling results. The same transport model, which has been used to reproduce density peaking in the radiative improved mode in TEXTOR, reveals a flat density profile in Ar seeded JET H-mode plasmas in agreement with the experimental observations. This behaviour is attributed to the rather flat profile of the safety factor in the bulk of H-mode discharges.Plasma Physics and Controlled Fusion 05/2004; 46(5A):241-247. · 2.42 Impact Factor
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2004–2010
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Forschungszentrum Jülich
Düren, North Rhine-Westphalia, Germany
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