| 1. |
- Angioni, C., et al.
(författare)
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Gyrokinetic study of turbulent convection of heavy impurities in tokamak plasmas at comparable ion and electron heat fluxes
- 2017
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Ingår i: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 57:2
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Tidskriftsartikel (refereegranskat)abstract
- In tokamaks, the role of turbulent transport of heavy impurities, relative to that of neoclassical transport, increases with increasing size of the plasma, as clarified by means of general scalings, which use the ITER standard scenario parameters as reference, and by actual results from a selection of discharges from ASDEX Upgrade and JET. This motivates the theoretical investigation of the properties of the turbulent convection of heavy impurities by nonlinear gyrokinetic simulations in the experimentally relevant conditions of comparable ion and electron heat fluxes. These conditions also correspond to an intermediate regime between dominant ion temperature gradient turbulence and trapped electron mode turbulence. At moderate plasma toroidal rotation, the turbulent convection of heavy impurities, computed with nonlinear gyrokinetic simulations, is found to be directed outward, in contrast to that obtained by quasi-linear calculations based on the most unstable linear mode, which is directed inward. In this mixed turbulence regime, with comparable electron and ion heat fluxes, the nonlinear results of the impurity transport can be explained by the coexistence of both ion temperature gradient and trapped electron modes in the turbulent state, both contributing to the turbulent convection and diffusion of the impurity. The impact of toroidal rotation on the turbulent convection is also clarified.
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| 2. |
- Batistoni, P., et al.
(författare)
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Technical preparations for the in-vessel 14 MeV neutron calibration at JET
- 2017
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Ingår i: Fusion engineering and design. - : ELSEVIER SCIENCE SA. - 0920-3796 .- 1873-7196. ; 117, s. 107-114
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Tidskriftsartikel (refereegranskat)abstract
- The power output of fusion devices is measured from their neutron yields which relate directly to the fusion yield. In this paper we describe the devices and methods that have been prepared to perform a new in situ 14 MeV neutron calibration at JET in view of the new DT campaign planned at JET in the next years. The target accuracy of this calibration is 10% as required for ITER, where a precise neutron yield measurement is important, e.g., for tritium accountancy. In this paper, the constraints and early decisions which defined the main calibration approach are discussed, e.g., the choice of 14 MeV neutron source and the deployment method. The physics preparations, source issues, safety and engineering aspects required to calibrate directly the JET neutron detectors are also discussed. The existing JET remote-handling system will be used to deploy the neutron source inside the JET vessel. For this purpose, compatible tooling and systems necessary to ensure safe and efficient deployment have been developed. The scientific programme of the preparatory phase is devoted to fully characterizing the selected 14 MeV neutron generator to be used as the calibrating source, obtain a better understanding of the limitations of the calibration, optimise the measurements and other provisions, and to provide corrections for perturbing factors (e.g., anisotropy of the neutron generator, neutron energy spectrum dependence on emission angle). Much of this work has been based on an extensive programme of Monte-Carlo calculations which provide support and guidance in developing the calibration strategy.
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| 3. |
- Bernert, M., et al.
(författare)
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Power exhaust by SOL and pedestal radiation at ASDEX Upgrade and JET
- 2017
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 12, s. 111-118
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Tidskriftsartikel (refereegranskat)abstract
- Future fusion reactors require a safe, steady state divertor operation. A possible solution for the power exhaust challenge is the detached divertor operation in scenarios with high radiated power fractions. The radiation can be increased by seeding impurities, such as N for dominant scrape-off-layer radiation, Ne or Ar for SOL and pedestal radiation and Kr for dominant core radiation. Recent experiments on two of the all-metal tokamaks, ASDEX Upgrade (AUG) and JET, demonstrate operation with high radiated power fractions and a fully-detached divertor by N, Ne or Kr seeding with a conventional divertor in a vertical target geometry. For both devices similar observations can be made. In the scenarios with the highest radiated power fraction, the dominant radiation originates from the confined region, in the case of N and Ne seeding concentrated in a region close to the X-point. Applying these seed impurities for highly radiative scenarios impacts local plasma parameters and alters the impurity transport in the pedestal region. Thus, plasma confinement and stability can be affected. A proper understanding of the effects by these impurities is required in order to predict the applicability of such scenarios for future devices.
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| 4. |
- Binda, Federico, 1987-, et al.
(författare)
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Calculation of the profile-dependent neutron backscatter matrix for the JET neutron camera system
- 2017
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Ingår i: Fusion engineering and design. - : ELSEVIER SCIENCE SA. - 0920-3796 .- 1873-7196. ; 123, s. 865-868
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the dependence of the backscatter component of the neutron spectrum on the emissivity profile. We did so for the JET neutron camera system, by calculating a profile-dependent backscatter matrix for each of the 19 camera channels using a MCNP model of the JET tokamak. We found that, when using a low minimum energy for the summation of the counts in the neutron pulse height spectrum, the backscatter contribution can depend significantly on the emissivity profile. The maximum variation in the backscatter level was 24% (8.0% when compared to the total emission). This effect needs to be considered when a correction for the backscatter contribution is applied to the measured profile. (C) 2017 The Authors. Published by Elsevier B.V.
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| 5. |
- Bisoffi, Andrea, et al.
(författare)
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Hybrid cancellation of ripple disturbances arising in AC/DC converters
- 2017
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Ingår i: Automatica. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0005-1098 .- 1873-2836. ; 77, s. 344-352
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Tidskriftsartikel (refereegranskat)abstract
- In AC/DC converters, a peculiar periodic nonsmooth waveform arises, the so-called ripple. In this paper we propose a novel model that captures this nonsmoothness by means of a hybrid dynamical system performing state jumps at certain switching instants, and we illustrate its properties with reference to a three phase diode bridge rectifier. As the ripple corrupts an underlying desirable signal, we propound two observer schemes ensuring asymptotic estimation of the ripple, the first with and the second without knowledge of the switching instants. Our theoretical developments are well placed in the context of recent techniques for hybrid regulation and constitute a contribution especially for our second observer, where the switching instants are estimated. Once asymptotic estimation of the ripple is achieved, the ripple can be conveniently canceled from the desirable signal, and thanks to the inherent robustness properties of the proposed hybrid formulation, the two observer schemes require only that the desirable signal is slowly time varying compared to the ripple. Exploiting this fact, we illustrate the effectiveness of our second hybrid observation law on experimental data collected from the Joint European Torus tokamak.
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| 6. |
- Boltruczyk, G., et al.
(författare)
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Development of MPPC-based detectors for high count rate DT campaigns at JET
- 2017
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Ingår i: Fusion engineering and design. - : ELSEVIER SCIENCE SA. - 0920-3796 .- 1873-7196. ; 123, s. 940-944
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Tidskriftsartikel (refereegranskat)abstract
- The products of fusion reactions at JET are measured using different diagnostic techniques. One of the methods is based on measurements of gamma-rays, originating from reactions between fast ions and plasma impurities. During the forthcoming deuterium-tritium (DT) campaign a particular attention will be paid to 4.44 MeV gamma-rays emitted in the Be-9(alpha,n gamma)C-12 reaction. Gamma-ray detectors foreseen for measurements in DT campaigns have to be able to register spectra at high count rates, up to approximately 500 kHz. For the Gamma-ray Camera at JET a new setup will be based on scintillators with a short decay time, e.g., CeBr3, and a multi-pixel photon counter (MPPC). We present two methods of output signal shortening in modules based on MPPC. A short detector output signal is necessary in order to minimize the number of pile up events at high count rates. One method uses a passive RC circuit with a pole zero cancellation, whereas an active transimpedance amplifier is used in the other one. Due to the strong dependence of MPPC properties on temperature variation, a special device MTCD@NCBJ was designed and produced to stabilize the gain in MPPC-based scintillation detectors. We show that this device guarantees stable working conditions.
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| 7. |
- Borodkina, I., et al.
(författare)
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An analytical expression for ion velocities at the wall including the sheath electric field and surface biasing for erosion modeling at JET ILW
- 2017
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 12, s. 341-345
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Tidskriftsartikel (refereegranskat)abstract
- For simulation of plasma-facing component erosion in fusion experiments, an analytical expression for the ion velocity just before the surface impact including the local electric field and an optional surface biasing effect is suggested. Energy and angular impact distributions and the resulting effective sputtering yields were produced for several experimental scenarios at JET ILW mostly involving PFCs exposed to an oblique magnetic field. The analytic solution has been applied as an improvement to earlier ERO modelling of localized, Be outer limiter, RF-enhanced erosion, modulated by toggling of a remote, however magnetically connected ICRH antenna. The effective W sputtering yields due to D and Be ion impact in Type-I and Type-III ELMs and inter-ELM conditions were also estimated using the analytical approach and benchmarked by spectroscopy. The intra-ELM W sputtering flux increases almost 10 times in comparison to the inter-ELM flux.
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| 8. |
- Chankina, A. V., et al.
(författare)
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Possible influence of near SOL plasma on the H-mode power threshold
- 2017
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Ingår i: Nuclear Materials and Energy. - : Elsevier. - 2352-1791. ; 12, s. 273-277
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Tidskriftsartikel (refereegranskat)abstract
- A strong effect of divertor configuration on the threshold power for the L-H transition (P-LH) was observed in recent JET experiments in the new ITER-like Wall (ILW) [1-3]. Following a series of EDGE2D-EIRENE code simulations with Be impurity and drifts a possible mechanism for the P-LH variation with the divertor geometry is proposed. Both experiment and code simulations show that in the configuration with lower neutral recycling near the outer strike point (OSP), electron temperature (T-e) peaks near the OSP prior to the L-H transition, while in the configuration with higher OSP recycling T-e peaks further out in the scrape-offlayer (SOL) and the plasma stays in the L-mode at the same input power. Code results show large positive radial electric field (E-r) in the near SOL under lower recycling conditions leading to a large E x B shear across the separatrix which may trigger earlier (at lower input power) edge turbulence suppression and lower P-LH. Suppressed T-e's at OSP in configurations with strike points on vertical targets (VT) were observed earlier and explained by a geometrical effect of neutral recycling near this particular position, whereas in configurations with strike points on horizontal targets (HT) the OSP appears to be more open for neutrals (see e.g. review paper [4]).
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| 9. |
- Chiariello, Andrea Gaetano, et al.
(författare)
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A 3D electromagnetic model of the iron core in JET
- 2017
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Ingår i: Fusion engineering and design. - : ELSEVIER SCIENCE SA. - 0920-3796 .- 1873-7196. ; 123, s. 527-531
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Tidskriftsartikel (refereegranskat)abstract
- The Magnet and Power Supplies system in JET includes a ferromagnetic core able to increase the transformer effect by improving the magnetic coupling with the plasma. The iron configuration is based on an inner cylindrical core and eight returning limbs; the ferromagnetic circuit is designed in such a way that the inner column saturates during standard operations [1]. The modelling of the magnetic circuit is a critical issue because of its impact on several applications, including equilibrium and reconstruction analysis required for control applications. The most used model in present applications is based on Equivalent Currents (ECs) placed on the iron boundary together with additional specific constraints, in a 2D axisymmetric frame. The (circular) ECs are chosen, by using the available magnetic measurements, to best represent the magnetic polarization effect [1]. Due to the axisymmetric assumption such approach is not well suited to deal with significant 3D effects, e.g. arising in operations with Error Field Correction Coils (EFCC). In this paper a new methodology is proposed, based on a set of 3D-shaped ECs and able to better model the actual 3D magnetization giving rise to a linear system to be solved. According to a well assessed approach [2], the 3D shape of ECs is represented by a set of elementary sources. The methodology has been successfully validated in a number of JET dry-run experiments where 3D effects are generated by EFCC currents. The new procedure has been designed to be easily coupled with equilibrium or reconstruction codes such as EFIT/V3FIT. The proposed model resulted to be very effective in representing 3D iron magnetization, especially if compared with typical 2D models.
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| 10. |
- Conroy, Sean, et al.
(författare)
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The global build-up to intrinsic ELM bursts and comparison with pellet triggered ELMs seen in JET
- 2017
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Ingår i: Nuclear Fusion. - : IOP PUBLISHING LTD. - 0029-5515 .- 1741-4326. ; 57:2
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Tidskriftsartikel (refereegranskat)abstract
- We focus on JET plasmas in which ELMs are triggered by pellets in the presence of ELMs which occur naturally. We perform direct time domain analysis of signals from fast radial field coils and toroidal full flux azimuthal loops. These toroidally integrating signals provide simultaneous high time resolution measurements of global plasma dynamics and its coupling to the control system. We examine the time dynamics of these signals in plasmas where pellet injection is used to trigger ELMs in the presence of naturally occurring ELMs. Pellets whose size and speed are intended to provide maximum local perturbation for ELM triggering are launched at pre-programmed times, without correlation to the occurrence times of intrinsic ELMs. Pellet rates were sufficiently low to prevent sustained changes of the underlying plasma conditions and natural ELM behaviour. We find a global signature of the build-up to natural ELMs in the temporal analytic phase of both the full flux loops and fast radial field coil signals. Before a natural ELM, the signal phases align to the same value on a similar to 2-5 ms timescale. This global build up to a natural ELM occurs whilst the amplitude of the full flux loop and fast radial field coil signals are at their background value: it precedes the response seen in these signals to the onset of ELMing. In contrast these signals do not clearly phase align before the ELM for ELMs which are the first to occur following pellet injection. This provides a direct test that can distinguish when an ELM is triggered by a pellet as opposed to occurring naturally. It further supports the idea [1-4] of a global build up phase that precedes natural ELMs; pellets can trigger ELMs even when the signal phase is at a value when a natural ELM is unlikely to occur.
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