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C F Maggi,
S Brezinsek,
M F Stamp,
S Griph,
P Heesterman,
C Hogben,
A Horton,
A Meigs,
C Morlock,
W Studholme,
K-D Zastrow
[show abstract]
[hide abstract]
ABSTRACT: In preparation for ITER, JET has been upgraded with a new ITER-like wall (ILW), whereby the main plasma facing components, previously of carbon, have been replaced by mainly Be in the main chamber and W in the divertor. As part of the many diagnostic enhancements, a new, survey, visible spectroscopy diagnostic has been installed for the characterization of the ILW. An array of eight lines-of-sight (LOS) view radially one of the two JET neutral beam shine through areas (W coated carbon fibre composite tiles) at the inner wall. In addition, one vertical LOS views the solid W tile at the outer divertor. The light emitted from the plasma is coupled to a series of compact overview spectrometers, with overall wavelength range of 380-960 nm and to one high resolution Echelle overview spectrometer covering the wavelength range 365-720 nm. The new survey diagnostic has been absolutely calibrated in situ by means of a radiometric light source placed inside the JET vessel in front of the whole optical path and operated by remote handling. The diagnostic is operated in every JET discharge, routinely monitoring photon fluxes from intrinsic and extrinsic impurities (e.g., Be, C, W, N, and Ne), molecules (e.g., BeD, D(2), ND) and main chamber and divertor recycling (typically Dα, Dβ, and Dγ). The paper presents a technical description of the diagnostic and first measurements during JET discharges.
The Review of scientific instruments 10/2012; 83(10):10D517. · 1.52 Impact Factor
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K D Lawson,
R Barnsley, C F Maggi,
S Tyrrell,
M Beldishevski,
J Brzozowski,
M Buckley,
G Cass,
T Elevant,
S Griph,
P Heesterman,
C Hogben,
M Jennison,
M F Stamp,
J Williams,
K-D Zastrow
[show abstract]
[hide abstract]
ABSTRACT: Enhancements to the JET poloidally scanning spectrometers are presented, which will aid the exploitation of the recently installed ITER-like wall in JET. They include the installation of visible filter∕photomultiplier tube assemblies and spectrometers and the replacement of large rotating mirrors in the JET vacuum with small oscillating mirrors outside. The upgrade has resulted in a more robust and reliable diagnostic than before, which is described. Drifts in the mirror angle reconstructed from quadrature encoder signals are found, a reference signal being required. The use of the small scanning mirrors necessitated the inclusion of focusing mirrors to maintain throughput into the vacuum ultraviolet spectrometers. The mirror design has taken account of the extreme sensitivity of the focusing to the grazing angle of incidence, an aspect of importance in the design of grazing incidence focusing components on future machines, such as ITER. The visible system has been absolutely calibrated using an in-vessel light source.
The Review of scientific instruments 10/2012; 83(10):10D536. · 1.52 Impact Factor
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V. Bobkov,
G. Arnoux,
S. Brezinsek,
J. W. Coenen,
L. Colas,
M. Clever,
A. Czarnecka,
F. Braun,
R. Dux,
A. Huber, [......],
D. Milanesio,
I. Monakhov,
R. Neu,
J.-M. Noterdaeme,
T. Puetterich,
F. Rimini,
G.J. van Rooij,
Sergienko G,
D. van Eester,
JET EFDA Contributors
20th International Conference on Plasma Surface Interaction in Fusion Devices, to be published in Journal of Nuclear Materials. 01/2012;
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M Groth,
S. Brezinsek,
P. Belo,
G. Corrigan,
D. Harting,
S. Iwesen,
M.N.A. Beurskens,
M. Brix,
M. Clever,
J. W. Coenen, [......], C. F. Maggi,
S. Marsen,
A.G. Meigs,
Sergienko G,
B. Sieglin,
C. Silva,
A. Sirinelli,
M. F. Stamp,
G. van Rooij,
JET EFDA Contributors
20th International Conference on Plasma Surface Interaction in Fusion Devices, to be published in Journal of Nuclear Materials. 01/2012;
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[show abstract]
[hide abstract]
ABSTRACT: A source describing the injection of fast ions due to the ionization of high-energy neutral beams has been added to the surface-averaged quasilinear Fokker–Planck code SSFPQL (Brambilla 1994 Nucl. Fusion 34 1121). For this purpose, the multiple-beam NBI code SINBAD (Feng et al 1995 Comput. Phys. Commun. 88 161) has been included as a module in SSFPQL, with the modifications required to handle arbitrary axisymmetric equilibria. Alternatively, the neutral beam injection (NBI) source can be built using the output of a Monte Carlo NBI code. We have also added a term describing losses of fast ions during thermalization, and a subroutine evaluating the neutron production rate by nuclear reactions. With these extensions, iterations between SSFPQL and the full-wave solver TORIC (Brambilla 1999 Plasma Phys. Control. Fusion 41 1) can now be used to investigate the strong interplay between NBI and ion cyclotron (IC) heating.By comparing the predicted and measured neutron production rates from D–D reactions in a discharge with combined NBI and IC heating in ASDEX Upgrade we obtain a plausible estimate of the importance of fast-ion losses (FILs), even if their cause cannot be identified. We find, however, that the plasma composition, in particular the presence of low Z impurities, plays a more critical role than FILs in limiting the efficiency of this heating scheme.
Nuclear Fusion 09/2011; 51(10):103034. · 4.09 Impact Factor
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R Neu,
M Balden,
V Bobkov,
R Dux,
O Gruber,
A Herrmann,
A Kallenbach,
M Kaufmann, C F Maggi,
H Maier, [......],
L Giannone,
S Gori,
S da Graca,
H Greuner,
A Gude,
S Günter,
G Haas,
J Harhausen,
B Heinemann,
N Hicks
Plasma Physics and Controlled Fusion 07/2011; · 2.42 Impact Factor
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A. Loarte,
A. Chankin,
S. Clement,
S. J. Davies,
K. Günther,
J. Lingertat, C. F. Maggi,
G. F. Matthews,
P. D. Morgan,
M. F. Stamp,
R. D. Monk,
H. Y. Guo,
A. Tabasso
[show abstract]
[hide abstract]
ABSTRACT: In this paper, results from the array of fixed Langmuir probes in the JET Mark I divertor are presented. During low density discharges there is good agreement between single and triple probes. At higher density, near the onset of divertor detachment, very low values of the electron to ion saturation current (even < 1) are observed which results in an overestimation of the electron temperature, particularly by the triple probe. Alternative methods of probe interpretation are applied and comparisons carried out with independent measurements. The consistency of probe measurements during H-mode discharges are also briefly discussed.
Beiträge aus der Plasmaphysik 11/2010; 36(S1):37 - 44.
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C.F. Maggi,
R.J. Groebner,
C. Angioni,
T. Hein,
L.D. Horton,
C. Konz,
A.W. Leonard,
C.C. Petty,
A.C.C. Sips,
P.B. Snyder,
J. Candy,
R.E. Waltz,
ASDEX Upgrade,
DIII-D Teams
[show abstract]
[hide abstract]
ABSTRACT: Pedestal and core confinement of hybrid discharges in ASDEX Upgrade (AUG) and DIII-D are studied in dedicated power scan experiments. The H98(y,2) confinement factor increases with total βN in both tokamaks and it is higher in DIII-D with higher δ plasma shape at a given βN. The pedestal beta, , increases linearly with total beta in AUG hybrid discharges, while it is roughly constant with βN at fixed shape in the DIII-D power scans. The confinement enhancement with power observed with respect to the IPB98(y,2) scaling is due to an increase in pedestal confinement in AUG hybrid discharges and to an increase in core confinement in the DIII-D hybrid power scans. The increase in pedestal pressure with power in AUG hybrid discharges is primarily due to an increase in the width of the edge transport barrier at constant pressure gradient. In the DIII-D discharges the widths of the Te and ne pedestals, and , are consistent with a scaling. In the AUG hybrid power scans a dependence of on βpol,PED cannot be excluded, while shows no dependence on βpol,PED In both machines increases with β. The maximum pedestal pressure achieved in the experiment prior to the onset of type I ELMs is consistent with predictions from ideal MHD; however, a physics model explaining the increase in the pedestal width with β is still missing. The increase in with β in the core of DIII-D is consistent with predictions by linear gyrokinetic simulations. In the plasma core, E × B shearing rate stabilization of the ITG modes is significant in both machines as beta is increased. Inclusion of electromagnetic effects in the gyrokinetic calculations provides additional stabilization at βN values achieved in the experiment. In AUG, proximity to the kinetic ballooning threshold and/or a stronger reduction in normalized ion heat flux with increasing input power are possible explanations for the constancy of at mid-radius as beta is increased.
Nuclear Fusion 01/2010; 50(2):025023. · 4.09 Impact Factor
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P.B. Snyder,
N. Aiba,
M. Beurskens,
R.J. Groebner,
L.D. Horton,
A.E. Hubbard,
J.W. Hughes,
G.T.A. Huysmans,
Y. Kamada,
A. Kirk, [......], C.F. Maggi,
R. Maingi,
T.H. Osborne,
N. Oyama,
A. Pankin,
S. Saarelma,
G. Saibene,
J.L. Terry,
H. Urano,
H.R. Wilson
[show abstract]
[hide abstract]
ABSTRACT: The pressure at the top of the edge transport barrier (or 'pedestal height') strongly impacts fusion performance, while large edge localized modes (ELMs), driven by the free energy in the pedestal region, can constrain material lifetimes. Accurately predicting the pedestal height and ELM behavior in ITER is an essential element of prediction and optimization of fusion performance. Investigation of intermediate wavelength MHD modes (or 'peeling–ballooning' modes) has led to an improved understanding of important constraints on the pedestal height and the mechanism for ELMs. The combination of high-resolution pedestal diagnostics, including substantial recent improvements, and a suite of highly efficient stability codes, has made edge stability analysis routine on several major tokamaks, contributing both to understanding, and to experimental planning and performance optimization. Here we present extensive comparisons of observations to predicted edge stability boundaries on several tokamaks, both for the standard (Type I) ELM regime, and for small ELM and ELM-free regimes. We further discuss a new predictive model for the pedestal height and width (EPED1), developed by self-consistently combining a simple width model with peeling–ballooning stability calculations. This model is tested against experimental measurements, and used in initial predictions of the pedestal height for ITER.
Nuclear Fusion 07/2009; 49(8):085035. · 4.09 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The toroidal rotation of H-mode plasmas in ASDEX Upgrade is studied in the outermost 5 cm of the confined plasma. The projection of the rotation velocity along the line of sight (approximately toroidal) is measured using charge exchange recombination spectroscopy, with a radial resolution of up to 3 mm and a temporal resolution of 1.9 ms. At about 1 cm inside the separatrix the rotation exhibits a local minimum. From there, the rotation in codirection increases towards the plasma center and towards the separatrix. The latter increase is the focus of this work. It is situated in the region of the edge transport barrier and amounts to 10-20 km/s. It is observed for D+, He2+, B5+, and C6+. The described rotation feature at the edge is not visible during an ELM crash and is probably connected to the occurrence of steep gradients in this plasma region.
Physical Review Letters 02/2009; 102(2):025001. · 7.37 Impact Factor
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R Neu,
M Balden,
V Bobkov,
R Dux,
O Gruber,
A Herrmann,
A Kallenbach,
M Kaufmann, C F Maggi,
H Maier, [......],
D Wagner,
M Wischmeier,
E Wolfrum,
E Würsching,
D Yadikin,
Q Yu,
D Zasche,
T Zehetbauer,
M Zilker,
H Zohm
[show abstract]
[hide abstract]
ABSTRACT: ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.
Plasma Physics and Controlled Fusion 11/2007; 49(12B):B59. · 2.42 Impact Factor
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A C C Sips,
G Tardini,
C B Forest,
O Gruber,
P J McCarthy,
A Gude,
L D Horton,
V Igochine,
O Kardaun, C F Maggi,
M Maraschek,
V Mertens,
R Neu,
A G Peeters,
G V Pereverzev,
A Stabler,
J Stober,
W Suttrop,
ASDEX Upgrade Team
Nuclear Fusion. 11/2007; 47(1):1485-1498.
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A.C.C. Sips,
G. Tardini,
C.B. Forest,
O. Gruber,
P.J. Mc Carthy,
A. Gude,
L.D. Horton,
V. Igochine,
O. Kardaun, C.F. Maggi,
M. Maraschek,
V. Mertens,
R. Neu,
A.G. Peeters,
G.V. Pereverzev,
A. Stäbler,
J. Stober,
W. Suttrop,
the ASDEX Upgrade Team
[show abstract]
[hide abstract]
ABSTRACT: Since 1998 ASDEX Upgrade has developed stationary H-modes that routinely obtain confinement enhancement factors H98(y,2) > 1 and normalized beta, βN = 2–3. These discharges are characterized by a q-profile with low magnetic shear in the centre and q(0) ~ 1. New results presented here concentrate on extending the operational range of these improved H-modes at ASDEX Upgrade and extrapolating the results to ITER. Discharges are obtained at high density, over a wide range of plasma collisionality and with a first wall predominantly covered by tungsten coated carbon tiles. The performance is optimized for q95 ranging from 3 to 5. At q95 ~ 3 real time control of βN is used and in some cases ECCD to suppress NTM activity at low βN ~ 2. For the extrapolation to ITER, the fusion power is calculated using the same thermal beta (βN,th) and kinetic profile shapes as obtained in ASDEX Upgrade and setting ne/nGW = 0.85. The fusion gain that could be obtained is evaluated using different confinement scaling expressions. The results indicate that improved H-modes are a candidate for an ITER hybrid scenario or could extend ITER operation beyond what is currently foreseen using standard H-modes.
Nuclear Fusion 10/2007; 47(11):1485. · 4.09 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: ICRF heated plasmas with low momentum input were carried out on ASDEX Upgrade with reversed ∇B drift direction with counter‐Ip beam injection blips for diagnostic. Corotation has been observed in both L‐ and H‐modes with no significant difference in the magnitude of Vϕ. The rotation profiles were found to be flat suggesting that the toroidal momentum could be driven from the plasma edge.
AIP Conference Proceedings. 09/2007; 933(1):103-106.
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G. Pautasso,
C.J. Fuchs,
O. Gruber, C.F. Maggi,
M. Maraschek,
T. Pütterich,
V. Rohde,
C. Wittmann,
E. Wolfrum,
P. Cierpka,
M. Beck,
the ASDEX Upgrade Team
[show abstract]
[hide abstract]
ABSTRACT: The massive injection of impurity gas into a plasma has been proved to reduce forces and localized thermal loads caused by disruptions in tokamaks. This mitigation system is routinely used on ASDEX Upgrade to shut down plasmas with a locked mode. The plasma response to impurity injection and the mechanism of reduction of the mechanical forces is discussed in the paper.
Nuclear Fusion 07/2007; 47(8):900. · 4.09 Impact Factor
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J. Stober,
A.C.C. Sips,
C. Angioni,
C.B. Forest,
O. Gruber,
J. Hobirk,
L.D. Horton, C.F. Maggi,
M. Maraschek,
P. Martin,
P.J. Mc Carthy,
V. Mertens,
Y.-S. Na,
M. Reich,
A. Stäbler,
G. Tardini,
H. Zohm,
the ASDEX Upgrade Team
[show abstract]
[hide abstract]
ABSTRACT: Recent studies at ASDEX Upgrade aim to further characterize and understand the physics of the improved H-mode scenario. The main focus is on the influence of the ramp-up phase of the plasma current and heating on energy confinement and MHD-activity during the subsequent flat-top phase. Depending on the ramp-up scenario two different stationary plasmas can be generated, which show different equilibrated current profiles, although external control parameters are the same in the flat-top phase. The difference of the current profiles in the flat-top phase seems to be due to different MHD-modes. These MHD-modes set in during relaxation of the current profile, which itself depends on the ramp-up scenario. Also the stored energy is different in the two cases as is the peaking of the temperature profiles.Three mechanisms seem to play a role in linking the observed changes in MHD-behaviour and current profile to the changes of the kinetic profiles: the increased transport due to the MHD-modes themselves, the variation of the ratio of magnetic shear s to safety factor q, which modifies the critical temperature gradient length for the onset of ion temperature gradients, and effects on the H-mode pedestal pressure.
Nuclear Fusion 07/2007; 47(8):728. · 4.09 Impact Factor
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C.F. Maggi,
R.J. Groebner,
N. Oyama,
R. Sartori,
L.D. Horton,
A.C.C. Sips,
W. Suttrop,
the ASDEX Upgrade Team,
A. Leonard,
T.C. Luce, [......],
Y. Kamada,
H. Urano,
the Team,
Y. Andrew,
C. Giroud,
E. Joffrin,
E. de la Luna,
EFDA-JET Contributors for the Pedestal,
Edge Physics,
the Steady State Operation Topical Groups of the ITPA
[show abstract]
[hide abstract]
ABSTRACT: Pedestal and global plasma parameters are compared in conventional ELMy H-modes and improved confinement discharges from ASDEX Upgrade (AUG), DIII-D, JET and JT-60U with varying net input power. Both electron and ion pedestal pressures are studied. The pedestal top pressure pPED increases moderately with power in all tokamaks, in broad agreement with the power dependence of the IPB98(y, 2) scaling. Higher pedestal pressures are observed in AUG improved H-modes and in JT-60U high βpol discharges at q95 ~ 6.5 and high triangularity. For all machines and all scenarios a robust correlation between the total and the pedestal thermal stored energy is observed, with the ratio of the two varying between ~0.3 and 0.5. However the relative importance of pedestal and core confinement varies from regime to regime. In AUG the confinement improvement with respect to the IPB98(y, 2) scaling is due to improved pedestal confinement in improved H-modes with early heating and to both improved pedestal and core confinement in improved H-modes with late heating. In DIII-D hybrid discharges the increase in confinement factor compared with conventional H-modes is due to improved confinement in the plasma core. JT-60U reversed shear H-modes have strong internal transport barriers and thus improved core performance. In all four tokamaks improved edge stability is correlated with increasing total βpol and H98(y,2) increases with pedestal βpol. The analysed multimachine data set supports a scaling expression for the pedestal stored energy derived under the assumption that the dominant loss term for the pedestal is by thermal conduction in the edge transport barrier region.
Nuclear Fusion 06/2007; 47(7):535. · 4.09 Impact Factor
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L. Vermare,
F. Ryter,
C. Angioni,
A.G. Peeters,
J. Stober,
R. Bilato,
L.D. Horton,
B. Kurzan, C.F. Maggi,
H. Meister,
J. Schirmer,
G. Tardini,
the ASDEX Upgrade Team
[show abstract]
[hide abstract]
ABSTRACT: First β scan experiments have been made in ASDEX Upgrade in H-mode plasmas with type-I ELMs. Two sets of discharges, performed in different density ranges, scan β between βN = (1.4–2.2) and βN = (1.4–2). Global analysis of these dedicated experiments exhibits a strong unfavourable β scaling as Bτth ∝ β−0.9. This tendency is confirmed by a local analysis which shows an increase in the thermal heat diffusivity with increasing β as χeff/B ∝ β0.65. These results are discussed and compared with studies made in DIII-D, JET and JT-60U.
Nuclear Fusion 05/2007; 47(5):490. · 4.09 Impact Factor
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G. Tardini,
J. Hobirk,
V.G. Igochine, C.F. Maggi,
P. Martin,
D. McCune,
A.G. Peeters,
A.C.C. Sips,
A. Stäbler,
J. Stober,
the ASDEX Upgrade Team
[show abstract]
[hide abstract]
ABSTRACT: Internal transport barriers (ITBs) in the ion channel in the tokamak ASDEX Upgrade allow for high energy confinement but collapse after only several energy confinement times. In this paper we show that in most cases the ITB phase is terminated clearly before the first ELM burst, thereby ruling out the ELMs as the main trigger of the ITB collapse. For the first time, the ITB formation and sustainment are found to be associated with a mechanism of transport suppression based on thermal ions dilution by the injected fast ions. Interestingly, such ITBs do not require reversed magnetic shear. The linear growth rate of the ion temperature gradient driven mode is computed as a function of the fast ion fraction with gyrokinetic stability analysis. Monte Carlo simulations predict the fast ion population to be above the gyrokinetic critical fraction in a region consistent with the experimental ITB width. The density threshold documented for the onset of ASDEX Upgrade ion ITBs is explained. The role of Ti/Te and of the plasma sheared rotation for ITB sustainment are analysed. The stabilization mechanism presented here is consistent with the observed ITB lifetime of the order of the beam slowing down time. A possible runaway mechanism leading to ITB collapse is described. Finally, the relevance of this particular ITB scheme for ITER is discussed.
Nuclear Fusion 03/2007; 47(4):280. · 4.09 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: This paper describes and further develops the understanding of toroidal momentum transport. Nonlinear gyro-kinetic simulations of the ion temperature gradient mode with adiabatic electrons show a strong coupling between the momentum and ion heat transport, with the ratio of the transport coefficients close to 1. Linear theory using a global description predicts an off-diagonal contribution to the momentum flux even in the absence of a radial electric field. The influence of the toroidal velocity gradient on the ion temperature profile is found to be small in the H-mode. These predictions are in qualitative agreement with experimental observations.
Plasma Physics and Controlled Fusion 11/2006; 48(12B):B413. · 2.42 Impact Factor
