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Nuclear Fusion 01/2013; 53(3):033006. · 4.09 Impact Factor
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T. Tala,
A. Salmi,
C. Angioni,
F.J. Casson,
G. Corrigan,
J. Ferreira,
C. Giroud,
P. Mantica,
V. Naulin,
A.G. Peeters,
W.M. Solomon,
D. Strintzi,
M. Tsalas,
T.W. Versloot, P.C. de Vries,
K.-D. Zastrow,
JET-EFDA contributors
[show abstract]
[hide abstract]
ABSTRACT: Several parametric scans have been performed to study momentum transport on JET. A neutral beam injection modulation technique has been applied to separate the diffusive and convective momentum transport terms. The magnitude of the inward momentum pinch depends strongly on the inverse density gradient length, with an experimental scaling for the pinch number being -Rvpinch/χ = 1.2R/Ln + 1.4. There is no dependence of the pinch number on collisionality, whereas the pinch seems to depend weakly on q-profile, the pinch number decreasing with increasing q. The Prandtl number was not found to depend either on R/Ln, collisionality or on q. The gyro-kinetic simulations show qualitatively similar dependence of the pinch number on R/Ln, but the dependence is weaker in the simulations. Gyro-kinetic simulations do not find any clear parametric dependence in the Prandtl number, in agreement with experiments, but the experimental values are larger than the simulated ones, in particular in L-mode plasmas. The extrapolation of these results to ITER illustrates that at large enough R/Ln > 2 the pinch number becomes large enough (>3–4) to make the rotation profile peaked, provided that the edge rotation is non-zero. And this rotation peaking can be achieved with small or even with no core torque source. The absolute value of the core rotation is still very challenging to predict partly due to the lack of the present knowledge of the rotation at the plasma edge, partly due to insufficient understanding of 3D effects like braking and partly due to the uncertainties in the extrapolation of the present momentum transport results to a larger device.
Nuclear Fusion 11/2011; 51(12):123002. · 4.09 Impact Factor
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T.W. Versloot,
R. Sartori,
F. Rimini, P.C. de Vries,
G. Saibene,
V. Parail,
M.N.A. Beurskens,
A. Boboc,
R. Budny,
K. Crombé, [......],
V. Kiptily,
T. Johnson,
P. Mantica,
M.-L. Mayoral,
D.C. McDonald,
I. Monakhov,
M.F.F. Nave,
I. Voitsekhovitch,
K.-D. Zastrow,
JET EFDA Contributors
[show abstract]
[hide abstract]
ABSTRACT: The experiment described in this paper is aimed at characterization of ELMy H-mode discharges with varying momentum input, rotation, power deposition profiles and ion to electron heating ratio obtained by varying the proportion between ion cyclotron (IC) and neutral beam (NB) heating. The motivation for the experiment was to verify if the basic confinement and transport properties of the baseline ITER H-mode are robust to these changes, and similar to those derived mostly from dominant NB heated H-modes. No significant difference in the density and temperature profiles or in the global confinement were found. Although ion temperature profiles were seen to be globally stiff, some variation of stiffness was obtained in the experiment by varying the deposition profiles, but not one that could significantly affect the profiles in terms of global confinement. This analysis shows the thermal plasma energy confinement enhancement factor to be independent of the heating mix, for the range of conditions explored. Moreover, the response of the global confinement to changes in density and power were also independent of heating mix, reflecting the changes in the pedestal, which is in agreement with globally stiff profiles. Consistently, the pedestal characteristics (pressure and width) and their dependences on global parameters such as density and power were the same during NB only or with predominant IC heating.
Nuclear Fusion 09/2011; 51(10):103033. · 4.09 Impact Factor
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X. Litaudon,
Y. Sakamoto, P.C. de Vries,
A. Salmi,
T. Tala,
C. Angioni,
S. Benkadda,
M.N.A. Beurskens,
C. Bourdelle,
M. Brix, [......],
T. Suzuki,
M. Takechi,
H. Takenaga,
T. Takizuka,
H. Urano,
I. Voitsekhovitch,
M. Yoshida,
ITPA Transport Group,
the team,
JET EFDA contributors
[show abstract]
[hide abstract]
ABSTRACT: The paper compares the transport properties of a set of dimensionless identity experiments performed between JET and JT-60U in the advanced tokamak regime with internal transport barrier, ITB. These International Tokamak Physics Activity, ITPA, joint experiments were carried out with the same plasma shape, toroidal magnetic field ripple and dimensionless profiles as close as possible during the ITB triggering phase in terms of safety factor, normalized Larmor radius, normalized collision frequency, thermal beta, ratio of ion to electron temperatures. Similarities in the ITB triggering mechanisms and sustainment were observed when a good match was achieved of the most relevant normalized profiles except the toroidal Mach number. Similar thermal ion transport levels in the two devices have been measured in either monotonic or non-monotonic q-profiles. In contrast, differences between JET and JT-60U were observed on the electron thermal and particle confinement in reversed magnetic shear configurations. It was found that the larger shear reversal in the very centre (inside normalized radius of 0.2) of JT-60U plasmas allowed the sustainment of stronger electron density ITBs compared with JET. As a consequence of peaked density profile, the core bootstrap current density is more than five times higher in JT-60U compared with JET. Thanks to the bootstrap effect and the slightly broader neutral beam deposition, reversed magnetic shear configurations are self-sustained in JT-60U scenarios. Analyses of similarities and differences between the two devices address key questions on the validity of the usual assumptions made in ITER steady scenario modelling, e.g. a flat density profile in the core with thermal transport barrier? Such assumptions have consequences on the prediction of fusion performance, bootstrap current and on the sustainment of the scenario.
Nuclear Fusion 06/2011; 51(7):073020. · 4.09 Impact Factor
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A T Salmi,
T Tala,
G Corrigan,
C Giroud,
J Ferreira,
J Lönnroth,
P Mantica,
V Parail,
M Tsalas,
T W Versloot, P C de Vries,
K-D Zastrow,
EFDA JET Contributors
[show abstract]
[hide abstract]
ABSTRACT: Accurate and validated tools for calculating toroidal momentum sources are necessary to make reliable predictions of toroidal rotation for current and future experiments. In this work we present the first experimental validation of torque profile calculation from neutral beam injection (NBI) under toroidal field ripple. We use discharges from a dedicated experimental session on JET where neutral beam modulation technique is used together with time-dependent torque calculations from ASCOT code for making the benchmark. Good agreement between simulations and experimental results is found.
Plasma Physics and Controlled Fusion 05/2011; 53(8):085005. · 2.42 Impact Factor
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[show abstract]
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ABSTRACT: A survey has been carried out into the causes of all 2309 disruptions over the last decade of JET operations. The aim of this survey was to obtain a complete picture of all possible disruption causes, in order to devise better strategies to prevent or mitigate their impact. The analysis allows the effort to avoid or prevent JET disruptions to be more efficient and effective. As expected, a highly complex pattern of chain of events that led to disruptions emerged. It was found that the majority of disruptions had a technical root cause, for example due to control errors, or operator mistakes. These bring a random, non-physics, factor into the occurrence of disruptions and the disruption rate or disruptivity of a scenario may depend more on technical performance than on physics stability issues. The main root cause of JET disruptions was nevertheless due to neo-classical tearing modes that locked, closely followed in second place by disruptions due to human error. The development of more robust operational scenarios has reduced the JET disruption rate over the last decade from about 15% to below 4%. A fraction of all disruptions was caused by very fast, precursorless unpredictable events. The occurrence of these disruptions may set a lower limit of 0.4% to the disruption rate of JET. If one considers on top of that human error and all unforeseen failures of heating or control systems this lower limit may rise to 1.0% or 1.6%, respectively.
Nuclear Fusion 04/2011; 51(5):053018. · 4.09 Impact Factor
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V.G. Kiptily,
G. Gorini,
M. Tardocchi, P.C. de Vries,
F.E. Cecil,
I.N. Chugunov,
T. Craciunescu,
M. Gatu Johnson,
D. Gin,
V. Goloborod'ko, [......],
A. Pietropaolo,
S.D. Pinches,
I. Proverbio,
P.G. Sanchez,
S.E. Sharapov,
A.E. Shevelev,
D.B. Syme,
V. Yavorskij,
V.L. Zoita,
JET-EFDA contributors
[show abstract]
[hide abstract]
ABSTRACT: The Doppler broadening of individual γ-ray lines was measured with a high purity germanium detector in JET plasma experiments. High-resolution γ-ray spectrometry of nuclear reactions between energetic D, 3He and 4He ions accelerated by ion cyclotron resonance heating and main plasma impurities such as carbon and beryllium has been used. The nuclear reactions giving rise to γ-rays have been identified and an effective temperature of the heated ions has been obtained in JET discharges. This technique could be used for fast ion and fusion alpha-particle studies in ITER.
Nuclear Fusion 07/2010; 50(8):084001. · 4.09 Impact Factor
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L Barrera,
E de la Luna,
L Figini,
M N A Beurskens,
M Brix,
F Castejón, P C de Vries,
D Farina,
M Kempenaars,
P Lomas,
J Mailloux,
I Nunes,
E R Solano,
JET-EFDA Contributors
[show abstract]
[hide abstract]
ABSTRACT: In this paper a novel use of the heterodyne radiometer in JET is presented. The operation of the diagnostic has been extended to allow simultaneously measurement of the pedestal temperature profile at the inboard and outboard midplane with high spatial and temporal resolution. Access to the inboard midplane region is obtained by measuring ordinary mode electron cyclotron emission (ECE) (no harmonic overlap) using an antenna located on the low-field side of the torus. To assess the potential of the ECE temperature measurements in the inboard pedestal region, a detailed analysis of ECE spectrum will be discussed. The measured electron temperature profiles are presented, including a discussion of the main uncertainties in the analysis. The comparison of inboard and outboard temperature pedestal profiles in a variety of H-mode plasmas shows a reasonable agreement in both shape and magnitude.
Plasma Physics and Controlled Fusion 07/2010; 52(8):085010. · 2.42 Impact Factor
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M Baruzzo,
B Alper,
T Bolzonella,
M Brix,
P Buratti,
C D Challis,
F Crisanti,
E de la Luna, P C de Vries,
C Giroud,
N C Hawkes,
D F Howell,
F Imbeaux,
E Joffrin,
H R Koslowski,
X Litaudon,
J Mailloux,
A C C Sips,
O Tudisco,
JET-EFDA contributors
[show abstract]
[hide abstract]
ABSTRACT: An experimental study on the poloidal mode number (m) spectrum produced by a single toroidal mode number (n) neoclassical tearing mode (NTM) in the JET tokamak is presented. Clear evidence of the existence of more than one significant m component is given. The analysis is performed comparing several methods and diagnostics; among the latter we mention high frequency magnetic pick-up coils and an electron cyclotron emission radiometer, which measures detailed electron temperature radial profiles at high time resolution. The two diagnostics are also used together in a cross coherence calculation technique. The issue of the interaction of this multiple m structure with the plasma is addressed as well, with particular attention paid to plasma toroidal rotation and rotation shear, obtained from charge exchange spectroscopy data. This effect has been studied under two different operational plasma scenarios on JET in order to investigate both dependences on plasma parameters and consequences on the scenario itself.
Plasma Physics and Controlled Fusion 05/2010; 52(7):075001. · 2.42 Impact Factor
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T W Versloot, P C de Vries,
C Giroud,
M-D Hua,
M N A Beurskens,
M Brix,
T Eich,
E de la Luna,
T Tala,
V Naulin,
K D Zastrow,
JET-EFDA Contributors
[show abstract]
[hide abstract]
ABSTRACT: The loss of plasma toroidal angular momentum and thermal energy by edge localized modes (ELMs) has been studied in JET. The analysis shows a consistently larger drop in momentum in comparison with the energy loss associated with the ELMs. This difference originates from the large reduction in angular frequency at the plasma edge, observed to penetrate into the plasma up to r/a ~ 0.65 during large type-I ELMs. As a result, the time averaged angular frequency is lowered near the top of the pedestal with increasing ELM frequency, resulting in a significant drop in thermal Mach number at the edge. An increase in profile peaking of ion temperature and angular frequency is observed. At the same time the plasma confinement is reduced while the ratio of confinement times (Rτ = τE/τ) increases noticeably with ELM frequency. This change could be explained by the relatively larger ELM induced losses for momentum in combination with the observed longer build-up time for the momentum density at the plasma edge.
Plasma Physics and Controlled Fusion 03/2010; 52(4):045014. · 2.42 Impact Factor
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[show abstract]
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ABSTRACT: Disruptions are sudden and unavoidable losses of confinement that may put at risk the integrity of a tokamak. However, the physical phenomena leading to disruptions are very complex and non-linear and therefore no satisfactory model has been devised so far either for their avoidance or their prediction. For this reason, machine learning techniques have been extensively pursued in the last years. In this paper a real-time predictor specifically developed for JET and based on support vector machines is presented. The main aim of the present investigation is to obtain high recognition rates in a real-time simulated environment. To this end the predictor has been tested on the time slices of entire discharges exactly as in real world operation.Since the year 2000, the experiments at JET have been organized in campaigns named sequentially beginning with campaign C1. In this paper results from campaign C1 (year 2000) and up to C19 (year 2007) are reported. The predictor has been trained with data from JET's campaigns up to C7 with particular attention to reducing the number of missed alarms, which are less than 1%, for a test set of discharges from the same campaigns used for the training. The false alarms plus premature alarms are of the order of 6.4%, for a total success rate of more than 92%. The robustness of the predictor has been proven by testing it with a wide subset of shots of more recent campaigns (from C8 to C19) without any retraining. The success rate over the period between C8 and C14 is on average 88% and never falls below 82%, confirming the good generalization capabilities of the developed technique. After C14, significant modifications were implemented on JET and its diagnostics and consequently the success rates of the predictor between C15 and C19 decays to an average of 79%. Finally, the performance of the developed detection system has been compared with the predictions of the JET protection system (JPS). The new predictor clearly outperforms JPS up to about 180 ms before the disruptions.
Nuclear Fusion 01/2010; 50(2):025005. · 4.09 Impact Factor
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P C de Vries,
Y Sakamoto,
X Litaudon,
M N A Beurskens,
M Brix,
K Crombé,
T Fujita,
C Giroud,
N C Hawkes,
N Hayashi, [......],
T Suzuki,
M Takechi,
H Takenaga,
T Tala,
M Tsalas,
H Urano,
I Voitsekhovitch,
M Yoshida,
JET EFDA contributors,
the team
[show abstract]
[hide abstract]
ABSTRACT: A series of experiments have been carried out in 2008 at JT-60U and JET to find common characteristics and explain differences between internal transport barriers (ITBs). The identity experiments succeeded in matching the profiles of most dimensionless parameters at the time ITBs were triggered. Thereafter the q-profile development deviated due to differences in non-inductive current density profile, affecting the ITB. Furthermore, the ITBs in JET were more strongly influenced by the H-mode pedestal or edge localized modes. It was found to be difficult to match the plasma rotation characteristics in both devices. However, the wide range of Mach numbers obtained in these experiments shows that the rotation has little effect on the triggering of ITBs in plasmas with reversed magnetic shear. On the other hand the toroidal rotation and more specifically the rotational shear had an impact on the subsequent growth and allowed the formation of strong ITBs.
Plasma Physics and Controlled Fusion 11/2009; 51(12):124050. · 2.42 Impact Factor
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P Mantica,
D Strintzi,
T Tala,
C Giroud,
T Johnson,
H Leggate,
E Lerche,
T Loarer,
A G Peeters,
A Salmi,
S Sharapov,
D Van Eester, P C de Vries,
L Zabeo,
K-D Zastrow
[show abstract]
[hide abstract]
ABSTRACT: Experiments were carried out in the JET tokamak to determine the critical ion temperature inverse gradient length (R/LTi=R|nablaTi|/Ti) for the onset of ion temperature gradient modes and the stiffness of Ti profiles with respect to deviations from the critical value. Threshold and stiffness have been compared with linear and nonlinear predictions of the gyrokinetic code GS2. Plasmas with higher values of toroidal rotation show a significant increase in R/LTi, which is found to be mainly due to a decrease of the stiffness level. This finding has implications on the extrapolation to future machines of present day results on the role of rotation on confinement.
Physical Review Letters 06/2009; 102(17):175002. · 7.37 Impact Factor
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[show abstract]
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ABSTRACT: The disruption rate (the percentage of discharges that disrupt) in JET was found to drop steadily over the years. Recent campaigns (2005–2007) show a yearly averaged disruption rate of only 6% while from 1991 to 1995 this was often higher than 20%. Besides the disruption rate, the so-called disruptivity, or the likelihood of a disruption depending on the plasma parameters, has been determined. The disruptivity of plasmas was found to be significantly higher close to the three main operational boundaries for tokamaks; the low-q, high density and β-limit. The frequency at which JET operated close to the density-limit increased six fold over the last decade; however, only a small reduction in disruptivity was found. Similarly the disruptivity close to the low-q and β-limit was found to be unchanged. The most significant reduction in disruptivity was found far from the operational boundaries, leading to the conclusion that the improved disruption rate is due to a better technical capability of operating JET, instead of safer operations close to the physics limits. The statistics showed that a simple protection system was able to mitigate the forces of a large fraction of disruptions, although it has proved to be at present more difficult to ameliorate the heat flux.
Nuclear Fusion 04/2009; 49(5):055011. · 4.09 Impact Factor
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M.N.A. Beurskens,
G. Arnoux,
A.S. Brezinsek,
C.D. Challis, P.C. de Vries,
C. Giroud,
A. Huber,
S. Jachmich,
K. McCormick,
R.A. Pitts, [......],
R. Pasqualotto,
V. Pericoli-Ridolfini,
R. Pugno,
E. Rachlew,
S. Saarelma,
E. Solano,
M. Walsh,
L. Zabeo,
K.-D. Zastrow,
JET-EFDA Contributors
[show abstract]
[hide abstract]
ABSTRACT: Advanced scenario plasmas must often be run at low densities and high power, leading to hot edge temperatures and consequent power handling issues at plasma–surface interaction zones. Experiments at JET are addressing this issue by exploring the use of extrinsic impurity seeding and D2 puffing to reduce heat fluxes.The experiments presented in this paper continue the line of advanced tokamak (AT) scenario studies at high triangularity in JET by concentrating on the characterization of the edge pedestal and the ELM behaviour with deuterium and/or light impurity fuelling (neon, nitrogen). Both injection of extrinsic impurities and D2 puffing are shown to have a significant impact on the edge pedestal in typical JET AT conditions. The ELM energy loss, ΔWELM/Wdia, can be reduced to below 3% and the maximum ELM penetration depth can be limited to r/a > 0.7, thus enhancing the possibility for sustainable internal transport barriers at large plasma radius. These conditions can be achieved in two separate domains, either at a radiated power fraction (Frad) of 30% or at a fraction of >50%. At the lower Frad the ELMs are type I and a high pedestal pressure is maintained, but the occasional large ELM may still occur. At Frad > 50% the pedestal pressure is degraded by 30–50%, but the ELMs are degraded to type III. The intermediate regime at Frad ~ 40% is unattractive for ITB scenarios because large type I ELMs occur intermittently during the predominantly type III ELM phases (compound type I/III). Frad = 30% can be obtained with D2 fuelling alone, whereas neon or nitrogen seeding is needed to achieve Frad > 50%. Only a limited number of tests have been carried out with nitrogen seeding, with the preliminary conclusion that the plasma edge behaviour is similar to that with neon seeding once the radiated fraction is matched.
Nuclear Fusion 08/2008; 48(9):095004. · 4.09 Impact Factor
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[show abstract]
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ABSTRACT: An extensive database to study the scaling of rotation and momentum transport has been constructed at JET. The database contains information from various operational scenarios, amongst them H-mode discharges, and parameters that characterize the rotation, as well as those that describe the general plasma conditions. JET plasmas are predominantly heated by neutral beam injection which is also the main source for the observed toroidal rotation. Dimensionless Mach numbers are introduced to quantify rotation. The scaling of plasma rotation and the Mach numbers in particular has been studied. The thermal and Alfvén Mach numbers were found to scale inversely with q and with the ratio of torque and additional heating power. Although the momentum and energy confinement times were found to be of the same magnitude, the ratio was found to vary. Regression analyses showed a dependence of both the energy and momentum confinement times on plasma rotation. If rotation was included in the scaling model of energy and momentum confinement the quality of the fits substantially improved. Detailed analysis of the core and edge (pedestal) confinement showed that momentum confinement was improved in the core of the plasma compared with the energy confinement. However, the pedestal proved to be less confining for the momentum than for the energy.
Nuclear Fusion 04/2008; 48(6):065006. · 4.09 Impact Factor
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P C De Vries,
A Salmi,
V Parail,
C Giroud,
Y Andrew,
T M Biewer,
K Crombé,
I Jenkins,
T Johnson,
V Kiptily,
A Loarte,
J Lönnroth,
A Meigs,
N Oyama,
R Sartori,
G Saibene,
H Urano,
K.-D Zastrow
[show abstract]
[hide abstract]
ABSTRACT: Dedicated experiments on TF ripple effects on the performance of tokamak plasmas have been carried out at JET. The TF ripple was found to have a profound effect on the plasma rotation. The central Mach number, M, defined as the ratio of the rotation velocity and the thermal velocity, was found to drop as a function of TF ripple amplitude (δ) from an average value of M = 0.40–0.55 for operations at the standard JET ripple of δ = 0.08% to M = 0.25–0.40 for δ = 0.5% and M = 0.1–0.3 for δ = 1%. TF ripple effects should be considered when estimating the plasma rotation in ITER. With standard co-current injection of neutral beam injection (NBI), plasmas were found to rotate in the co-current direction. However, for higher TF ripple amplitudes (δ ∼ 1%) an area of counter rotation developed at the edge of the plasma, while the core kept its co-rotation. The edge counter rotation was found to depend, besides on the TF ripple amplitude, on the edge temperature. The observed reduction of toroidal plasma rotation with increasing TF ripple could partly be explained by TF ripple induced losses of energetic ions, injected by NBI. However, the calculated torque due to these losses was insufficient to explain the observed counter rotation and its scaling with edge parameters. It is suggested that additional TF ripple induced losses of thermal ions contribute to this effect.
Fusion. 01/2008; 4850.
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P C De Vries,
E Joffrin,
N C Hawkes,
X Litaudon,
C D Challis,
Y Andrew,
M Beurskens,
M Brix,
J Brzozowski,
K Crombé,
C Giroud,
J Hobirk,
T Johnson,
J Lönnroth,
A Salmi,
T Tala,
V Yavorskij,
K-D Zastrow,
Jet Efda Contributors
[show abstract]
[hide abstract]
ABSTRACT: The effect of a toroidal field (TF) ripple on the formation and performance of internal transport barriers (ITBs) has been studied in JET. It was found that the TF ripple had a profound effect on the toroidal plasma rotation. An increased TF ripple up to δ = 1% led to a lower rotation and reduced the rotational shear in the region where the ITBs were formed. ITB triggering events were observed in all cases and it is thought that the rotational shear may be less important for this process than, for example, the q-profile. However, the increase in the pressure gradient following the ITB trigger was reduced in discharges with a larger TF ripple and consequently a lower rotational shear. This suggests that toroidal rotation and its shear play a role in the growth of the ITB once it has been triggered. 10 See the appendix of Watkins M L et al 2006 Proc. 21st Int Conf. on Fusion Energy 2006 (Chengdu) (Vienna: IAEA).
Plasma Phys. Control. Fusion. 01/2008; 50:65008-12.
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Fusion Energy. 01/2008;
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T Tala,
K Crombé, P C de Vries,
J Ferreira,
P Mantica,
A G Peeters,
Y Andrew,
R Budny,
G Corrigan,
A Eriksson, [......],
M F F Nave,
V Parail,
K Rantamäki,
B D Scott,
P Strand,
G Tardini,
A Thyagaraja,
J Weiland,
K-D Zastrow,
JET-EFDA Contributors
[show abstract]
[hide abstract]
ABSTRACT: The present status of understanding of toroidal and poloidal momentum transport in tokamaks is presented in this paper. Similar energy confinement and momentum confinement times, i.e. τE/τ ≈ 1 have been reported on several tokamaks. It is more important though, to study the local transport both in the core and edge plasma separately as, for example, in the core plasma, a large scatter in the ratio of the local effective momentum diffusivity to the ion heat diffusivity χeff/χi,eff among different tokamaks can be found. For example, the value of effective Prandtl number is typically around χeff/χi,eff ≈ 0.2 on JET while still τE/τ ≈ 1 holds. Perturbative NBI modulation experiments on JET have shown, however, that a Prandtl number χ/χi of around 1 is valid if there is an additional, significant inward momentum pinch which is required to explain the amplitude and phase behaviour of the momentum perturbation. The experimental results, i.e. the high Prandtl number and pinch, are in good qualitative and to some extent also in quantitative agreement with linear gyro-kinetic simulations. In contrast to the toroidal momentum transport which is clearly anomalous, the poloidal velocity is usually believed to be neo-classical. However, experimental measurements on JET show that the carbon poloidal velocity can be an order of magnitude above the predicted value by the neo-classical theory within the ITB. These large measured poloidal velocities, employed for example in transport simulations, significantly affect the calculated radial electric field and therefore the E × B flow shear and hence modify and can significantly improve the simulation predictions. Several fluid turbulence codes have been used to identify the mechanism driving the poloidal velocity to such high values. CUTIE and TRB turbulence codes and also the Weiland model predict the existence of an anomalous poloidal velocity, peaking in the vicinity of the ITB and driven dominantly by the flow due to the Reynold's stress. It is worth noting that these codes and models treat the equilibrium in a simplified way and this affects the geodesic curvature effects and geodesic acoustic modes. The neo-classical equilibrium is calculated more accurately in the GEM code and the simulations suggest that the spin-up of poloidal velocity is a consequence of the plasma profiles steepening when the ITB grows, following in particular the growth of the toroidal velocity within the ITB.
Plasma Physics and Controlled Fusion 11/2007; 49(12B):B291. · 2.42 Impact Factor
