Hot spot phenomena on Tore Supra ICRF antennas investigated by optical diagnostics

Colas, L.; Costanzo, L.; Desgranges, C.; Brémond, S.; Bucalossi, J.; Agarici, G.; Basiuk, V.; Beaumont, B.; Bécoulet, A.; Nguyen, Frédéric

doi:10.1088/0029-5515/43/1/301

Cited by 52 publications

(29 citation statements)

References 23 publications

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“…[28], JET [30], ALCATOR C-mod [31] and Tore-Supra [32]. Amongst the reported effects, RF sheaths rectification [27] is invoked to explain enhanced impurity production and local hot spots when using ICRF.…”

Section: Mechanisms Leading To Localized Density Increasementioning

confidence: 99%

Maximization of ICRF power by SOL density tailoring with local gas injection

et al. 2016

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Experiments have been performed under the coordination of the International Tokamak Physics Activity (ITPA) on several tokamaks, including ASDEX Upgrade (AUG), JET and DIII-D, to characterize the increased Ion cyclotron range of frequency (ICRF) antenna loading achieved by optimizing the position of gas injection relative to the RF antennas. On DIII-D, AUG and JET (with the ITER-Like Wall) a 50% increase in the antenna loading was observed when injecting deuterium in ELMy H-mode plasmas using mid-plane inlets close to the powered antennas instead of divertor injection and, with smaller improvement when using gas inlets located at the top of the machine. The gas injection rate required for such improvements (~0.7 × 10 22 el s −1 in AUG, ~1.0 × 10 22 el s −1 in JET) is compatible with the use of this technique to optimize ICRF heating during the development of plasma scenarios and no degradation of confinement was observed when using the mid-plane or top inlets compared with divertor valves. An increase in the scrape-off layer (SOL) density was measured when switching gas injection from divertor to outer mid-plane or top. On JET and DIII-D, the measured SOL density increase when using main chamber puffing is consistent with the antenna coupling resistance increase provided that the distance between the measurement lines of sight and the injection location is taken into account. Optimized gas injection was also (2016) 046001 (14pp) P. Jacquet et al 2 found to be beneficial for reducing tungsten (W) sputtering at the AUG antenna limiters, and also to reduce slightly the W and nickel (Ni) content in JET plasmas. Modeling the specific effects of divertor/top/mid-plane injection on the outer mid-plane density was carried out using both the EDGE2D-EIRENE and EMC3-EIRENE plasma boundary code packages; simulations indeed indicate that outer mid-plane gas injection maximizes the density in the mid-plane close to the injection point with qualitative agreement with the AUG SOL density measurements for EMC3-EIRENE. Field line tracing for ITER in the 15 MA Q DT = 10 reference scenario indicates that the planned gas injection system could be used to tailor the density in front the antennas. Benchmarking of EMC3-EIRENE against AUG and JET data is planned as a first step towards the ITER SOL modelling required to quantify the effect of gas injection on the SOL density in front of the antennas.

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Section: Mechanisms Leading To Localized Density Increasementioning

confidence: 99%

Maximization of ICRF power by SOL density tailoring with local gas injection

et al. 2016

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“…20 In Tore Supra the up/down heat flux asymmetry on the antenna was interpreted as arising from a largescale rf-sheath driven convection roll pattern in front of the antenna. [23][24][25] This convection occurs because the antenna acts like a giant biased probe, charging positive all the field lines in front of it. The tokamak magnetic field gives a preferred direction to the E×B drift pattern and is responsible for convecting plasma preferentially into the bottom of the antenna.…”

Section: Fw Launch Antenna-edge Interactions: Rf Sheathsmentioning

confidence: 99%

Nonlinear ICRF-plasma interactions

et al. 2006

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Abstract. The well developed linear theory of ICRF (including FW, HHFW and IBW) interactions with plasma has enjoyed considerable success in describing antenna coupling and wave propagation, and provides a well-known framework for calculating power absorption, current drive, etc. In some situations, less well studied nonlinear effects are of interest, such as flow drive, ponderomotive forces, rf sheaths, parametric decay and related interactions with the edge plasma. Standard ICRF codes have begun to integrate this physics to achieve improved modeling capabilities. This paper concentrates on basic rf-plasma-interaction physics with illustrative applications to tokamaks. For FW antennas, the parallel electric field near launching structures is known to drive rf-sheaths which can give rise to convective cells, interaction with plasma "blobs", impurity production, and edge power dissipation. In addition to sheaths, IBW waves in the edge plasma are subject to strong ponderomotive effects and parametric decay. In the core plasma, slow waves can sometimes induce nonlinear effects. Mechanisms by which these waves can influence the radial electric field and its shear are summarized, and related to the general (reactive-ponderomotive and dissipative) force on a plasma from rf waves. It is argued that there are significant opportunities now for new predictive capabilities by advances in integrated simulation.

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“…For megawatt power coupling, the rectified potential is generally large (~ kV) and has important consequences. In fusion experiments, rf sheaths cause a variety of deleterious interactions, such as impurity generation, 7 convective transport, [8][9][10] hot spots 11 , 12 and damage to the antenna and surrounding structures, and power dissipation, 13 caused by both the near-field 7 and far-field 14 waves. In many experiments, the heating efficiency with low-k || waves is poor, and sheath power dissipation is a likely candidate to explain this phenomenon.…”

Section: Introductionmentioning

confidence: 99%

A radio-frequency sheath boundary condition and its effect on slow wave propagation

2006

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Predictive modeling of radiofrequency wave propagation in high-power fusion experiments requires accounting for nonlinear losses of wave energy in the plasma edge and at the wall. An important mechanism of "anomalous" power losses is the acceleration of ions into the walls by rf sheath potentials. Previous work computed the "sheath power dissipation" non-self-consistently by post-processing fields obtained as the solution of models which did not retain sheaths. Here, a method is proposed for a self-consistent quantitative calculation of sheath losses by incorporating a sheath boundary condition (SBC) in antenna coupling and wave propagation codes. It obtains the self-consistent sheath potentials and spatial distribution of the time-averaged power loss in the solution for the linear rf fields. It can be applied for ion cyclotron and (in some cases) lower hybrid waves. The use of the SBC is illustrated by applying it to the problem of an electron plasma wave propagating in a waveguide. This model problem is relevant to understanding the low heating efficiency in direct ion-Bernstein wave launch.Implications for calculating sheath voltages driven by fast-wave antennas are also discussed.

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Hot spot phenomena on Tore Supra ICRF antennas investigated by optical diagnostics

Cited by 52 publications

References 23 publications

Maximization of ICRF power by SOL density tailoring with local gas injection

Maximization of ICRF power by SOL density tailoring with local gas injection

Nonlinear ICRF-plasma interactions

A radio-frequency sheath boundary condition and its effect on slow wave propagation

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