Various divertor biasing configurations and improved divertor performance with biasing on Tokamak de Varennes (TdeV)*

Decoste, R.; Lachambre, J. L.; Abel, G.; Boileau, A.; Boucher, C.; Cote, A.; Fall, T.; Gauvreau, J. L.; Gregory, B.C.; Haddad, E.; Janicki, Christian; Liu‐Hinz, C.; H., H.; Martín, F.; Michaud, Daniel; Richard, Nicolas; Ross, G.G.; Sarkissian, A.; Stansfield, B.L.; Terreault, B.; Zuzak, W.

doi:10.1063/1.870699

Cited by 28 publications

(13 citation statements)

References 10 publications

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“…The electrical biasing of divertors provides a separate category of information on the effect of drifts. The Tokamak de Varennes can be operated with electrical biasing of the divertor plates such as to permit external manipulation of the radial electric field in the SOL [24]. The resulting poloidal drifts lead to measured pressure asymmetries [24] between the two divertors, which are consistent with the analysis of Section 3.…”

Section: Discussionmentioning

confidence: 58%

Simple models for the radial and poloidal E × B drifts in the scrape-off layer of a divertor tokamak: Effects on in/out asymmetries

Stangeby¹,

Chankin²

1996

Nucl. Fusion

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The influence of radial and poloidal E × B drifts in the scrape-off layer (SOL) of a divertor tokamak is considered. For B in the `forward' direction (the ∇B ion drift towards a lower X point, single null divertor configuration), the poloidal drift tends to increase the plasma pressure and the recycling rates at the outer target, lowering the temperature there - as compared with the inner target. When the radial drift dominates, these asymmetries are reversed. The radial drift is estimated to be more important than the poloidal drift for high ne operation (the `high recycling' regime). The poloidal drift case is analysed by using a simple, isothermal model and boundary conditions at the targets which allow for the effect of drift. The radial drift case is modelled on the basis of a heuristic analysis of the viscous drag experienced by the return parallel flow along the SOL which is induced by the radial drift fluxes. Poloidal drifts can result in substantially reduced plasma contact at one target (`detachment'); simultaneous detachment at both targets due to poloidal drifts appears to be less likely. Radial drifts could result in detachment at one or both targets, but the type of detachment involved does not appear to be consistent with that experimentally observed. The experimental observations made in the Tokamak de Varennes, with divertor biasing arranged to increase/decrease the radial electric field in the SOL, relative to its natural level, are broadly consistent with the poloidal drift model

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Section: Discussionmentioning

confidence: 58%

Simple models for the radial and poloidal E × B drifts in the scrape-off layer of a divertor tokamak: Effects on in/out asymmetries

Stangeby¹,

Chankin²

1996

Nucl. Fusion

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“…On TUMAN-3 [14] and TORTUS [15] enhanced particle confinement was also observed for both electrode biasing polarities. Divertor biasing on TdeV and improvement of particle exhaust were described in [16]. Biasing a fully toroidal belt limiter has been done with the purpose of improving the ALT-II pump performance as well as of enhancing the core plasma confinement on TEXTOR.…”

Section: Introductionmentioning

confidence: 99%

Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing

Cabral¹,

Varandas²,

Alonso³

et al. 1998

Plasma Phys. Control. Fusion

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Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through timeresolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the H α radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

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“…Toroidally asymmetric divertor biasing was tested in two tokamaks, and some promising results were reported [16,17]. Apart from the novel divertor biasing experiments, the effects of toroidally symmetric divertor biasing were intensively studied experimentally and theoretically, with particular focus on generation of plasma rotation and the associated electric field in the SOL, the L-H confinement transition and divertor exhaust/pumping of fuel and impurity gases [18][19][20][21][22][23][24][25][26].…”

Section: Introductionmentioning

confidence: 99%

Effects of toroidally-distributed-divertor biasing on scrape-off-layer (SOL) current drive, divertor particle flux and fast electron confinement in the QUEST spherical tokamak

et al. 2023

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A novel divertor biasing by four biasing plates distributed toroidally (TDDB) on the upper divertor target plate is applied to low density tokamak plasmas started-up by 28 GHz 2nd harmonic electron cyclotron current drive (ECCD) in a quasi-double null configuration of the QUEST spherical tokamak (ST). In the ST plasmas of line averaged electron density <n_e>~0.7-1x1018 m-3, about 20 to 40 % of the current I_biasdriven by ~85 V sawtooth bias voltage reaches the lower divertor plate along the biased scrape-off-layer (SOL) flux tube as the SOL current I_SOL. The fact I_SOLis noticeably lower than I_bias indicates an appreciable leakage of parallel current from the biased SOL flux tube. The leakage currents in the toroidal and radial directions are confirmed by detection of them using the unbiased plates in the TDDB experiments. From the ion saturation current density profile obtained by a divertor Langmuir probe array, the fall-off lengths of divertor particle flux are estimated together with the strike line position. Total particle flux to the upper divertor, evaluated by the integrated ion saturation current density profile is reduced by up to 45% during positive biasing of the TDDB, depending on the position of the strike line to the biased plate. In addition, the TDDB also induces a noticeable loss of fast electrons produced by ECCD, leading to an ~2 % reduction in the maximum toroidal current of the ST plasma compared to a shot without the TDDB. Reduction of the divertor particle flux and enhancement of the fast electron losses are thought to be dominantly caused by E×B drift induced by the TDDB. In the present experimental conditions, the effects of magnetic perturbations produced by the SOL currents on the fast electron losses can be neglected because of a too small SOL current.

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Various divertor biasing configurations and improved divertor performance with biasing on Tokamak de Varennes (TdeV)*

Cited by 28 publications

References 10 publications

Simple models for the radial and poloidal E × B drifts in the scrape-off layer of a divertor tokamak: Effects on in/out asymmetries

Simple models for the radial and poloidal E × B drifts in the scrape-off layer of a divertor tokamak: Effects on in/out asymmetries

Enhancement of the ISTTOK plasma confinement and stability by negative limiter biasing

Effects of toroidally-distributed-divertor biasing on scrape-off-layer (SOL) current drive, divertor particle flux and fast electron confinement in the QUEST spherical tokamak

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