Charged particle flows in the beam extraction region of a negative ion source for NBI

Geng, Suiyan; Tsumori, K.; Nakano, H.; Kisaki, M.; Ikeda, K.; Osakabe, M.; Nagaoka, K.; Takeiri, Y.; Shibuya, Masayuki; Kaneko, O.

doi:10.1063/1.4931796

Cited by 19 publications

(15 citation statements)

References 10 publications

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“…Electron current shows a deviation of about 30% between the bottom and top apertures. In particular, except the two extreme apertures, all the others figure 1), the H − flow direction is opposite to that of positive ions (shown in figure 3(b)), a fact that has recently found experimental evidences [15]. As a consequence, the The asymmetry detected around every single aperture has an impact on the quality of the single beamlet extracted.…”

Section: Extraction Through the Multiaperture Gridmentioning

confidence: 82%

“…All these parameters are arbitrary and artificial and often them are chosen on the basis of results considered acceptable and meaningful. At the same time, experimental evidences [13] and expansion region models [14] show that 5 cm from PG, plasma is not Maxwellian, the axial flux is not ambipolar (locally ¹ + -j j z z , , , where z is the direction normal to the grid) and the electron current has a strong transversal (to the grid) component [15]. The electron transport across the magnetic filter field is anomalous and non-local.…”

Section: Critical Revision Of Existing Numerical Modelsmentioning

confidence: 98%

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PIC modeling of negative ion sources for fusion

Taccogna

Minelli

2017

New J. Phys.

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This work represents the first attempt to model the full-size ITER negative ion source prototype including expansion, extraction and part of the acceleration regions keeping the resolution fine enough to resolve every single aperture of the extraction grid. The model consists of a 2.5-dimensional Particle-in-Cell/Monte Carlo Collision representation of the plane perpendicular to the filter field lines. Both the magnetic filter and electron deflection fields have been included. A negative ion current density of = --j 500 A m H 2 produced by neutral conversion from the plasma grid is used as fixed parameter, while negative ions produced by electron dissociative attachment of vibrationally excited molecules and by ionic conversion on plasma grid are self-consistently simulated. Results show the non-ambipolar character of the transport in the expansion region driven by electron magnetic drifts in the plane perpendicular to the filter field. It induces a top-bottom asymmetry detected up to the extraction grid which in turn leads to a tilted positive ion flow hitting the plasma grid and a tilted negative ion flow emitted from the plasma grid. As a consequence, the plasma structure is not uniform around the single aperture: the meniscus assumes a form of asymmetric lobe and a deeper potential well is detected from one side of the aperture relative to the other side. Therefore, the surfaceproduced contribution to the negative ion extraction is not equally distributed between both the sides around the aperture but it come mainly from the lower side of the grid giving an asymmetrical current distribution in the single beamlet.

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Section: Extraction Through the Multiaperture Gridmentioning

confidence: 82%

Section: Critical Revision Of Existing Numerical Modelsmentioning

confidence: 98%

PIC modeling of negative ion sources for fusion

Taccogna

Minelli

2017

New J. Phys.

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“…If we consider the movement of a H − ion in only y-z plane shown in Fig. 2, a H − ion experiences gyration in filter magnetic field, since the filter magnetic field exists in large area in x direction with a strength of around 0.006 T. The H − ion temperature was measured to be around 0.1 eV [15,16]. The gyroradius is calculated to be around 9 mm or the diameter of the gyration is around 18 mm.…”

Section: Discussionmentioning

confidence: 99%

Depth of Influence on the Plasma by Beam Extraction in a Negative Hydrogen Ion Source for NBI

Geng

Tsumori

Nakano

et al. 2016

Plasma and Fusion Research

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Experimental measurements with Langmuir probe and laser photodetachment in beam extraction region of a cesium-seeded negative ion source for NBI has been conducted in order to investigate the response of charged particles to applied external field. The profiles of probe saturation current and H − ion density in the direction normal to the plasma grid (PG) surface were measured by scanning the tip position. By comparing the results before and during beam extraction, the charged particle responses due to the beam extraction have been analyzed. During beam extraction, probe saturation current increases since H − ions are extracted and electrons flow into the extracting region for charge neutrality. The maximum increment of the probe saturation current due to electron flow appears in the range of 15 -25 mm apart from the PG surface. Meanwhile, the maximum decrement of the H − ion density is at around 18 mm from the PG. In the region far from the PG, probe saturation current increment is low as well as the decrement of the H − ion density. The extrapolations of the profiles suggest that the depth of the influence on the plasma by beam extraction is 42 mm from the plasma grid surface.

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“…Temperature of H − ions determines the recovery time of the photodetachment signal and the probe diagnostics results indicate the H − ion temperature in the RNIS extraction region is in the order of 0.1 eV [92]. The H − ion temperature of 0.1 eV was considered too low to explain measure H − ion current density, as H − ions stop at the region of any negative potential barrier in the plasma before reaching the extraction hole.…”

Section: Negative Ion Temperaturementioning

confidence: 99%

“…Presence of Cs in the discharge also affects the electric field, or the plasma potential gradient in the beam extraction region [88]. The potential difference between the plasma grid and the plasma potential in the extraction region shows a strong correlation against the electron current co-extracted with H − ion current.…”

Section: Plasma Parametersmentioning

confidence: 99%