Space Charge Neutralization in the ITER Negative Ion Beams

Surrey, E.

doi:10.1063/1.2773666

Cited by 5 publications

(3 citation statements)

References 8 publications

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“…Due to the relevance of space charge compensation in ruling the beam performances, usually very detailed numerical models are employed solving the Poisson equation for the detailed computation of the behaviour of the plasma in the beam drift region. Several 1D [68], 2D [69][70][71] and 3D [72] models were applied to the investigation of this issue for ITER HNBs. The main result of these computations is that space charge compensation should be guaranteed within a short distance from the exit of the accelerator; actually slight overcompensation is expected, which is not known whether it might result in focussing of the ITER beam [68] as usually happens in the relativistic limit [32].…”

Section: Secondary Plasma Formation and Space Charge Compensationmentioning

confidence: 99%

“…Several 1D [68], 2D [69][70][71] and 3D [72] models were applied to the investigation of this issue for ITER HNBs. The main result of these computations is that space charge compensation should be guaranteed within a short distance from the exit of the accelerator; actually slight overcompensation is expected, which is not known whether it might result in focussing of the ITER beam [68] as usually happens in the relativistic limit [32]. This plasma might be severely affected by the fringe field of the accelerator and a positive ion current towards the beam source might be drawn [72].…”

Section: Secondary Plasma Formation and Space Charge Compensationmentioning

confidence: 99%

See 1 more Smart Citation

Neutralisation and transport of negative ion beams: physics and diagnostics

Serianni¹,

Agostinetti²,

Agostini³

et al. 2017

New J. Phys.

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Neutral beam injection is one of the most important methods of plasma heating in thermonuclear fusion experiments, allowing the attainment of fusion conditions as well as driving the plasma current. Neutral beams are generally produced by electrostatically accelerating ions, which are neutralised before injection into the magnetised plasma. At the particle energy required for the most advanced thermonuclear devices and particularly for ITER, neutralisation of positive ions is very inefficient so that negative ions are used. The present paper is devoted to the description of the phenomena occurring when a high-power multi-ampere negative ion beam travels from the beam source towards the plasma. Simulation of the trajectory of the beam and of its features requires various numerical codes, which must take into account all relevant phenomena. The leitmotiv is represented by the interaction of the beam with the background gas. The main outcome is the partial neutralisation of the beam particles, but ionisation of the background gas also occurs, with several physical and technological consequences. Diagnostic methods capable of investigating the beam properties and of assessing the relevance of the various phenomena will be discussed. Examples will be given regarding the measurements collected in the small flexible NIO1 source and regarding the expected results of the prototype of the neutral beam injectors for ITER. The tight connection between measurements and simulations in view of the operation of the beam is highlighted. operating in the existing tokamaks. Consequently, PRIMA, the ITER Neutral Beam Test Facility [2], was set up to constitute a test-bed, where the solutions to all the issues related to the achievement of full performances in the heating NBI system for ITER are going to be addressed and optimised, particularly regarding critical aspects like density and uniformity of the extracted negative ion current, high voltage holding and heat loads on the components [3]. Megavolt ITER Injector Concept Advancement (MITICA) is the full-scale prototype of the ITER NBIs [4]; it includes an RF-driven plasma source for the production of negative ions and should operate at a pressure as low as 0.3 Pa in hydrogen or deuterium gasses. The negative ions are produced on the surface of the grid (Plasma Grid, PG) that closes the plasma region; their production is enhanced thanks to a thin caesium layer continuously deposited over the PG by evaporation. Negative ions are extracted through the 1280 apertures in the PG by application of a suitable positive voltage to the extraction grid (EG), located just downstream of the PG. The RF plasma source operates at an applied electric potential of about −1 MV. Five additional acceleration grids (AG1, AG2, AG3, AG4, GG), at intermediate electric potential increasing by 200 kV steps, are located downstream with respect to the EG, thus constituting a 5-stage electrostatic accelerator. The resulting negative ion beam at 1 MeV, after passing through a gas cell neutraliser and an electro...

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Section: Secondary Plasma Formation and Space Charge Compensationmentioning

confidence: 99%

Section: Secondary Plasma Formation and Space Charge Compensationmentioning

confidence: 99%

Neutralisation and transport of negative ion beams: physics and diagnostics

Serianni¹,

Agostinetti²,

Agostini³

et al. 2017

New J. Phys.

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“…This work has concentrated on modelling of ITER ion sources and components relating to both ITER and DEMO. Examples of this are understanding virtual cathode formation [15] due to the negative ion space charge at the caesiated wall of the ion source affecting the amount of negative ions which could be extracted, gas heating and target depletion due to plasma formation in the ITER neutraliser [16], the effect of plasma formation in the ITER beamline Residual Ion Dump [17] and space charge effects in the ITER ion beams [18].…”

Section: Introduction and Historical Perspective Of Negative Ion Devementioning

confidence: 99%

Negative ion research at the Culham Centre for Fusion Energy (CCFE)

McAdams

Holmes²,

King

et al. 2016

New J. Phys.

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A summary of negative ion development work being presently undertaken at the Culham Centre for Fusion Energy is given. The small negative ion facility has an RF driven volume ion source with beam extraction at energies up to 30 keV. The extracted beam of H − ions has an associated co-extracted electron beam with an electron to ion ratio of <1 over the whole range of operating parameters. In order to understand this performance spectroscopic investigations have been undertaken using the Balmer series line to determine the electron temperature. In addition a 1D fluid model of an RF driven ion source is also under development. This model is based on a successful model for both arc discharge positive and negative ion sources. Additional system studies of neutral beam injection systems for future fusion machines beyond ITER are being carried out. This is required to understand the limits of various neutralisation and energy recovery systems in order to maximise overall electrical efficiency.

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Analytical and numerical studies of positive ion beam expansion for surface treatment applications

Lounes-Mahloul

Bendib

Oudini

2018

Eur. Phys. J. Appl. Phys.

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The aim of this work is to study the expansion in vacuum, of a positive ion beam with the use of one dimensional (1D) analytic model and a two dimensional Particle-In-Cell (2D-PIC) simulation. The ion beam is extracted and accelerated from preformed plasma by an extraction system composed of two polarized parallel perforated grids. The results obtained with both approaches reveal the presence of a potential barrier downstream the extraction system which tends to reflect the ion flux. The dependence of the critical distance for which all extracted ions are reflected, is investigated as a function of the extracted ion beam current density. In particular, it is shown that the 1D model recovers the well-known Child-Langmuir law and that the 2D simulation presents a significant discrepancy with respect to the 1D prediction. Indeed, for a given value of current density, the transverse effects lead to a greater critical distance.

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Space Charge Neutralization in the ITER Negative Ion Beams

Cited by 5 publications

References 8 publications

Neutralisation and transport of negative ion beams: physics and diagnostics

Neutralisation and transport of negative ion beams: physics and diagnostics

Negative ion research at the Culham Centre for Fusion Energy (CCFE)

Analytical and numerical studies of positive ion beam expansion for surface treatment applications

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