Dynamics of the transport of ionic and atomic cesium in radio frequency-driven ion sources for ITER neutral beam injection

Gutser, R.; Wünderlich, D.; Fantz, U.; Team, Nnbi

doi:10.1088/0741-3335/53/10/105014

Cited by 34 publications

(19 citation statements)

References 24 publications

(56 reference statements)

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“…the beneficial role of having the temperature of the source at or well above 35°C and the grid between 125°C and 200°C, the relevance of building up a caesium reservoir in the vacuum phases for the next plasma pulse, etc, the caesium conditioning procedure of this large ion source is still not optimized. The experimental efforts are supported by a 3D trajectory code capable of modeling the caesium flow from the oven, the caesium transport and its redistribution in vacuum and plasma phases [51]. A large part of the required input parameters (sticking coefficient of caesium as a function of temperature, impurities in the background gas in vacuum, duration of plasma interaction) were not available in literature and are thus taken from measurements in lab scale experiments operating at ion source relevant parameters [52].…”

Section: Scaling Of Magnetic Field Topology Bias Area and Cs Consumpmentioning

confidence: 99%

Towards large and powerful radio frequency driven negative ion sources for fusion

et al. 2017

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The ITER neutral beam system will be equipped with radio-frequency (RF) negative ion sources, based on the IPP Garching prototype source design. Up to 100 kW at 1 MHz is coupled to the RF driver, out of which the plasma expands into the main source chamber. Compared to arc driven sources, RF sources are maintenance free and without evaporation of tungsten. The modularity of the driver concept permits to supply large source volumes. The prototype source (one driver) demonstrated operation in hydrogen and deuterium up to one hour with ITER relevant parameters. The ELISE test facility is operating with a source of half the ITER size (four drivers) in order to validate the modular source concept and to gain early operational experience at ITER relevant dimensions. A large variety of diagnostics allows improving the understanding of the relevant physics and its link to the source performance. Most of the negative ions are produced on a caesiated surface by conversion of hydrogen atoms. Cs conditioning and distribution have been optimized in order to achieve high ion currents which are stable in time. A magnetic filter field is needed to reduce the electron temperature and coextracted electron current. The influence of different field topologies and strengths on the source performance, plasma and beam properties is being investigated. The results achieved in short pulse operation are close to or even exceed the ITER requirements with respect to the extracted ion currents. However, the extracted negative ion current for long pulse operation (up to 1 h) is limited by the increase of the co-extracted electron current, especially in deuterium operation.

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Section: Scaling Of Magnetic Field Topology Bias Area and Cs Consumpmentioning

confidence: 99%

Towards large and powerful radio frequency driven negative ion sources for fusion

et al. 2017

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“…However, in addition, Cs is the most reactive alkali metal and its high vapor pressure leads to a high volatility of the surface layer. Hence, under conditions of negative ion sources with non-negligible amounts of impurities from the background gases (vacuum of 10 −6 mbar) and interaction with the low pressure hydrogen plasma in front of the surface, high dynamics including deterioration [5] and redistribution [6] occur. The expected Cs consumption as well as a desired stable and homogenous work function thus drive the question for alternative materials to caesium [7].…”

Section: Introductionmentioning

confidence: 99%

Work function of Cs-free materials for enhanced H− surface production

Friedl

Cristofaro

Fantz

2018

AIP Conference Proceedings

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Materials with low work function (WF), such as the well-known caesiated surfaces, are most efficient in producing H − ions. In view of the applicability in future negative ion sources, alternative materials to Cs evaporation are studied in a dedicated experiment. Lanthanated (WL10, WL2, MoLa, LaB 6 ) and bariated materials (TDC) as well as molybdenum implanted with Cs are investigated regarding their achievable WF at ion source relevant conditions, in particular for temperatures below 500 • C. In contrast to their usual application as electron emitters at temperatures above 1000 • C, the work functions of the investigated materials under these conditions do not decrease below 3.6 eV (measured global minimum, achieved with LaB 6 ). For the lanthanated materials, the obtained WF values are stable under plasma exposure times of several hours. However, for all the materials the work function is subject to degradation in absence of heating or plasma exposure. Compared to Cs evaporation, with WF values measured down to 2.1 eV, none of the materials tested so far can be regarded as an actual alternative.

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“…It is deposited at the inner surfaces of the source from which it is redistributed mainly by the influence of the plasma [19]. This redistribution process is strongly affected by the wall surface temperatures and the caesium chemistry, i.e.…”

Section: Introductionmentioning

confidence: 99%

A collisional radiative model for low-pressure hydrogen–caesium plasmas and its application to an RF source for negative hydrogen ions

Wünderlich

Wimmer

Friedl

2014

Journal of Quantitative Spectroscopy and Radiative Transfer

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a b s t r a c tA collisional radiative (CR) model for caesium in low-temperature, low-pressure hydrogen-caesium plasmas is introduced. This model includes the caesium ground state, 14 excited states, the singly charged caesium ion and the negative hydrogen ion. The reaction probabilities needed as an input are based on literature data, using some scaling and extrapolations. Additionally, new cross sections for electron collision ionization and threebody recombination have been calculated.The relevance of mutual neutralization of positive caesium ions and negative hydrogen ions is highlighted: depending on the densities of the involved particle species, this excitation channel can have a significant influence on the population densities of excited states in the caesium atom. This strong influence is successfully verified by optical emission spectroscopy measurements performed at the IPP prototype source for negative hydrogen ions.As a consequence, population models for caesium in electronegative low-temperature, low-pressure hydrogen-caesium plasmas need to take into account the mutual neutralization process. The present CR model is an example for such models and represents an important prerequisite for deducing the total caesium density in such plasmas.

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Dynamics of the transport of ionic and atomic cesium in radio frequency-driven ion sources for ITER neutral beam injection

Cited by 34 publications

References 24 publications

Towards large and powerful radio frequency driven negative ion sources for fusion

Towards large and powerful radio frequency driven negative ion sources for fusion

Work function of Cs-free materials for enhanced H− surface production

A collisional radiative model for low-pressure hydrogen–caesium plasmas and its application to an RF source for negative hydrogen ions

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