Negative hydrogen ion production mechanisms

Bacal, M.; Wada, M.

doi:10.1063/1.4921298

Cited by 239 publications

(181 citation statements)

References 245 publications

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“…4 The negative hydrogen ion source is based on surface production of negative ions on a caesiated grid: hydrogen atoms and positive ions, produced in a low pressure, low temperature hydrogen plasma are converted to negative hydrogen ions on a surface with low work function. 5 As a consequence of the vacuum conditions in the source (background pressure in the order of 1 × 10 −6 mbar) in combination with the high chemical reactivity of Cs, a repetitive or even continuous evaporation of Cs into the source is required for maintaining a low work function of the converter surface. A critical parameter is the amount of co-extracted electrons, which often limits the performance of the prototype source: co-extracted electrons are removed out of the extracted particle beam by magnets, bending them onto the second grid of the three-grid extraction system prior full acceleration.…”

Section: Introductionmentioning

confidence: 99%

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Wimmer

Fantz

NNBI-Team³

2016

Journal of Applied Physics

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Large-scale sources for negative hydrogen ions, capable of delivering an extracted ion current of several ten amperes, are a key component of the neutral beam injection system of the upcoming ITER fusion device. Since the created heat load of the inevitably co-extracted electrons after magnetic separation from the extracted beam limits their tolerable amount, special care must be taken for the reduction of coextracted electrons -in particular in deuterium operation, where the larger amount of co-extracted electrons often limits the source performance. By biasing the plasma grid (PG, first grid of the extraction system) positively with respect to the source body the plasma sheath in front of the PG can be changed from an electron repelling towards an electron attracting sheath. In this way, the flux of charged particles onto the PG can be varied, thus changing the bias current and inverse to it the amount of co-extracted electrons. The PG bias affects also the flux of surface-produced H − towards the plasma volume as well as the plasma symmetry in front of the plasma grid, strongly influenced by an ⃗ E × ⃗ B drift. The influence of varying PG sheath potential profile on the plasma drift, the negative hydrogen ion density and the source performance at the prototype H − source is presented, comparing hydrogen and deuterium operation. The transition in the PG sheath profile takes place in both isotopes, with a minimum of co-extracted electrons formed in case of the electron attracting PG sheath. The co-extracted electron density in deuterium operation is higher than in hydrogen operation, which is accompanied by an increased plasma density in deuterium.

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Section: Introductionmentioning

confidence: 99%

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Wimmer

Fantz

NNBI-Team³

2016

Journal of Applied Physics

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“…A long-pulsed N-NBI is required to produce the beam energy of ∼ 1 MeV and beam current of ∼ 200 A/m 2 for ITER. There are two processes of negative ion production, called volume and surface production processes [7]. In the volume production process, hydrogen molecules are vibrationally excited by collisions with high energy electrons.…”

Section: Introductionmentioning

confidence: 99%

“…The negative hydrogen ions are produced via a dissociative attachment process with low energy electrons. To increase the dissociative attachment process and to avoid the destruction of negative ions, it is necessary to decrease the electron temperature to less than 1 eV [7]. In the surface production, hydrogen ions or atoms capture electrons on the metal surface having a low work function.…”

Section: Introductionmentioning

confidence: 99%

Characteristics of a Large Diameter Radio-Frequency Negative Hydrogen Ion Source

Sasaki

Takayama

Nakano

et al. 2016

Plasma and Fusion Research

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The characteristics of a 230-mm-diameter radio-frequency (rf) negative hydrogen ion source are investigated by the measurements of electron density and temperature. A hydrogen plasma is produced by an inductively coupled discharge operated with an rf frequency of 300 kHz and a power of a few tens of kW, where a fieldeffect-transistor-based inverter power supply is used as an rf generator. The ion source has a magnetic filter for reduction of the electron temperature near the plasma grid used for the extraction of a negative ion beam. A high electron density greater than 10 18 m −3 is successfully obtained for an operating gas pressure of 0.3 Pa. The electron temperature near the plasma grid is observed to decrease to about 1 eV with increasing magnetic field strength of the magnetic filter.

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“…The efficiency of this process significantly depends on the work function χ of the surface. For this purpose the converter is covered with the alkali metal caesium which lowers its surface work function and thus significantly increases the negative ion yield 1,2 . Hence, quantification of the work function and identification of its influencing factors are mandatory for optimization of negative ion sources; especially in view of the powerful large-scale negative ion sources required for the next generation of fusion devices like ITER, where negative ion beams of several ten amperes have to be generated homogeneously over an area of several m 2 and stable for several thousand seconds.…”

Section: Introductionmentioning

confidence: 99%

Influence of H2 and D2 plasmas on the work function of caesiated materials

Friedl

Fantz

2017

Journal of Applied Physics

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Caesium-covered surfaces are used in negative hydrogen ion sources as a low work function converter for H–/D– surface production. The work function χ of the converter surface is one of the key parameters determining the performance of the ion source. Under idealized conditions, pure bulk Cs has 2.14 eV. However, residual gases at ion source background pressures of 10−7–10−6 mbar and the plasma surface interaction with the hydrogen discharge in front of the caesiated surface dynamically affect the actual surface work function. Necessary fundamental investigations on the resulting χ are performed at a dedicated laboratory experiment. Under the vacuum conditions of ion sources, the incorporation of impurities into the Cs layer leads to very stable Cs compounds. The result is a minimal work function of χvac ≈ 2.75 eV for Cs evaporation rates of up to 10 mg/h independent of substrate material and surface temperature (up to 260 °C). Moreover, a distinct degradation behavior can be observed in the absence of a Cs flux onto the surface leading to a deterioration of the work function by about 0.1 eV/h. However, in a hydrogen discharge with plasma parameters close to those of ion sources, fluxes of reactive hydrogen species and VUV photons impact on the surface which reduces the work function of the caesiated substrate down to about 2.6 eV even without Cs supply. Establishing a Cs flux onto the surface with ΓCs ≈ 1017 m−2 s−1 further enhances the work function obtaining values around 2.1 eV, which can be maintained stable for several hours of plasma exposure. Hence, Cs layers with work functions close to that of pure bulk Cs can be achieved for both H2 and D2 plasmas. Isotopic differences can be neglected within the measurement accuracy of about 0.1 eV due to comparable plasma parameters. Furthermore, after shutting down the Cs evaporation, continuing plasma exposure helps against degradation of the Cs layer resulting in a constant low work function for at least 1 h.

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Negative hydrogen ion production mechanisms

Cited by 239 publications

References 245 publications

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Characteristics of a Large Diameter Radio-Frequency Negative Hydrogen Ion Source

Influence of H2 and D2 plasmas on the work function of caesiated materials

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