Electron density profile measurements at a self-focusing ion beam with high current density and low energy extracted through concave electrodes

Fujiwara, Yukio; Hirano, Y.; Kiyama, S.; Nakamiya, A.; Koguchi, Haruhisa; Sakakita, Hajime

doi:10.1063/1.4827680

Cited by 5 publications

(6 citation statements)

References 5 publications

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“…The experimental apparatus (figure 1) includes an ion source, extracting electrodes, an ion-beam propagation chamber, and measurement systems including a pair of double probes and three Faraday cups. The setup is almost the same as that used in our previous experimental work [4][5][6][7][8].…”

Section: Experimental Methodsmentioning

confidence: 99%

“…A transition from a rather broad profile to a focused profile takes place when the beam energy exceeds a threshold, which is dependent of I ib . For example, a rapid transition takes place between 60 and 80 eV for I ib ∼30 mA [6] and an abrupt transition between 140 and 160 eV for I ib ∼100 mA [4,5]. Electrostatic double probes were installed in a beam plasma region and, with I ib =30 mA, profiles of the electron density (n e ) and temperature (T e ) were measured [6].…”

Section: Introductionmentioning

confidence: 99%

“…For example, a rapid transition takes place between 60 and 80 eV for I ib ∼30 mA [6] and an abrupt transition between 140 and 160 eV for I ib ∼100 mA [4,5]. Electrostatic double probes were installed in a beam plasma region and, with I ib =30 mA, profiles of the electron density (n e ) and temperature (T e ) were measured [6]. The value of n e was found to be quite high, almost ten times larger than the beam-ion density (n ib ).…”

Section: Introductionmentioning

confidence: 99%

“…To survey the possible mechanisms underpinning the rapid transition to the highly focused state, we improved to some extent measurement taking and the methods of analysis from previous work [4][5][6][7][8]. We tried selective use of a double probe, in which one electrode (P 3 ) is shielded by another electrode (P 1 ) with respect to the ion beam trajectory.…”

Section: Introductionmentioning

confidence: 99%

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Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

Hirano¹,

Fujiwara²,

Kiyama³

et al. 2019

Plasma Sources Sci. Technol.

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Using an electrostatic double probe, profiles of the ion beam current density, electron density, and electron temperature were measured in the background plasma that appears by injecting a hydrogen ion beam into low-pressure hydrogen gas in a propagation chamber. The hydrogen ion beam is extracted using concave-shaped electrodes at extremely low energies (E ib =60-120 eV). Focusing of the beam occurs when E ib exceeds a certain threshold. One probe electrode P 3 is far from the beam source and is set in the shadow of another electrode in the beam trajectory. The ion saturation current in P 3 is then estimated without considering the beam contribution. The measured electron densities are much larger than those of the ion beam density, the electron temperatures are very low (<1 eV), and the ion beam current densities exhibit fairly good agreement with those measured by Faraday cups. The profiles of the space potential are also estimated by measuring the potential of P 3 with respect to ground with a voltage divider having an extremely high resistance. The space potentials obtained are quite low at <10 eV with focusing and ∼23 eV without focusing. The data with and without focusing are compared and conditions for focusing are examined. Focusing achieved through additional electron beam injection in the ground electrode is also examined. The results obtained indicate that a large electron source is required to balance the ion charges. Secondary electron emissions and/or small electron beam injection are effective sources.

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Section: Experimental Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

Hirano¹,

Fujiwara²,

Kiyama³

et al. 2019

Plasma Sources Sci. Technol.

View full text Add to dashboard Cite

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“…An example of the use of movable emissive probes is reported by Sherman and Allison [152]. More recently in low-energy positive beams, movable 4-pin double probes are used [153]¸the probe support is a conductor to avoid charge-up of the internal insulator up to the beam potential. Voltage oscillations were measured using capacitive probes [154] or arrays of Langmuir probes suitably located to detect also the frequency-wavenumber spectrum [62]: diagnostic systems of this kind allowed the identification of the features of the waves excited during beam-gas interaction.…”

Section: Measurement Of Beam Space Charge Compensation or Beam Plasmamentioning

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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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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Self-focusing of a high current density ion beam extracted with concave electrodes in a low energy region around 150 eV

Hirano

Kiyama

Koguchi

et al. 2013

Review of Scientific Instruments

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Spontaneous self-focusing of ion beam with high current density (Jc ∼ 2 mA/cm(2), Ib ∼ 65 mA) in low energy region (∼150 eV) is observed in a hydrogen ion beam extracted from an ordinary bucket type ion source with three electrodes having concave shape (acceleration, deceleration, and grounded electrodes). The focusing appears abruptly in the beam energy region over ∼135-150 eV, and the Jc jumps up from 0.7 to 2 mA/cm(2). Simultaneously a strong electron flow also appears in the beam region. The electron flow has almost the same current density. Probably these electrons compensate the ion space charge and suppress the beam divergence.

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Electron density profile measurements at a self-focusing ion beam with high current density and low energy extracted through concave electrodes

Cited by 5 publications

References 5 publications

Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

Neutralisation and transport of negative ion beams: physics and diagnostics

Self-focusing of a high current density ion beam extracted with concave electrodes in a low energy region around 150 eV

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