First operations with caesium of the negative ion source SPIDER

The negative-ion based neutral beam injector for heating and current drive of the ITER plasma (ITER HNB) is under development, at present focusing on the optimization of the full-scale plasma source in the SPIDER test stand. The production of H- or D- ions in the ion source is based on the low work function surfaces obtained by caesium evaporation. This paper describes the caesium conditioning procedure and the corresponding beam performances during the first operation of SPIDER with caesium. Technical solutions to overcome present limitations of the test stand are described. The influence of source parameters on the caesium effectiveness was investigated in short beam pulse operation; with total radio-frequency (RF) power of 400 kW and filling pressure below 0.4 Pa, and a limited number of extraction apertures, a negative ion current density of about 200 A m(-2) was extracted in hydrogen, with beam energy lower than 60 keV. Beam optics and beam uniformity were assessed thanks to the acceleration of isolated ion beamlets. A possible procedure to accelerate a uniform beam was demonstrated at low RF power. The results obtained in this first investigation provided key indications on the operation of one of the largest existing sources of accelerated negative hydrogen-like ions.

Negative hydrogen ion dynamics inside the plasma volume of a linear device: Estimates from particle-in-cell calculations

Riccardo Agnello, Ivo Furno, Philippe Frédérique Bruno Guittienne, Alan Howling, Rémy Jacquier

Negative hydrogen or deuterium ions are the precursor particles used to generate a high power beam of neutrals in order to heat the tokamak plasma core of magnetic fusion devices, inject current, and to some extent control instabilities. In the case of ITER, for instance, the negative ions are produced inside a high power large volume low-pressure tandem type magnetized ion source and extracted toward an electrostatic accelerator which accelerates them to 1 MeV before entering a neutralizer converting the ions into a neutral beam. This so-called neutral beam injector relies on the production of negative ions on the surface facing the plasma of the ion source extraction electrode. The latter is covered by a cesium layer in order to increase the negative ion yield. The use of cesium is currently an issue as it may diffuse outside of the source and induce secondary particle production or voltage breakdowns inside the accelerator vessel requiring a regular maintenance in a nuclear environment. In this work, we analyze numerically with a 2.5D particle-in-cell model the production rate and transport of negative ions in a linear device used as an ion source. The negative ions are generated via a dissociative attachment process with a hydrogen molecule in the volume of a magnetized cesium-free plasma. The linear device in the model has a large aspect ratio with a radius of 5 and a length of 100 cm and the magnetic field strength ranges from 100 to 400 G. We show that the shape and depth of the plasma potential profile may be controlled by biasing the end-plates which in turn strongly influence the residence time of the electrons and hence the negative ion yield. We observe the formation of large-scale rotating structures when the positive ions become magnetized with a rotation velocity in the kHz range.

AIP Publishing2021

Nitriding of tetragonal zirconia in a high current d.c. plasma source

Thierry Delachaux, Christoph Hollenstein

Surface nitriding of tetragonal zirconia (t-ZrO2) has been performed with the aim of combining the optical properties of zirconium nitride (ZrN) with the hardness and toughening properties of tetragonal zirconia. A high current d.c. arc plasma source has been used to create a reactive environment of atomic hydrogen and NH radicals. Plasma analysis showed that a highly reductive atmosphere is necessary to transform the tetragonal zirconia. X-ray diffraction and transmission electron microscopy analysis show an efficient transformation of the t-ZrO2 at the surface which exhibits the typical yellow-gold color of ZrN and high wear resistance. For a 2 h treatment at 750 degreesC the nitriding is accompanied by the formation of a 250 nm thick gradient layer of ZrN structure and a partial transformation of t-ZrO2 into cubic zirconia deeper in the bulk. (C) 2002 Elsevier Science B.V. All rights reserved.

2003

Negative hydrogen ion dynamics inside the plasma volume of a linear device: Estimates from particle-in-cell calculations

Riccardo Agnello, Ivo Furno, Philippe Frédérique Bruno Guittienne, Alan Howling, Rémy Jacquier

AIP Publishing2021