The ITER NBI is based on negative hydrogen ions extracted from caesiated ion sources. The 3D particle-in-cell Monte Carlo collision code Orsay negative ion extraction (ONIX) models the beamlet formation of negative ions in such sources where surface production plays an important role. A coupling scheme between ONIX and the ion-optics code ion beam simulator (IBSimu) has been developed and compared to other particle simulation approaches. This extends the computational domain such that the complete grid system can be included while only marginally increasing the computational cost. The properties of the accelerated ONIX beamlet are studied and compared to standalone IBSimu calculations, which are based on a simplified plasma model. The comparison provides insight about the effect of approximations made in ion-optics codes, which were also used to design the ITER NBI grid systems. ONIX volume and surface produced negative ions have a different angular distribution in the accelerated beamlet. The ONIX volume produced particles have a similar core divergence compared to standalone IBSimu calculations, but there is more halo in the IBSimu angular distribution. In the ONIX simulations, a Debye sheath is formed between the plasma and the grid, which repels negatively charged particles. The sheath decreases the extracted current density at the edge of the aperture for volume produced ions. Contrarily, surface produced particles are directly extracted near the edge of the aperture. Particles extracted near the edge of the aperture are highly divergent at the end of the grid system, independent of their initial angle. To summarize, the presence of the plasma sheath around the apertures in the plasma grid as calculated by ONIX decreases the halo from volume produced particles compared to standalone IBSimu.

https://iopscience.iop.org/article/10.1088/1741-4326/ac9c6f/pdf

From meniscus formation to accelerated H− beam: coupling of 3D-PIC and ion-optics simulations

A caesiated RF driven source delivers H- ions that, after stripping at the end of the 160 MeV H- linear injector, provides protons to CERN's accelerator complex including LHC, where the protons reached a record energy of 6.8 TeV. In Caesiated RF sources, H- ions are produced via dissociative attachment of electrons onto roto-vibrationally excited H2-molecules (volume) and re-emission as negative ions of protons or hydrogen atoms colliding on a low work function caesiated molybdenum plasma electrode (surface). During initial caesiation, the production mechanism evolves from the initial Cs-free volume production to a predominant surface production mode; the observed stunning reduction of co-extracted electrons is concomitant to an increase of the H- ion current to RF-power yield. This paper describes the evolution of the beam-profile at today's operational beam intensities of 35 mA for various ratios of volume and surface ion-origin. The presence of surface produced ions occurring on a conical plane is characterized by the electron to ion ratio and by measurement of the Cs-coverage of the molybdenum plasma electrode down to a fraction of a monolayer. Angular distributions are extracted from beam profile and Beam Emission Spectroscopy (BES) measurements. These experimental results provide an initial comparison to beam formation simulation that, at a later stage, could be coupled to beam transport software packages. The paper focuses on the caaesiation transient to present experimental evidence for 3D beam formation studies, it provides insight into the mixing of volume and surface production modes, reduction of co-extracted electrons and Cs-coverage. The paper also establishes magnetic field induced asymmetries in the beam's current density.

Correlation of H- beam properties to Cs-coverage

The CERN accelerator complex is served by several different types of hadron sources, producing both high- and low-intensity negative H- beams, highly charged positive stable ions, but also transforming low-charge radioactive ions to higher charges before post-acceleration. Apart from the operational sources, research and development is carried out at dedicated test stands for H- production and pre-acceleration, ECRIS oven experiments and general EBIS development. In this paper, we will report on the performance and the latest development related to the sources. Efforts to enhance the theoretical understanding of the source physics will be briefly reviewed.

CERN hadron sources: status and innovation overview

Abstract A caesiated RF driven source delivers H - ions that, after stripping at the end of the 160 MeV H - linear injector, provides protons to CERN's accelerator complex including LHC, where the protons reached a record energy of 6.8 TeV. In Caesiated RF sources, H - ions are produced via dissociative attachment of electrons onto roto-vibrationally excited H 2 -molecules (volume) and re-emission as negative ions of protons or hydrogen atoms colliding on a low work function caesiated molybdenum plasma electrode (surface). During initial caesiation, the production mechanism evolves from the initial Cs-free volume production to a predominant surface production mode; the observed stunning reduction of co-extracted electrons is concomitant to an increase of the H - ion current to RF-power yield. This paper describes the evolution of the beam-profile at today's operational beam intensities of 35 mA for various ratios of volume and surface ion-origin. The presence of surface produced ions occurring on a conical plane is characterized by the electron to ion ratio and by measurement of the Cs-coverage of the molybdenum plasma electrode down to a fraction of a monolayer. Angular distributions are extracted from beam profile and Beam Emission Spectroscopy (BES) measurements. These experimental results provide an initial comparison to beam formation simulation that, at a later stage, could be coupled to beam transport software packages. The paper focuses on the caaesiation transient to present experimental evidence for 3D beam formation studies, it provides insight into the mixing of volume and surface production modes, reduction of co-extracted electrons and Cs-coverage. The paper also establishes magnetic field induced asymmetries in the beam's current density. 

Correlation of H- beam properties to Cs-coverage

H- Beam formation simulation in negative ion source for CERN's Linac4 accelerator

An H- ion source is being operated at the new 160 MeV linear injector (Linac4) of the CERN accelerator complex. The source’s plasma is of the Radio Frequency Inductively Coupled Plasma type (RF-ICP), without magnetic cusp and runs with Cs-loss compensation [1]. Vertical downward oriented filter- and electron dump-dipolar magnetic fields expand over the plasma chamber, beam-formation, beam-extraction and electron dump regions and generate horizontal asymmetry and beam angular deflection partially compensated by mechanical alignment of the front-end. The H- beam is generated via volume and caesiated plasma surface modes, the latter inducing a radial asymmetry characterized by an increased current density close to the plasma electrode surface [2]. Asymmetries affecting the meniscus shape, or its current density have to be simulated via 3D Particle In Cell Monte Carlo (PIC-MC) solvers, such as the Orsay Negative Ion eXtraction code (ONIX) [3]. Validation of these simulations require dedicated measurements. This contribution describes experimental methods newly implemented at CERN’s ion source test stand and initial results for Optical and Beam Emission Spectroscopy (OES, BES), emittance and beam profile measurements. In a later stage, the gathered data sets can be compared to source plasma parameters and extracted beam parameters from PIC-MC simulations, once coupled to the Ion Beam Simulation (IBSimu) [4] beam transport code. The experimental parameter space includes RF-power, density of neutrals, position of the RF coil and extraction field. Beams of H-, D- and protons were produced; examples of measured data are presented in this contribution.

Beam Formation Studies on the CERN IS03b H Source

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Beam Formation Studies on the CERN IS03b H Source

Correlation of H- beam properties to Cs-coverage

H- Beam formation simulation in negative ion source for CERN's Linac4 accelerator