Comparison of ONIX simulation results with experimental data from the BATMAN testbed for the study of negative ion extraction

Abstract

The development of negative ion (NI) sources for the ITER neutral beam injector is strongly accompanied by modelling activities. The ONIX (Orsay Negative Ion eXtraction) code simulates the formation and extraction of negative hydrogen ions and co-extracted electrons produced in caesiated sources. The 3D geometry of the BATMAN extraction system and the source characteristics such as the extraction and bias potential, 3D magnetic field were integrated in the model. Calculations were performed using plasma parameters experimentally obtained on BATMAN. Comparison of the ONIX calculated extracted NI density with experimental results suggests that predictive calculations of the extraction of negative ions are possible. The results show that for an ideal status of the Cs conditioning the extracted hydrogen NI current density could reach about ~30 mAcm -2 at 10 kV and about ~20 mAcm -2 at 5 kV extraction potential with an electron/NI current density ratio of about 1, as measured in the experiments under the same plasma and source conditions. The dependency of the extracted NI current on the negative ion density in the bulk plasma region from both the modeling and the experiment was investigated. The separate distributions composing the negative ion beam originating from the plasma bulk region and the PG surface are presented for different NI plasma volume densities and NI emission rates from the plasma grid wall respectively. The extracted current from NI produced at the Cs covered plasma grid (PG) surface, initially moving towards the bulk plasma and then being bent towards the extraction surfaces is lower compared to the extracted NI current from directly extracted surface produced ions.

Kinetic sheath in presence of multiple positive ions, negative ions, and particle wall emission

“…For the simplest case of an electropositive hydrogen plasma with one positive ion specie (protons), potential sheath drops and sheath sizes of the presented analytical model were successfully benchmarked in 51 against two particle-in-cell codes ONIX 52,53 and BACON 21 and one analytical model 28 . Both ONIX and BACON enforce the Bohm criterion and the presheath voltage drop within 20 % when compared to fluid sheath results.…”

Negative Hydrogen Ion Sources for Fusion: From Plasma Generation to Beam Properties

“…For covering this aspect, 3D simulation codes resolving the plasma sheath are required. Among them is the ONIX code [46][47][48], which is a 3D particle-in-cell code using a self-consistent description of the plasma-wall interface in the presence of surface-emitted negative ions. Since 3D PIC codes resolving the plasma sheath are extraordinary computational demanding, only a small simulation domain can be used (one aperture, plasma a few mm upstream of the PG until the EG surface).…”

Comment on ‘Issues in the understanding of negative ion extraction for fusion’

“… Co-extracted electrons ONIX is the only code aiming to calculate and predict the co-extracted electron current. A dedicated benchmark shows good agreement for different extraction potentials and source conditioning [5]. As pointed out several times, the 3D topology of the magnetic field, in particular the deflection field in the extraction grid, plays a crucial role for such simulations.…”

“…The Cs conditioning from experiments was modelled in ONIX by variation of the emission rate of negative ions from the plasma grid surface. It was found that the simulations were able to reproduce experimental results [5] by using the emission rate in the realistic range (50-550 A/m 2 ).…”