Topic/Type: 1.6 Plasma-based devices, Poster

Modeling of a high power large volume magnetized negative ion source

G. Hagelaar1, 2, 3, P. Sarrailh1, 2, G. Fubiani1, 2, N. Kohen1, 2, J.P. Boeuf1, 2

1 Universit? de Toulouse; UPS, INPT; LAPLACE (Laboratoire Plasma et Conversion d'Energie), Toulouse, France
2 CNRS; LAPLACE; F-31062 Toulouse, France

A negative ion source for the ITER neutral beam injector (NBI) is intended to deliver a high negative ion current density, typically of the order of 30 mA/cm. The source may be separated in three distinct regions, that is, (i) the inductively coupled plasma discharge (driver) that generates the plasma and dissociate the deuterium gas, (ii) an expansion chamber, and (iii) a magnetic filter designed to create a low temperature electronegative plasma close to the extraction grid. A low electron temperature is necessary in order to enhance negative ion production and survival in the extraction region. In addition, production of a high negative ion current density requires the use of cesium in the source. The latter lowers the work function of the source walls and consequently significantly enhances surface production of negative ions, which accounts for most of the current extracted toward the electrostatic accelerator. The physics involved in such a source is relatively complex and, as a result, providing a precise design requires a substantial modeling effort.

A physical and a numerical model of the driver, expansion chamber, and extraction regions have been developed for the simulation of the negative ion source. The aim of the model is to provide a better understanding of the source and realistic predictive capabilities at a reasonable computational cost. The model is based on a fluid description of electrons and ions coupled with Poisson?s equation in a semi-implicit way. The drift-diffusion approximation is used for the momentum equations of electrons and negative ions, while inertia is properly taken in to account in the positive ion momentum equation). The neutral particles are described either as a fluid, using Navier Stokes equations, or in a fully kinetic way, using a Direct Simulation Monte-Carlo (DSMC) algorithm. The main features of the model will be presented, with emphasis on the specific difficulties associated with the presence of the magnetic filter.

This work is supported by ANR (National Research Agency) under contract BLAN08-2\_310122, by the ITER Federation and by the French Atomic Energy Commission (CEA).