Modeling of the negative ion extraction from a hydrogen plasma source. Application to ITER Neutral Beam Injector

The development of a high performance negative ion (NI) source constitutes a crucial step in the construction of Neutral Beam Injector of the future fusion reactor ITER. NI source should deliver 60 A of H (or 40 A D), which is a technical and scientific challenge, and requires a deep understanding of the underlying physics of the source including the role of the magnetic filter. The present knowledge of the ion extraction mechanism from the negative ion source is mainly based on experimental work involving among other difficulties the complex magnetized plasma sheaths used to avoid electrons being co-extracted from the plasma together with NI. Moreover, due to the asymmetry induced by the crossed magnetic configuration used to filter and further to deflect the electrons, any realistic study of this problem must consider the three space dimensions. To properly address this problem, 3D Particle-in-Cell electrostatic collisional code was developed within the framework of this thesis. Binary collisions between the particles are introduced using Monte Carlo Collision approach. The code estimates the solution of Boltzmann equation, bringing detailed information about the density and energy distribution of the different species. The code uses Cartesian coordinate system, but it can deal with curve boundary geometry as it is the case of the extraction apertures. Complex orthogonal magnetic fields are also taken into account. This code, called ONIX (Orsay Negative Ion eXtraction), was used to investigate the plasma properties and the transport of the charged particles close to a typical extraction aperture. The main results obtained from this code are presented in this thesis. They include: • negative ions and electrons 3D trajectories in the extraction region of ITER Neutral Beam Injector plasma source system; • the meniscus formation in the plasma in front of the extraction aperture by the repealing of positive ions; • negative ion and electron current density profiles for different local magnetic field configurations; • production, destruction, and transport of H in the plasma volume, close to the extraction region. Production of H in the volume is investigated via electron dissociative attachment to the vibrationally exited molecules H2(v) collision. The negative ion destruction processes are:(i) mutual neutralization,(ii) electron detachment and (iii) associative and non-associative detachment with H ; • negative ion surface production is examined via (i) interaction of the positive ions H and H 2 with the aperture surface and (ii) collisions of the neutral gas H with plasma grid wall. 1 te l-0 06 72 14 0, v er si on 1 20 F eb 2 01 2 • extraction efficiency of the negative ion produced from the volume and the plasma grid surface; • the role of sheath behavior in the vicinity of the aperture and the NI extraction limitation due to the double layer structure induced by negative ion flux from the surface; • influence of the external extracted potential value on the formation of negative sheath and the strength of the magnetic filter on the total extracted NI and coextracted electron current; • the suppression of the electron beam by the negative ion produced at the plasma grid wall. Most of these results are in good agreement with experimental data obtained by IPP group (Garching, Germany). The performance predicted for the extractor fulfills the ITER NBI requirements in terms of extracted NI and electron currents and current densities. Thus, it opens a large field of possible configurations being a valuable tool for future optimizations of the NI source. 2 te l-0 06 72 14 0, v er si on 1 20 F eb 2 01 2