Plasma properties of the cybele negative ion source for fusion applications: PIC simulations and experiments

Summary form only given. Neutral beam injection (NBI) is an essential part of the initial plasma heating that is necessary for fusion reactors to enter in the burn phase. For ITER, the neutral beam will be generated by collisional detachment of negative ions produced in and extracted from an inductively coupled plasma, and accelerated to 1 MeV. The efficiency of negative ion neutralization by collisions with neutrals is modest (about 55%) and more efficient solutions are sought for future reactors (DEMO). The IRFM team a CEA Cadarache has proposed a more efficient concept of neutralization of the negative ion beam based on photodetachment¹. This concept (called Siphore) requires that the ion source and accelerator provide a thin (~3 cm wide) ion beam sheet (30A of D^- at 1 or 2 MeV) in the neutralization region.A new negative ion source (called Cybele) has been designed at IRFM in order to produce an intense ion beam sheet of negative ions1,2. This source is a magnetized plasma column (~1 m long) with a uniform axial magnetic field B. Extracting grids with a relatively thin aperture are distributed along the walls of the plasma column. In this paper we focus on the Cybele negative ion source. In a first experimental campaign at IRFM, the plasma of Cybele has been generated by heated filaments distributed along the axis of the plasma column. We present model results of the Cybele source obtained with Particle-In-Cell Monte Carlo Collision (PIC MCC) simulations compared with Langmuir probe measurements of the plasma properties. Good agreement between simulations and experiments is obtained for time averaged quantities such as plasma density and potential distributions. The simulations predict the development of rotating plasma instabilities that are consistent with the critical ionization concept3 and with the measurement of the time averaged radial electric field Er in the plasma (plasma rotation at a velocity Er/B).