Negative Ion Extraction With Finite Element Solvers and Ray Maps

The negative ion production and extraction are critical aspects of the neutral beam injector for ITER, so that improvements in their physical understanding are expected to contribute to the optimization of the injector design. A self-consistent 2-D model of the formation and acceleration of a beam of negative ions (including the sheath) is presented here, with plasma presheath replaced by a boundary condition (on a so-called start line) consistent with the 1-D complete plasma equations. An equivalent volume production before the start line is assumed. Negative beam space charge is expressed in terms of a current modulus j_Sigma(z,x), coupled to the adimensional electric potential u. Solution is obtained with a partial differential equation finite element solver for u and with an iterative approach for j_Sigma. A continuous map of particle orbits or rays (efficiently generated by interpolation) is used to improve the traditional discrete ray tracing; moreover, beam edge resolution is refined. Results include detailed information on plasma meniscus and beam skin structure.