Negative Ion Extraction With Finite Element Solvers and Ray Maps

Abstract

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 Σ (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 Σ . 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.

Moderately converging ion and electron flows in two-dimensional diodes

“…For example, with R 1 = 2 cm and |q|j a = 300 A/m 2 of H − ions, which are typical values for ion extraction, 11,16 we get K E0 = 16.9 keV, which is a scale kinetic energy for these ion diodes. For electrons with same R 1 and current density, we get K E0 = 1.38 keV.…”

“…Note that many useful numerical codes exist [1][2][3][4] for diode and triode design, with faster computations for 2D cases; 11 typical trajectory plots confirm that flow is not rectilinear also in the first gap, see Fig. 2.…”

Conceptual design and circuit analyses for the power supplies of the NIO1 experiment

“…As an example, beam optics analyses [4] on aiming and focusing have to be translated also in the specification of the maximum allowable ripple of the output voltage of the AGPS and EGPS.…”

Exploration of operational scenarios of the SPIDER accelerator

“…To this scope the 8ypo code [12] was used, being the only self-consistent code able to cope with both electrons and ions also in the meniscus shape calculation. Once the SPIDER characteristics were implemented in the code, (including the magnetic field distribution, calculated externally), the value of RJ was tested in the range 0.5-2, and its effect on the beam optics was checked.…”