Modeling of negative ion transport in a plasma source (invited)

A code called NIETZSCHE has been developed to simulate the negative ion transport in a plasma source, from their birth place to the extraction holes. The H(-)/D(-) trajectory is calculated by numerically solving the 3D motion equation, while the atomic processes of destruction, of elastic collision with H(+)/D(+) and of charge exchange with H(0)/D(0) are handled at each time step by a Monte Carlo procedure. This code can be used to calculate the extraction probability of a negative ion produced at any location inside the source. Calculations performed with NIETZSCHE have been allowed to explain, either quantitatively or qualitatively, several phenomena observed in negative ion sources, such as the isotopic H(-)/D(-) effect, and the influence of the plasma grid bias or of the magnetic filter on the negative ion extraction. The code has also shown that, in the type of sources contemplated for ITER, which operate at large are power densities (>1 W cm(-3)), negative ions can reach the extraction region provided they are produced at a distance lower than 2 cm from the plasma grid in the case of volume production (dissociative attachment processes), or if they are produced at the plasma grid surface, in the vicinity of the extraction holes. (C) 1998 American Institute of Physics.