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A coupled PIC-Poisson-solver code for the extraction of charged particles from a negative ion source
Abstract: Negative ion sources have since long been proposed as suitable production means of fast particle beams to be used in International Thermonuclear Experiment Reactor (ITER) as additional heating mechanisms. The numerical simulation of the particle production and extraction mechanisms within these sources is a valuable tool towards their optimisation. A challenging aspect of the problem of producing a realistic consistent model for the external applied fields, extracted particles and plasma boundary has been the large difference between the microscopic characteristic Debye length and the macroscopic size of the actual geometry. Therefore, a lot of work has been focussed on describing the extraction region over distances of the order of Debye sheath length. As a consequence, any macroscopical description-aimed at describing details over spatial scales of order of 10-2 m and larger-cannot be obtained within the same models by using realistic computer resources. Here, instead, a numerical code has been developed ex-novo by decoupling the problem: the model has been reduced to one of electrostatics coupled self-consistently to plasma dynamics, in which electric fields are both applied from the outside as well as partly generated by the motion of the charged particles. Non-trivial boundary conditions are to be supplemented, in order to describe the complicated geometry of the actual sources. A realistic geometry has been used, attempting to model with quite detail the extraction geometry of the sources under development for the ITER injector. This geometry includes values of electrical and magnetic fields foreseen for the ITER injector. The code is designed to work in cases where the plasma density acts as a perturbation over the potential profiles or, equivalently, in the approximation of very strong applied extracting potentials compared to the plasma potential. In this work the present stage of development of such a numerical code is presented. Present results include: an assessment of the relative effectiveness of surface production versus volume production for extracting negative ions; and a study of the effectiveness of a magnetic filter in confining electrons away from the extraction region.
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