Abstract
The negative hydrogen ion current that can be extracted from ion sources for neutral beam heating in fusion experiments can be strongly restricted by the amount of co-extracted electrons and their increase over time, particularly during long pulses (up to 1 h). Models describing the underlying physics of particle extraction from a low-temperature plasma with a high amount of negative ions are essential for identifying measures for reducing and stabilizing the co-extracted electrons. In this work, the 3D PIC-MCC code ONIX (Orsay Negative Ion eXtraction) for the plasma volume around one extraction aperture in the first grid of the extraction system is used for analyzing the effect of the magnetic field configuration on the co-extracted electrons and the extracted negative ions. The magnetic field topology is the result of superimposing two different fields that are perpendicular to each other, the filter field (dominant in the ion source volume) and the electron deflection field (dominant in the extraction system). A parametric study changing the relative intensity of these two fields is performed. It is demonstrated that on the local scale of the simulation, the strength of the filter field does not affect the amount of co-extracted electrons, while a significant reduction of the co-extracted electron current is observed when strengthening the electron deflection field.
Highlights
The international fusion experiment ITER will use two neutral beam injection (NBI) systems with a power of 16.5 MW each for heating and current drive.[1]
The negative hydrogen ion current that can be extracted from ion sources for neutral beam heating in fusion experiments can be strongly restricted by the amount of co-extracted electrons and their increase over time, during long pulses
The magnetic field topology is the result of superimposing two different fields that are perpendicular to each other, the filter field and the electron deflection field
Summary
The international fusion experiment ITER will use two neutral beam injection (NBI) systems with a power of 16.5 MW each for heating and current drive.[1]. It is essential to perform general physical investigations that can help develop measures for reducing and stabilizing the co-extracted electrons One candidate for such measures is modifications of the strength and the topology of the total magnetic field used to tune the plasma parameters in front of the extraction system: the magnetic field topology is given, on the one hand, by the magnetic filter field aligned in the horizontal direction of the ion source to reduce the electron temperature and density close to the grid system. To check the influence of the deflection field on the particle trajectories, particle tracking codes can be used,[7,11,12] but these codes do not contain a self-consistent description of the plasma Both fluid models and particle tracking codes fail to describe the physics of the meniscus, which is defined as the equipotential surface dividing plasma and beam. Possible measures for reducing the co-extracted electron current without reducing too much the extracted negative ions are identified
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