RF-Driven Ion Sources for Neutral Beam Injectors for Fusion Devices

Abstract

Inductively coupled plasma sources at a frequency of 1 MHz and at power levels of up to 100 kW per cylindrically shaped plasma generation chamber (driver) are used for neutral beam injection (NBI) systems of fusion devices. The modular concept, having several drivers for plasma generation arranged at the backplate of a large expansion chamber, is the baseline for the NBI systems of the international fusion experiment ITER. The reliability in terms of operation and the proven technology of an RF-driven ion source demonstrated for positive hydrogen ions and initiated a development line with size-scaling for negative ions (hydrogen and deuterium) being mandatory for ITER NBI. The cornerstones of the development route toward the ion source of an area of 1 m × 2 m to illuminate with a uniform plasma the multi-aperture extraction system and the generation of negative hydrogen ion densities in the order of 300 A/m² are described. Addressed are the RF-coupling scheme, reliable operation at low pressure (0.3 Pa for ITER), and the plasma parameters achieved in the ion source. Another building block is the interplay of the magnetic filter field with the biasing of surfaces, the consequences on the plasma uniformity, and in particular, the consequences on the reduction and uniformity of the inevitable co-extracted electrons in such large sources. The cesium coverage of the plasma grid surface, where the negative ions are formed, and the role of cesium redistribution in the ion source come along with the temporal stability of the co-extracted electrons which react, due to the presence of a plasma dominated by negative and positive ions (ion–ion plasma) in front of the plasma grid, much more sensitive than the negative ion current. Of particular challenge is the achievement of high negative ion current densities in long pulses, meaning steady-state operation up to 1 h. The limitation originates from the strong temporal dynamics of the co-extracted electrons as their heat load on the extraction grid reaches the tolerable value, with the consequence that the ion source parameters like RF-power and extraction voltage need to be reduced to avoid damages of the grid system. For a general reduction, the interplay of magnetic filter field and biasing of surfaces is important, whereas cesium management is of utmost relevance for the temporal stabilization. Finally, the activities in the development of the RF-driven ion source for fusion devices beyond ITER, like a DEMOnstration power plant, are addressed as well.