Abstract
Neutral beam injection is one of the most important methods of plasma heating in thermonuclear fusion experiments, allowing the attainment of fusion conditions as well as driving the plasma current. Neutral beams are generally produced by electrostatically accelerating ions, which are neutralised before injection into the magnetised plasma. At the particle energy required for the most advanced thermonuclear devices and particularly for ITER, neutralisation of positive ions is very inefficient so that negative ions are used. The present paper is devoted to the description of the phenomena occurring when a high-power multi-ampere negative ion beam travels from the beam source towards the plasma. Simulation of the trajectory of the beam and of its features requires various numerical codes, which must take into account all relevant phenomena. The leitmotiv is represented by the interaction of the beam with the background gas. The main outcome is the partial neutralisation of the beam particles, but ionisation of the background gas also occurs, with several physical and technological consequences. Diagnostic methods capable of investigating the beam properties and of assessing the relevance of the various phenomena will be discussed. Examples will be given regarding the measurements collected in the small flexible NIO1 source and regarding the expected results of the prototype of the neutral beam injectors for ITER. The tight connection between measurements and simulations in view of the operation of the beam is highlighted.
Highlights
The history of negative ion beams is already several decades long and their field of use is still growing, including several scientific and technological applications, from sources for accelerators and spallation neutron sources, to nuclear fusion and industrial applications [1]
Particular attention will be devoted to the case of high power beams made of negative ions, of the type to be employed in neutral beam injectors (NBI) for plasma heating and current drive in ITER
The section will deal with the physics of beam neutralisation and transport, whereas section 3 will be devoted to the experimental characterisation of the beam and of the aforementioned phenomena
Summary
The history of negative ion beams is already several decades long and their field of use is still growing, including several scientific and technological applications, from sources for accelerators and spallation neutron sources, to nuclear fusion and industrial applications [1]. Progress is recently driven by the use of negative lightion beams for heating and current drive of thermonuclear fusion plasmas. Such beams are required to have huge energy and current as well as low divergence and high uniformity. Particular attention will be devoted to the case of high power beams made of negative ions, of the type to be employed in neutral beam injectors (NBI) for plasma heating and current drive in ITER. The section will deal with the physics of beam neutralisation and transport, whereas section 3 will be devoted to the experimental characterisation of the beam and of the aforementioned phenomena
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