LAUSR

Neutral beam injection is one of themost importantmethods of plasma heating in thermonuclear fusion experiments, allowing the attainment of fusion conditions aswell as driving the plasma current. Neutral beams are generally produced by electrostatically accelerating ions, which are neutralised before injection into themagnetised plasma. At the particle energy required for themost 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 occurringwhen a high-powermulti-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, whichmust take into account all relevant phenomena. The leitmotiv is represented by the interaction of the beamwith the background gas. Themain outcome is the partial neutralisation of the beamparticles, but ionisation of the background gas also occurs, with several physical and technological consequences. Diagnosticmethods capable of investigating the beamproperties and of assessing the relevance of the various phenomenawill be discussed. Examples will be given regarding themeasurements collected in the smallflexibleNIO1 source and regarding the expected results of the prototype of the neutral beam injectors for ITER. The tight connection betweenmeasurements and simulations in view of the operation of the beam is highlighted.