An electrostatic steering system to focus the bea mlets in advanced neutral beam injectors

Neutral beam injection is the main method to heat the plasma in magnetic confinement fusion devices. In high energy injector (E>100 KeV/amu), it is convenient to produce neutrals by conversion of negative ions via electron detachement reactions. In the c ase of large fusion devices , as ITER, several A mpere of H ? or D ? , current are required to satisfy the heating power demand, calling for a large extraction area on the ion source, hence for the use of gridded electrodes in the accelerator, allowing the acceleration of thousands of beamlets together. A careful aiming system is required to merge these beamlets, and to deliver their power in the plasma chamber, usually placed far from the the accelerator: in the case of the ITER, the focal point is 24.5 met ers away from the beam source. In nowadays injectors, the aiming is realized by aperture offset tecnique or by grid shaping. This paper discuss an alternative concept of beamlets aiming, based on an electrostatic "steerer" to be placed at the end of the ac celerator. A feasibility study of this component is also presented, and its main advantages and drawbacks with respect to other methods are discussed.