Abstract: A system for deflecting a beam of particles having different momenta, preferably through a 90.degree. angle, so as to cause the beam to impinge upon a moving target and to scan across the target. The system includes a means responsive to a beam from a suitable source for causing the beam to periodically scan in a scanning plane and further means for deflecting the periodically scanned beam through the desired angle in a deflection plane so that the deflected beam impinges on the target. Means are included in the system for reducing the momentum dispersion at the target in both the deflection and the scanning planes and for spatially focussing the beam so as to produce a desired beam diameter at the target.