**9. Extendable mass-Analyzer for meter-scale ribbon beams**

Conventional magnetic mass analysis of ribbon beams broader than 500 mm is difficult and does not scale well. The gap between the poles of a conventional dipole magnet is much smaller than the bending radius of the trajectories in the magnet, to avoid enormous aberrations, stray fields and related difficulties. As the beam breadth is increased, the cost, weight, power of the magnet all increase more steeply than with the square of the beam breadth. The fringe field grows in proportion to the pole gap and is a source of aberrations and crosstalk with other system components, which also increase in magnitude with the square of the pole gap.

A number of partial solutions have been developed, as described above, but these have been incremental improvements with significant tradeoffs. Aitken proposed a radically new analyzer in 2002 [20] which used a new approach, from which the present design has taken inspiration; however, it was more complex and larger than the present device for given beam parameters. I attempted a design using an E-shaped magnetic yoke [21], which was successfully tested, but the present design is simpler and lighter, with better resolution. In a conventional magnet the focal length scales with the bending radius, and the bending radius of necessity scales with the gap between the poles, so for a beam 2 m broad, one can expect the focal length to be several meters, while the aberrations would be severe, and the weight to exceed a hundred tons.

The magnet design presented here addresses these issues, and differs from conventional dipole magnet configurations in the following ways:


A system meeting these goals can have no component of magnetic field in the beam breadth direction, or it must violate these requirements. But if a field component normal to the breadth deflects the beam, then after a finite short distance the opposite field restores the beam direction, there must be a component of field in the gap between these zones in which a component of field is orthogonal to the s-shaped beam trajectories. Thus the solution is a magnet which deflects initially in the breadth direction, and produces a 3D bent S-shaped set of trajectories, hence the acronym U3DS [22].
