Slow positron beam extraction from high magnetic fields

The high intensity positron beam at PSI will be produced in a high magnetic field region (∼ 1 T). On the other hand, micro-beam formation (through brightness enhancement operations) has to be carried out in field-free space. This requires the extraction of the positron beam into field-free space, an operation which becomes possible when symmetry breaking elements are introduced. In this contribution a method is presented, based on B-field termination through a magnetic shield with an aperture furnished by a magnetically conductive internal structure. The positron beam is first guided to a low field region where the shield is placed and then extracted into field-free space by letting it pass through the aperture. The designed aperture has a diameter of 120 mm, an 80% open area and a thickness of 10 mm. The simulated results of the field distributions show that a factor of nearly two orders of magnitude reduction of the field across the aperture and a spread of B⊥ in the plane of the aperture opening can be achieved. This reduction factor has been confirmed experimentally. Measurements of transmission efficiency and divergency will be discussed. Two parameters can be varied in order to achieve reasonably small transverse energy of the extracted positrons which makes the following focusing performance highly efficient.