BEAM DISTRIBUTION FLATTENING TECHNIQUES 2. Distribution shaping by octupoles The principle of the method is to use a magnetic optics including two octupoles, that sequentially fold the horizontal and vertical tails of the beam profile back onto the core, in order to achieve a nearly-rectangular beam distribution. The beam line including these non-linear elements has been optimized by the TRACEWIN/PARTRAN code, starting from the beam description at the linac output, as given from AccSys ----- DTL OUTPUT BEAM PARAMETERS AND PHASE SPACES ----- ----- ENVELOPES ----- ----- PHASE SPACES AT BEAMLINE OUTPUT ----- ----- Octupoles off ----- ----- Octupoles on ----- ----- BEAM EVOLUTION WITH OCTUPOLES ON ----- The beam folding in each plane is carried out in two stages: first the nonlinear magnetic fields fold the beam phase-ellipse in x’ (or y’) space and then when the beam drifts in a long field-free region the beam ellipse shears in x (or y) space, causing the tails in the transverse profile to fold back onto the core of the beam. This folding is done sequentially in the x-x’ and y-y’ space.The price is an emittance growth in the transverse planes. ----- Input Octupole 1 ----- -----Output Octupole 1 ----- ----- Input Octupole 2 ----- -----Output Octupole 2 ----- -Computed transverse emittances - One of the two octupoles to be installed