Beam extraction - 50th anniversary of the CERN Proton Synchrotron

Fig. 24: Evolution of the beam distribution during resonance crossing. The initial state is represented
by a bi-Gaussian beam (left), at resonance crossing some particles are trapped inside the moving
islands (centre), at the end of the process, the particles trapped in the islands are moved towards higher
amplitudes (right).
When the tune is changed, the islands move through the phase space region where the charged
particles sit and some are trapped inside the islands. At some stage a complete separation between the
beamlets and the central core occurs and the distance between the beamlets can be increased at will by
simply acting on the tune. It is worth while stressing that the beam after trapping has a peculiar structure,
i.e., it is made of two disconnected parts: the beamlets, which are indeed one single structure closing up
after four turns around the machine (see Fig. 25), and the central core.
The idea behind this process is that such a beam splitting in the transverse phase space can be used
to perform multi-turn extraction. In fact, once the various beamlets are separated, the whole structure can
be pushed towards an extraction septum by means of a closed slow bump. Then, kicker magnets generate
a fast closed bump and one island jumps beyond the septum blade so that the beamlets are extracted out
of the machine in four turns. The fifth beamlet, i.e., the beam core, is extracted using a classical singleturn
extraction. The advantage of this approach is that, at least for the first four turns, the optical
parameters are, by definition, the same. This is intrinsic to the method, as the same stable island is used to
extract the beam.
It is worth stressing that numerical simulations are performed by crossing the resonance from
above: this is the opposite of what is done in the experimental tests, where the resonance is crossed from
below. The choice of the resonance to be crossed is completely arbitrary as the use of a fourth-order
resonance is dictated only by the CERN-specific application (see Refs. [37, 38] for a generalization to
other types of resonance and to injection, respectively).
6.2 Measurement results
6.2.1 Overall measurement strategy
In parallel with the computational and theoretical analysis, an intense experimental campaign was
launched at the end of 2001 on the CERN PS (see Ref. [39] for the final account of the results achieved).
This entailed the development of new measurement systems, such as the turn-by-turn orbit measurement
system [40, 41], as well as the installation of sextupoles and octupoles to generate the stable islands.
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