Beam extraction - 50th anniversary of the CERN Proton Synchrotron

7 Implementation of MTE at the CERN PS
7.1 Design principle
The experimental campaign was completed at the end of 2004. The analysis of the required modifications
to implement the proposed multi-turn extraction took place during the 2005/2006 long shutdown of the
PS machine. The conceptual design of the proposed multi-turn extraction can be sketched as follows (see
Ref. [44] for more details and Ref. [45] for a complete account of the hardware commissioning):
– Beam splitting: two pairs of two sextupoles and one octupole each will separate the initial
single beam into the five beamlets prior to extraction. Contrary to the experimental set-up,
where only one set of two sextupoles and one octupole was used, the choice of two pairs is
mainly dictated by the need to control and adjust the phase of the islands at the extraction point.
– Extraction: the extraction point is in SS16, where the magnetic septum for the beam extraction
towards the SPS is located. In the proposed scheme, the electrostatic septum, currently used to
slice the beam in the context of the CT extraction in SS31, is not required, thus simplifying the
overall scheme. Two bumps will be used to displace the beam towards the magnetic septum
blade (slow bump) and to extract the beamlets over five turns (fast bump).
– Slow bump: a set of dipole magnets (bumpers) will be used to generate the slow bump around
the magnetic septum. Currently, four bumps powered with a series/parallel circuit are used to
extract the beams towards the SPS. In the proposed scheme, six independently powered
magnets are foreseen. The large number of bumpers is imposed by the aperture constraints, as it
will allow a careful shaping of the bump to overcome the potential aperture bottlenecks.
– Fast bump: three new kickers will generate the fast bump used to displace the beam beyond the
blade of the magnetic septum. The pulse length should correspond to five PS turns. Since the
centre core of the beam needs to be ejected, an additional kick will have to be imparted at the
fifth turn. For this the fast extraction kicker will be used.
– Trajectory correction in the transfer line towards the SPS: even though the extraction conditions
for the novel multi-turn extraction do not change from turn to turn, as one single island is used
to extract the beam, the feed-down effects of the machine non-linearities (particularly from the
pole face windings in the main magnets) due to the extraction bumps could generate turn-byturn
variation of the beamlet position at PS extraction. Such an effect could have a negative
impact on the emittance after filamentation in the SPS. Hence two kickers capable of generating
deflection changing from turn to turn will be used in the TT2 transfer line to correct for the
variation in the extraction conditions (position and angle). These two devices are already being
used for the CT extraction mode.
7.2 Implementation
The implementation of MTE required different interventions, such as design, production, and installation
of new octupoles; construction of new power converters for the new extraction bump; design,
construction, and installation of new vacuum chambers for increasing the beam aperture in particular in
the extraction region; construction of new extraction kickers and their Pulse Forming Network (PFN). A
large fraction of the PS circumference was affected by those interventions, summarized in Fig. 29, and
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