Beam extraction - 50th anniversary of the CERN Proton Synchrotron

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off and extracted as a continuous ribbon over four turns. Therefore, only a fraction of the beam core remains in the machine and is extracted last, during the fifth turn, by changing the beam trajectory, so as to jump over the septum blade. This shaved beam receives a horizontal kick by the electrostatic septum that produces the necessary displacement to jump over the thicker blade of the magnetic septum located in section 16 of the PS ring (see Fig. 19). Fig. 19: Sketch of the CT extraction from Ref. [27] To amplify the effect of the kickers and septa on the beam trajectory, the optics of the machine is perturbed by means of the so-called kick enhancement quadrupoles (QKE16), namely two quadrupoles located in SS05 and SS25, as can be seen in Fig. 20 (top). They increase the horizontal beta-function at the location of the electrostatic septum so as to reduce the local beam density and hence the losses on the device. At the same time, the value of the horizontal dispersion function is reduced at the electrostatic septum, thus making the whole extraction process less sensitive to the intrinsic momentum spread of the beam. In 2007, during a campaign of extraction loss studies, the quadrupoles in SS05 were displaced to SS73. The new optics, shown in Fig. 20 (bottom), displaced the losses produced by particles scattered by the electrostatic septum in better shielded zones of the tunnel. From the principle it is clear that the five extracted slices have different extraction conditions, i.e., positions and angles at extraction and have different shapes and emittances in transverse phase space. During normal operation, the difference in extraction trajectories is corrected by means of dedicated kickers installed in the TT2 transfer line, the DFAs, also called Emittance Reduction Dipoles (ERDs). It was already mentioned that one of the aims of CT extraction is to reduce the extracted beam emittance. Nevertheless, it is worth noting that the extraction shaving occurs in the horizontal plane, while the aperture limitation in the SPS is mainly in the vertical plane. Therefore, a special optics insertion was added in the TT10 part of the PS–SPS transfer line in order to exchange the transverse plane and hence increase the usefulness of the emittance reduction [28]. 16
80 70 60 β x β y D x SS31 50 40 SS05 SS25 30 20 10 0 -10 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Straight Section 80 70 60 β x β y D x 50 40 SS25 SS73 30 20 10 0 -10 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Straight Section Fig. 20: Optical parameters for the PS ring when the beam is sliced for CT. The standard optics is perturbed by means of special quadrupoles, the QKE16 (top, original extraction optics, bottom, optics used for the loss displacement). A typical time profile of the CT ejection is shown in Fig. 21, from the 2008 SPS physics run. The intensity is measured at injection in the SPS by means of a fast beam current transformer. The first injected batch (left part) remained in the machine for 1.2 s while waiting for the second batch. It is clear, however, that a number of drawbacks are present, namely: i) beam losses, especially at the electrostatic septum, are unavoidable. They amount to about 10–15% of the total beam intensity; ii) the extracted slices do not match the natural structure (circles) in normalized horizontal phase-space, thus generating a betatronic mismatch. This, in turn, induces emittance blow-up in the receiving machine; iii) the extracted slices have different horizontal transverse emittance. A detailed analysis of the properties of the extracted slices can be found in Ref. [29], where computation of the mismatch parameters for the CT as a function of the slicing was performed. For the CT extraction, the optical parameters and the beam emittance are different for the five slices, thus generating different emittance blow-up at SPS injection. Furthermore, because of the fancy shape of the slices, the mismatch can be rather large. The optical properties of the five extracted slices of the CT can be derived by performing integration over the beam distribution taking into account the shape of the slice due to the interaction with the electrostatic septum. Knowing that the relative position between the septum and the beam can be controlled on a turn-by-turn basis, it is rather straightforward to compute all relevant quantities for each slice (in some cases it is even possible to derive analytical expressions of the optical parameters vs. the septum position). Two approaches have been followed, corresponding to the equalization of the beam intensity or of the extracted emittance. The first approach corresponds to what is done in reality when 17
Page 1 and 2: Beam extraction 1 Introduction In t
Page 3 and 4: high-quality cable used for the PFN
Page 5 and 6: 3.1 Integer resonance extraction 3.
Page 7 and 8: A last improvement came in 1971 wit
Page 9 and 10: the semi-quadrupole began to raise
Page 11 and 12: Fig. 11: Left: electrostatic septum
Page 13 and 14: 3.3.3 Performance of the SE61 extra
Page 15: After the proposal, an experimental
Page 19 and 20: 3.3×10 13 protons per PS batch. Am
Page 21 and 22: Fig. 25: 3D view of the beamlets al
Page 23 and 24: Fig. 27: Sequence of beam profile d
Page 25 and 26: they were done over two consecutive
Page 27 and 28: Fast pulsed magnets (kickers), to b
Page 29 and 30: Fig. 34: Upper: Cross-section of th
Page 31 and 32: Fig. 36: Intensity of a MTE de-bunc
Page 33 and 34: [16] F. Galluccio, T. Risselada, Ch

Beam extraction - 50th anniversary of the CERN Proton Synchrotron

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