Beam extraction - 50th anniversary of the CERN Proton Synchrotron

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tuning the beam. The second one cannot be applied in practice on account of lack of an appropriate instrument to measure the beam emittance of each extracted slice. 5.0 4.5 4.0 3.5 Intensity (10 10 p) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0 5 10 15 20 25 Time (ms) Fig. 21: The two-batch injection in the SPS using a five-turn CT extraction from the PS. The beam intensity is measured with a SPS fast beam current transformer. The width of the line is due to the overlay over seven injections. The beam is then accelerated and extracted towards the CNGS target. As expected, it turns out that the two approaches are not compatible with each other. In other words, making the intensity equal generates very unequal emittances and vice versa. This fact is visualized in Fig. 22, where the shape of the five slices is shown for the two approaches. The difference is clearly visible. 4 4 3 3 2 2 1 1 0 -4 -3 -2 -1 0 1 2 3 4 -1 0 -4 -3 -2 -1 0 1 2 3 4 -1 -2 -2 -3 -3 -4 -4 Fig. 22: Shape of the five slices for the present CT in normalized phase space (x, x’) according to whether intensities (left) or emittances (right) have been equalized. The solid circle has a radius of 14ε where ε is the r.m.s. beam emittance before slicing. 5 Why replace the CT In the framework of the activities to prepare the future high-intensity proton beam for the CERN Neutrino to Gran Sasso (CNGS) Project [30], a critical review of the key processes used to generate such a beam was carried out [31], in view of a possible upgrade beyond the present nominal intensity value of about 18
3.3×10 13 protons per PS batch. Among other issues, efforts were devoted to the improvement of the present extraction scheme from the PS to the SPS, the Continuous Transfer. In the framework of the High Intensity Protons Working Group (HIP-WG) [32] a detailed analysis of the losses for the beam for CNGS was performed [33]. The outcome is rather striking: for an overall intensity of about 4.5×10 19 protons/year required by the neutrino experiments, approximately 1.7×10 19 are lost in the accelerator chain, corresponding to about 40% of the total intensity. A large fraction of beam losses, namely 0.7×10 19 , or 40% of the total intensity lost occurs in the electrostatic septum of the PS ring used to slice the beam. In the quest for an improved extraction mode, a novel approach was proposed, named multi-turn extraction (MTE). In the new scenario the beam is separated in transverse phase space by generating stable islands inside the region where the beam sits and by slowly (adiabatically) moving them towards higher amplitudes. By doing this, particles may get trapped inside islands thus generating well-separated beamlets [34]. This method is potentially superior to the present one as no intercepting device is used to split the beam; hence particle losses are limited to the fraction of the beam improperly deflected during the kicker rise time. Furthermore, the extracted beam should better match the phase space structure. 6 Novel Multi-Turn Extraction 6.1 Principle The novel technique relies on the use of non-linear magnetic fields (sextupolar and octupolar) to generate stable islands in the horizontal phase space [35]. By means of an appropriate tune variation, a specific resonance is crossed, the fourth-order in the case under study, and the beam is split by trapping inside the stable islands moving from the origin of the phase space towards higher amplitudes [34]. A good model consists in choosing a simple FODO cell with a sextupole and an octupole located at the same longitudinal position, both represented in the single-kick approximation [36]. An example of the change of the phase space topology during resonance crossing is shown in Fig. 23. 1 (a) 1 (b) 1 (c) X’ X’ X’ -1 -1 X 1 -1 -1 X 1 -1 -1 X 1 Fig. 23: Topology of the normalized phase space during resonance crossing The evolution of the beam distribution during the resonance crossing is shown in Fig. 24. 19
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 and 16: After the proposal, an experimental
Page 17: 80 70 60 β x β y D x SS31 50 40 S
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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