Beam extraction - 50th anniversary of the CERN Proton Synchrotron

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Fig. 13: Left: phase space plot at ES23. Right: phase space plot at TSM57. The de-bunching is kept unchanged from the Square extraction. When the beam reaches the flat top, the radial position is changed, using an offset on the beam control radial loop, so as to place the beam just outside the resonance. In order to suppress the RF component in the spill and enlarge Δp/p (to lower the effect of the low frequency ripple), de-bunching is then done as seen on Fig. 14. Starting from the stable beam in longitudinal phase space (a), the RF phase is changed by 180 o during about 500 μs so that bunches stretch along separatrices (b), then the phase is changed back to the initial value (c) so that bunches rotate during about 3 ms after which the RF power is turned off and cavities are short-circuited (d). A careful tune of these parameters results in a nearly homogeneous distribution of Δp/p and therefore a regular spill with almost rectangular shape versus time. Fig. 14: The RF gymnastic for de-bunching 12
3.3.3 Performance of the SE61 extraction Owing to East Area radiation limitations, the intensity is restricted to 2×10 11 protons per pulse (ppp) per each of the two external targets. The particle momentum is 24 GeV/c in normal operation, but other energies are also possible. When carefully adjusted, the efficiency can reach values above 95%. Losses are practically limited to the electrostatic septum straight section. For a 3×10 11 ppp beam, the typical physical emittance of the extracted beam is 0.1 π μrad in the horizontal plane and 0.8 π μrad in the vertical plane. The instantaneous momentum spread Δp/p is 0.08% and the total Δp/p is 0.3%. The maximum spill length is 500 ms, the standard value being 400 ms. 3.3.4 Ripple correction The residual undulation on the power supplies was responsible for a ripple in Q h which induced a ripple in the extracted beam intensity. The frequencies were usually 100, 300 and 600 Hz. Physicists often required a cure for these intensity fluctuations. A servo-system had already been used with the Square extraction to overcome this problem. A monitor in the extracted beam gives the intensity of the spill. It is compared with a fixed reference, the difference undergoes various corrections through filters and it is fed through a wide-band power amplifier to a quadrupole. Only the lower frequencies could be corrected this way owing to the slow response of the extraction to a change in Q h . This is due to the great number of revolutions in the machine before a particle is ejected. Another, more successful way to correct the low frequency ripple is the feed-forward method, as shown in Fig. 15. Fig. 15: Left: spill and circulating current without correction (1997). Right: spill (PE.MLSDSESPILL) and circulating (PE.TRA345E11) current after correction of the mains harmonics 2, 3, 4, 6, 8, 12 of 50 Hz. An air quadrupole is installed in the machine at a high β h location. An audio-frequency amplifier feeds it with the harmonics to be corrected. They are synthesized with a function generator, synchronized with the mains, variable in phase and amplitude. The optimization was done manually, optimizing the Fourier transform from cycle to cycle. This was a long process, since 7 frequencies were involved: 50, 100, 150, 200, 300, 400 and 600 Hz. The optimization had to be performed from time to time because of ageing and detuning of the various power supplies or changes in mains phase. A third possibility is the phase displacement (empty bucket channelling) method [19]. The beam is displaced away from the resonance and then de-bunched. RF cavities are excited at a harmonic of the revolution frequency corresponding to the position of the resonance. This increases dQ/dt of the particles which squeeze in between the empty buckets, and thus decreases efficiently the low-frequency ripple (Fig. 16). The latter is traded against a strong RF component. This method was tested experimentally only 13
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: Fig. 11: Left: electrostatic septum
Page 15 and 16: After the proposal, an experimental
Page 17 and 18: 80 70 60 β x β y D x SS31 50 40 S
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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