RF gymnastics in the PS - CERN

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Fig. 4: RF peak voltage and beam at the beginning of longitudinal recombination at 25 GeVIn that case there is no need to recapture pairs of bunches in the same bucket: the beam issimply ejected onto the target when both beam sets are azimuthally superimposed. The mountainrange display in Fig. 5 shows a typical result at 10 13 p/p.Fig. 5: Mountain range display of longitudinal recombination at 25 GeVThe practical implementation of this process was rather complex. Controls and low-level RFhad to provide all the necessary signals, and the ferrite cavities had to operate in three groups tuned atdifferent frequencies and with different voltage programmes. Gap relays were momentarily activatedon the four cavities used on h=19 and h=21 at the beginning of the flat top to reduce their impedancewhen they were changing tune.Another strongly non-adiabatic technique was successfully applied for a few years, incombination with the previous one. With a specially designed RF deflecting dipole in the transfer linebetween the PSB and the PS, bunches of two PSB rings were interlaced (Fig. 6) so that each PSbucket captured two of them [12]. This ‘funnelling’ process is intrinsically imperfect, both in thetransverse phase plane because all particles in a bunch do not get the same deflection, and in thelongitudinal phase plane because capture is strongly non-adiabatic, triggering an intense filamentationand a large blow-up. An acceptable operational compromise was nevertheless found — adjusting thedistance between bunches to optimize the overall result and minimize beam loss (Fig. 7).4
Fig. 6: ‘Funnelling’ with an RF dipole in the PSB–PS transfer lineFig. 7: Average and peak beam current during a complete acceleration cycle with the RF dipoleThe needs of the upgrade programme launched in 1983 and based on the Antiproton Collector(ACOL) could, however, not be met with the previous methods of preparation of the antiprotonproduction beam. Bunch rotation of the antiproton bunches entering ACOL was indeed foreseen, toquickly reduce their energy spread by a large factor, without resorting to any cooling. Hence theproton bunches had to have a reproducible and as short as possible bunch length. A two-step quasiadiabaticprocess was therefore proposed, using bunch merging and batch compression [13].Merging pairs of bunches is obtained at constant B field by changing the RF voltage seen by thebeam from h=20 to h=10, with the h=10 stable phase centred in the middle of the h=20 buckets (Fig.8).Fig. 8: RF voltages and buckets during bunch-pair merging5
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RF gymnastics in the PS - CERN

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