RF gymnastics in the PS - CERN

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All these measures and renewed low-level electronics are nowadays combined to provide theperformance regularly achieved for the Antiproton Decelerator (Fig. 15).Fig. 15: Mountain range display of batch compression (h=8 → h=20) at 25 GeV for the AD3.2 RF gymnastics for antiproton accelerationThe main difficulty with the acceleration of antiprotons in the PS resulted from the large longitudinalemittance of the bunch. To maximize the RF bucket acceptance, h=6 was therefore used as the lowestharmonic number compatible with the tuning range of the ferrite cavities. However, it remainedinsufficient for a ‘conventional’ non-adiabatic bunch rotation to provide a bunch shorter than 5 ns at25 GeV that could fit into a single 200 MHz RF bucket in the SPS. That was solved with a moreinvolved gymnastics combining RF on h=6 and h=12 and varying amplitudes and phase during bunchrotation to linearize the focusing voltage only over the instantaneous length of the bunch (Fig. 16).Synchronization with the SPS revolution frequency to about 0.5 ns remained a demanding challengebecause of the large voltage and phase transients due to the voltage and phase manipulations requiredby the bunch rotation gymnastics.Fig. 16: Bunch at end of bunch rotation on h=6 (left), h=6+12 (right) at 25 GeV8
For LEAR, the PS had for the first time to decelerate beam from 3.57 GeV/c down to609 MeV/c. To stay within the tuning range of the ferrite cavities, the RF system had to operate onh=10 instead of h=6. To deal with very low bunch intensities, the beam control was based on anaccurate frequency programme and an AC-coupled phase loop, without radial loop.4 Electrons and positrons for LEPIn the context of LEP, the suggestion was made in 1980 [17], and quickly adopted, to modify the SPSand PS as electron–positron injectors. The PS was charged with accelerating the electrons andpositrons provided by the EPA from 500 MeV up to the energy of 3.5 GeV at which they could betransferred to the SPS [18]. A new RF system with two cavities operating at 114 MHz was thereforeinstalled in 1986 [19]. To preserve the capability to accelerate high-intensity proton beams, thesecavities were equipped with HOM dampers and short-circuited by copper bars moved during thedead-time between lepton and proton cycles (Fig. 17).Contrary to most other lepton synchrotrons where damping times are short so that bunchesreach their equilibrium size soon after injection, the low radiation loss in the PS made bunches keep a‘memory’ of the early part of the cycle. Special gymnastics were implemented taking into account thiseffect [18] to tailor the bunches to the needs of the SPS by the combined action of RF and of twoRobinson wigglers controlling the damping partition numbers. Without these wigglers the horizontalbetatron oscillations would be antidamped (J x = -1) as the PS has a combined-function lattice [20].One scenario was to use the 114 MHz system alone on h=240: at the end of acceleration, bunchesneeded ‘expansion’ to the proper emittance which was obtained by reducing the damping partitionnumber J ε by 3.8 units. Another scenario was based on acceleration with the ferrite cavities on h=16or h=8: at 3.5 GeV, bunches needed compression which was obtained by a quasi-adiabatic increase ofvoltage at 114 MHz and a small reduction of J ε by 0.9 unit. The scenario finally preferred in operationused the same J ε as in the expansion case, but it provided bunches twice as long, focusing the beamwith one quarter of the RF voltage at 114 MHz and accelerating it on the crest of a 200 kV, h=8 RF at3.8 MHz.The two 114 MHz cavities were removed after the end of LEP, in December 2000.Fig. 17: 114 MHz cavity9
Page 1: RF gymnastics1 IntroductionDuring i
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Page 22: [16] F. Blas and R. Garoby, Design

RF gymnastics in the PS - CERN

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