RF gymnastics in the PS - CERN

5 Protons for the LHCThe luminosity goals in the LHC require proton beams with a brightness exceeding the existingcapabilities of the Linac2–PSB–PS–SPS cascade. An upgrade was therefore launched [21] based onthree main ingredients: (i) division by 2 of the beam brightness in the PSB by the use of two pulses tofill the PS, (ii) division by 1.5 of space-charge tune spread in the PS by increasing the injection energyto 1.4 GeV, and (iii) suppression of the transition bottle neck in the SPS by injecting at 25 GeV. Thelongitudinal stacking of two PSB batches in the PS is made possible by having a single bunch in eachPSB ring, operating the RF on h=1, and changing to the beam transfer scheme sketched in Fig. 18.The harmonic number also had to be changed in the PS, and the initial choice was for h=8.Fig. 18: PSB–PS beam transfer before and after modification for the LHCSingle-batch filling of the PS remains possible, with two bunches per PSB ring (RF on h=2)and RF on h=8 in the PS. Once implemented at the beginning of 1998, all beams delivered by the PShad to be modified to operate with new harmonic numbers and revised gymnastics [22]. In particular,the inverse process to merging pairs of bunches, splitting bunches into two, became routine operation[23].Beyond beam brightness, the PS also has to give its final time structure to the proton beam,for example transforming it into a 40 MHz bunch train (25 ns spacing between bunches) in thenominal case. Debunching on h=16 followed by rebunching on h=84 (40 MHz) at 25 GeV was theprocess initially foreseen for that purpose (Fig. 19) [21]. A fixed-tune 40 MHz RF system wastherefore built and installed in the PS in 1996 [24]. It was complemented in 1998 with an 80 MHz RFsystem [25] to allow for the bunch rotation necessary to reduce bunch length down to 4 ns, as requiredby the SPS. All of these cavities are equipped with a telescopic mechanical short-circuit which almostperfectly closes their gaps when they are not in use (Fig. 20). Although opening and closing takeapproximately one second, movements are planned to take place only a few times per day,immediately before and after filling the LHC. In addition, a fast RF feedback is implemented on bothsystems to minimize impedance when the gap is open.10