RF gymnastics in the PS - CERN

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Fig. 25: Generation of a bunch train with 75 ns between bunchesThe combination of the successive splittings from h=7 to h=84 with this bunch rotationprovides the nominal bunch emittance and length, while avoiding beam loss at PS ejection during therise-time of the ejection kicker. It has been adopted as the basic operational scheme for the generationof the proton beam for the LHC.An additional advantageous feature is that it easily allows for having gaps in the bunch trainby sending no beam from certain PSB rings, or for having a 50 ns time interval between bunches byremoving the last splitting step at 25 GeV.Using the same kind of gymnastics with different sets of harmonics, a 75 ns time intervalbetween bunches is also available. Starting from six bunches on h=7, as in the nominal case, twosplittings into two are applied. The first one is at injection energy (1.4 GeV), giving 12 bunches onh=14 which are then accelerated on that harmonic up to high energy. The second splitting takes placeat 25 GeV, giving 24 bunches on h=28 (13 MHz). The whole process is sketched in Fig. 25.The RF system that provides voltage at this frequency is the same one that can operate at 20 MHz[28]. This scheme is also operational and typical results are shown in Fig. 26. Beam can even bedelivered every 2.4 s by the PS, using a single batch from the PSB.a: Splitting in two at 1.4 GeV b: Splitting in two at 25 GeVFig. 26: Mountain range displays when generating a bunch train at the nominal intensity for the LHC, with a75 ns bunch distance14
6 Heavy ions for the LHCFor ions heavier than protons, the beam for the LHC does not pass through the PSB. It is prepared inthe Low Energy Ion Ring (LEIR) which sends it directly to the PS. The role of the PS is to accelerateand adapt the beam from LEIR to the needs of the downstream machines, imposing the 100 ns bunchspacing required by LHC experiments and the bunch charge, bunch length, and bunch frequencywhich are acceptable to the SPS.For that purpose ‘batch expansion’, the reverse process to ‘batch compression’ applied to theAD production beam, is applied twice at an intermediate energy, separated by a splitting into two[29]. Starting from two LEIR bunches captured and accelerated on h=16 up to a magnetic plateau, thebunch spacing is increased during the first batch expansion by progressively decreasing the harmonicnumber from h=16 down to h=12. Bunches are then split into two on h=24. The correct spacingbetween bunches is finally obtained by a second batch expansion decreasing the harmonic number toh=21. This full process was demonstrated experimentally in August 2009 (Fig. 27).Additional gymnastics take place at top energy (86.7 Tm), after acceleration on h=21 throughtransition. First, the four bunches in consecutive h=21 buckets are transferred from the ferrite cavitysystem to the fixed-frequency 80 MHz one (h=169), to precisely set a distance between bunches thatcorresponds to SPS buckets. Optionally, if space charge proves problematic at injection energy in theSPS, bunches could be split into two to fit into pairs of consecutive 200 MHz buckets. The finalbunch pattern sees 5 ns between the ‘bunchlets’ of each pair and 95 ns between the nearest neighboursof consecutive pairs.7 SummaryFig. 27: Batch expansions and splitting of lead ionsThe PS has progressed remarkably in capability since its start, fifty years ago. It is nowadaysequipped with a total of five multi-cavity RF systems (Table 1) which provide the means to cover abroad range of harmonic numbers at high beam intensity (Table 2). The low-level RF and the overall15
Page 1: RF gymnastics1 IntroductionDuring i
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RF gymnastics in the PS - CERN

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