Beam Observations with Electron Cloud in the Cern Ps & Sps Complex

With the start of the machine studies to characterize the behaviour of the LHC beam in the SPS in 1999, it became evident that electron multipacting was occurring in the SPS vacuum chambers in the presence of this beam. Multipacting induces dramatic pressure increases preventing stable operation, it limits the performance of beam instrumentation and high voltage electrostatic devices (e.g. electrostatic septa) and it induces strong transverse instabilities leading to emittance dilution. Although an increase of the threshold bunch population for multipacting can be obtained by beam conditioning (”scrubbing”), multipacting persists in the arcs for the nominal LHC bunch population and electron cloud instabilities remain an issue for the LHC beam. A programme of studies has been launched since 1999 to study the electron cloud build-up and related instabilities in the SPS and in the PS for the LHC and fixed target beams. The experimental tools and analysis developed so far are presented together with the results of the observations. The countermeasures applied in the PS Complex & SPS against the electron cloud instability are also briefly discussed. INTRODUCTION The main beam parameters for the LHC and fixed target beams are listed in Tables 1 and 2. Table 1: The LHC beam in CERN PS and SPS [1] PS@extr. SPS@inj. SPS@extr Momentum [GeV/c] 26 26 450 Revolution period [μs] 2.1 23.07 23.05 Tunes (H/V) 6.25 26.185/26.13 Gamma transition 6.1 22.8 Max. n. of batches 1 4 n. bunches/batch 72 72 Nominal Nb [10] 1.15 1.15 Bunch spacing [ns] 24.97 24.97 24.95 Full bunch length [ns] 4-16 4 <2 Batch spacing [ns] 224.7 224.6 r.m.s. εH,V [μm] 3 3 3.5 εL [eV s] 0.35 0.35 <0.8 * The bunch length is reduced from 16 to 4 ns by a non-adiabatic bunch compression prior to extraction. This process takes approximately 100 turns (~200 μs) Table 2: The Fixed Target beam in CERN SPS SPS@inj. SPS@extr Momentum [GeV/c] 14 400 Revolution period [μs] 23.11 23.05 Tunes (H/V) 26.62/26.58 Gamma transition 23.2 Max. n. of batches 2 n. bunches/batch 2100 Nominal Nb [10] 0.1 – 0.2 Bunch spacing [ns] 5.00 4.99 Full bunch length [ns] 4 <3 Batch spacing [ns] 105