The PS in the LEP injector chain - 50th anniversary of the CERN ...

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PS and for SPS injection. To modify and control the damping partition, Robinson wigglers had to be installed in the PS. Since the electron–positron emittances were smaller than the proton emittances, the leptons could be accepted by the SPS at a fairly low energy of 3.5 GeV. However, to prevent instabilities and beam losses, the total charge accelerated per pulse had to be spread over many bunches. This was the reason for choosing eight bunches throughout the injector chain as the basic mode of operation. Next, the eight positron bunches were transferred to the SPS in two batches of four bunches (spaced by 30 ms) via the existing proton beam line TT10, and the eight electron bunches were similarly sent the SPS in two batches of four bunches through the past antiproton beam line TT70. The leptons were originally accelerated to 20 GeV in the SPS with an extra 200 MHz RF system consisting of 32 single-cell cavities and, after extraction the SPS, they were sent to LEP via two new transfer lines. The lepton beams were injected in the arcs of LEP, where the dispersion function was non-zero. The circulating beam was brought close to the septum by means of a closed-orbit bump so that the incoming beam then followed an orbit similar to and near to that of the circulating beam. So, the injected bunch could be stacked in either betatron phase space (large-amplitude betatron oscillations about the central orbit integrating the stored beam density after synchrotron radiation damping) or synchrotron phase space (large-amplitude synchrotron oscillations about an off-momentum orbit, reaching the stored beam density after damping, as the dispersion function was non-zero at injection and the incoming lepton energy was a bit lower than the tuned LEP energy) next to the already circulating bunch of LEP. The same injection equipment was used for both betatron and synchrotron phase space stacking schemes, the preference among the two depending on the LEP operation mode. Quite wide-ranging modifications had to be made to use the PS as a 3.5 GeV electron–positron synchrotron. Almost all the existing equipment of the PS machine was compatible with the acceleration of electrons and positrons. However, the vacuum chamber was entirely changed; electron and positron transfer lines from EPA to PS and injections into the PS were added. New 114 MHz RF cavities, Robinson wiggler magnets, and some instrumentation were added too. The new injection system was installed without compromising the performance with proton beams. The extraction channels were the same as for proton–antiproton operation. Since the PS is a combined-function lattice synchrotron and has a long acceleration cycle, the implementation of Robinson wigglers to ensure the stability of the beams was essential. The wigglers also helped to match the longitudinal bunch size to the SPS. In 1986 leptons were injected and accumulated in the EPA at 500 MeV instead of 600 MeV as initially planned. Since then, 500 MeV remained the nominal lepton energy in EPA. First injection in the PS of electrons at 500 MeV and acceleration was achieved in 1986. By 1987 electron and positron beams were ready. 2 Operating modes of the PS and SPS injectors In the early years for the basic mode of operation the electrons or positrons were accelerated in the dead-time of the SPS; but in place of one acceleration cycle, four 1.2 s lepton magnetic cycles were inserted between the proton cycles to decrease the intensity of the lepton beams in the injectors for beam stability reasons. Positrons were accelerated in the first two cycles and electrons in the second two, which led to the PS and SPS supercycle pattern {p e + e + e - e - }. Both LEP beams consisted of four bunches. Figure 2 shows the magnet supercycle of the SPS and PS, and the total beam intensity in the EPA. PS and SPS supercycles were made of various magnetic cycles, one or several cycles being allocated to each user [4]. Within a SPS 14.4 s supercycle time period, the positrons were accumulated in the EPA during the quite long SPS proton cycles due to the low positron intensity of the linac (LIL-W). The linac 2
operated with a repetition frequency of 100 Hz, and each pulse was put one at a time into another one of the eight EPA stockpiling buckets by stacking the pulse in betatron phase space close to the stored bunch (the elapsed time between two injections into the same bucket was 80 ms). Hence, after the required beam intensity was reached, each of the eight positron bunches was moved onto a thin electrostatic septum which cut in half the bunch in betatron phase space and ejected one half to the PS on the first positron cycle. The remaining eight halves stayed in EPA waiting for the second positron cycle to be transferred to the PS. The eight bunches injected into the PS were then accelerated to 3.5 GeV and ejected to the SPS, accelerated to 20 GeV and sent to LEP. Once the positrons were extracted from EPA the linac began to inject electrons and filled up the eight EPA bunches to an intensity equal to half that of the positrons. The eight electron bunches were then transferred through the PS and SPS to LEP like the positrons. After the second electron cycle, the linacs resumed positron injection into the EPA. All circular injector accelerators ran with eight bunches to lessen the charge per bunch. Fig. 2: The 14.4 s PS and SPS supercycles and the EPA total beam intensity. Positrons and electrons were accelerated on the four last consecutive 1.2 s cycles of the PS supercycle Variant operating mode schemes were examined to cope with possible constraints due to the stability thresholds. Had the thresholds been less favorable than expected, the PS and SPS bunch intensity would have had to be decreased by a factor of, say, two and the number of cycles doubled. The EPA electrostatic septum would have cut the eight positron bunches such that one quarter of the initial intensity was extracted each time. The eight bunches remaining for the last positron cycle would have been transferred to the PS via standard extraction. The electrons would have been handled in the same way as in the basic scheme, except that there would have been four electron transfers during the supercycle instead of two. Later, once the new 100 MHz RF cavities were installed in the SPS, allowing the bunch intensity to be increased, a new mode of operation using only two magnetic cycles per PS supercycle was feasible, the first cycle for positron acceleration and the second for electron acceleration. 3 Lepton injection into the PS The ratio of the PS and EPA circumferences was chosen to be equal to the integer 5, and since the bunches were equidistant in EPA and PS the eight EPA bunches could be injected within a single turn of the PS. Single-turn injection was desirable because it makes the beam transfer very fast so that a slowly rising PS magnetic field could be allowed. The design of the PS injection channels took into account the constraints imposed by the position of the EPA and the operation of the PS with proton beams. Symmetric injection trajectories into the PS were chosen to standardize equipment, with a large angle of incidence of 160 mrad to reduce the magnet stray field effect. The injection positron septum was located in straight section SS92, that of the electron septum in SS74. Enough space was left in the straight sections for the installation of a vertical correction dipole and a position monitor [1]. 3
Page 1: The PS in the LEP injector chain 1
Page 5 and 6: 2 for a planar ring, where C q
Page 7 and 8: Results showed that from an initial
Page 9 and 10: constraints an operating point (Ej)
Page 11 and 12: Fig. 9: Longitudinal bunch profile
Page 13 and 14: equirements as the previous nominal
Page 15 and 16: 10 PS to SPS lepton transfer scheme
Page 17: [7] D. Boussard, E. Brouzet, R. Cap

The PS in the LEP injector chain - 50th anniversary of the CERN ...

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