SuperKEKB EMITTANCE GROWTH BY MISALIGNMENTS AND JITTERS IN SUPERKEKB INJECTOR LINAC Y. Seimiya, M. Satoh, T. Suwada, T. Higo, Y. Enomoto, F. Miyahara, K. Furukawa High Energy Accelerator Research Organization (KEK) Abstract In SuperKEKB injector linac, photocathode RF gun is used as electron source for low emittance high-charged beam. Main reason of the electron beam emittance blow-up is generally induced by wakefield in acceleration cavities. Offcenter charged beam in a acceleration cavity is affected by the wakefield depending on the offset size in the cavity and the beam emittance is increased. The emittance blow-up can be eliminated by appropriate steering magnet control so as to cancel the wake effect in the cavity. We perform particle tracking simulation with both misalignments (quadrupole magnet + acceleration cavity) and jitters (quadrupole and steering magnetic force + beam position). Emittance growth by the misalignments and the beam jitter is evaluated in this report. 1. SuperKEKB KEKB[1] 40 8 10 35 cm 2 /s SuperKEKB Phase1 2016 2 6 Phase1 2017 Phase2 Phase2 Fig. 1 SuperKEKB A, B, J-ARC, 1 5 Linac 20mm.mrad Linac 2 10nC 1 10nC damping ring (DR) DR Phase2 4GeV SuperKEKB LER(Low Energy Ring) RF 5nC (20mm.mrad ) SuperKEKB 7GeV SuperKEKB HER(High Energy Ring) 50Hz 96ns 2 HER DR [2, 3, 4] wake seimiya@post.kek.jp wake 4 4 4 Figure 1: Schematic layout of the SuperKEKB injector linac. 2. SuperKEKB C 5 wake [5] wake 96ns S-band wake Strategic Accelerator Design[6] 1. BPM 0
Table 1: Basic parameter set. Aperture values indicate the radius. δ is relative momentum deviation. Parameter Value Unit Initial emittance 10 mm.mrad Initial charge 5 nc Initial σ z 3/2.35 mm Initial δ 0.004 - # of initial particles 40000 - Distribution Gauusian - S-band accelerator aperture 10 mm 2. C 4 BPM Quad-BPM 4 [7] BPM 4 BPM 4 50µm 3BPM [8, 9] 2mm 50µm BPM [10] wake 0 x, x, y, y 4 wake 5 4 4 Table 1 END 3. END 20mm.mrad 4 3σ RMS ϵ x = γβ x 2 x 2 xx 2. (1) γ β β Fig. 2 RMS 0.3mm END β 4 K 1 Figure 2: An example of beam parameters from sector C 5 with 0.3mm RMS misalignment of quadrupole magnets and acceleration cavities. 3.1 RMS 0.1mm, 0.2mm, 0.3mm 60 Fig. 3 vs. 1 Phase2 2nC Phase3 5nC 2nC 0.3mm 5nC 5nC RMS 0.3mm 20mm.mrad 0.1, 0.2mm 20mm.mrad Phase3 0.2mm RMS
Figure 3: Emittance growth at the linac end for 60 random seed in each RMS misalignment in case of 2nC and 5nC. Fig. 4 4 RMS 0.1mm 0.1, 0.2, 0.3mm 4 4 4 Fig. 5 4 K 100 60 60 4 0.08% RMS 0.2mm 20mm.mrad Normalized EMIT [mm.mrad] 35 30 25 20 15 With jitters Average without jitters Average with jitters 10 0.1 0.15 0.2 0.25 0.3 Misalignment [mm] Figure 5: Emittance growth averaged for 100 kinds of jitters (K value) about 60 misalignment. Figure 4: Emittance growth at the linac end for 60 random seed in each RMS misalignment in case that quadrupole or accelerator cavity misalignment is fixed to 0.1mm. MA is MisAlignment. 3.2 100 4 or 60 K Q / K Q = 0.32% (peak-peak). K ST / K ST = 0.08% (peak-peak). = 100µm ( ). K Q K ST 4 K Fig. 6 Fig. 5 3 RMS 60 100 100 100µm RMS 0.2mm 0.1mm 20mm.mrad 4 20mm.mrad Fig. 7 2% Fig. 5 2% 3.3 Si (PD) PD
2 4 8 2 3 4 2 4 20mm.mrad 8 20mm.mrad 4 Figure 6: Emittance growth averaged for 100 kinds of beam position jitters in each 60 kinds of misalignments. Normalized EMIT [mm.mrad] 24 22 20 18 16 14 12 With jitters Average without jitters Average with jitters Figure 8: Frame position data measured by Photo-Diode (left-most) and emittance at the linac end in case of default measured misalignment, 2 times misalignment at the sector joint, 4times misalignment at the sector joint, and 8 times at the sector joint. 10 0.1 0.15 0.2 0.25 0.3 Misalignment [mm] Figure 7: Emittance growth averaged for 100 kinds of bunch charge jitter in each 60 kinds of misalignments. [11] Fig. 8 PD C 5 2015 4 2016 1 2 RMS 0.1mm RMS σ total = σframe 2 + σ2 ACC, (2) σ frame RMS σ ACC RMS 4 RMS 0.2mm 2 4 vs. 100 4. 4 4 20mm.mrad 4 RMS 0.2mm 4 0.2mm 0.3% 100µm 100 200 m 2% PD 5. This work was partly supported by JSPS KAKENHI Grant Number 16K17545.
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