Electron cloud in Final Doublet ILC Interaction Region
Electron cloud in Final Doublet ILC Interaction Region Engineering Design Workshop (IRENG 07) September 17 -21, 2007, SLAC Lanfa Wang
Motivation ØIn 14 mrad crossing angle (baseline), the two beams are separated at Final Doublet (SC) where electron cloud is one concern. ØQuadrupole field can trap electrons by mirror field trapping; There also strong solenoid field in QD 0 which also may confine electrons. QD 0: G=140 T/m QF 1: G=80 T/m Radius=10 mm to be prototyped during EDR
Solenoid field in QD 0 Bz Si. D field and FD with L*=3. 5 m GLD field and FD with L*=4. 5 m Br Constant solenoid field was used LDC field and FD with L*=4. 5 m § Use 2005 field maps for Si. D and GLD and older TESLA field for LDC § Use 14 mrad total crossing angle, L*=3. 51 m for Si. D detector and L*=4. 51 m for GLD and LDC § Use same FD structure, with FD quads and sextupoles rematched
Program & Model CLOUDLAND: 3 D PIC code for e-cloud § 3 D space charge solver (FEM) § Various Magnetic and electric field w. General 3 -dimensional fields given by expression. w. Fields can also be import from other program using table w. Field solver for electrode, solenoid, etc § Beam field w w Gaussian bunch in round chamber ( image charge is included) PIC method for general geometry § Clearing electrode, Solenoid, Grooved chamber, § Applied to various accelerators: SNS, PSR, KEKB, RHIC, ILC DR…
Trapping phenomenon---in quadrupole magnet 3 D orbit 2 D orbit Orbit of a trapped photoelectron in normal quadrupole magnet during the train gap (field gradient=0. 5 T/m) Field lines
Trapping mechanism (2) – Short bunch Invariation value of motion ILC bunch (300 m) is short Beam E-field enough for trapping! Positron bunch mirror field trapping Reflective Points: Trapping condition =0 Turning points || Trap requirement for positron bunch Bunch length should be shorter than period of gyration motion Trap factor is constant if no other force (except B force) disturbs the electron and smaller than 1. 0, no trapping (a) 4 mm (b) 4 cm Trapped electrons in Quad k=10. 3 T/m (KEKB beam)
Trapping mechanism (3) – resonance (bunch spacing) Resonance of the interaction between e-cloud and positron bunch: Bunch spacing = 1/2 of the period of electron longitudinal motion ILC has very long bunch spacing 175 ns, The probably of resonance is small. ~16 ns Bunch spacing=8 ns Distribution of period of the ~24 ns Bunch spacing=12 ns trapped electron cloud in quadrupole of KEKB LER during the train gap for 4 mm bunch length
E-cloud in Super. B Quadrupole (H. Fukuma) Multipacting At end of bunch train Trapped electrons At bunch gap (20 bunch spacing/40 ns after the last bunch in the train)
Electron seed (I): from surface of beam pipe (QD 0) Simulation Parameters § Photon electron; Beam loss; etc § Large Peak secondary emission yield =2. 0 § Quad filed +Constant solenoid field (0. 2 T/1 T) is used in QD 0 § Large initial electron seed: number electron per meter per e+ = 2. 40 E-3 § Bunch length 300 m § Beam size 639 nm/5. 7 nm § Bunch intensity 2 1010 Results ØThere is no multipacting due to the long bunch spacing ØThere is no trapping; Electron cloud build-up in QD 0
Electron Distribution & Orbit Electron distribution in transverse plane Energy gain 22 e. V; Speed of 5 e. V electron=1 e 6 m/s Drift distance during one bunch spacing 175 ns=230 mm Electron orbit
Electron seed (II): Ionization Simulation Parameters § § Peak SEY 2. 0 Constant solenoid field 0. 2/1 T 1 n. Torr Pressure Cross section of ionization 2 Mbarn Bz=1 T Results ØNo multipacting ØShort time trapping; ØThe accumulated electron density is small Bz=0. 2 T Electron cloud build-up in QD 0
Orbit of Trapped electron (QD 0) Electron’s orbit in QD 0 +1 T Solenoid
Escape from trapping (QD 0) Electron orbit Electron distribution
Electron in QF 1 seed: surface Simulation Parameters § § § Peak SEY 2. 0 1 n. Torr Pressure Cross section of ionization 2 Mbarn Results ØNo multipacting ØNo trapping; ØThe accumulated electron density is VERY small seed: Ionization Electron cloud build-up in QF 1
Summary § The e-cloud in FD is simulated with 3 D model § No multipacting due to long bunch spacing § No Mirror field trapping due to long bunch spacing § Some Trapped electrons in QD 0 by the solenoid, but the electron density is small § (To model realistic solenoid field? )
Acknowledgements Thanks to A. Seryi, M. Pivi and M. Woodley
- Slides: 16