Beam Physics Issue in BEPCII Commisionning Xu Gang Accelerator physics group

Outline Backup scheme for colliding mode and synchrotron radiation mode Injection Ramping for SR mode Analysis of bepcii optics and correction(by Y. Y. Wei) Impedence and Bunch length(later on by D. M. Zhou) First Collision of BEPCII(by C. H. Yu) Instability, lifetime and luminosity summary

Backup scheme for colliding mode Due to time delay of the superconducting system, the backup scheme must consider the colliding mode in missing the superconducting quadrupoles. A new scheme including to colliding mode were proposed. Feature: two BEPC 160Q quadrupoles are used for replacing two superconducting coils. Due to the geometry and strength of the quadrupoles, β * y cannot be small like the formal mode, as a colliding study, we take the β * y =5cm

Main parameters for backup colliding mode E=1.89GeV β * x =2m, β * y =0.05m ν * x =6.54, ν * y =5.59m α p =0.022, ε x0 =120nm Bunch bucket height 0.7% U 0 =0.12MeV

“New” backup scheme for synchrotron radiation mode Feauture: two superconducting dipole coils are replaced by one PEPII H-type magnet at IP, this is different from the “old” backup scheme using two BEPC bending magnets with new coils Main parameters are similar to the formal scheme

Main parameters for backup SR mode E=2.5GeV β xmax =25m, β ymax =25m, D xmax =1.95m ν * x =7.28, ν * y =5.18m α p =0.0177, ε x0 =140nm U 0 =0.337MeV

Injection Hardware problems in the beginning: wrong link for magnets, the wrong polarity of magnets(quadrupoles and even dipole), short circuit for magnets(sextupoles in ring and bending magnet in transport line), profile monitors and vacuum valves blocking First turn BPMs play the decisive role for first injection

Injection A good injection mode should be a complete set of optics mode, COD, KICKERs’ strength, timing about KICKERS, RF and LINAC gun, and injected beam quality(beam intensity, energy, energy spread, orbit, emittance). Any one in the set could lead to the difficulty or the lower rate of injection. For convenience to compear, the best way is to record all the parameters for the good rate.

Special attentions on injection Phase advance between two kickers must be considered in optics correction, non-Pi section will lead the inevitable remnant oscillation. The related study have been done, but there are conflict from different LIBERA-BPMs, this most like LIBERA- BPM problem

Special attentions on injection Uniformity of multi-bunch injection is not so easy, the negative effect could be from wave form of kicker or multi-bunch instability. Optimizing the kickers’ strength and timing even phase advance could helpful, and BCM even the feedback system also can be helpful, still need further studies.

Special attentions on injection Injection with collision is difficult when the bunch current of “1st” beam is more than 5mA, sometime the existing “1ST” beam can be squeezed out. Reducing the remnant oscillation or offset of collision could be helpful. Another way is completely separating the two beams on longitudinally. The related study need to do.

Ramping for SR mode E: 1.89GeV->2.5GeV Synchronization for power supplies is one of the most important aspect. After a great of efforts by control group the ramping synchronization has been improved. Working point is also important to avoid beam losing due to high order resonance during the ramping The hysteresis of the magnets can also lead the losing. This has been fixed with a series energy steps (2.0, 2.1, 2.2, 2.3, 2.4, 2.5GeV). The best way is table-ramping. We will do this experiment.

Instability, lifetime and luminosity Current limit for single bunch >40mA Multi-bunch instability (quadruple oscillation in the synchrotron light) has been observed, this oscillation can be eliminated by octupole(for SR mode). For colliding mode, the transverse feedback must be necessary. Beam-beam instability has been observed also. The instability behaves on beam blowup, lifetime reduction, difficulty of “2nd” beam injection or “1st” beam losing. The solution is optimizing the colliding condition.

Quadrupole oscillation for SR mode R4OOCT=0A R4OOCT=-1.5A

Lifetime Beam lifetime is main contributed by Touschek lifetime, The lifetime strongly relies on vertical COD The shaker strength of tune measurement has large effect on the lifetime because the beam size changed by shaker The reason that Touschek lifetime is smaller than 7hrs prediction need study

Luminosity BBS can verify whether the two beams are in collision, but how good are the collision only can be detected directly by luminosity detector or in- directly by tune-tuneshift. The luminosity detectors have two kinds 0-angle and bar-bar, but they are suffered by the background and still need calibration Temporarily the tuneshift are used for estimation the luminosity Reading out the tune shift is not so easy. The peak occur sometimes in e- ring sometimes in e+ ring. Sometimes only in horizontal plane.

Tuneshift measurement

Luminosity The shake strength has effect on the tune shift. If too strong, the tune shift will become smaller, if too weak the peak will not occur So far, the maximum vertical tune shift is about beam-beam parameter~0.012:0.017 the luminosity=2.17*10 34 *1.89GeV*(0.012~0.017)*0. 005A/5cm=(5~7)*10 29 (1/cm 2 /s)

Summary BEPCII backup scheme including to SR mode and colliding mode have successfully achieved beam storage and the two beams’ collision. There are huge works to be done in more detail