Commissioning Issues of BEPCII Sheng Wang AP group

Commissioning Issues of BEPCII Sheng Wang AP group, Accelerator Centre, IHEP, CAS Oct. 5 th, 2010

Outlines • Brief introduction to BEPC and BEPCII • Overview of the BEPCII performance • Luminosity commissioning • Problems in the luminosity enhancement

A Bird View of BEPCII 1988, first beam obtained from BEPC 2006, first beam from BEPCII

1. Overview of the BEPCII performance 2006 Oct. Installation completed 2006 Nov 18 2007 Mar 26 2007 May 14 First beam stored Phase 1 First collision Luminosity reached that of BEPC 2007 Oct. Installation completed without detector 2007 Oct. 24 2007 Nov. 18 2008 Jan. 29 First beam stored First collision Luminosity >1 1032 cm-2 s-1 Phase 2 2008 June. Installation completed with BESIII in the IR July 19, 2008 First event detected with BESIII April 8, 2009 Luminosity reached 2. 3 1032 cm-2 s-1 May 19, 2009 Luminosity reached 3. 3 1032 cm-2 s-1 Phase 3

Luminosity evolution in Phase 3 3. 3 1032 cm-2 s-1 @523 m. A× 529 m. A After screen monitor removed Tunes move to half integers

Main parameters achieved in beam collision Parameters design Achieved BER BPR Energy (Ge. V) 1. 89 Beam curr. (m. A) 910 ~630 Bunch curr. (m. A) 9. 8 ~8. 0 Bunch number 93 93(86) RF voltage 1. 5 (1. 35)* 1. 5 (1. 35) ns @1. 5 MV 0. 033 0. 032 y* (m) 0. 015 ~0. 013 3. 5@910 m. A ~2. 0@600 m. A ~2. 5@600 m. A Lifetime (hrs) Lum. ( 1033 cm-2 s-1) 1 * Calculated with the measured synchrotron tune 0. 33

3. Problems in the luminosity enhancement • High dark current of the detector --- bottleneck of the high current collision --- source of the dark current (Touscheck effect, beam-gas scattering, etc. ) --- way to lower the dark current

• Luminosity with multi-bunch collision is not linearly proportional to the single bunch collision at the same bunch current. e. g. Ls. b. = 4. 6 E 30@8. 5 m. A * 8. 5 m. A Lm. b. = 2. 8 E 32@580 m. A * 580 m. A (70 bunches) --- Reasons? Instability? Feedback tuning? Others?

• Beam-beam issue Low beam-beam parameter : peak y ~ 0. 02 @ Ibunch=0. 82 Ibunch 0

• Beam blow-up after collision Single bunch collision BPR BER Multi-bunch collision

--- How to reduce blow-up? --- Curing blow-up by changing tunes, IP offsets, etc. , but not repeatable after every collision --- The stronger the beam current, the more serious the blow-up

• Beam lifetime --- The real lifetime is much shorter than expected --- Lifetime of e is shorter than e+ (smaller coupling? ) --- Possible reasons for short beam lifetime: Ø vacuum pressure? Ø physical aperture? Ø dynamic aperture? Ø other reasons?

• More bunches Ø A reasonable way to enhance luminosity But Ø Serious instability observed if bunch number > 90 Ø How about 120 bunches or more with big bunch current? Ø Can feedback systems work well under more bunches?

Thanks for your attentions !