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Proposal of study for incoherent effects caused by electron cloud

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					Proposal of study for incoherent
effects caused by electron cloud
        K. Ohmi, J. Flanagan
                KEK
           10 June, 2008
Tune scan at 98 ns spacing in KEKB
• 100 bunches, 80 mA
• Coupling resonance and its sidebands, nx-
  ny+kns=n (k=+-2) were seen.
• Coupling chromaticity induces the sidebands.
          LER ns=-0.0240            HER ns=-0.0209

ny=0.64




ny=0.62




ny=0.60 nx-ny-ns=n     nx-ny+ns=n

sx
     sy


ny=0.58




                nx                       nx
                                                     3
Beam size measurement with changing
           filling and tune
• Beam size was measured for changing fill pattern,
  which means changing electron density.
   – 100 bunches, 50 mA, single beam.
   – H. tune = 0.52
   – V. tune = 0.55, 0.56
• Sychro-beta sideband caused by electron cloud fast
  head-tail was seen at filling by 3 and 4.
• Synchro-beta sideband caused by electron cloud fast
  head-tail was not seen at filling by 6(12ns), 8(16ns),
  and 12(24ns), but emittance growth caused by
  coupling chromaticity resonance nx-ny+2ns=n disappear
  and weakened.
• Emittance growth far from the coupling the resonances
  was not seen.
   Error from tune shift along the
                train
• Tune shift along the bunch train was
  measured (by T. Ieiri). 0.006, 0.004, 0.003 for
  6(12ns), 8(16ns) and 12(24ns) spacings,
  respectively.
• Beam size measurement was performed 0.002
  step. A leak due to the tune shift (spread) is
  negligible.
• The tune spread of the resonance is wider
  ~0.01.
                Beam Size vs. Fill Pattern
Bunches mA mA        Bucket mA       Head-Tail   σ y (μ m) @ σ y (μ m)@
#       Total /Bunch Spacing /Bucket Sideband    ν y = 0.55 ν y = 0.56
   100    50     0.5      3   0.167 ○ 進行方向振動            3.3        3.5
   100    50     0.5      4   0.125 ○                   2.3        2.5
   100    50     0.5      5   0.100 ? 進行方向振動            1.7
   100    50     0.5      6   0.083 ×                   1.2        1.2
   100    50     0.5      8   0.063 ×                   1.3        1.2
   100    50     0.5     12   0.042 ×                   1.4        1.2
   100    50     0.5     48   0.010 ×                   1.2        1.6



    Emittance growth seen at nx-ny+2ns=n (ny=0.56) disappear
     narrower spacing than 24 ns.
    Emittance growth far from the coupling (ny=0.55) the
     resonances was not seen.
                  H. Tune Scan (at ny=0.6)

• No e-cloud
   (5/14)
 (98ns spacing)



• With e-
   cloud (5/27)
 (16ns spacing)
 Summary for KEKB measurement
• The coherent electron cloud fast head-tail
  instability was dominant in KEKB: that was,
  emittance growth was not seen at operating
  point far from chromatic synchro-beta
  resonance below the threshold of the
  coherent instability.
• The chromatic synchro-beta was weakened by
  electron cloud rather than emittance growth.
     Proposal for Cesr experiment
• Beam size measurement of bunch by bunch in
  tune space. Tune measurement should be done
  simultaneously.
• As a first step, average beam size measurement
  can be done. Tune shift along the train was
  0.0026 in Cesr. Average beam size measurement
  is feasible for a coarser scan with step of ~0.002
  at the tune shift condition.
• Measurement of coupled bunch instability caused
  by electron cloud. Bunch oscillation is recorded
  after turning off the bunch by bunch feedback.
 (M. Tobiyama et al., Phys. Rev. ST-AB 9, 012801 (2006).

				
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