1. Electron cloud studies at CESR
Mark Palmer
Cornell University
Laboratory for Elementary-Particle Physics

2. Measurements made at CESR
Investigation of possible electron cloud effects
Look for vertical tune shift for positrons (coherent)
Look for positron beam size blow up (incoherent)
Comparison measurements taken with electrons
Other phenomena
Onset of instability in vertical beam size and vertical position
Tune split (betatron sideband) vertical and horizontal
Other effects on positrons and electrons
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

3. Measurement conditions at CESR
Beam Energy
Bunch Patterns
Wiggler States
Feedback
Bunch Currents
Low:
2 GeV
1t x 45b
6t x 5b
8t x 3b
12 on vs. 6 on
12 on
on/off on
0.25 to 1.5 mA
1.0 to 3.0 mA
0.5 to 4.0 mA
High:
5.3 GeV
9t x 6b
all off at high energy
all on
1.0 to 8.0 mA
1.0 to 5.0 mA
Single beam (either e+ or e-)
Pretzel off (separators off)
14 ns bunch spacing, 280 ns train spacing
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

4. ILC DR Electron Cloud and Impedance/Instabilites
Primary Diagnostics
BSM (Beam Size Monitor)
shuttered, 32 channel linear PMT array looking at synchrotron light
BPM (Beam Position Monitor)
uses four beam buttons, four channels per beam
BSM and BPM use similar DAQ
DAQ is based on a 72 MHz Digital Signal Processor (DSP)
turn by turn and bunch by bunch data acquisition
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

5. Turn-by-Turn Beam Profiles : Optics
BSM synchrotron light optics line for positrons
(optics line for electrons is similar)
Off-axis Parabolic Mirror, diameter 4.5” , f=445mm
Neutral Density Filters
32 CHANNEL PMT ARRAY
TV CAMERA
Gauss Filter 500 ± 40 nm
Ground Glass Grid
0.3
0.1
0.01
Splitter T/R=50/50
Eyepiece, f=63mm
Shutter
Expander Lens f=-50 mm
Beryllium Mirror in Beam Pipe
CCD
Hamamatsu linear PMT array (H7260K) followed by a 72 MHz Digital Signal Processor
Quartz Vacuum Window
Splitter T/R=50/50
Optics Box
UV Filter
Adjustable Mirror
Adjustable Mirror
Vertical Periscope
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

6. Evidence of beam blow-up for e+
2 GeV vertical beam size for 1x45 pattern, positrons
0.25 mA
0.35 mA
1 mA
Notice advancing onset of beam size blow up as a function of bunch current
Qualitative comparison of theory and measurement of instability onset (L.Schachter)
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

7. Evidence of tune shift for positrons found
5.3 GeV vertical tune for 1x45 pattern, positrons
(2 kHz Df ~ DQ)
1.5 mA e+, 21 bunches
(trailing bunches fell out)
0.5 mA e+, 45 bunches
2 kHz full scale
Bunch #
Bunch #
5.3 GeV vertical tune for 1x45 pattern, electrons
0.5 mA e-, 45 bunches
1.5 mA e-, 45 bunches
e- tune change is much smaller than for e+ (but there is an effect)
Bunch #
Bunch #
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

8. Transverse feedback, 2 GeV,1t x 45b
positrons position and beam size FB on 0.75mA FB off 0.75 mA FB off 0.25 mA
Vertical bunch by bunch feedback reduces the coherent motion of the beam and reduces the beam size at the end of the train
electrons position and beam size FB off, 0.25 mA FB on 0.25 mA FB on 0.75 mA
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

9. Spectra 2 GeV, e+, 1t x 45b, 0.8 mA/bunch
bunch 1, BPM FFT data
bunch 45, BPM FFT data
horizontal spectrum
horizontal spectrum
vertical spectrum
vertical spectrum
Tune split coincides with beam blow-up, complicates tune extraction
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

10. ILC DR Electron Cloud and Impedance/Instabilites
Summary
Effects seen: Electron cloud
See clear evidence in tune versus bunch for positrons vs. electrons
See clear evidence in beam size versus bunch
No effects seen: 12 vs. 6 wigglers
6 of 12 wigglers turned off
Other phenomena
Electron beam size blow up and instability (ions?) – conditioning dependent
Tune splitting, betratron sideband require explanation
Vertical tune vs. bunch dependence for electrons
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites

11. ILC DR Electron Cloud and Impedance/Instabilites
Plans
Next
Explore different bunch spacings in trains
Employ witness bunch trailing trains
Distinguish impedance, ion effects from ECE
Detailed comparisons with simulation (SLAC/LBNL/FNAL collaboration)
May 2007 Down
Install RFAs (APS design) in North IR straight
Solenoid wound pipe
Longer term
Start to prepare special wiggler vacuum chambers
Electrodes and grooved
Segmented RFAs
SLAC/LBNL collaboration
Installation April 2008
outside
of ring
Oct 31, 2006
ILC DR Electron Cloud and Impedance/Instabilites