1. First Results on Electron Cloud Generation and Trapping in a PSR Quadrupole Magnet R. J. Macek, A. A. Browman & L. J. Rybarcyk, 9/26/06
Acknowledgements:
LANL: J. O’Hara and mechanical section, J. Ledford, M. Borden, D. Martinez,
R. McCrady
Special thanks to M. Pivi, SLAC for collaboration on simulations
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RJM_EC in PSR Quad.ppt

2. Outline Motivation Diagnostic Concept and Design First Results
Find dominant source of e-cloud driving the e-p instability at PSR
Benchmark simulations
Diagnostic Concept and Design
First Results
Signals for “prompt” and “swept” electrons
Lifetime of trapped electrons
Energy spectra of “prompt” electrons
EC-signal amplitudes as function of intensity
Summary & Future Work
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RJM_EC in PSR Quad.ppt

3. PSR Layout with EC & e-p Diagnostics
ROED/ES1
ED51
ED02
ED92
rf buncher
ES41
WM41 and WC41
Pinger plates
Circumference = 90m
Beam energy = 798 MeV
Revolution frequency =2.8 MHz
Bunch length ~ 275 ns (~70 m)
Accumulation time ~ 750 ms
~2000 turns
ED22
ES43Q
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RJM_EC in PSR Quad.ppt

4. Motivation for e-cloud diagnostic in quadrupole
Need to identify dominant sources of electrons driving the e-p instability in PSR: Drift spaces, dipoles or quads?
Clearing fields, TiN coatings and weak solenoids in drift spaces have had little or no effect on instability threshold.
However, beam scrubbing has been effective.
Why suspect quadrupoles even though simulations show “multipactor gain” down factor of ~20?
Expect “seed” electrons born at the wall to be strongest in quads by as much as a factor of compared with drifts or dipoles
Grazing angle beam losses from foil scattering are largest in quads where b-functions are largest
Quads can trap electrons during the passage of the beam-free gap
Simulations show long lifetime after beam has left the quad
Collection of electrons from biased BPM electrodes gave largest signal in a quadrupole compared with drift and dipole.
At the CERN SPS, strip detector in quadrupole gave larger signal than for dipole or drift.
In addition, simulations show ExB drifts in 3D-quads eject significant numbers of electrons into adjacent drift space which can seed more multipacting and may be the dominant source of seeds for drift spaces.
Data for benchmarks of the simulation codes
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RJM_EC in PSR Quad.ppt

5. Animations
Play *.mov in Quicktime or *.avi in Windows media player etc
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RJM_EC in PSR Quad.ppt

6. Distribution of Zcoll in simulations
slength =.7 m centered
on quad (Leff=0.5m)
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7. Schematic Layout in PSR Quadrupole
Diagnostic Concept
Schematic Layout in PSR Quadrupole
Cross-section
●Electrons striking the RFA entrance holes with E>Vrep pass through repeller grid and create signal
at collector
●Trapped electrons swept in RFA by applying negative HV pulse to HV electrode
●RF shielding of Collector:----Entrance holes (2.7 mm) covered with 40 mesh Cu screen and repeller grid (40 mesh Cu) has rf capacitors to provide high frequency AC ground
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8. Sweeping Feature
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9. Distribution of Phi_coll with z_cut
RFA chamber opening
covers 27 to 45 degrees
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10. Detector Assembly 8/21/06 RFA components:
E-filter (with 2.7 mm holes), repeller grid and standoffs for collector plate
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RJM_EC in PSR Quad.ppt

11. Detector Interior Surfaces
RFA entrance plate
HV Electrode
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12. Histogram of t_collision with Z_cut
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13. First Results: electron signals, 4.6 mC/pulse beam
Beam current
and electron
signal during
beam accumulation
Last 3 turns
before
extraction
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RJM_EC in PSR Quad.ppt

14. Electron signal (cont’d)
Sweeping electrode pulsed (-500V) for 100ns ~16 ms after last beam bunch
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15. Dissipation of trapped electrons at 2 beam intensities
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16. Energy spectrum of “prompt” electrons
Amp is all e’s with E>-Vrep
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17. Dependence on beam intensity (4-8 mC/pulse)
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18. Comparison with simulations
Comparisons are just beginning
Two major inputs have large uncertainties
Peak SEY, have been using 1.5
Seed electrons from beam losses
Hope to determine them by fits to data
Some general features of data in rough agreement with simulations
Pulse shapes and timing
Order of magnitude agreement on signal amplitudes
Cumulative energy spectrum
Long decay time for trapped electrons
Influence of nearby transport elements not yet included in simulations
Dipole fringe field (< 1m away)
Another quad ~4.5 m away
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RJM_EC in PSR Quad.ppt

19. Example of simulation of prompt and swept e-signal
Swept signal
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RJM_EC in PSR Quad.ppt

20. Example of simulated prompt electron energy spectrum
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RJM_EC in PSR Quad.ppt

21. Summary & Future Studies
The first results demonstrate a useful electron cloud diagnostic for studying electron cloud generation and trapping in the PSR quadrupole magnets
Long decay times ( ms decay constant) observed for electrons trapped after beam is extracted
Prompt and trapped electrons are both strong functions of beam intensity
Comparisons with simulations are just beginning
A number of beam experiments are planned in the near future
Vary beam losses (local closed orbit bumps, increase foils fits)
Vary vacuum pressure
Look at unstable beams
Single turn kick to beam to generate betatron oscillation of beam centroid ……
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22. Backups
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23. Electronics Block Diagram
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24. 9/22/2018
RJM_EC in PSR Quad.ppt

25. Swept electron pulse shape
What causes the second peak or shoulder 16 ns from main peak?
Check of electronics with pulser, TRD and network analyzer shows it is not reflection or other instrument problem
Could be due to unexpected distribution of trapped electrons, e.g., higher density near chamber walls
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RJM_EC in PSR Quad.ppt