1. Collimator Wakefields
Igor Zagorodnov
BDGM, DESY

2. References Codes ECHO CST MS ABCI MAFIA GdfidL VORPAL PBCI Tau3P Used
by me
K. Yokoya, CERN-SL/90-88, 1990
F.-J.Decker et al., SLAC-PUB-7261, 1996
G.V. Stupakov, SLAC-PUB-8857, 2001
P. Tenenbaum et al, PAC’01, 2001
B. Podobedov, S. Krinsky, EPAC’06, 2006
I. Zagorodnov, K.L.F. Bane, EPAC‘06, 2006
K.L.F. Bane, I.A. Zagorodnov, SLAC-PUB-11388, 2006
I. Zagorodnov et al, PAC‘03, 2003
and others

3. Outline Round collimators 3D collimators (rectangular, elliptical)
Inductive regime
Diffractive regime
Near wall wakefields
Resistive wakefields
3D collimators (rectangular, elliptical)
Simulation of SLAC experiments
XFEL collimators
Effect of tapering and form optimization
Kick dependence on collimator length

4. Effect of the kick

7. Round collimator. Inductive
 [mrad]
a [mm]
b [mm]
l [mm]
Q [nC]
 [mm]
TESLA 20
17.5
0.4 1.
0.3
NLC
0.2
0.1

8. Round collimator. Inductive

10. Round collimator. Diffractive
Figure 2:Kick factor vs. collimator length. A round collimator
(left), a square or rectangular collimator (s = 0.3 mm, right).

12. Round collimator

14. Round collimator. Resistive wall wakes
Analytical estimations are available. To be studied.
Can the total wake be treated as the direct sum of the geometric
and the resistive wakes? Numerical modeling is required.
Discrepancy between analytical estimations and measurements.
Transverse geometric kick for long collimator is approx. two times larger as for the short one.

15. 3D collimators. Regimes

16. 3D collimators. Regimes

17. 3D Collimators. Diffractive Regime
S.Heifets and S.Kheifets, Rev Mod Phys 63,631 (1991)
I. Zagorodnov, K.L.F. Bane, EPAC‘06, 2006

18. 3D collimators. Diffractive Regime

19. 3D collimators. Inductive Regime

20. 3D collimators. Inductive Regime
ECHO
GdfidL

21. 3D collimators. Inductive Regime
ECHO
GdfidL

22. 3D collimators. Inductive Regime
Round: 2D vs.3D
Blue-3D
Green-2D accurate

23. 3D collimators. Inductive Regime
Dipolar
Quadrupolar
I estimate that error in this numbers is about 5%

24. Simulations of SLAC experiment 2001

25. Simulations of SLAC experiment 2001

27. XFEL collimators. Tapering
Inductive
When a short bunch passes by an out-transition, a significan reduction in the wake will not happen until the tapered walls cut into the cone of radiation, i.e until

28. XFEL collimators. Tapering
Geometry of the “step+taper” collimator for TTF2
Collimator geometry optimization.
Optimum d ~ 4.5mm

29. Geometry of the “step+taper” absorber for XFEL
XFEL collimators. Tapering
Geometry of the “step+taper” absorber for XFEL
d mm
10 / 20 mm
4.5 mm
3.4 mm
taper 20 mm
step (d=3.4mm)
taper (d=4.5mm)
step+taper
(d=4.1mm)
Loss,
V/pC
Spread,
Peak,
step
110 43
-156
taper 20mm 38 42
-83
taper 20mm +step
50 (45%)
29 (67%)
-82 (53%)

31. Click here to see a movie
Acknowledgements to
K. Bane,
M. Dohlus,
B. Podobedov
G. Stupakov,
T. Weiland
for helpful discussions on wakes and impedances, and to
CST GmbH
for letting me use CST MICROWAVE STUDIO for the meshing.
Click here to see a movie