1. Roger M. Jones Cockcroft Institute and The University of Manchester
Progress on a Prototype CLIC Manifold Damped and Detuned Structure
Roger M. Jones Cockcroft Institute and The University of Manchester R
t/2 b a1 Rc a a a L
a+a1

2. Wake Function Suppression for CLIC -Staff
2. FP420 –RF Staff
Roger M. Jones (Univ. of Manchester faculty)
Alessandro D’Elia (Dec 2008, Univ. of Manchester PDRA based at CERN)
Vasim Khan (PhD student, Sept 2007)
Part of EuCARD ( European Coordination for Accelerator Research and Development) FP7 NCLinac Task 9.2
Collaborators: W. Wuensch, A. Grudiev, I. Syrachev, R. Zennaro, G. Riddone (CERN)
A. D’Elia, CI/Univ. of Manchester PDRA based at CERN (former CERN Fellow).
V. Khan, CI/Univ. of Manchester Ph.D. student pictured at EPAC 08

3. Overview Three Main Parts:
Review of salient features of manifold damped and detuned linacs.
Initial designs (three of them).  CLIC_DDS_C.
Further surface field optimisations  CLIC_DDS_E(R).
Finalisation of current design. Based on moderate damping on strong detuning. Single-structure based on the eight-fold interleaved for HP testing  CLIC_DDS_A
Concluding remarks and future plans.

4. 1. Introduction –Present CLIC baseline vs. alternate DDS design
The present CLIC structure relies on linear tapering of cell parameters and heavy damping with a Q of ~10.
Wake function suppression entails heavy damping through waveguides and dielectric damping materials in relatively close proximity to accelerating cells.
Alternative scheme, parallels the DDS developed for the GLC/NLC, entails:
1. Detuning the dipole bands by forcing the cell parameters to have a precise spread in the frequencies –presently Gaussian Kdn/df- and interleaving the frequencies of adjacent structures.
2. Moderate damping Q ~

5. 1. Features of CLIC DDS Accelerating Structure
SLAC/KEK RDDS structure (left ) illustrates the essential features of the conceptual design
Each of the cells is tapered –iris reduces (with an erf-like distribution –although not unique)
HOM manifold running alongside main structure removes dipole radiation and damps at remote location (4 in total)
Each of the HOM manifolds can be instrumented to allow: 1) Beam Position Monitoring 2) Cell alignments to be inferred
High power
rf coupler
HOM coupler
Beam tube
Acceleration cells
Manifold

6. 1. Features of DDS Accelerating Structure –GLC/NLC

7. Dots indicate power minimisation
1. Determination of Cell Offset From Energy Radiated Through Manifolds –GLC/NLC
Dots indicate power minimisation
Refs: ??????

8. 1. GLC/NLC Exp vs Cct Model Wake
DDS3
(inc 10MHz rms errors)
DDS1 DS
Qcu
RDDS1
ASSET
Data
H60VG4SL17A/B
-2 structure interleaved
RDDS1
Conspectus of GLC/NLC Wake Function Prediction
and Exp. Measurement (ASSET dots)
Refs: 1. R.M. Jones,et al, New J.Phys.11:033013,2009.
2. R.M. Jones et al., Phys.Rev.ST Accel. Beams 9:102001, 2006.
3. R.M. Jones, Phys.Rev.ST Accel. Beams, Oct.,2009.

9. 1. CLIC Design Constraints
Beam dynamics constraints
For a given structure, no. of particles per bunch N is decided by the <a>/λ and Δa/<a>
Maximum allowed wake on the first trailing bunch
Wake experienced by successive bunches must also be below this criterion
1) RF breakdown constraint
2) Pulsed surface temperature heating
3) Cost factor
Ref: Grudiev and Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08

10. 2.0 Initial CLIC_DDS Designs
Three designs
Initial investigation of required bandwidth to damp all bunches (~3GHz)
New design, closely tied to CLIC_G (similar iris a), necessitates a bandwidth of ~ 1 GHz. Geometry modified to hit bunch zero crossings in the wakefield .
Relaxed parameters, modify bunch spacing from 6 to 8 rf cycles and modify bunch population. Wake well-suppressed and seems to satisfy surface field constraints. CLIC_DDS_C (f ~ 3.6, 13.75%).

11.  2.1 Initial CLIC_DDS Design –f determination
Structure
CLIC_G
Frequency (GHz) 12
Avg. Iris radius/wavelength <a>/λ
0.11
Input / Output iris radii (mm)
3.15, 2.35
Input / Output iris thickness (mm)
1.67, 1.0
Group velocity (% c)
1.66, 0.83
No. of cells per cavity 24
Bunch separation (rf cycles) 6
No. of bunches in a train
312
Bandwidth Variation
 Variation
Lowest dipole
∆f ~ 1GHz
Q~ 10 
CLIC_DDS Uncoupled Design
CLIC_G

12. 2. Initial design for CLIC DDS
First dipole Uncoupled, coupled. Dashed curves: second dipole
8-fold interleaving employed
Finite no of modes leads to a recoherance at ~ 85 ns.
For a moderate damping Q imposed of ~1000, amplitude of wake is still below 1V/pc/mm/m
Red is uncoupled and blue is coupled… check this!!
3.3 GHz structure does satisfy the beam dynamics constraints
However, it fails to satisfy RF breakdown constraints as surface fields are unacceptable.

13. 2.2 Gaussian linked to CLIC_G parameters –Zero Crossings
Uncoupled params:
<a>/λ=0.102
∆f = 3σ = 0.83 GHz
∆f/<f>= 4.56%
Coupled
Uncoupled
Systematically shift cell parameters (aperture and cavity radius) in order to position bunches at the zero crossing in the amplitude of the wake function.
Efficacy of the method requires a suite of simulations in order to determine the manufacturing tolerances.
Amplitude Wake
Q = 500

14. 2.3 Relaxed parameters tied to surface field constraints
(f/<f> = %)
Uncoupled parameters
Cct Model Including Manifold-Coupling
Cell 1
Cell 24
Iris radius = 4.0 mm
Iris thickness = 4.0 mm ,
ellipticity = 1
Q = 4771
R’/Q = 11,640 Ω/m
vg/c = 2.13 %c
Iris radius = 2.13 mm
Iris thickness = 0.7 mm,
ellipticity = 2
Q = 6355
R’/Q = 20,090 Ω/m
vg/c = 0.9 %c
Employed spectral function and cct model, including Manifold-Coupling, to calculate overall wakefunction.

15. 2.3 Structure Geometry: Cell Parametersb Rc a
a+a1 R
Iris radius
Iris radius
amin, amax= 4.0, 2.13 a1
t/2 a L
Cavity radius
Cavity radius
bmin, bmax= 10.5, 9.53
Fully Interleaved
8-structures
Sparse Sampled HPT (High Power Test)

16. 2.3 Relaxed parameters –full cct model
Avoided crossing
Uncoupled 1st mode
Uncoupled manifold mode
Coupled 3rd mode
Light line
Uncoupled 2nd mode
Avoided crossing
Uncoupled 2nd mode
Uncoupled 1st mode
Uncoupled manifold mode
Coupled 3rd mode
Light line
Mid-Cell
First Cell
Dispersion curves for select cells are displayed (red used in fits, black reflects accuracy of model)
Provided the fits to the lower dipole are accurate, the wake function will be well-represented
Spacing of avoided crossing (inset) provides an indication of the degree of coupling (damping Q)
Avoided crossing
Uncoupled 2nd mode
Uncoupled 1st mode
Uncoupled manifold mode
Coupled 3rd mode
Light line
End Cell

17. 2.3 Summary of CLIC_DDS_C ∆f=3.6 σ =2.3 GHz ∆f/fc=13.75% Meets design
192 cells
8-fold interleaving
Dipole mode
Manifold mode
Manifold
Coupling slot
∆f=3.6 σ
=2.3 GHz
∆f/fc=13.75%
24 cells
No interleaving
Meets
design
Criterion?
∆fmin = 8.12 MHz
∆tmax =123 ns
∆s = m
∆fmin = 65 MHz
∆tmax =15.38 ns
∆s = 4.61 m
192 cells
8-fold interleaving

18. 3. CLIC_DDS_E
Enhanced H-field on various cavity contours results in unacceptable T (~65° K).
Can the fields be redistributed such that a ~20% rise in the slot region is within acceptable bounds?
Modify cavity wall
Explore various ellipticities (R. Zennaro, A. D’Elia, V. Khan)

19. 3. CLIC_DDS_E Elliptical Design
Circular
Square
Elliptical
Convex
Concave b a
ε = a/b

20. 3. CLIC_DDS_E Elliptical Design –E Fields
Circular
Square
Single undamped cell
Iris radius=4.0 mm
Convex ellipticity
Concave ellipticity
ε=20
ε=5
ε=10

21. 3 CLIC_DDS_E Elliptical Design, Single Undamped Cell Dependence of Fields on 
Circular
Rectangular
Elliptical
(Convex)
(Concave)
 of cavity 10 5
3.33
2.0 1 20
facc(GHz)
12.24
12.09
11.98
12.0
11.99
Eacc(V/m)
0.43
0.42
Hsur max /Eacc (mA/V)
3.64
4.86
4.71
4.54
4.29
3.75 3
4.94
4.99
5.11
Esur max /Eacc
2.27
2.33
2.28
Iris radius = 4. 0 mm
Iris thickness = 4.0 mm
Chosen design

22. Manifold-damped single cell
3. CLIC_DDS_E Single-Cell Surface Field Dependence on ε
Optimisation of cavity shape for min
Iris radius ~4mm. For both geometries
Averaging surface H over contour  =3
Rectangular
Circular
( ~ 0.8)
Circular cell
ε=3
ε=2
ε=2
Manifold-damped single cell
ε=3
ε=1
Optimised parameters for DDS2
ε=5
ε=10
Undamped cell

23. 3. CLIC_DDS_E, Optimisation of:
, ∆f and Efficiency
Efficiency ∆T
Optimisation of parameters based on manifold damped structures.
Vary half-iris thickness.
3-cell simulations, with intermediate parameters obtained via interpolation.
Choose parameters with minimal surface E-field, pulse temperature rise, and adequate efficiency.
Pin
∆f dipole
Chosen optimisation (CLIC_DDS_E)

24. 3. CLIC_DDS_E: Detailed GeometryRc h r1
2*r2 r2 h1
r1+h+2r2
Radius = 0.5 mm
Constant parameters (mm)
All cells L
8.3316 r1
0.85 h
4.5 h1
1.25
cavity and manifold joint
1.0
Rounding of cavity edge
0.5
Variable
parameters (mm)
Cell #1
Cell#24 a
4.0
2.3 b
11.01
10.039 a2
2.0
0.65 a1
2a2 Rc
6.2
6.8 r2
3.25
g=L - t a2 a1
t= 2a2 a L

25. CLIC_DDS_C to CLIC_DDS_E
3. Impact on Parameters:
CLIC_DDS_C to CLIC_DDS_E
DDS1_C
DDS2_E
DDS1_C
DDS2_E
Parameter
DDS1_C
DDS2_E
Modified to achieve
Shape
Circular
Elliptical
Min. H-field
<Iris thickness> (mm)
2.35
2.65
Min. E-field
Rc 1to 24 (mm)
Critical coupling

26. -Fundamental Mode Parameters
3. CLIC_DDS_E
-Fundamental Mode Parameters
DDS_E
DDS1_C Vg
R/Q Q
DDS1_C
DDS1_C
DDS_E
DDS_E
Group velocity is reduced due increased iris thickness
R/Q reduced slightly
Surface field and T reduced significantly by using elliptical cells
DDS_E
DDS1_C
DDS1_C Es Hs
DDS_E

27. 3. Wake Function for CLIC_DDS_E -Dipole Circuit Parameters
Cct  
DDS_C
DDS_E
DDS_E
DDS1_C
Avoided crossing x is significantly reduced due to the smaller penetration of the manifold.
Some re-optimisation could improve this
DDS_C
DDS_E
Avoided Crossing 
∆f=3.5 σ
=2.2 GHz
∆f/fc=13.75%
a1=4mm
a24=2.3mm

28. 3. Consequences on Wake Function
Spectral Function
Wake Function
DDS1_C
DDS2_E

29. 4. CLIC_DDS_E: Modified Design Based on Engineering Considerations
DDS2_ER
DDS2_E 
Rounding necessitates reducing this length (moves up)  Rc
Rounding
To facilitate machining of indicated sections, roundings are introduced (A. Grudiev, A. D’Elia).
In order to accommodate this, Rc needs to be increased  DDS2_ER.
Coupling of dipole modes is reduced and wake-suppression is degraded. How much?

30. 4. CLIC_DDS_ER Dispersion Curves
Uncoupled Dipole mode
Uncoupled manifold mode
Cell # 1
Uncoupled 2nd Dipole Mode
Cell # 1 
Avoided crossing
Light Line
Light line
Cell # 24
Cell # 24 

31. 4. CLIC_DDS_E vs CLIC_DDS_ER Wakefield
Spectral Function
Wakefunction
CLIC_DDS_E :Rc= mm (optimised penetration)
CLIC_DDS_ER : Rc=6.8 mm const (a single one of these structures constitutes CLIC_DDS_A, being built for HP testing)
Wakefield suppression is degraded but still within acceptable limits.

32. 4. CLIC_DDS_A: Structure Suitable for High Power Testing
Info. on the ability of the 8-fold interleaved structure to sustain high e.m. fields and sufficient T can be assessed with a single structure.
Single structure will be fabricated this year CLIC_DDS_A, to fit into the schedule of breakdown tests at CERN.
Design is based on CLIC_DDS_ER
To facilitate a rapid design, the HOM couplers will be dispensed with in this prototype.
Use mode launcher design
Status: rf design for main cells complete, matching cells in mode launcher almost complete.
Mechanical drawings, full engineering design EuCARD partners + CERN

33. 4. CLIC_DDS_A: Structure Suitable for High Power Testing
Non-interleaved 24 cell structure
High power (~71MW I/P) and high gradient testing
To simplify mechanical fabrication, uniform manifold penetration chosen
Cell # 24
Cell # 1
Illustration of extrema of the end cells of a 24 cell structure

34. 4. CLIC_DDS_A: Manifold Geometry Highlighted
mc=
14.45
mr = 2.2mm
Rc=6.8
mc = 14.45mm
Cell 1
mc = 15mm
Cell 24
mc = 14.45mm
mc = 15mm
Vary manifold radius and penetration
Choose radius and separation of manifold centre, such the monopole accelerating mode is cut-off (minimises impact on R/Q degradation).
In order to facilitate straightforward fabrication, a uniform manifold has been chosen throughout the structure (const. mc and mr)

35. 4. Surface EM Fields and Sc Parameter: Cell 1
E-field
H-field
40 o K
6.75 W/μm2
50 o K
Ref: A. Grudiev et al, PRST-AB 2009.
Where S= Poynting vector
Opposite sides
Selected contours
@ 95 MV/m

36. 4. Surface EM Fields and Sc Parameter: Cell 24
E-field
H-field Sc
220 MV/m
@ 138 MV/m
Opposite sides
Selected contours

37. 4. CLIC_DDS_A Parameter Variation∆b

38. 4. CLIC_DDS_A Surface Fields

39. 4. CLIC_DDS_A Fundamental Mode Parameters

40. 4. HPT CLIC_DDS_A Parameters
Max. Values
Esur=220 MV/m
∆T = 51 K
Pin= 70.8
Eacc_UL=131 MV/m
Sc=6.75 W/μm2
RF-beam-eff=23.5% ∆T
35*Sc
Esur
Eacc
Pin
Dashed curves : Unloaded condition
Solid curves: Beam loaded condition
CLIC_G Values
Esur=240 MV/m
∆T = 51 deg.
Pin= 63.8
Eacc_UL=128 MV/m
Sc=5.4 W/μm2
RF-beam-eff=27.7%

41. 4. HPT CLIC_DDS_A Wake Qavg ~1700 24 cells No interleaving Undamped

42. 4. Matching Cell Procedure
Mode launcher
Regular cell
N=1-3
Regular iris
Matching iris mg mb
ma =
Two options
Vary ma and mb : very small room to vary mb
vary ma and mg

43. 4. Matching First Cell of CLIC_DT_A
Incident
Power
For each side of the structure (cell♯1 and cell♯24):
Simulate a structure with one standard cell (noc=1) and two identical matching cell at either end.
Investigate minimization of S11 as a function of ‘a’ (iris radius) and ‘g’ (gap length) of matching cells
Follow same procedure with two and then three middle cells (noc=2 and 3)
Matching condition is the one common to all three cases (noc=1, 2 and 3)
ma= matching ‘a’
mg= matching ‘g’

44. 4. Matching Last Cell of CLIC_DT_A

45. First matching cell
Last matching cell
GHz
GHz

46. Summary
CLIC_DDS_C : Achieved excellent wakefield suppression. Recent simulations indicated unacceptably high surface fields, which prompted overall geometry to be reconsidered (R. Zennaro, A. Grudiev, A. D’Elia, V. Khan).
CLIC_DDS_E : Surface fields within acceptable limits. There still exists the potential to reduce coupling slightly (reduce manifold penetration), hence reduce T. Also, potential exists to reduce the average iris radius, increase the overall shunt impedance, and hence recover some additional efficiency.
CLIC_DDS_A : A single structure, out of 8, will be fabricated and a schedule is underway (G. Riddone). The rf design for DDS_E is complete. and will be built ~
The sparse sampled structure will enable the hp rf properties to be tested –includes max and min values of distribution. This will be a representative single-structure test of the features of the complete 8-fold interleaved structure! Schedule for fabrication of CLIC_DDS_A

47. Future
CLIC_DDS_B : Full structure including HOM couplers and ports –A. D’Elia is investigating suitable couplers and will report on initial results in August We anticipate a full design by 2011 to be high power tested.
Lossy manifolds? Interesting discussions with I. Syrachev indicate the potential for using SiC rods within the manifold for example. Will provide a lower Q, relax tolerances, reduce requirement on slots (smaller) and reduce T
Move to a 5/6 phase advance to reduce vg and in this way recover the Rsh and hence reduce the input power?
These new designs should be verified with experimental testing of wake function (revive ASSET!)

48. Future Plans Task May Jun Jul Aug Sept. onwards CLIC_DDS_A
Matching end cells
Almost complete
S11 of full structure
In Progress
GdfidL simulations
Initial Investigations
Mechanical design
EuCARD + CERN
CLIC_DDS_B
Schedule TBD
CLIC_DDS Lossy Manifold
Discuss

49. Acknowledgements
I am pleased to acknowledge a strong and fruitful collaboration between many colleagues and in particular, from those at CERN, University of Manchester, Cockcroft Inst., SLAC and KEK.
Several at CERN within, the CLIC programme, have made critical contributions: W. Wuensch, A. Grudiev, I. Syrachev, R. Zennaro, G. Riddone (CERN).

50. Extra Slides

51. Extra Slides: Matching First Cell

52. Extra Slides: Matching Complete Structure –In Progress
mb1
mb24
b1.5
b24.5
First matching cell
Last matching cell
Mode launcher
b2.5
mb1 = b1
mb24 = b25

53. Extra Slides: Lossy Manifold
Motivation : Enhanced wakefield suppression
CLIC_DDS Geometry : SiC running through manifold
Possible advantage(s) :
1) Relaxed tolerances
2) Short coupling slots → reduced ∆T
Possible disadvantage :
Need to think about locating BPM’s

54. Extra Slides: Lossy Manifold
–Dispersion Curves
Monopole
Sic
εr = 10
μr = 1
tanδ = 0.06
mr = 3.2 mm
r = 2.0 mm
Iris radius = 4. 0 mm
Iris thickness = 4.0 mm
Dipole

55. CLIC_DDS_E Parameters
Cell
Eacc Q
R’/Q
vg/c
Es/Eacc
Hsur/Eacc
Sc/Eacc2 #
(V/m)
kΩ/m
(%)
mA/m
W/(x10-4) 1
0.4488
5020
10.179
2.07
2.23
4.9
7.3 2
0.4563
5091
10.649
1.85
2.2
4.82
6.9 5
0.4882
5325
11.723
1.62
2.05
4.5
5.5 9
0.5255
5604
12.899
1.51
1.89
4.15 13
0.5555
5838
13.952
1.42
1.8
4.0 17
0.5828
6061
15.049
1.34
1.7
3.75
3.4 21
0.6021
6307
16.416
1.22
1.66
3.5
2.9 23
0.4217
6451
17.411
1.11
1.65
3.3 24
0.5749
6534
18.130
1.0
1.67
2.98

56. Extra Slides: Dipole Mode Propertiesfx
fsyn, fπ f0

57. DDS1 revised simulations with surface approximation 5micron+aspect ratio of 5
Cells
esur
hsur
eacc
ttf
Eacc*ttf
Esur/eaccttf
hsur./eaccttf 1
0.67
1.5
0.2748
0.96
0.26
2.54
5.76 5
0.94
2.5(2.7)
0.4963
0.953
0.473
1.987
5.28 9
0.93
2.2(2.3)
0.538
0.946
0.509
1.827
4.32 13
0.94(0.97)
0.5532
0.939
0.519
1.81
4.23 17
0.78
1.7(1.8)
0.4742
0.932
0.442
1.765
3.85 21
0.89
1.88(1.9)
0.5574
0.925
0.516
1.725
3.64 24
0.67(0.72)
1.0
0.3272
0.92
0.3
2.23
3.33

58. DDS1 : Plain curve ; DDS2 : Curve with dots

59. DDS2 Cell # 1 Rc=6.2 Cell # 1 Rc=6.5 Cell # 1 Rc=6.8 Cell # 13 Rc=6.8

60. DDS2 Cell # 24 Rc=7.2 Cell # 24 Rc=7.0 Cell # 24 Rc=6.8 Cell # 24

62. ∆f=3.5 σ =2.2 GHz ∆f/fc=13% a1=4mm a24=2.3mm DDS1_C : Plain curve
DDS2_E : Curve with dots
∆f=3.5 σ
=2.2 GHz
∆f/fc=13%

63. Dipole mode : circuit parameters
DDS1 : Plain curve ; DDS2 : Curve with dots

64. 5. Extra Slides CLIC 30 GHz TDS Prediction vs Exp ASSET data
Good agreement achieved up to ~ 2 ns
Resonance, not included in prediction simulations, at 7.6 GHz, external to structure leads discrepancy between theory/exp.
Ref: I. Wilson et al., Proceedings of the 2000 European Particle Accelerator Conference (EPAC00), Vienna, Austria, 2000

65. 2. Relaxed parameters (RP)–
Decoupling 2 End Cells G Wt S
SRMS