Progress on a Prototype CLIC Manifold Damped and Detuned Structure

1. Progress on a Prototype CLIC Manifold Damped and Detuned Structure Roger M. JonesCockcroft Institute andThe University of Manchester R t/2 b a1 Rc a L a a 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 ~ 500-1000

5. 1. Features of CLIC DDSAccelerating Structure Acceleration cells Beam tube Manifold HOM coupler High power rf coupler • 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 Monitoring2) Cell alignments to be inferred

6. 1. Features of DDSAccelerating Structure –GLC/NLC

7. 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 Qcu DS ASSET Data RDDS1 DDS3 (inc 10MHz rms errors) DDS1 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 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 • 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> = 13.75 %) 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 Parameters Structure GeometryCell parameters Iris radius Iris radius R amin, amax= 4.0, 2.13 b t/2 Cavity radius Cavity radius a1 Rc a bmin, bmax= 10.5, 9.53 a Fully Interleaved 8-structures Sparse Sampled HPT (High Power Test) a+a1 L

16. 2.3 Relaxed parameters –full cct model Coupled 3rd mode Coupled 3rd mode Uncoupled 2nd mode Uncoupled 2nd mode Uncoupled 1st mode Uncoupled 1st mode Avoided crossing Avoided crossing Uncoupled manifold mode Light line Light line Uncoupled manifold mode Uncoupled 2nd mode Coupled 3rd mode Uncoupled 1st mode Avoided crossing Light line Uncoupled manifold mode 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) End Cell

17. 2.3 Summary of CLIC_DDS_C Dipole mode Manifold mode Manifold ∆f=3.6 σ =2.3 GHz ∆f/fc=13.75% Coupling slot 24 cells No interleaving Meets design Criterion? ∆fmin = 8.12 MHz ∆tmax =123 ns ∆s = 36.92 m ∆fmin = 65 MHz ∆tmax =15.38 ns ∆s = 4.61 m 192 cells 8-fold interleaving 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 b a Circular Elliptical Convex Concave Square 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  Iris radius = 4. 0 mm Iris thickness = 4.0 mm Chosen design

22. 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 Geometry r2 Radius = 0.5 mm h1 2*r2 r1 a2 h g=L - t r1 r1+h+2r2 a1 Rc t= 2a2 a L

25. 3. Impact on Parameters: CLIC_DDS_C to CLIC_DDS_E DDS2_E DDS1_C DDS1_C DDS2_E

26. 3. CLIC_DDS_E -Fundamental Mode Parameters DDS1_C DDS_E DDS_E Vg Q DDS1_C R/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 DDS1_C Es Hs DDS_E

27. 3. Wake Function for CLIC_DDS_E -Dipole Circuit Parameters DDS_C DDS_E 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 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. Cell # 1 Uncoupled Dipole mode Uncoupled manifold mode Cell # 24 4. CLIC_DDS_ER Dispersion Curves Uncoupled 2nd Dipole Mode Cell # 1  Avoided crossing Light Line Light line Cell # 24 

31. 4. CLIC_DDS_E vs CLIC_DDS_ER Wakefield Spectral Function Wakefunction CLIC_DDS_E :Rc=6.2 - 6.8 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. mr = 2.2mm mc= 14.45 Rc=6.8 4. CLIC_DDS_A: Manifold Geometry Highlighted 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 Sc 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 H-field Sc E-field 220 MV/m @ 138 MV/m Opposite sides Selected contours

37. 4. CLIC_DDS_A Parameter Variation ∆t ∆a ∆b

38. 4. CLIC_DDS_A Surface Fields

39. 4. CLIC_DDS_A Fundamental Mode Parameters

40. 4. HPT CLIC_DDS_A Parameters Esur 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% 35*Sc Eacc Pin ∆T 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% Dashed curves : Unloaded condition Solid curves: Beam loaded condition

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

42. 4. Matching Cell Procedure mb mg Matching cell Matching cell Mode launcher Regular cell ma = Mode launcher N=1-3 Matching iris Matching iris Regular iris • 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 a g 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 11.994 GHz 11.994 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 ~11-2010. • 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 2010. 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

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