Damped and Detuned Structures for CLIC The University of Manchester The Cockcroft Institute 4/10/2013I.Nesmiyan1 Acknowledgements to colleagues at CERN, The University of Manchester, The Cockcroft Institute, SLAC and KEK

4/10/2013I.Nesmiyan2 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 An alternative is presented by our CLIC_DDS design - parallels the DDS design developed for the GLC/NLC, and 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 ~ Present CLIC baseline vs. alternate DDS design

4/10/2013I.Nesmiyan3 High power rf coupler HOM coupler Beam tube Acceleration cells Manifold NLC/G(J)LC SLAC/KEK RDDS structure (right) illustrates the essential features of the conceptual design Each of the cells is tapered –iris reduces (with an erf-like distribution) HOM manifold running alongside main structure removes dipole radiation and damps at remote location (4 in total) Features of CLIC DDS Linac

4/10/2013I.Nesmiyan4 1) RF breakdown constraint 2) Pulsed surface temperature heating 3) Cost factor Beam dynamics constraint 1)Maximum allowed wake on the first trailing bunch Wake experienced by successive bunches must also be below this criterion Ref: Grudiev and Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08 CLIC Design Constraints

4/10/2013I.Nesmiyan5 b Rc a a+a1 R Structure Geometry:Cell Parameters a1 t/2 a L Iris radius Cavity radius Sparse Sampled HPT (High Power Test) Fully Interleaved 8-structures a min, a max = 4.0, 2.13 b min, b max = 10.5, 9.53 V.Khan

192 cells 8-fold interleaving 4/10/2013I.Nesmiyan6 24 cells No interleaving 192 cells 8-fold interleaving Manifold Coupling slot Dipole mode Manifold mode ∆fmin = 65 MHz ∆tmax =15.38 ns ∆s = 4.61 m ∆fmin = 8.12 MHz ∆tmax =123 ns ∆s = m ∆f=3.6 σ =2.3 GHz ∆f/fc=13.75% Meets design Criterion? Summary of CLIC_DDS_C V.Khan

7 Water pipes for cooling Vacuum flange Power input Power output Tuning holes Cutaway-view Bea m G. Riddone, V.Soldatov, CERN ~ 8mm Non-interleaved 24 cell structure –first structure of 8-fold interleaved structure chosen. To simplify mechanical fabrication, uniform manifold penetration chosen Mechanical Eng. Design of CLIC_DDS_A Modified bunch spacing (from 6 to 8 rf cycles) and modified bunch population. Wake well-suppressed and seems to satisfy surface field constraints. CLIC_DDS_C and subsequently CLIC_DDS_E–SUCCESS (on suppressing wakes and meeting breakdown criteria). CLIC_DDS design is successfully finished (incl. PhD Khan) 4/10/2013I.Nesmiyan

4/10/2013I.Nesmiyan8 Full production of all cells was done by MORIKAWA Cells are bonded and couplers are brazed The structure was shipped to CERN in September 2012 RF tests have to be scheduled at CERN Fabrication Status of CLIC_DDS_A A. D’Elia Structures successfully built, tuned and measured! Measurements Simulations

4/10/2013I.Nesmiyan99 Particular thanks to R. Wegner, V. Khan and A. Olyunin who participated in measurements Thanks to CLIC RF Structure Development Group October 2012 T. Higo (KEK) guided DDS_A production DDS_A Prototype –Measured at CERN A. D’Elia Many thanks to all collaborators for their contribution on CLIC_DDS_A production

4/10/2013I.Nesmiyan10 An analytical matrix-based technique (D. Schulte) which allows a rapid determination of worst case regimes of emittance dilution as wakefields and other parameters are varied Beam dynamics simulations with the tracking code PLACET Beam Dynamics Study I.Nesmiyan

To rapidly analyse the effect of the wakefield of the beam under various conditions, we utilize an analytical model : D. Schulte, “Multi-bunch calculations in the CLIC main linac”, PAC09, The analysis starts by considering a bunch k with an initial displacement y N,k 0 which kicks bunch j. The final offset of bunch j is: where L is the length of the linac, N e is the number of e - in the bunch, W(z j -z k ) is transverse wakefield exited by bunch k and experienced by bunch j. This analysis describes the direct impact of the bunch on another. However, to include the influence of succeeding bunches on one other – which we refer to as the indirect effect – is included as follows: A jk Matrix A jk includes both the direct and indirect effect. 114/10/2013I.Nesmiyan

Definitions: Fc, Frms To study the impact of the long-range wakefield on the beam we use the following variables: F c which describes the coherent jitter and F rms which describes the random bunch-to bunch jitter, where Influence of the transverse wakefield (W) at at the first trailing bunch F c as a function of the wakefield at the first trailing bunch obtained analytically and by using the code PLACET for the point-like bunches at the end of the CLIC main linac for an initial 2  offset in the bunch train Some validation. Calculations for the CLIC baseline design (CLIC_G) CLIC_G 124/10/2013I.Nesmiyan I.Nesmiyan et al, Beam dynamics studies for the CLIC Main linac, IPAC12, pp wakefield in

CLIC_DDS. Beam Dynamics Results Based on the Analytical Model 13 Comparison of the figure of merit* DesignFcFrms DDS, n=8, Q=2000, uncoupled wake3222 DDS, n=8, Q=700, uncoupled wake CLIC_G, Q~ * I.Nesmiyan et al, Beam dynamics studies for the CLIC Main linac, IPAC12, pp /10/2013I.Nesmiyan F c coherent jitter F rms random bunch-to bunch jitter Transverse Wakefield

14 Beam Dynamics Based on the tracking code PLACET 4/10/2013I.Nesmiyan ParameterValueParameterValue Initial beam energy, E 0 9 GeVBunch length,  z 44  m Final beam energy, E 1500 GeV Initial  x 550 nm Particles per bunch3.72×10 9 Final  x 660 nm Bunch spacing0.15/0.2m(6/8 rf periods) Initial  y 10 nm Bunch train length156 ns Final  y 20 nm Number of bunches 312 Initial  E /E 2% Selected beam parameters 8-fold interleaved structure is insufficient to provide adequate wakefield suppression Hence in order to obtained the requisite wakefield suppression -12 and 16 fold interleaving (corresponding to an equivalent structure of 288 and 384 cells, respectively) is used bunch train initially offset by  y (0.5  m)

15 Here we focus on CLIC_DDS_A interleaved 12-fold (8 rf) and 16-fold (6rf) designs Bunch train initially offset by  y (0.5  m) Systematic frequency errors are investigated by small changes in the bunch spacing We note that emittance dilution occurs relatively close to the nominal bunch spacing Realistic random frequency errors (~2MHz) from cavity to cavity are particularly beneficial in reducing the impact of the wakefield on the beam quality Impact of Systematic and Random Errors on Emittance Dilution 4/10/2013I.Nesmiyan

16 Perfect linacs Vertical random offsets of 10 µm rms were added to all the structures and this is compared to the baseline design CLIC_G Case II consists of 16-fold interleaving of mode frequencies and bunches spaced 6 rf cycles from their neighbours Preliminary studies on the dilution due to structure misalignments indicate similar values to the baseline design However, the implications of not being able to accommodate 16 structures on a girder are not taken into account, as it is a considerable task to redesign the associated beam optics Misalignment Study 4/10/2013I.Nesmiyan

4/10/2013I.Nesmiyan17 CLIC_DDS design is successfully finished (incl. PhD Khan) Structures successfully built, tuned and measured High power/high gradient tests have to be scheduled at CERN Beam dynamics study has shown that a damped and detuned structure, with appropriately interleaved dipole mode frequencies can be used for the CLIC main linacs Conclusions