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ABSTRACT We report on the suppression of long-range wakefields in the main linacs of the CLIC collider. This structure operates with 2π/3 phase advance per cell. The wakefield is damped using a combination of detuning the frequencies of beam excited higher order modes (HOMs) and by light damping through slot-coupled manifolds. This serves as an alternative to the present baseline CLIC design which relies on heavy damping. We report on fabrication details of a structure consisting of 24 cells diffusion bonded together. This design known as CLIC_DDS_A takes into practical mechanical engineering issues and is the result of several optimisations since CLIC_DDS designs. This structure is due to be tested for its capacity to sustain high gradients at CERN. RF PARAMETERS A manifold damped and strongly detuned structure has been studied [1]-[2] as an alternative to the present heavily damped main linac structure for CLIC [3]. In order to satisfy the stringent rf breakdown and beam dynamics constrains two main modifications are performed. • Firstly, the cavity walls of the DDS cells are changed to elliptical shape from the conventional circular . It was necessary to reduce the pulsed temperature rise by re-distributing the surface fields. • Secondly, a moderate bandwidth of the lowest dipole frequencies was chosen so as to suppress the wakefields within allowable limit for a revised inter bunch spacing of 8 rf cycles (0.67 ns). Considering the mechanical challenges imposed due to an X-band operation (small structures) and breakdown issues pertaining to high gradient, a test structure known as CLIC_DDS_A has been designed to study the fundamental mode properties of this structure at high power operation. For a beam loading of 4.2 x 10 9 particles per bunch at an inter bunch spacing of 8 cycles, the peak input power requirement of this structure is 71 MW to maintain an average accelerating gradient of 100 MV/m. FABRICATION AND DESIGN OF THE MAIN LINACS FOR CLIC WITH DAMPED AND DETUNED WAKEFIELD SUPPRESSION AND OPTIMISED SURFACE FIELDS V. F. Khan †* , A. D’Elia †*‡ , A. Grudiev ‡ , R. M. Jones †* , G. Riddone ‡ , V. Soldatov ‡ , W. Wuensch ‡ † School of Physics and Astronomy, The University of Manchester, Manchester, U.K. * The Cockcroft Institute of Accelerator Science and Technology, Daresbury, U.K. ‡ CERN, Geneva, Switzerland. JINR, Dubna, Russia ACKNOWLEDGEMENTS This research has received funding from the European Commission under FP7 Research Infrastructure grant agreement no. 227579. V. Khan acknowledges receipt of funding from the Cockctoft Institute. REFERENCES [1] V.F. Khan, et. al, IPAC10, 2010. [2] V.F. Khan, et. al, To be published, NIM-A, 2011. [3] H. Braun, et. al, CLIC-Note764, 2008. [4] I. Syratchev, CLIC-Note503, 2002. [5] C. Nantista, PRST-AB, 7, 072001, 2004. [6] N. M. Kroll, et. al, LINAC00, 2000. RF properties Unit Value Accelerating mode properties Shunt impedance R’ (In, Out) MΩ/m 51, 118 Group velocity v g /c (In, Out) % 2.07, 1.0 Bunch population (N b ) 10 9 4.2 Peak input power (P in ) MW 70.8 Pulse length (t c p , t p p ) ns 251, 208 Pulsed temperature rise o K 51 Surface electric field MV/m 220 Modified Poynting vector W/μm 2 6.75 RF-beam efficiency (η) % 23.5 P in (t p p ) 1/3 /C in MWns 1/3 /mm 16.93 Luminosity / bunch cross m -2 1.36 x 10 34 Figure of merit arb. units 7.6 Lowest dipole mode properties Frequency spread ∆f GHz 2.0 Standard deviation - σ =∆f/3.48 Detuning spread ∆f/f c % 11.8 The long-range wakefield,W t , in a non-interleaved structure as CLIC_DDS_A, is non-adequately damped. However, a fully interleaved structure (8-fold interleaved) will suppress the wakefields well within the acceptable limits [2]. Purpose of this structure is to demonstrate the capability to sustain high powers. Envelope of wakefield in an 8-fold interleaved structure Transverse wakefield comparison DS Q cu ~ 5860 Beam dynamics constraint Recoherence position Spectral function(G) of CLIC_DDS_A Envelope of wakefield in CLIC_DDS_A CLIC_DDS_A : RF properties END CELL MATCHING PROCEDURE Full structure simulations are employed to account for fine tuning to get accurate accelerating frequency and phase advance per cell. An already existing mode launcher [4]-[5] will be used to feed the structure and then it is necessary to match input and output of CLIC_DDS_A to this device. The matching procedure consists in three basic steps: Parameters Value Remarks Φ acc (Deg.) 120° ∆Φ max =6° f acc [GHz] 11.994 S 12 0.705 S 11 0.004 t f [ns] 57.15 Q Cu 6165 Gradient averaged over 24 regular cells V 24 [V]@P in = 1 W 2678 L=198.6 mm G 24 [V/m]@P in = 1 W 13481 P in [MW]@<G 24 =100MV/m> 55.03 1 st step (For input and output cells): a) One simulates a structure as in Figure 1 with one regular cell (first or last) and two matching cells at each end. The minimum value of S 11 as a function of the geometrical parameters of the matching cells (ma and mg) is sought. b) One does the same for a structure with two and three identical regular cells in the middle. c) The matching is the one coincident to all three cases (Figure 2). Figure 1: Structure used for the matching procedure; ma, mb and mg are the parameters that might be varied: we opted for ma and mg because of the presence of the manifold which limits b variations. Figure 2: Matching condition for the output: only one minimum (encircled in red) is common to the three cases (noc is the number of regular cells in the structure) 2 nd step: Use Kroll method [6] to minimize the SWR from output to input (see Figure 3) Optimized Ez profile Phase Figure 3: Minimization of the SWR from output to input using Kroll Method: outlined in red the optimized Ez field profile and the respective phase advance (polar plot). 3 rd step: Finally, one refines the input match of the full structure (Figure 4 and Table) MECHANICAL DETAILS CLIC_DDS_A will consist of 24 discs and 2 output guide rings as shown in Figure 5). Accelerating frequency is tuned using a dedicated push-pull system inside each single disc. High precision discs (± 2.5 μm iris shape accuracy) will be machined with conventional technology (milling and turning) and finally diffusion bonded (required flatness accuracy is 1 μm) under hydrogen atmosphere. Four qualification discs have been produced by VDL with shape accuracy and surface quality within requirements (Figure 6a). Three of them have been successfully bonded by Bodycote (Fig. 6b). The full structure will be machined by Morikawa in Japan in collaboration with CERN and KEK. It is also foreseen to produce a first stack of ten all equal cells to be metrological measured and cold RF measured. CLIC_DDS_A : 3D cut- view Figure 5: CLIC_DDS_A disc subdivision Figure 6: CLIC_DDS_A: a) Test Discs by VDL, b) bonded Discs by Bodycote. a) b) Figure 4: a) Results of the input matching; b) transverse wakepotential from Gdfidl; c) group delay and Q factor a) c ) THP083 FINAL REMARKS A structure able to meet the dual requirements of minimal surface e.m. fields and well-damped wakefield (with 8-fold interleaving) has been designed. A single structure, CLIC_DDS_A, is being fabricated to test the high power performance at 71 MW peak input power and will be tested at CERN in 2012. GdfidL vs Circuit model
