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## CLIC MAIN LINAC DDS DESIGN AND FORTCOMING

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**CLIC MAIN LINAC DDSDESIGN AND FORTCOMING**Vasim Khan & Roger Jones V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 1/14**Wakefield suppression in CLIC main linacs**The present main accelerating structure (WDS) for the CLIC relies on linear tapering of cell parameters and heavy damping with a Q of ~10. The wake-field suppression in this case entails locating the dielectric damping materials in relatively close proximity to the location of the accelerating cells. Ref: A. Grudiev, W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08 V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 2/14**Wakefield suppression in CLIC main linacs**The present main accelerating structure (WDS) for the CLIC relies on linear tapering of cell parameters and heavy damping with a Q of ~10. The wake-field suppression in this case entails locating the dielectric damping materials in relatively close proximity to the location of the accelerating cells. To minimise the breakdown probability and reduce the pulse surface heating, we are looking into an alternative scheme for the main accelerating structures: Ref: A. Grudiev, W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08 V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 2/14**Wakefield suppression in CLIC main linacs**The present main accelerating structure (WDS) for the CLIC relies on linear tapering of cell parameters and heavy damping with a Q of ~10. The wake-field suppression in this case entails locating the dielectric damping materials in relatively close proximity to the location of the accelerating cells. To minimise the breakdown probability and reduce the pulse surface heating, we are looking into an alternative scheme for the main accelerating structures: • Detuning the first dipole band by forcing the cell parameters to have Gaussian spread in the frequencies • Considering the moderate damping Q~500 Ref: A. Grudiev, W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08 V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 2/14**Constraints**• Beam dynamics constraints [1],[2] • 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 • Rest of the bunches should see a wake less than this wake(i.e. No recoherence). RF breakdown constraint [1],[2] 1) 2) Pulsed surface heating 3) Cost factor Ref: [1]: A. Grudiev and W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08 . [2]: H. Braun, et al. , Updated CLIC Parameters, CLIC-Note 764, 2008. V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 3/14**Uncoupled (designed) distribution of Kdn/dffor a four fold**interleaved structure dn/df K dn/df Mode separation An erf distribution of the cell frequencies (lowest dipole) with cell number is employed. In order to provide adequate sampling of the uncoupled Kdn/df distribution cell frequencies of the neighbouring structures are interleaved (4xN where N = 24) . V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 4/14**Damped and detuned structures**3.3 GHz structure* 1.0 GHz structure* <a>/λ=0.102, ∆f = 0.83 GHz, ∆f = 3σ, ∆f/favg= 4.56 % <a>/λ=0.155 ∆f = 3.3 GHz , ∆f = 3.6σ, ∆f/favg= 20 % • 3.3 GHz structure does satisfy beam dynamics constraints but does not satisfies • RF breakdown constraints. • 1.0 GHz structure satisfies RF constraints but does not satisfy beam dynamics • constraints and hence relies on zero crossing scheme which is subjected to very tight • fabrication tolerances. * This is a detuned structure i.e. no manifold geometry employed and a damping Q is artificially put in the calculations. V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 5/14**A 2.3 GHz Damped-detuned structure**Cell mode Manifold Manifold mode ∆f = 3.6 σ = 2.3 GHz ∆f/fc =13.75 % <a>/λ=0.126 Coupling slot V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 6/14**Spectral function -----(IFT) Wake function**∆fmin = 65 MHz ∆tmax =15.38 ns ∆s = 4.61 m 24 cells No interleaving 24 cells No interleaving V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 7/14**Spectral function -----(IFT) Wake function**∆fmin = 65 MHz ∆tmax =15.38 ns ∆s = 4.61 m ∆fmin = 32.5 MHz ∆tmax =30.76 ns ∆s = 9.22 m 48cells 2-fold interleaving 24 cells No interleaving 48cells 2-fold interleaving 24 cells No interleaving V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 8/14**Spectral function -----(IFT) Wake function**∆fmin = 16.25 MHz ∆tmax = 61.52 ns ∆s = 18.46 m 96 cells 4-fold interleaving 96 cells 4-fold interleaving V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 9/14**Spectral function -----(IFT) Wake function**∆fmin = 16.25 MHz ∆tmax = 61.52 ns ∆s = 18.46 m ∆fmin = 8.12 MHz ∆tmax =123 ns ∆s = 36.92 m 96 cells 4-fold interleaving 192 cells 8-fold interleaving 96 cells 4-fold interleaving 192 cells 8-fold interleaving V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 9/14**CLIC_G Vs CLIC_DDS**For CLIC_G structure <a>/λ=0.11, considering the beam dynamics constraint bunch population is 3.72 x 10^9 particles per bunch and the heavy damping can allow an inter bunch spacing as compact as ~0.5 ns. This leads to about 1 A beam current and rf –to-beam efficiency of ~28%. For CLIC_DDS structure (2.3 GHz) <a>/λ=0.126, and has an advantage of populating bunches up to 4.5x10^9 particles but a moderate Q~500 will require an inter bunch spacing of 8 cycles (~ 0.67 ns). Though the bunch spacing is increased in CLIC_DDS, the beam current is compensated by increasing the bunch population and hence the rf-to-beam efficiency of the structure is not affected alarmingly. V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 10/14**CLIC_G Vs. CLIC_DDS**[1] A. Grudiev, CLIC-ACE, JAN 08 [2] H. Braun, CLIC Note 764, 2008 * Averaged values of structure #1 & #8 V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 11/14**Single structure vs. Interleaved structure**Interleaved structures Interleaved structures Single structure Single structure Mean of interleaved structures Mean of interleaved structures Single structure Single structure V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 12/14**Features of manifold geometry**• Manifolds running parallel to the main • structure removes higher order mode and • damp at remote location • Each manifold can be used for : • Beam position monitoring • Cell alignments Potential structure for CTF3 module 8 structures in each CTF3 module V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 13/14**We would like to thank W. Wuensch, A. Grudiev,**D. Schulte, J. Wang and T. Higo for their involvement in discussions and many useful suggestions. Thank you ........ V. Khan LC-ABD, Cockcroft Institute 22.09.09 14/14