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Choke-mode Damped X-band Structure for CLIC Main Linac Hao ZHA, Jiaru SHI CERN Sep 27, 2011 Jiaru Shi, LCWS11 Workshop, Granada1

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Acknowledgement Collabration between CERN and Tsinghua University in China Ph.D Student visiting CERN for 6 months, ZHA Hao Manufacturing capability at Tsinghua Outline The Idea of choke-mode-damping Parameters Comparison with waveguide damping Wakefield damping study Future plans 2

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The choke mode cavity Progress at KEK and SLAC Applications at Japanese XFEL 3

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Radial Choke [1] Jiaru Shi, LCWS11 Workshop, Granada [1] A. Grudiev 4

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Parameters [1] Jiaru Shi, LCWS11 Workshop, Granada CellNDSWDSCDS1mmCDS1.5mmCDS2mm Q Cu68255635(0.83)4720(0.7)5129(0.75)5617(0.82) Vg / c (%)1.321.2(0.9)1.12(0.85)1.04(0.79)0.97(0.73) R/Q (kOhm/m)17.816.1(0.91)15.2(0.85)14.1(0.79)13.2(0.74) Rs (Mohm/m)121.590.7 (0.75)71.7 (0.59)72.3 (0.59)74.1 (0.60) E surfmax /E acc 1.93 H surfmax /E acc [mA/V]2.54.0(1.6)2.5 Pulsed T rise (K)133213 Plused T rise in first cell. @ 118MV/m 204720 [1] Based on CLIC-G middle cell [2] 250 ns, 100MV/m 5

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Advantages Lower pulsed surface heating, no magnetic field enhancement Bonding joint at low magnetic field, no problem on the chamfer Easy manufacturing, turning only Considerations Knowledge from SLAC [1], small gap (1mm) severe breakdown / 4mm gap OK different gap size Prototype structures for high power test 6Jiaru Shi, LCWS11 Workshop, Granada

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Damping simulation with Gdfidl/HFSS (Model in HFSS) Gap 1mm 2mm Good damping for first dipole Mode reflected by the choke, to be studied… E field, fundamental mode E field of a dipole mode that is reflected by the choke Impedance and wakefield simulated in Gdfidl 7

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EM field Jiaru Shi, LCWS11 Workshop, Granada High H, Low E Zero H, High E Fundamental mode at 12GHz High order dipole at 30GHz 8

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EM field after detuning with “bubble” Jiaru Shi, LCWS11 Workshop, Granada H H E E Fundamental mode tuned back to 12GHz By changing choke-length 9

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Smith Chart 10 A B C A B C B C Bubble,+j∆ No effect when adding to infinity Off from open point Red: fundamental mode Blue; 3h-HOM

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Model in HFSS for optimization 11 Port 1 Port 2 Absorber in radial line absorb in the radial line: sqrt( 1-S11^2-S21^2 )

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Radial line absorption 12 Choke reflection

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Results of “bubble-choke” s0.15m origin20~30 “Bubble- choke” 20~25 WDS6~8 13 31GHz 34GHz(in V/pC/m/mm) Push the second peak, more and more

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Smith chart “double” 14 A B C A B C B double 2*z C Red: fundamental mode Blue; 3h-HOM

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“double choke”, “bottle-choke” 15 40GHz!50GHz!

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First dipole 16 s0.15m origin20~30 CDS50~60 WDS6~8 (in V/pC/m/mm)

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“Thin-neck bottle” Blue: “1:1:3 bottle-choke” Red: “3:1:3 thin-neck bottle-choke” 17 12GHz 52GHz Narrow passband!

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Result of “thin-neck bottle” 18 s0.150.30 CDS- origin 20~2510~15 CDS2~40.5~0.8 WDS6~81~1.5 (in V/pC/m/mm) Note: applied to Tapered structure

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Problem of the “thin neck” 220MV/m electric field in the choke (@100MV/m gradient) 19

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Impedance matching Reduced field in choke: 1:1 to gradient; ½ as max surface field @ iris. Optimizing the depth, length, position of the step, try to match the 16G,18G mode 20

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Absorption after impedance matching Blue: “1:1:3 bottle-choke” Red: “1.5:1:3 bottle-choke” with impedance matching 21

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Result: 22 s0.150.30 CDS- origin 20~2510~15 CDS6~81~2.2 WDS6~81~1.5 (in V/pC/m/mm)

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Possible design 23 Pin: 61.3MW Loaded gradient: 100MV/m CLIC-G TD24-Choke same irises Pin: 61.3MW Loaded gradient: 96MV/m

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Summary Jiaru Shi, LCWS11 Workshop, Granada Study choke as transmission line, with smith chart Simple model in HFSS for optimization, also possible for a prototype to do RF measurement Choke mode damping with comparable result to waveguide damping Future plans Finalize the alternative RF design with choke-mode damping that could possibly replace the baseline design Propose wakefield measurement at SLAC Prototype for high power test 24

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Thank you! Jiaru Shi, LCWS11 Workshop, Granada25

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