1. Wakefield suppression in the CLIC main accelerating structures Vasim Khan & Roger Jones

2. Wakefield suppression in CLIC main linacs We are looking into alternative scheme in order to suppress the wakefield in the main accelerating structures: 1) Detuning the first dipole band by forcing the cell parameters to have Gaussian spread in the frequencies 2) Considering the moderate damping Q~500 12/11/20082

3. StructureCLIC_G Frequency (GHz)12 Avg. Iris radius/wavelength /λ0.11 Input / Output iris radii (mm)3.15, 2.35 Input / Output iris thickness (mm)1.67, 1.0 Group velocity (% c)1.66, 0.83 No. of cells per cavity24 Bunch separation (rf cycles)6 No. Of bunches in a train312 Cella (mm)b (mm)t (mm)Vg/c (%)f1 (GHz) 1st3.159.91.671.6317.45 Ref 12.979.861.51.4217.64 Ref 22.759.791.341.217.89 Ref 32.549.751.181.018.1 24th2.359.711.00.8618.27 Cell parameters of a modified CLIC_G structure: Gaussian distribution Uncoupled values: /λ=0.11 ∆f = 0.82 GHz ∆f = 3σ i.e.(σ=0.27 GHz) ∆f/favg= 4.5 % Then we calculate the coupled mode frequencies and kicks using double band circuit model CLIC_G structure 312/11/2008

4. Modified CLIC_G structure Uncoupled Coupled Q = 500 Undamped 4 Envelope Wake-field Amplitude Wake-field 12/11/2008

5. Cell #a (mm)b (mm)t (mm)Vg/c (%)f1 (GHz) 12.999.881.61.4917.57 42.849.831.41.3817.72 82.729.801.31.2917.85 122.619.781.21.1817.96 162.519.751.11.0618.07 202.379.730.960.9818.2 242.139.680.70.8318.4 Cell parameters of seven cells of CLIC_ZC structure having Gaussian distribution Uncoupled values: /λ=0.102 ∆f = 0.83 GHz ∆f = 3σ i.e.(σ=0.27 GHz) ∆f/favg= 4.56% ∆a1=160µm and ∆a24= 220µm. The first trailing bunch is at 73% of the peak value (W max =180 V/pC/mm/m). ∆f=110 MHz. There is a considerable difference in the actual wake-field experienced by the bunch, which is 1.7 % of peak value which was otherwise 27%. Zero crossing of wake-field The oscillations in the wake-field amplitude are due to a corresponding sin term. We adjust the mode frequencies to force the bunches to be located at the zero crossing in the wake- field. We adjust the zero crossing by systematically shifting the cell parameters (aperture and cavity radius). 512/11/2008

6. CLIC_ZC structure Coupled Uncoupled Undamped Q = 500 6 Envelope Wake-field Amplitude Wake-field 12/11/2008

7. Interleaved cells & S RMS Q = 500 24 cells 192 cells 712/11/2008

8. Thank you 12/11/20088