Update of CLIC accelerating structure design Hao Zha, Alexej Grudiev 18/01/2015

Could be further optimized CLIC base line design So called CLIC-G design: [A. Grudiev et al., Design of the clic main linac accelerating structure for clic conceptual design report, LINAC 2010, MOP068]. Could be further optimized Working frequency[GHz] 11.994 Shunt impedance [MΩ/m] 92.0 Number of Cell 26+2 Peak input power [MW] 63.4 Active length[mm] 230 RF to beam efficiency 27.9% Average Gradient [MV/m] 100 Total pulse length [ns] 242 Phase advance per cell 120° Filling time [ns] 67 Bunch charge 3.72*109 Maximum electric field [MV/m] 248 Bunch train number 312 Maximum Sc [MW/mm2] 5.70 Bunch separation [ns] 0.5 Maximum temperature rise [K] 47

HOM wakefield suppression of CLIC-G Wakefield suppression requirement: transverse kick at 0.5 ns < 6.6 V/pC/m/mm The design was proved by the Wakefield measurement at FACET facility. Transverse offset deflected orbit Downstream BPMs Dump e+ e- CLIC-G TD26cc Dipole e+, Driver bunch e-, Witness bunch Charge: 3 nC; Energy: 1.19GeV; Bunch length: 0.7mm (RMS) EXPERIMENTAL VERIFIED! !

New-CLIC-G design We found that: the profile of wall geometry: elliptical arc -> polynomial line, surface H-field ↓ ; Optimize the waveguide geometry: cell diameter ↓,surface H-field ↓ ; Larger rounding: input power ↓, manufacturing cost ↓ R = 0.5mm R = 2.5mm elliptical arc Width=11mm ->10.1mm Opening=8mm->7.8mm polynomial line

Maximum temperature rise [K] New-CLIC-G design Now we have a lower surface field , lower input power , lower cost  design: CLIC-G* . Manufacturing is now undergoing (anticipate high power test in 2016). CLIC-G CLIC-G* Rounding[mm] 0.5 1.0 Manufacturing cost reduction -- 7%↓ Shunt impedance [MΩ/m] 92.0 95.4↑ Peak input power [MW] 63.4 62.3↓ RF to beam efficiency 27.9% 28.4%↑ Filling time [ns] 67 66↓ Maximum electric field [MV/m] 248 250↑ Maximum Sc [MW/mm2] 5.70 5.65↓ Maximum temperature rise [K] 52 41↓

Load design for CLIC structure Cell, higher manufacturing accuracy ~1um Cell diameter: 20 cm. Use manifold design to reduce cost Manifold, lower manufacturing accuracy 5~10um 20 cm 4 cm 5 cm : safety distance

Load design for CLIC-G structure Cell diameter is limited by : Safety distance (Maintain Low accelerating mode loss) RF load design (HOM suppression) 200 W peak loss per load 2.4 mW average loss

Load design for new CLIC-G* structure New design diameter: 17 cm Safety distance : 3.3 cm, Load Length: 4.2 cm 17 cm 4.2 cm 3.3 cm

Load design details Length is dominated by the tapered section. L1 = 38 mm, L2 = 4mm (old CLIC-G, L1 = 30 mm, L2 = 10 mm). Smaller waveguide -> longer propagate wavelength -> L1↑ . More powerful damped material? -> L1 ↓ , safety distance ↑, total length change very little. power SiC: R. EICHHORN, CORNELL 5 mm L2 = 4 mm : Normal section L1 = 38 mm : Tapered section 0.5 mm

Change the waveguide geometry Change waveguide geometry to compensate the reflection in the load. Matching step in the waveguide could match the RF load cell by cell. Tapered waveguide design Matching step design

Final RF load design Load length = 3.3 cm, Safety length = 3.2 cm. Full diameter decrease by 5 cm! Wakefield suppression keep same. Old CLIC-G design : 20 cm New CLIC-G design : 17 cm New CLIC-G design with matching step: 15 cm

Coupler design CLIC-G structure has double feed coupler. CLIC PETS 3 dB hybrid High power RF load 62.5 MW 135 MW CLIC-G structure has double feed coupler. A power splitter is usually use to feed the structure in two branches. Wakefield dipole mode? A power splitter here ? Coupler waveguide HOM waveguide

HOM suppression in the coupler Wakefield TM011x mode go though the coupler waveguide to the power splitter. Reflected? Absorbed? HOM suppression Magic-T: Power splitter and HOM coupler are isolated + - Accelerating TM010 mode Wakefield TM011y mode Wakefield TM011x mode Coupler waveguide HOM waveguide + Coupler waveguide HOM waveguide + - Coupler waveguide HOM waveguide - +

HOM suppression in the coupler Do Fourier

Design of the HOM-free Magic-T Straightforward to match the working frequency (bandwidth: 130 MHz). For HOM mode matching? Need very large bandwidth (> 1 GHz): tune the waveguide dimensions. Mismatch in working frequency Mismatch in dipole frequency Trapped modes

Trapped modes Change the waveguide width to tune the frequency of trapped modes. Optimum width: 25 mm.

Wakefield simulations The HOM free magic decrease at least 1/2 of the reflected wakefield in the coupler.

I hate trapped modes! Bend waveguide excited extra TE20 modes. TE20 mode is allowed for 17 Ghz in the WR 90 waveguides. Need more work!

Summary We’ve almost finished the new CLIC accelerating structure design: Better RF parameters of the structure Lower machining cost Compact transverse dimensions and RF loads. HOM-free magic-T for the power coupling.