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

2. 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

3. 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! !

4. 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

5. 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↓

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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 - +

14. HOM suppression in the coupler
Do Fourier

15. 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

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

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

18. 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!

19. 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.