1. Progress in the design of a damped an
tapered accelerating structure for CLIC
Jean-Yves Raguin
CERN AB/RF

2. The Tapered Damped Structure
2p/3 travelling-wave mode, 150 cells – Av. Acc. Grad. of 150 MV/m
Strong damping
Each cell coupled to four rectangular identical waveguides
Cutoff frequency of the fundamental waveguide mode between the p-mode frequency of the fundamental passband and the lowest frequency of the first dipole band
Trapped fundamental mode whereas the E-M energy of the higher modes propagates out of the cells and is absorbed by SiC loads terminating the waveguides
Light detuning
Linear variation of the irises radius from 2.25 mm at the head of the structure to 1.75 mm at the end, each cell being tuned to have the same fundamental frequencies
Dipole frequency spread (5.4 %) which contributes to a further reduction of the transverse wakefields
Demonstrated suppression of the transverse wakefields by two orders of magnitude within 0.67 ns (time between two consecutive bunches) on a 15 GHz-scaled version (ASSET experiment)

3. Accelerating mode: electric field magnitude (Log)
SiC load
Dipole mode: Poynting vector

4. Performance limits and peak surface electric fields
L= 50 cm
Pin/section = 248 MW
h = 23.8 %

5. Temperature increase for the middle cell of the TDS with
Pulsed surface heating
Temperature increase for the middle cell of the TDS with
= 150 MV/m and tp=130ns
Due to the configuration of the cell-waveguide coupling iris (3.3 mm wide) the local magnetic field is too high,
leading to excessive maximum temperature rise

6. Design of a new 2p /3 damped and tapered structure
Criteria to design new CLIC
damped accelerating structure
Solutions
To prevent electric breakdown, low peak surface electric field
To prevent excessive pulse heating, low peak surface magnetic field
Good damping of higher order modes and, in particular, of the first and second dipole modes
Use of an elliptical profile for irises
Appropriate profile of the cells wall
Damping of higher order modes by coupling each cell to four T-cross waveguides

8. Topology of the cell and of the damping waveguides

9. Field pattern of the first two waveguide modes
Fundamental mode
Damping for the first dipole band
Second mode
Damping for the second dipole band
Electric field
Electric field
Perfect Elec. wall
Perfect Mag. wall
Magnetic field
Magnetic field

10. First cell Iris thickness: 0.55 mm Q=3744
with s = Mhos/m (Cu)
R’/Q= 24.5 kW /m
vg/c = 8.1 %
fcutoff,TE10-e = 32.1 GHz
Epeak / Eacc = 2.55
Hpeak / Eacc = 4.50 mA/V
3.0 mm
5.25 mm

11. Hsurf/Eacc (mA/V) on the walls of the first cell

12. Last cell Iris thickness: 1.00 mm Q=3373 with s = 5.80.107 Mhos/m (Cu)
R’/Q= 30.0 kW /m
vg/c = 2.6 %
fcutoff,TE10-e = 32.4 GHz
Epeak / Eacc = 1.75
Hpeak / Eacc = 4.38 mA/V
3.0 mm
5.25 mm

15. For 84 cells (L= 28 cm) Epeak=400 MV/m Pin/section = 130 MW h = 26.1 %
DT=154 K
DT=120 K

16. Transverse wakefields analysis
First cell
Middle cell
Last cell
Real part of the transverse impedance vs. frequency computed with GdfidL for the first, middle and last cells
Transverse wakefields
|W t | = 90 V/pC/mm/m at the 2nd bunch
Q dip,first= Q dip,middle= 51
Q dip,last= Dfdip /f dip,first = 3.3 %

17. THERE IS ROOM FOR IMPROVEMENT…

18. Design of a damped structure - b d = 110 deg.
Working at a lower phase
advance allows to decrease
Epeak / Eacc
Decrease iris thickness along
the structure
Increase the coupling cell-
waveguide along the structure
for better damping

19. First cell Iris thickness: 0.80 mm Q=3387
with s = Mhos/m (Cu)
R’/Q= 24.1 kW /m
vg/c = 7.7 %
fcutoff,TE10-e = 32.3 GHz
Epeak / Eacc = 2.21
Hpeak / Eacc = 4.50 mA/V
3.0 mm
5.25 mm

20. Last cell Iris thickness: 0.55 mm Q=3365 with s = 5.80.107 Mhos/m (Cu)
R’/Q= 33.2 kW /m
vg/c = 3.8 %
fcutoff,TE10-e = 32.3 GHz
Epeak / Eacc = 2.00
Hpeak / Eacc = 4.17 mA/V
3.2 mm
5.32 mm

23. For 83 cells (L= 25.4 cm) Epeak=355 MV/m Pin/section = 128 MW
h = 24.8 %
DT=122 K
DT=122 K

24. Transverse wakefields analysis
First cell
Middle cell
Last cell
Real part of the transverse impedance vs. frequency computed with GdfidL for the first, middle and last cells
Transverse wakefields
|W t | = 58 V/pC/mm/m at the 2nd bunch
Q dip,first= Q dip,middle= 31
Q dip,last= Dfdip /f dip,first = 4.9 %

25. There is room for improvement (2)…

26. Design of a damped structure - b d = 110 deg. (2)
First cell
Iris thickness: 0.80 mm
2.0 mm
3.867 mm
Q=3266
with s = Mhos/m (Cu)
R’/Q= 24.0 kW /m
vg/c = 7.6 %
fcutoff,TE10-e = 32.3 GHz
Epeak / Eacc = 2.20
Hpeak / Eacc = 4.51 mA/V
3.0 mm
5.25 mm

27. Last cell Iris thickness: 0.55 mm Q=3252 with s = 5.51.107 Mhos/m (Cu)
R’/Q= 32.5 kW /m
vg/c = 3.8 %
fcutoff,TE10-e = 32.3 GHz
Epeak / Eacc = 1.95
Hpeak / Eacc = 4.10 mA/V
3.2 mm
5.32 mm

30. For 83 cells (L= 25.4 cm) Epeak=355 MV/m Pin/section = 130 MW
h = 24.4 %
DT=119 K
DT=128 K

31. For 77 cells (L= 23.5 cm) Epeak=348 MV/m Pin/section = 125 MW
h = 23.8 %
DT=121 K
DT=122 K

32. Transverse wakefields analysis 83-cell structure
First cell
Middle cell
Last cell
Real part of the transverse impedance vs. frequency computed with GdfidL for the first, middle and last cells
Transverse wakefields
|W t | = 45 V/pC/mm/m at the 2nd bunch
Q dip,first= Q dip,middle= 27
Q dip,last= Df dip/f dip,first = 5.4 %

33. Shall we dare to lower the accelerating gradient?…

34. Average accelerating gradient of 125 MV/m
For 75 cells (L= 22.9 cm)
Epeak< 300 MV/m
Pin/section = 89 MW
h = 27.2 %
DT=88 K
DT=87 K
with the same
beam current…

35. Investigation of new materials
Conclusions
Design of copper structure with average accelerating gradient of 150 MV/m, peak surface electric field lower that 300 MV/m and maximum temperature rise lower than 100 K seems unrealistic – would lead to smaller iris radius
Copper structure with average accelerating gradient of 125 MV/m, peak surface electric field lower that 300 MV/m and maximum temperature rise lower than 100 K seems feasible
Investigation of new materials
Design of the latest structure with molybdenum irises – calls for a reassessment of the fundamental mode characteristics
Material for the cell outer walls which would solve the pulsed surface heating problem?
Transverse wakefields suppression
Need for tuning the dipole frequencies