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Progress in the design of a damped an tapered accelerating structure for CLIC

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Progress in the design of a damped an

tapered accelerating structure for CLIC

Jean-Yves Raguin

CERN AB/RF

- 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

- Each cell coupled to four rectangular identical 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)

Accelerating mode: electric field magnitude (Log)

SiC load

Dipole mode: Poynting vector

Performance limits and peak surface electric fields

L= 50 cm

Pin/section = 248 MW

h = 23.8 %

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

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

Solutions

Criteria to design new CLIC

damped accelerating structure

Topology of the cell and of the damping waveguides

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

First cell

Iris thickness: 0.55 mm

2.0 mm

3.792 mm

Q=3744

with s = 5.80.107 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

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

Last cell

Iris thickness: 1.00 mm

1.5 mm

3.661 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

For 84 cells (L= 28 cm)

Epeak=400 MV/m

Pin/section = 130 MW

h = 26.1 %

DT=154 K

DT=120 K

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= 52 Q dip,middle= 51

Q dip,last= 44 Dfdip /f dip,first = 3.3 %

THERE IS ROOM FOR IMPROVEMENT…

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

First cell

Iris thickness: 0.80 mm

2.0 mm

3.870 mm

Q=3387

with s = 5.80.107 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

Last cell

Iris thickness: 0.55 mm

1.5 mm

3.539 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

For 83 cells (L= 25.4 cm)

Epeak=355 MV/m

Pin/section = 128 MW

h = 24.8 %

DT=122 K

DT=122 K

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= 53 Q dip,middle= 31

Q dip,last= 24 Dfdip /f dip,first = 4.9 %

There is room for improvement (2)…

First cell

Iris thickness: 0.80 mm

2.0 mm

3.867 mm

Q=3266

with s = 5.51.107 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

Last cell

Iris thickness: 0.55 mm

1.5 mm

3.532 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

For 83 cells (L= 25.4 cm)

Epeak=355 MV/m

Pin/section = 130 MW

h = 24.4 %

DT=119 K

DT=128 K

For 77 cells (L= 23.5 cm)

Epeak=348 MV/m

Pin/section = 125 MW

h = 23.8 %

DT=121 K

DT=122 K

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= 43 Q dip,middle= 27

Q dip,last= 21 Df dip/f dip,first = 5.4 %

Shall we dare to lower the accelerating gradient?…

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…

- 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