CLIC MAIN LINAC DDS DESIGN AND FORTCOMING Vasim Khan & Roger Jones V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 1/14

CLIC MAIN LINAC DDSCLIC MAIN LINAC DDSDESIGN AND FORTCOMINGDESIGN AND FORTCOMING

Vasim Khan & Roger Jones

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 1/14

Wakefield suppression in CLIC main linacsThe present main accelerating structure (WDS) for the CLIC relies on linear tapering of cell parameters and heavy damping with a Q of ~10. The wake-field suppression in this case entails locating the dielectric damping materials in relatively close proximity to the location of the accelerating cells.

Ref: A. Grudiev, W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08


Wakefield suppression in CLIC main linacs

To minimise the breakdown probability and reduce the pulse surface heating, we are looking into an alternative scheme for the main accelerating structures:

The present main accelerating structure (WDS) for the CLIC relies on linear tapering of cell parameters and heavy damping with a Q of ~10. The wake-field suppression in this case entails locating the dielectric damping materials in relatively close proximity to the location of the accelerating cells.



Wakefield suppression in CLIC main linacs

To minimise the breakdown probability and reduce the pulse surface heating, we are looking into an alternative scheme for the main accelerating structures:

• Detuning the first dipole band by forcing the cell parameters to have Gaussian spread in the frequencies

• Considering the moderate damping Q~500

The present main accelerating structure (WDS) for the CLIC relies on linear tapering of cell parameters and heavy damping with a Q of ~10. The wake-field suppression in this case entails locating the dielectric damping materials in relatively close proximity to the location of the accelerating cells.



Constraints RF breakdown constraint [1],[2]

1)

2) Pulsed surface heating

3) Cost factor

Beam dynamics constraints [1],[2]

1)For a given structure, no. of particles per bunch N is decided by the <a>/λ and Δa/<a>2)Maximum allowed wake on the first trailing bunch

Rest of the bunches should see a wake less than this wake(i.e. No recoherence).

260MV/mEmaxsur

K 56ΔTmax

mmnsMW 18CτP 3in

3pin

N

10 4 mm/m6.667V/pC/W

9

t1

Ref: [1]: A. Grudiev and W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08 . [2]: H. Braun, et al. , Updated CLIC Parameters, CLIC-Note 764, 2008.


Uncoupled (designed) distribution of Kdn/df for a four fold interleaved structure

In order to provide adequate sampling of the uncoupled Kdn/df distribution cell frequencies of the neighbouring structures are interleaved (4xN where N = 24) .

An erf distribution of the cell frequencies (lowest dipole) with cell number is employed.

df

dnK α WT

Mode separation

K dn/dfdn/df


Damped and detuned structures

• 3.3 GHz structure does satisfy beam dynamics constraints but does not satisfies RF breakdown constraints.• 1.0 GHz structure satisfies RF constraints but does not satisfy beam dynamics constraints and hence relies on zero crossing scheme which is subjected to very tight fabrication tolerances.

<a>/λ=0.102, ∆f = 0.83 GHz,∆f = 3σ, ∆f/favg= 4.56 %


<a>/λ=0.155 ∆f = 3.3 GHz , ∆f = 3.6σ, ∆f/favg= 20 %

* This is a detuned structure i.e. no manifold geometry employed and a damping Q is artificially put in the calculations.

3.3 GHz structure*

1.0 GHz structure*

Manifold

Coupling slot

Cell mode

Manifold mode


Cell parameters Cell # 1 Cell # 24

Iris radius (mm) 4.0 2.15

Iris thickness (mm) 4.0 0.7

Ellipticity 1.0 2.0

Q 4771 6355

R’/Q (kΩ/m) 11.64 20.09

vg/c (%) 2.13 0.9

∆f = 3.6 σ = 2.3 GHz∆f/fc =13.75 %<a>/λ=0.126

A 2.3 GHz Damped-detuned structure

24 cellsNo interleaving

Spectral function -----(IFT) Wake function


∆fmin = 65 MHz∆tmax =15.38 ns∆s = 4.61 m




48cells2-fold interleaving

∆fmin = 32.5 MHz∆tmax =30.76 ns∆s = 9.22 m


∆fmin = 65 MHz∆tmax =15.38 ns∆s = 4.61 m

48cells2-fold interleaving


96 cells4-fold interleaving




∆fmin = 16.25 MHz∆tmax = 61.52 ns∆s = 18.46 m






∆fmin = 16.25 MHz∆tmax = 61.52 ns∆s = 18.46 m


∆fmin = 8.12 MHz∆tmax =123 ns∆s = 36.92 m

A 1.19

GHz 11.9942

6101.61072.3

I19-9

For CLIC_G structure <a>/λ=0.11, considering the beam dynamics constraint bunch population is 3.72 x 10^9 particles per bunch and the heavy damping can allow an inter bunch spacing as compact as ~0.5 ns. This leads to about 1 A beam current and rf –to-beam efficiency of ~28%.

For CLIC_DDS structure (2.3 GHz) <a>/λ=0.126, and has an advantage of populating bunches up to 4.5x10^9 particles but a moderate Q~500 will require an inter bunch spacing of 8 cycles (~ 0.67 ns).

A 1.13

GHz 11.9942

8101.6104.75

I19-9

V/pc/mm/m 1.71072.3150

10410010W

9

9limitT

V/pc/mm/m 5.6104.75150

10410010W

9

9limitT

Though the bunch spacing is increased in CLIC_DDS, the beam current is compensated by increasing the bunch population and hence the rf-to-beam efficiency of the structure is not affected alarmingly.

CLIC_G Vs CLIC_DDS


Parameters CLIC_G (Optimised)

[1,2]

CLIC_DDS(Single

structure)

CLIC_DDS*(8-fold

interleaved)

Bunch space (rf cycles/ns) 6/0.5 8/0.67 8/0.67

Limit on wake (V/pC/mm/m) 7.1 5.6 5.3

Number of bunches 312 312 312

Bunch population (109) 3.72 4.7 5.0

Pulse length (ns) 240.8 273 272.2

Fill time (ns) 62.9 42 40.8

Pin (MW) 63.8 72 75.8

Esur max. (MV/m) 245 232 224

Pulse temperature rise (K) 53 47.3 50

RF-beam-eff. 27.7 26.6 26.7

[1] A. Grudiev, CLIC-ACE, JAN 08[2] H. Braun, CLIC Note 764, 2008* Averaged values of structure #1 & #8

CLIC_G Vs. CLIC_DDS



Single structure vs. Interleaved structure

Interleaved structures

Single structure

Interleaved structures

Single structure

Single structureSingle structure

Mean ofinterleaved structures

Mean ofinterleaved structures

Manifolds running parallel to the main structure removes higher order mode and damp at remote location Each manifold can be used for :

o Beam position monitoringo Cell alignments

Features of manifold geometry


Potential structure for CTF3 module

8 structures in each CTF3 module

Thank you ........Thank you ........

V. Khan LC-ABD, Cockcroft Institute 22.09.09 14/14

We would like to thank W. Wuensch, A. Grudiev, D. Schulte, J. Wang and T. Higo for their involvement in discussions and many useful suggestions.

Documents

CLIC MAIN LINAC DDS DESIGN AND FORTCOMING Vasim Khan & Roger Jones V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 1/14