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CLIC MAIN LINAC DDS DESIGN AND FORTCOMING

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CLIC MAIN LINAC DDSDESIGN AND FORTCOMING

Vasim Khan & Roger Jones

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

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.

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

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 2/14

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.

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

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

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 2/14

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.

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

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

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 2/14

- Beam dynamics constraints [1],[2]
- For a given structure, no. of particles per bunch N is decided by the <a>/λ and Δa/<a>
- Maximum allowed wake on the first trailing bunch
- Rest of the bunches should see a wake less than this wake(i.e. No recoherence).

RF breakdown constraint [1],[2]

1)

2) Pulsed surface heating

3) Cost factor

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.

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 3/14

dn/df

K dn/df

Mode separation

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

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

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 4/14

3.3 GHz structure*

1.0 GHz structure*

<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 %

- 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.

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

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 5/14

A 2.3 GHz Damped-detuned structure

Cell mode

Manifold

Manifold mode

∆f = 3.6 σ = 2.3 GHz

∆f/fc =13.75 %

<a>/λ=0.126

Coupling slot

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 6/14

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

∆fmin = 65 MHz

∆tmax =15.38 ns

∆s = 4.61 m

24 cells

No interleaving

24 cells

No interleaving

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 7/14

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

∆fmin = 65 MHz

∆tmax =15.38 ns

∆s = 4.61 m

∆fmin = 32.5 MHz

∆tmax =30.76 ns

∆s = 9.22 m

48cells

2-fold interleaving

24 cells

No interleaving

48cells

2-fold interleaving

24 cells

No interleaving

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 8/14

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

∆fmin = 16.25 MHz

∆tmax = 61.52 ns

∆s = 18.46 m

96 cells

4-fold interleaving

96 cells

4-fold interleaving

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 9/14

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

∆fmin = 16.25 MHz

∆tmax = 61.52 ns

∆s = 18.46 m

∆fmin = 8.12 MHz

∆tmax =123 ns

∆s = 36.92 m

96 cells

4-fold interleaving

192 cells

8-fold interleaving

96 cells

4-fold interleaving

192 cells

8-fold interleaving

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 9/14

CLIC_G Vs CLIC_DDS

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).

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.

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 10/14

CLIC_G Vs. CLIC_DDS

[1] A. Grudiev, CLIC-ACE, JAN 08

[2] H. Braun, CLIC Note 764, 2008

* Averaged values of structure #1 & #8

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 11/14

Single structure vs. Interleaved structure

Interleaved structures

Interleaved structures

Single structure

Single structure

Mean of

interleaved structures

Mean of

interleaved structures

Single structure

Single structure

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 12/14

Features of manifold geometry

- Manifolds running parallel to the main
- structure removes higher order mode and
- damp at remote location
- Each manifold can be used for :
- Beam position monitoring
- Cell alignments

Potential structure for CTF3 module

8 structures in each CTF3 module

V. Khan LC-ABD 09, Cockcroft Institute 22.09.09 13/14

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.

Thank you ........

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