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DDS design status

Alessandro D’Elia

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Outline

1. General overview of DDS working principle2. CLIC_DDS_A design status3. Towards CLIC_DDS_B:

a) wakefield simulations and impedancesb) HOM coupler design

4. Some conclusion

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Damped and detuned design

• Detuning: A smooth variation in the iris radii spreads the dipole frequencies. This spread does not allow wake to add in phase

• Error function distribution to the iris radii variation results in a rapid decay of wakefield.

• Due to limited number of cells in a structure (truncated Gaussian) wakefield recoheres.

• Damping: The recoherence of the wakefield is suppressed by means of a damping waveguide like structure (manifold).

• Interleaving neighbouring structure frequencies help enhance the wake suppression

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NLC/GLC DDS design

High powerrf coupler

HOM coupler

Beam tube

Acceleration cells

Manifold

Ref: R. Jones, et al. , PRSTAB 9, 102001, (2006).

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Large bandwidth structure

dfdnK α WT

Error function distribution

Re erf n 4i t / 2 2where : (t, f )

erf n / 2 2

22 tt

Truncated Gaussian :

W 2Ke (t, f )

Courtesy of R. M. Jones

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Interleaving

Courtesy of V. Khan

Why a Detuning Damping Structure (DDS) for CLIC

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• Huge reduction of the absorbing loads: just 4x2 loads per structure

• Inbuilt Wakefield Monitors, Beam Position Monitors that can be used as remote measurements of cell alignments

• In principle, lower pulse temperature rise• Consequently (still in principle) lower probability

of breakdown events• Huge reduction of the outer diameter of the

machined disks

CLIC_DDS_A

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• In October 2009 it has been decided to produce a first prototype to be tested at input power of 62 MW to ascertain the suitability of the structure to sustain high e.m. field gradients

• RF and mechanical design completed in Summer 2010• 4 qualification disks machined by VDL received in Oct

2010• The 4 disks have been successfully bonded by Bodycote• The whole structure will be machined in Japan by

Morikawa under the supervision of KEK• High Power Tests are foreseen as soon as we will get the

full structure

NOW

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CLIC_DDS_A: regular cell optimizationThe chose of the cell geometry is crucial to meet at the same time:1. Wakefield suppression2. Surface fields in the specs

Consequences on wake function

Cell shape optimization for fields

DDS1_C DDS2_E

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Further information:

• Manifold dimensions are uniform throughout the structure• The manifold radius is now parameterised in order to keep the lowest manifold mode above 12 GHz.

CLIC_DDS_A: regular cells, final design

Roundings enhance the magnetic field however a reduction of the slot size mitigates this enhancement and RF parameters are back within required limits. Also the wake damping is still in the limits with margins for a further improvement.

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CLIC_DDS_A full structure

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CLIC_DDS_A some further detail

VDL

BODYCOTE

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CLIC_DDS_B

• The study of a further structure (CLIC_DDS_B) is already started

• This structure will be based on CLIC_DDS_A but will be provided with HOM couplers and with a compact coupler for fundamental mode

• Both wakefield suppression and high power performances will be tested

Next future

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First steps toward CLIC_DDS_BWakefield calculations for DDS are, in the early design stage, based on single infinitely periodic cells. Though cell-to-cell interaction is taken into account to calculate the wakefields, it is important to study full structure properties using computational tools.

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Comparison between GdfidL and Circuit Model

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Impedance

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Recalculation of Kicks and Q’s from the impedance

Lorentzian fit of the peaks

QdipKick factor

Procedure

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Comparison between GdfidL and reconstructed wake

)(2

12)(1

sQi

csiExpKsW

n

nN

nn

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Comparison of fsyn, Q’s and Kicks

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As a first approach I decided to reproduce the same as done at for NLC/JLC:1) HOM coupler attached at first and last regular cells2) Only Matching cells uncoupled3) How much is the bandwidth?

PEC PML

Port1Port2

Port3

Port4

SLAC

A possible geometry for the HOM CouplerJ. W. Wang and al. “Progress toward NLC/JLC prototype accelerator structure”, LINAC04

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A first naÏf consideration

CLIC_DDS_A Impedance

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Build up of the wake signalW0(t)

….

Wn(t+nt)

bN

n

tnt0

nt )(WW

Obviously this analysis is qualitative and in the build up we are neglecting the effect of the bunches on the following ones

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Some exampleNb=1 Nb=10 Nb=28

Nb=100 Nb=260 Nb=312

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Nb=312Bunch train “Built wake”

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A last interesting consideration

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Preliminary HOM coupler thoughts

Same technique as for matching cells

No common

mini

ma yet

@ 18

GHz

My understanding is that, for first dipole band the most dangerous frequency is ~18GHz. Then I’m trying to match the HOM coupler at this frequency.

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Conclusions

• A first prototype, CLIC_DDS_A, has been fully designed and is going to be produced (hopefully at the end of the year)

• The study for the HOM coupler which is the fundamental device for CLIC_DDS_B has started

• New ideas to improve DDS performances are under investigation