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DDS design status . Alessandro D’Elia. Outline. General overview of DDS working principle CLIC\_DDS\_A design status Towards CLIC\_DDS\_B: wakefield simulations and impedances HOM coupler design Some conclusion. Damped and detuned design.

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dds design status

DDS design status

Alessandro D’Elia

  • General overview of DDS working principle
  • CLIC_DDS_A design status
  • Towards CLIC_DDS_B:
    • wakefield simulations and impedances
    • HOM coupler design
  • Some conclusion
damped and detuned design
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) wakefieldrecoheres.
  • 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
nlc glc dds design
Acceleration cells

Beam tube


HOM coupler

High power

rf coupler

NLC/GLC DDS design

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

large bandwidth structure
Large bandwidth structure

Error function distribution

Courtesy of R. M. Jones


Courtesy of V. Khan

why a detuning damping structure dds for clic
Why a Detuning Damping Structure (DDS) for CLIC
  • 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
  • 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


CLIC_DDS_A: regular cell optimization

The chose of the cell geometry is crucial to meet at the same time:

Wakefield suppression

Surface fields in the specs



Consequences on wake function

Cell shape optimization for fields

clic dds a regular cells final design
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.

  • 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 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

first steps toward clic dds b
First steps toward CLIC_DDS_B

Wakefield 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.

recalculation of kicks and q s from the impedance
Recalculation of Kicks and Q’s from the impedance

Lorentzian fit of the peaks



Kick factor

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








As a first approach I decided to reproduce the same as done at for NLC/JLC:

HOM coupler attached at first and last regular cells

Only Matching cells uncoupled

How much is the bandwidth?

a first na f consideration
A first naÏf consideration

CLIC_DDS_A Impedance

build up of the wake signal
Build up of the wake signal




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

some example
Some example







nb 312

“Built wake”

Bunch train

preliminary hom coupler thoughts
Preliminary HOM coupler thoughts

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.

Same technique as for matching cells

No common minima yet @ 18GHz

  • 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