Alternate Means of Wakefield
Suppression in CLIC Main Linac
Roger M. Jones,
Vasim Khan,
Alessandro D’Elia
Cockcroft Institute, UK and
The University of Manchester, UK
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Overview of Wakefield Suppression
 Alternate method entails heavy detuning and moderate damping
of a series of interleaved structures (known as CLIC_DDS).
This is a similar technique to that experimentally verified and
successful employed for the NLC/GLC program.
 Integration of Task 9.2 within NC WP 9 -anticipate test of
CLIC_DDS on modules
 Potential benefits include, reduced pulse temperature heating,
ability to optimally locate loads, built-in beam and structure
diagnostic (provides cell to cell alignment) via HOM radiation.
Provides a fall-back solution too!
 Initial studies encouraging. However, the challenge remains to
achieve adequate damping at 0.5 ns intra-bunch spacing
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Alternate Design CLIC
Accelerating Structure
Acceleration
cells
 DDS
(NLC/GLC
design)
illustrates the essential features of
the conceptual design
Beam tube
 Each of the cells is tapered –iris
reduces
with
an
Erf-like
distribution
Manifold
 HOM manifold running alongside
main structure remove dipole
radiation and damp at remote
location (4 in total)
 Each of the HOM manifolds can
be
instrumented
to
allow:
1) Beam Position Monitoring
Damped and Detuned Structure (DDS)
2) Cell alignments to be inferred
HOM coupler
High power
rf coupler
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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FP7 CLIC_DDS -Staff
Roger M. Jones (Univ. of Manchester faculty)
Alessandro D’Elia (Dec 2008, Univ. of Manchester PDRA based
at CERN)
Vasim Khan (Ph.D. student, Sept 2007)
V. Khan, CI/Univ. of
Manchester Ph.D. student
pictured at EPAC 08
A. D’Elia, CI/Univ. of
Manchester PDRA based
at CERN (former CERN
Fellow).
Collaborators: W. Wuensch, A. Grudiev (CERN)
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Integration of Task 9.2 within NC WP 9
Abstract of the planned activity
This work package will explore HOM damping in single multi-cell
cavities and in groups of thereof. The features of both the long-range
and short-range wake-fields will be explored. The consequences of the
short-range wake-field on cavity alignment will be delineated. For the
long-range wake-fields, trapped modes in particular will be focused
upon. Global scattering matrix analysis will be employed in addition to
current electromagnetic codes. The frequency sensitivity of the modes
will be explored by exploiting a circuit analysis of the electromagnetic
field and this will enable the sensitivity of the wake-field to fabrication
errors to be evaluated over the complete collider.
At the University of Manchester and the Cockcroft Institute we are
actively involved in simulating higher order modes of accelerating
cavities and experimentally determining the structure of these modes
with a purpose built stretched wire measurement set-up. We are
actively involved in using intensive computer codes coupled with
cascading of individual sections in order to rapidly compute the modal
structure.
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Integration of Task 9.2 within NC WP 9
List of Goals and Milestones
Goal 1. Develop a circuit model and a generalized scattering matrix technique to obtain
accurate calculations on the global electromagnetic field from small segments thereof.
This is a study of mode excitation.
Milestones
1.1 Sep 09: Write report on circuit model and globalised scattering matrix
technique. This will include an analysis of the partitioning of dipole modes in
CLIC structures
1.2 Apr 110: Produce a report for the design of damping and detuning a CLIC
module
Goal 2. Make an accurate simulation of the wake-fields and HOMS. This is expected
to be broadly verified with initial experiments on CTF3 and more precisely verified with
an experiment at the SLAC FACET facility and stretched wire measurements.
Milestones
2.1 Apr 10: Experiments on the measurement of HOMs on CTF3. This will
enable the predicted features of HOM damping to be verified although only the
broad characteristics of the modes are expected to be measurable.
2.2 Aug 10: Perform additional measurements on the wake-field at the SLAC
FACET facility. This will facilitate a detailed comparison between the
predicted decrement in the wake envelope and experimentally determined
values. ASSET typically is accurate to ~ 0.01 V/pC/mm/m.
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Integration of Task 9.2 within NC WP 9
2.3 Sept 10: Write up a report on the experimental measurement of
modes.
2.4 April 11: Conduct wire measurement on CLIC cavities to verify
the distribution of frequencies and kick factors
Goal 3. Undertake beam dynamics simulations with Placet. These
simulations will take into account both the long-range and short-range wakefields. Simulations will be performed both with the baseline design and with
relaxed fabrication tolerances. In addition to the standard wake-field the
influence of x-y coupling of wake-fields from possible cavity distorsions will
also be investigated.
Milestones
3.1 April 11: Initial result on baseline beam dynamics simulations
3.2 June 11: Results on beam dynamics simulations with relaxed
tolerances and initial simulations on transverse mode coupling
3.3 August 11: Report on beam dynamics simulations including long
and short range wakefields.
3.3 Sept 11: Report on beam dynamics simulations including
transverse mode coupling
Major goal: Design and measure wakefield suppression in
module
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Wealth of Experience on Detuned Structure
and Manifold Wakefield Suppression
More than one and half decades of experience in this area
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Wealth of Experience on Detuned Structure
and Manifold Wakefield Suppression
DDS1
RDDS1
DDS3
H60VG4SL17A/B
1. Influence of fabrication errors on wake function suppression in NC X-band accelerating structures for linear colliders, R.M. Jones et al. 2009 New J. Phys. 11
033013 (13pp).
2. Wakefield damping in a pair of X-band accelerators for linear colliders.R.M. Jones et al., Phys.Rev.ST Accel.Beams 9:102001,2006.
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Circuit Model of CLIC_DDS
Coupled 3rd mode
Uncoupled 2nd mode
Uncoupled 1st mode
Avoided crossing
Light line
Uncoupled manifold mode
Cell 1 E-Field and Dispersion
Curves for CLIC_DDS
Three cells in the chain are
illustrated. TM modes couple to the
beam. Both TM and TE modes are
excited (hybrid dipole mode).
Coupling to the manifold takes place
via slot-coupled TE modes. Manifold
is modelled as a transmission line
Refs:
1. Wakefield damping in a pair of X-band accelerators for linear colliders.R.M. Jones , et al., Phys.Rev.ST Accel.Beams 9:102001,2006.
2. Influence of fabrication errors on wake function suppression in NC X-band accelerating structures for linear colliders, R.M. Jones et al 2009 New J. Phys. 11
033013 (13pp).
th
th
4th CLIC Advisory Committee (CLIC-ACE), 26 - 28
May 2009
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Wake-field Suppression in
CLIC_DDS Main Linac -Initial design
 Circuit model provides
rapid determination of
Uncoupled
optimal wakefield
dn/df
suppression results in a
bandwidth of 3.6 (3.36
GHz) and f/ fc =20%.
Kdn/df
 Leftmost indicates the
Coupled
modal distribution and
rightmost the coupled
Envelope of Wakefield for Single
and uncoupled wakefield
Kick Factor Weighted
 Four-fold interleaving of
25-Cell Structure (Q ~ ∞)
Density Function
successive structures
results in excellent wakefield suppression at the
location of each bunch
 Meets CLIC beam
dynamics requirements!
 However, breakdown
considerations require a
Wakefield for 8-Fold
redesign with additional
Envelope of Wakefield for 4-Fold
Interleaved
Structure
(Q
~
∞)
constraints imposed
Interleaved Structure (Finite Q )
11
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
2. Parameters of WDS-120 protos
@14.4Wu
Alexej Grudiev (CERN)
Structure number
maxFoM
2(minCost
)
4
6
CLIC_14W
u
RF phase advance per cell: Δφ [o]
120
120
120
120
120
Average iris radius/wavelength:
<a>/λ
0.115
0.105
0.11
0.125
0.12
Input/Output iris radii: a1,2 [mm]
3.33, 2.4
2.85, 2.4
3.15, 2.35
3.84, 2.4
3.87, 2.13
Input/Output iris thickness: d1,2
[mm]
3.33, 0.83
1.5, 0.83
1.67, 1.0
2.00, 0.83
2.66, 0.83
Group velocity: vg(1,2)/c [%]
1.44, 1.0
1.28, 1.0
1.66, 0.83
2.93, 1.0
2.39, 0.65
N. of cells, structure length: Nc, l
[mm]
12, 112
23, 204
24, 230
24, 212
24, 229
Bunch separation: Ns [rf cycles]
6
6
6
7
7
Number of bunches in a train: Nb
278
106
312
77
120
Pulse length, rise time: τp , τr [ns]
188.2, 17.3
126.9, 17.7
240.8,
22.4
101.5,
17.6
160, 30
Input power: Pin [MW], P/C1,2
[GW/m]
54, 2.6, 2.4
61, 3.4, 2.6
63.8, 3.22
87, 3.6
76, 3.1, 2.7
Max. surface field: Esurfmax [MV/m]
262
274
245
323
323
Max. temperature rise: ΔTmax [K]
55
30
53
30
37
Efficiency:
η [%]
25.9 (CLIC-ACE),
19.0
19.3
4th CLIC
Advisory Committee
26th -27.7
28th May 2009
21.5
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Wake-field Suppression in CLIC
Main Linac -Redesign
Non-interleaved
Kdn/df
Q ~ 300
Interleaved –
(uncoupled-undamped)
dn/df
Interleaved
 Interleaving improves sampling of wake
function and enhances falloff
 Initial redesign tied to present (CLIC_G))
 Bandwidth restricts the rate of decay of the
cell parameters –cell 1 and 24
wake function
 Frequencies and kick factor weighted
 Additional work entails:
density function Kdn/df from 4-fold
1. relaxing the bandwidth restriction, enabling
interleaving of structures shown
better sampling (more cells/structure)
 This restricts the bandwidth /2 ~1 GHz
2. reoptimisation of Kdn/df distribution
~ 3
3. non-linear positioning of interleaved
frequencies
Investigation of an alternate means of wakefield suppression in the main linacs of CLIC, V. Khan and R.M. Jones, Proceedings of PAC09.
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Overall Goals
 Provide proof-of-principle of manifold damped and
detuned design and structure test at CTF3
 Overall properties of wakefield suppression to be
tested in modules at CTF3 (SLAC FACET?)
 Provide typical tolerances/alignments for practical
multi-structure operation (from PLACET beam
dynamics simulations) –CLIC!
 N.b. this structure has the potential for a significantly
smaller pulse temperature rise than the present
baseline design for CLIC
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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Summary
 Analytical truncated Gaussian is a useful design
tool to predict wakefield suppression.
 Initial design provides a well-damped wakefield.
 Including the constraints imposed by breakdown
forces a consideration of zero-point crossing.
 Beam dynamics study including systematic and
random errors in progress –will provide detailed
answer.
 Manifold damping provides useful characteristics
of built-in BPM together with a direct indication
of internal alignments
4th CLIC Advisory Committee (CLIC-ACE), 26th - 28th May 2009
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