4th CLIC Advisory Committee (CLIC-ACE), 26 th - 28 th May 2009 1 Alternate Means of Wakefield Suppression in CLIC Main Linac Roger M. Jones, Vasim Khan,

4th CLIC Advisory Committee (CLIC-ACE), 264th CLIC Advisory Committee (CLIC-ACE), 26thth - 28 - 28thth May 2009 May 2009 1

Alternate Means of Wakefield Alternate Means of Wakefield Suppression in CLIC Main LinacSuppression in CLIC Main Linac

Roger M. Jones,

Vasim Khan,

Alessandro D’Elia

Cockcroft Institute, UK andThe University of Manchester, UK


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


High powerrf coupler

HOM coupler

Beam tube

Acceleration cells

Manifold

Alternate Design CLIC Accelerating Structure

DDS (NLC/GLC design) illustrates the essential features of the conceptual design

Each of the cells is tapered –iris reduces with an Erf-like distribution

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 Monitoring2) Cell alignments to be inferred

Damped and Detuned Structure (DDS)


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)

Collaborators: W. Wuensch, A. Grudiev (CERN)

FP7 CLIC_DDS -Staff

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).


Abstract of the planned activityThis 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.

Integration of Task 9.2 within NC WP 9


List of Goals and MilestonesGoal 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.

Milestones1.1 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 structures1.2 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.

Milestones2.1 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.



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 wake-fields. 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 coupling3.3 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


Wealth of Experience on Detuned Structure and Manifold Wakefield Suppression

More than one and half decades of experience in this area


Wealth of Experience on Detuned Structure and Manifold Wakefield Suppression

DDS1 DDS3

RDDS1 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.


Circuit Model of CLIC_DDS

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).

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

Light lineUncoupled manifold mode

Avoided crossing

Coupled 3rd mode

Uncoupled 2nd mode

Uncoupled 1st mode


Wake-field Suppression in CLIC_DDS Main Linac -Initial design

Circuit model provides rapid determination of optimal wakefield suppression results in a bandwidth of 3.6 (3.36 GHz) and f/ fc =20%.

Leftmost indicates the modal distribution and rightmost the coupled and uncoupled wakefield

Four-fold interleaving of successive structures results in excellent wake-field suppression at the location of each bunch

Meets CLIC beam dynamics requirements!

However, breakdown considerations require a redesign with additional constraints imposed

Envelope of Wakefield for Single 25-Cell Structure (Q ~ ∞)

Wakefield for 8-Fold Interleaved Structure (Q ~ ∞)

Kick Factor Weighted Density Function

Envelope of Wakefield for 4-Fold Interleaved Structure (Finite Q )

dn/df

Kdn/dfCoupled

Uncoupled


2. Parameters of WDS-120 protos 2. Parameters of WDS-120 protos @[email protected]

Structure number maxFoM 2(minCost) 4 6 CLIC_14Wu

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 19.0 27.7 19.3 21.5

Luminosity per bunch X-ing: Lb× [m-2] 2.4×1034 2.0×1034 1.22×1034 2.8×1034 2.6×1034

Bunch population: N 5.3×109 4.2×109 3.72×109 6.5×109 5.8×109

Figure of merit: ηLb× /N [a.u.] 11.6 8.8 9.1 8.3 9.5

Alexej Grudiev (CERN)


Wake-field Suppression in CLIC Main Linac -Redesign

Initial redesign tied to present (CLIC_G)) cell parameters –cell 1 and 24

Frequencies and kick factor weighted density function Kdn/df from 4-fold interleaving of structures shown

This restricts the bandwidth /2 ~1 GHz ~ 3

dn/dfKdn/df

Interleaving improves sampling of wake function and enhances falloff

Bandwidth restricts the rate of decay of the wake function

Additional work entails: 1. relaxing the bandwidth restriction, enabling

better sampling (more cells/structure) 2. reoptimisation of Kdn/df distribution 3. non-linear positioning of interleaved

frequencies

Non-interleaved

Interleaved

Q ~ 300

Investigation of an alternate means of wakefield suppression in the main linacs of CLIC, V. Khan and R.M. Jones, Proceedings of PAC09.

Interleaved –(uncoupled-undamped)


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


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

Documents

4th CLIC Advisory Committee (CLIC-ACE), 26 th - 28 th May 2009 1 Alternate Means of Wakefield Suppression in CLIC Main Linac Roger M. Jones, Vasim Khan,