FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 1

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

Roger M. JonesContribution from

Cockcroft Institute andThe University of Manchester

X-Band Structures and Beam Dynamics Workshop1st – 4th December 2008

The Cockcroft Institute, Daresbury

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 2

Overview of FP7 Wakefield Suppression Kickoff Meeting

Integration of Task 9.2 within NC WP 9

Overall methodology and main goal

Possible goals and milestones

Very brief overview of past achievements

Review of some benefits of manifold damped and detuning wakefield suppression

Achievements and prospects for CLIC structure to date

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 3

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 Wakefield Suppression -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).

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 4

Integration of Task 9.2 within NC WP 9

J-P Delahaye, XB08 Workshop, Cockcroft Inst., UK, 1 Dec 2008

E. Jensen, EuCARD Kickoff CERN, 5 Dec 2008

R.M. Jones attended as Univ. Manchester rep ofEuCARD Governing Board

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 5

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

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 6

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 better than 0.01 V/pC/mm/m.

Integration of Task 9.2 within NC WP 9

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 7

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

Integration of Task 9.2 within NC WP 9

Major goal: Design and measure wakefield suppression in module

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 8

Wealth of Experience on Detuned Structure and Manifold Wakefield Suppression

More than one and half decades of experience in this area

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 9

Circuit Model of DDS

**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.

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 10

Wealth of Experience on Detuned Structure and Manifold Wakefield Suppression

DDS1 DDS3

RDDS1 H60VG4SL17A/B

1. SLAC-PUB 7287 (1996), 2. SLAC-PUB 8174 (1999)3. 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.

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 11

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 12

Determination of Modes in Structure via Stretched Wire Measurement

Measurement of RDDS1

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Determination of Cell Offset From Energy Radiated Through Manifolds

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 14

Summary of Manifold Suppression of Wakefields in Detuned Structures

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 15

Beam Dynamics and Relaxed Tolerances

Emittance dilution (illustrated by the red dashed curve) versus the percentage change in the bunch spacing. Also shown is the corresponding rms of the sum wake-field (by the solid blue curve).

Emittance we incorporate random frequency errors into a set of 50 accelerating structures and randomly distribute them along the entire linac. In all cases the beam is injected into the linac with an offset of approximately one y, with an energy of 5 GeV and the progress of the

beam is monitored as it traverses the entire linac. The final emittance dilution, together with the rms of the sum wake-field, is illustrated for small changes in the bunch spacing. The particular simulation illustrated includes a cell-to-cell frequency error with an rms value of 20 MHz. We chose this rather large frequency error in order to gain an understanding of the impact of relaxed

**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.

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 16

Summary of Manifold Wakefield Suppression

Detuning along with moderate damping has been shown to be well-predicted by the circuit model.

Interleaving of successive structures allows the detuning to be effective.

Manifold wakefield suppression has added benefits:1. Serves as built-in beam diagnostic2. Allows internal alignment of cells to be obtained from

manifold radiation3. Serves as vacuum pump-outs.

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 17

Achievements and Prospects for CLIC Structure

Current CLIC Baseline Design Heavily Damped Structure (Q ~ 10)

CAD of Conceptual Design for Alternate CLIC Moderately Damped Structure (Q ~ 500)

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 18

2. Parameters of WDS-120 protos 2. Parameters of WDS-120 protos @14.4Wu@14.4Wu

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 0.115 0.125 0.12

Input/Output iris radii: a1,2 [mm] 3.33, 2.4 2.85, 2.4 3.33, 2.4 3.84, 2.4 3.87, 2.13

Input/Output iris thickness: d1,2 [mm] 3.33, 0.83 1.5, 0.83 1.83, 0.83 2.00, 0.83 2.66, 0.83

Group velocity: vg(1,2)/c [%] 1.44, 1.0 1.28, 1.0 1.93, 1.0 2.93, 1.0 2.39, 0.65

N. of cells, structure length: Nc, l [mm] 12, 112 23, 204 25, 221 24, 212 24, 229

Bunch separation: Ns [rf cycles] 6 6 7 7 7

Number of bunches in a train: Nb 278 106 83 77 120

Pulse length, rise time: τp , τr [ns] 188.2, 17.3 126.9, 17.7 115.1, 17.3 101.5, 17.6 160, 30

Input power: Pin [MW], P/C1,2 [GW/m] 54, 2.6, 2.4 61, 3.4, 2.6 73, 3.5, 2.7 87, 3.6 76, 3.1, 2.7

Max. surface field: Esurfmax [MV/m] 262 274 277 323 323

Max. temperature rise: ΔTmax [K] 55 30 23 (const) 30 37

Efficiency: η [%] 25.9 19.0 18.4 19.3 21.5

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

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

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

Alexej Grudiev (CERN)

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 19

Bunch spacing is 6/7 cycles (depending on specific design) and this corresponds to 0.5003/0.5837 ns at a wavelength of 25 mm (0/2 = 11.9942 GHz).

C.f. NLC/JLC in which 0/2= 11.424 GHz and the bunch spacing was 1.4/2.8 ns –i.e. CLIC is ~ 3 times smaller in bunch spacing

Thus, it is clear the detuning must demand a more rapid fall-off in the wake-field.

In practise the bandwidth of the Gaussian needs to be increased.

C.f. NLC/JLC in which we investigated a bandwidth in terms of sigma: 4/5 sigma. We also investigated various bandwidths (in the range >9 % to <12 %)

Ambitious/demanding requirements!

Application to CLIC StructureApplication to CLIC Structure

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 20

Band partitioning of kick factors in 206 cell DDS1 X-band structure (facc=11.424 GHz). Largest kick factors located in the first band. Third and sixth bands although, an order of magnitude smaller, must also be be detuned along with the 1st band.

CLIC design facc =11.9942 GHz shifts the dipole bands up in frequency.

Band PartitioningBand Partitioning

Ref: Jones et. al, 2003, SLAC-PUB 9467

The partitioning of bands changes with phase advance. Choosing a phase advance close to pi per cell results in a diminution of the kick factor of the first band and and enhancement of the 2nd and 3rd bands. A similar effect occurs close to pi/2.

Kick factors versus phase advance for cells with an iris radius of ~ 4.23 mm.

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 21EPAC, 26 June 2008 W. Wuensch, CERN

HOM damping waveguides

Magnetic field concentration –pulsed surface heating

High electric field and powerflow region - breakdown

Short range wakefields

Cooling Vacuum pumping

Alignment

Beam and rf

11.9942 GHz, 2π/3 so 8.332 mm period

Current CLIC Baseline Accelerating Structure

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 22

High powerrf coupler

HOM coupler

Beam tube

Acceleration cells

Manifold

Alternate Design CLIC Accelerating Structure

RDDS structure 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

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 23

Wake-field Suppression in CLIC 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

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 24

Wake-field Suppression in CLIC Main Linac -Modified design

Electrical breakdown considerations at a CLIC gradient of 100 MV/m restricts the group velocity of the fundamental mode

This forces the initial and final iris radii to be restricted and enforces a severe restriction on the bandwidth of the accompanying dipole modes.

The envelope of the wake-field is not sufficiently suppressed.

The option of forcing the bunches to be located at the zero-crossing is explored

Beam dynamics study with PLACET in progress

Results of initial study shown

See Khan & Jones, Proc. EPAC 2008 and LINAC 2008

Parameters of detuned CLIC structure CLIC_ZC Envelope of Wakefield for Single

24-Cell Structure (Q ~ 500)

Envelope of Wakefield for 4-Fold Interleaved Structures

Wakefield for 4-Fold Interleaved CLIC_ZC Structure

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 25

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

FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 26

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