CLIC Accelerating Structure Development

CLIC Accelerating Structure Development

W. WuenschXB08

2-12-2008

• Introduction• Combined RF and linac design and optimization• High-power limits and breakdown simulation• Fabrication capabilities• High-power testing program• Breakdown and pulsed surface heating experiments• Instrumentation• Other collaborations and X-band projects

Introduction:

Our mandate is to develop accelerating structures for CLIC. The key challenges:

100 MV/m, order of 10-7/pulse/m, is the obvious, high-profile, feasibility challenge which we face. The high gradient also creates a life-time issue through pulsed surface heating.

Demanding beam dynamics requirements which include low short-range transverse wakefields (consequently micron alignment tolerances), strong long-range wakefield suppression and tight phase stability.

These issues are all deeply interrelated as we will see.

The subject of my talk is the structure development program - but what is the point?

To introduce important issues and highlight the interrelations amongst them.

While doing this I hope to convince you that we have a coherent, thorough and efficient approach.

To encourage you to participate in our collaboration!

Major effort lies ahead in many areas along with the opportunity for new ideas to make a major impact on the study.

I am personally convinced that the perception of feasibility of CLIC will be significantly enhanced by the existence of a large and vibrant X-band community.

Hence the importance of this workshop. And my sincere thanks to Roger for having taken the initiative in organizing it.

Combined RF and linac design and optimization

W. Wuensch, CERNEPAC, 26 June 2008

Baseline accelerating structure features

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

11.994 GHz, 2π/3 so 8.332 mm period

C L I CC L I C

CLIC-ACE, 16 Jan. 2008Alexej Grudiev, Structure optimization.

Optimization procedure

Bunch population

Structureparameters

Cell parameters

Bunch separation

Beamdynamics

<Ea>, f, ∆φ, <a>, da, d1, d2

N

Ns

Q, R/Q, vg, Es/Ea, Hs/Ea Q1, A1, f1

η, Pin, Esmax, ∆Tmax

Ls, Nb

rfconstraints

Cost function minimization

YES

NO

Beamdynamics

C L I CC L I C

CLIC-ACE, 16 Jan. 2008Alexej Grudiev, Structure optimization.

Beam dynamics input

High-power RF optimum aperture: <a>/λ =

0.1 ÷ 0.12

Why X-band ?Crossing gives

optimum frequency

FoM =Lbx/N · η

BD RF

BD optimum aperture: <a> = 2.6 mm

Lbx/N

CLIC_ZC structure

Coupled

CoupledUncoupled

Uncoupled

Undamped Undamped

Q = 500 Q = 500

9

Envelope Wake-field

Amplitude Wake-field

12/11/2008

V. Khan

High-power limits and breakdown simulation

We can gain enormous benefits with an accurate knowledge of the scaling of high-power limits.

One objective we have is a reasonably accurate phenomenological description of the limits to use in high-power structure design.

We also are trying to determine the limits from first principles and a theory and simulation effort has begun.

CLIC Breakdown Workshop Sergio Calatroni TS/MMECLIC Breakdown Workshop Sergio Calatroni TS/MME 12

Time constant to reach the copper melting point (cylinders, =30)

10-4

10-3

10-2

10-1

100

101

102

10-12

10-10

10-8

10-6

10-4

0.01

1

101 102 103 104 105

Parameters to attain the melting point of the tipof a Cu cylinder of given radius and =30

I peak [A]

P peak [W]time constant [sec]

Energy [J]

radius [nm]

The tips which are of interest for us are extremely tiny, <100 nm (i.e. almost invisible even with an electron microscope)

100 ns

CLIC Breakdown Workshop Sergio Calatroni TS/MMECLIC Breakdown Workshop Sergio Calatroni TS/MME 13

Power density at the copper melting point (cylinders, =30)

102

103

108

109

1010

1011

1012

1013

1014

101 102 103 104 105

Parameters to attain the melting point of the tipof a Cu cylinder of given radius and =30

E peak [MV/m]Power density [W/m2]Current density [A/m2]

radius [nm]

Power density of about 0.5 W/μm2

C L I CC L I C

May. 2008Alexej Grudiev, New RF Constraint.

P

FN

Field emission and rf power flow

Ploss

Prf

S

rf

FN

S

FNFN

V

loss

rfFNloss

dsHEP

IEdsHEP

dvJP

PPPT

FN

~

~

2

There are two regimes depending on the level of rf power flow

1. If the rf power flow dominates, the electric field remains unperturbed by the field emission currents and heating is limited by the rf power flow (We are in this regime)

2. If power flow associated with field emission current PFN dominates, the electric field is reduced due to “beam loading” thus limiting field emission and heating

P’rf

C L I CC L I C


Field emission and power flow

tE

mGVtAEEI

ttHEtHEHE

FN

SWTW

sin

62expsin

cossinsin

0

330

002

00

EIFN·E E·HSW E·HTW

C L I CC L I C


Field emission and rf power coupling

What matters for the breakdown is the amount of rf power coupled to the field emission power flow.

SWSWTWTW

T

rf

T

FN

T

FNrfcoup

HECHEC

dtPdtPdtPPP

0000

4

0

4

0

4

0

Assuming that all breakdown sites have the same geometrical parameters the breakdown limit can be expressed in terms of modified Poynting vector Sc.

SS ImRe0000 cSW

TW

SWTW

c gHEC

CHES

Our new design constraint. Must be less than 6 W/μm2 at 100 ns.

0 5 10 15 20 25 30

0

20

40

60

80

100

120

140

160

Average gradient for different geoemtries at 100ns,BDR=1e-6

geometry number

Av

era

ge

gra

die

nt

[MV

/m]

0 5 10 15 20 25 30

0

50

100

150

200

250

300

350

400

450

Surface gradient for different geoemtries at 100ns,BDR=1e-6

geometry number

Su

rfa

ce

gra

die

nt

[MV

/m]

0 5 10 15 20 25 30

0

1

2

3

4

5

6

7

8

9

Sqrt(P/C) for different geoemtries at 100ns,BDR=1e-6

geometry number

Sq

rt(P

*f/C

) [S

qrt

(MW

*GH

z/m

m)]

0 5 10 15 20 25 30

0

0.5

1

1.5

2

2.5

3

3.5

Sqrt(Sc6) for different geoemtries at 100ns,BDR=1e-6

geometry number

Sq

rt(S

c6

) [s

qrt

(MW

)/m

m]

Sc = Re{S} + Im{S}/6

Summary of high-gradient tests

A. Grudiev

Max-Planck-Institut für Plasmaphysik, EURATOM Association

Helsinki Institute of Physics, University of Helsinki

A Multiscale Model of Arc Discharge

Simulations

combining Particle-

in-Cell and

Molecular Dynamics

Coupling:

PIC gives particle

fluxes and energy

distributions for MD

Max-Planck Institut für Plasmaphysik, Greifswald, Germany

Konstantin Matyash and Ralf Schneider

Department of Physics and Helsinki Institute of Physics – University of Helsinki, Finland

Helga Timkó, Flyura Djurabekova and Kai Nordlund

Fabrication capabilities

Structure production

RF design Mechanical design

Tendering

Assembly QC

Fabrication

CERN-KEK-SLAC collaboration

20

Mechanical design

21

RF CHECK

RF INPUT 3D Part Generated 3D Parts

Assembly Design

S TSmarTeam

Project structure

22

Damped quadrants

• machining by 3D milling (carbide or diamond tools)• alignment of the quadrants by pins or balls and gooves (plastic deformation of copper)• assembly by “brazing” or by bolting• damping implemented in the design

160 mm

Quadrants: machining by milling

23

300 mm

30 GHz 11.4 GHz

Disks: machining by turning and milling

24

• Disks: machining by diamond turning• Adding damping features Needs milling (no circular

symmetry) with smooth transition between milled and turned surfaces.

• Alignment of the disks on V-shaped marble before assembly in a stack: use external “cylinder” surface as reference.

• Assembly by vacuum brazing

Damped disksVG1

Accelerating structure CLIC G (undamped)

Achieved accuracy (disk)30 GHz [accuracy in mm]

Specified Achieved Specified Achieved

11.4 GHz [accuracy in mm]

SA: iris shape accuracyOD: outer diameterID: inner diameterTh: iris thicknessRa: roughness

Speed bump TD18_disk

Dimensional control (TD18)

26

TD18 (VDL)

4 mm

Assembly by brazing

27

Before brazing

After brazing

After brazing , ready for RF check

Holes/grooves for brazing

High-power testing program

We have high-power testing program which is made in close collaboration with a number of laboratories, in particular KEK and SLAC .

They play a substantial role in not only in structure fabrication and testing but also in analysis/discussion/ideas/ advice/etc. I would like acknowledge their contributions.

I will continue here with a focus on CLIC specific testing. Again, we also benefit greatly from other high-gradient and X-band activities some of which are covered by other talks by Valerey, Micha, etc.

The principle elements of our test structure program

CLIC prototype: Nominal CLIC accelerating structure, CLIC_G with damping and made from brazed copper disks. This test is scheduled for spring 2009.

Near CLIC prototype: Other structures to approach CLIC_G one step at a time to isolate the influence of individual features. One example is a CLIC_G without damping, to determine the effect of the damping features.

Quadrant structures: More adaptable to alternative materials for both breakdown and especially pulsed surface heating and shares fabrication technology with the PETS. BUT quadrant accelerating structures have worked poorly. We still don’t understand why.

The principle elements of our test structure program

Ten-cell structures: Smaller, cheaper, faster structures using SLAC re-useable couplers for investigations to test new ideas and support theory and simulation. Examples include alternative damping features and iris scaling.

Dedicated experiments: To support breakdown theory and simulation using various structures. Examples include staircase pulse-shape, bi-periodic pulse train, long-pulse induced fatigue, reversed structures.

Collaboration structures: Not made to CLIC targets but can provide important experience and information. H75 for PSI and Trieste are an example.

T18 – Successful fabrication and test of the first CLIC X-band test structure designed for 100 MV/m, low breakdown rate operation.

18 undamped cells, 29 cm long, 2.6 to 1% tapered group velocity and manufactured by diamond turning and diffusion bonding.

The result – After steady improvement, operation below the 4x10-7 CLIC breakdown rate specification with above 105 MV/m unloaded gradient.

95 100 105 110 11510

-7

10-6

10-5

10-4

Unloaded Gradient: MV/m

BK

D R

ate

: 1/p

uls

e/m

BKD Rate for 230ns

250hrs

500hrs

1200hrs

900hrs

Lines o

f breakd

own rate vs

gradient tradeoff

CLIC breakdown rate specification

Excellent collaboration - designed by CERN, manufactured by KEK and bonded and tested by SLAC.

T18tested to 105 MV/m, 230 ns, 2x10-7/(mxpulse)

TD18

CLIC_G undamped

CLIC_G with damping, full prototype

Add damping

Add damping

From T18 to prototype

Move to design iris range

Move to design iris rangeMove to design iris range and add damping

Supporting tests:Quadrant fabricationCD10 Choke mode CD10

Supporting tests:C10 seriesT23

Today early 2009 mid to late 2009

Achieved results to prototype CLIC structure

T18 test structure CLIC prototype

C L I CC L I C

CLIC2008, 16 Oct. 2008Alexej Grudiev, Pulse shape dependence.

Staircase pulse shape

0 0.5 1 1.50

0.2

0.4

0.6

0.8

1

t/tp

T

/Tm

ax,

P/P

max

111 MV/m

93 % in gradient

0 0.5 1 1.50

0.2

0.4

0.6

0.8

1

t/tp

T

/Tm

ax,

P/P

max

119 MV/m

100 %

• No influence on BDR for gradients below 80%•Strong influence on the BDR for gradients above 90%•Very good agreement with SLAC data

0 500 1000 1500 20000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

sub pulse

main pulse

after pulse

0 20 40 60 80 100 12010

-6

10-5

Unloaded gradient for sub pulse: MV/m

Bre

akdow

n r

ate

: 1/p

uls

e

0 20 40 60 80 100 1200

20

40

60

80

100

Unloaded gradient for sub pulse: MV/m

Perc

ent

of

bre

akdow

n e

vents

Sub pulse breakdown

Main pulse breakdown

After pulse breakdown

SLED output pulse

*:Max gradient in the structure

Chris, Faya

04.12.2008 36

MASTER SCHEDULE (1/2)

04.12.2008 37

MASTER SCHEDULE (2/2)

Breakdown and pulsed surface heating experiments

CLIC08 Workshop – CERN, 16th October 2008 39 / 18

Experimental set-up : ‘‘ the spark system ’’

Vpower supply (up to 15 kV) C (28 nF)

vacuum chamber (UHV 10-10 mbar)

anode(rounded tip,

Ø 2.3 mm)

cathode(plane)

HV switch HV switch

spark

m-displacementgap 10 - 50 mm

(±1 mm)20 mm typically

• Two similar systems are running in parallel now

• Types of measurements : 1) Field Emission ( b)

2) Conditioning ( breakdown field Eb)

3) Breakdown Rate ( BDR vs E)


Conditioning curves of pure metals


Breakdown Rate : DC & RF (30 GHz)

DC RF

Cu 10 - 15 30

Mo 30 - 35 20

g = power in the fit

BDR ~ Eg

Same trend in DC and in RF, but difficult to compare ‘slopes’


no visible increase in outgassing just before a breakdown cluster

H2 outgassing in Breakdown Rate mode (Cu)

43 / 18

Evolution of b during BDR measurements

spark

• quiet period low b

• b seems to increase (a few %) during a quiet period if E is sufficiently high

Are small tips pulled by the field? (we need more data)

RF team meeting - 12.11.08 44 / 4

DC spark test on oxidized copper

• We still need to

characterize the layer

control the layer

understand what’s going on

Cu

… but the effect is real !

effect of the oxide

layer thickness = ‘‘15 – 20 sparks’’

breakdown field mutliplied by 2 at least

conditioning observed

Pulse Heating Samples RF Tested

May 2008 – September 2008

HIP2 (KEK) 33 E-Deposited Cu (KEK) 62Single Crystal Cu (KEK) ??

KEK3 (KEK) 19Cu101 (SLAC) 10

CuZr3-2 (CERN) 77 CuCr102 (SLAC) 83 Hardness Test Value

Lisa Laurent, SLAC

Markus Aicheler 16.10.2008

CLIC08 Workshop

SLAC_Slash5

Instrumentation

X-Band workshop – Daresbury – Dec. 2008

Objectives:• Accelerating structure performances with beam: 11.9942 GHz, 100 MV/m,

150ns bunch train …• Precise structure alignment for bunch train emittance preservation

→ determination of the accelerating structure position with respect to

the beam by the EM fields excited by the beam = Wakefield Monitors (WFM)

CEA Saclay collaboration in CLIC Module

Kick-off meeting at CERN: 8 Dec. 2008

Development plan:1) Detailed design of WFM and procurement of a prototype for integration in a

CERN accelerating structure and test on the Two Beam Test Stand with CALIFES

probe beam (2009)

In the framework of the Clic French Contribution (CFC)

First beam in RF gun: 27 Nov. 2008 !

2) Design and procurement of complete accelerating structures including WFM (2010)

1/3


Accelerating Structure

RF Design• CLIC_G structure: R/Q=, Q=, 24 cells, 120° advance per cell, 230mm

active length, tapered iris for constant gradient, Heavy damping by

orthogonal waveguides + RF absorbers

HFSS model of one cell

Fabrication• Technology: precise machining of copper disks, high temp. brazing, sealed structure

→ Mecachrome for machining ? → Bodycotte for brazing ?

GdFidl simulation of complete structure (CERN result)

2/3


Electronics• 180° hybrid coupler in waveguide or striplines• Fast digital scope (feasibility demonstration)

Performances• Good linearity, resolution ~ 1 µm, precision ~ 10 µm• Should not low cost ( quantity of 140000 WFM for CLIC!)

Wakefield Monitors

RF Design• One transverse position detector per structure, located at the middle cell• Four couplers in orthogonal position integrated in the damped waveguides

D port for offset measurement (dipole signals have opposite phase at the two opposite waveguides)

S port for beam intensity measurement (monopole signals have the same phase)

3/3

Or loop, antenna…

Other collaborations and X-band projects

The CLIC/CERN rf structure activity is now committed to a number of rf and X-band activities which are not directly CLIC.

This important step has been taken to encourage the spread of X-band technology. We of course benefit from increased experience, new perspectives, increased supplier base etc. I believe the feasibility of CLIC requires a solid X-band technology base.

• Accelerating structures for X-FELs: PSI, Trieste• Accelerating structures, distribution and sources for medical accelerators: TERA Foundation• rf absorbing materials: CEA/IRFU, CIEMAT, Cornell, PSI, Thales

RF

MS

-V

févr

ier

0 72/

23/2

007D

:\TE

Ddy

\DG

\Mod

eles

\MO

DE

L\P

r es_

f.p p

t

Components & Subsystems

Gyrotron - RF Absorber Properties –

CERN 18th October 2008

54

RF

MS

-V

Components & Subsystems

23 /02/20 D:\TEDdy\DG\Modeles\MODEL\Pres_f.ppt 5407

TH1507/Beam Tunnel

Beam Tunnel absorbers :Be0SiC (60/40) is used from many years for all our gyrotrons : TH1504, TH1506, TH1507.

Series of tests have been made with different absorbers from 1997 to 1998 :

BeOSiC (40%)

AlNSiC (20%, 40%)

MgOSiC (5%, 40%)

Al2O3/SiC (40%,60%)

Vector Measurements made by ABmm on disks (2x2inches) in free space propagation, in the range 27 to 190GHz. (Abmm Vector Network Analyser)

M. Dehler, XBWS 08, Dec. 2008

A CLIC/PSI-XFEL accelerating structure

CLIC view:

•Another data point in high gradient test program

•Validation of design and fabrication procedures

•A true long term test in another accelerator facility

PSI view:

•An X band structure to compensate long. phase space nonlinearities

•High gradient/power requirements of CLIC = a design for safe operation at the more relaxed parameters of the PSI X-FEL

RF design (mostly) by PSI, fabrication & LL RF test (mostly) at CERN

M. Dehler, XBWS 08, Dec. 2008

The generic approach

•Use 5π/6 phase advance:

• Longer cells: smaller transverse wake

• Intrinsically lower group velocity: Good gradient even for open design with large iris

•Long constant gradient design (efficiency!)•No HOM damping•Wake field monitors to insure optimum structure alignment

Do a castrated NLC type H75 without damping manifolds!

[email protected] 5757

LINACS FOR HADRONTHERAPY:a 12 GHz

CArbon BOoster for Therapy In Oncology

Ugo Amaldi

TERA FOUNDATION – Novara - Italy

[email protected] 5858

CABOTO at 12 GHz would be shorter and would consume less power (CNAO consumes 3-4 MW!)

This is the « CABOTO 12 GHz » design project

proposed for a collaboration withCLIC, EPFL, PSI

SC EBIS source byDREEBIT – Dresden

300-400 Hz – 107 C/pulse

SC Synchrocyclotron230 MeV/uH2

+ and C+6

@ 300-400 Hz (TERA + EPFL+ A. Laisné

and P. Mandrillon + IBA)

400 MV – 12 GHz linac≤ 1 μs pulses

5-6 m

≤ 15 m

230 MeV/u 430 MeV/u

04.12.2008 59

RF structure development project – List of collaborations

CLIC Accelerating Structure Development

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