ILC Wakefest Workshop, Dec 11-13, 2007

WakeFest Summary 2

Subject Areas

• Wakefield with cavity imperfection

• Beamline HOM absorber

• Large system simulation

• Multipacting simulation

A total of 9 talks

Modeling Imperfection Effects on Dipole

Modes in TESLA Cavity

Liling Xiao

Advanced Computations Department, SLAC

1. TESLA Cavity Dipole Mode Measurement Data

Dipole mode frequencies shift and Qext scatter

TTF module 5: 1st/2nd dipole band

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1600 1650 1700 1750 1800 1850 1900

F (MHz)

Qe

xt

1.E+03

1.E+04

1.E+05

1700 1701 1702 1703 1704 1705 1706

1st band 6th pair

1.E+04

1.E+05

1.E+06

1877 1878 1879 1880 1881 1882

2nd band 6th pair

• Omega3Po Measured data in colour

x

z

2. TESLA Cavity Imperfection Model

Red: ideal cavity, Blue: deformed cavity

Distorted mesh for the cavity imperfection model

a) Cell length error

b) Cell radius error

c) Cell surface deformed

d) Elliptical cell shape

1st ILC Workshop at KEK

A. Matheisen

TDR cavity with elliptical cell shape

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.60E+09 1.65E+09 1.70E+09 1.75E+09 1.80E+09 1.85E+09 1.90E+09

F (Hz)

Qex

t

2nd band: 6th pair

1.E+04

1.E+05

1704.0 1705.0 1706.0 1707.0 1708.0

F (MHz)

Qex

t

defor cell1 along xdefor cell4 along xdefor cell1 and 4 along xidea cavity

• ideal cavity

O elliptically deformed cell shape cavity

Example 2: Cell shape elliptical deformed (dr=0.25mm)

- cause mode polarization change and mode splitting

Elli. deformed cavity

1st band 6th pair 2nd band 6th pair

ideal cavity

1.E+03

1.E+04

1.E+05

1787.0 1787.5 1788.0 1788.5 1789.0 1789.5 1790.0

defor cell1 along x

defor cell4 along x

defor cell1 and 4 along x

idea cavity

x

y

Beamline HOM Absorber

Talks:

• Beamline HOM Absorber – Oleg Nezhevenko, FNAL

• Beamline HOM absorber simulation using T3P – Liling Xiao, SLAC

T.Higo, et al, IId ILC Workshop

8

O.N.

Absorption efficiency estimations.

Simple model of HOM absorber: the ring has the length of 50mm, internal diameter of 90 mm, and the thickness of 10mm (DESY style).

9

O.N.

S. Nagaitsev 10

•Overall Plan: Test ILC RF units•3 CM, Klystron, Modulator, LLRF•Move A0 Injector to provide ILC like beam

•New bldg: diagnostic, new cryo plant•ILC Twin tunnel design to allow 2nd RF unit and to study tunnel layout and maintenance issues

Bunch Compressor

ILC RF unitDiagnosticsGun

CC I,II

Laser

3rd har

Test Area

~ 22 M72 M

Existing Building

New Building

New ILC like tunnel

2nd ILC RF unit

Test Areas

new 300 W cryo plant

RF Equipment

RF Unit Test FacilityRF Unit Test Facility

10

O.N.

• Simulation Results

1) 3D single cavity with beamline absorber (εr=15, σeff=0.6s/m)

A Gaussian bunch with σz=10mm, Q=3.2nc on axis.

3.5million mesh elements, 2nd basis function run on Franklin at Nersc.

512 processors 24000 time steps within 12 hours

Copper coated wall

TM0n modes

L.X.

One bunch Q=3.2nc, bunch length=10mm

Loss factor (V/pc)=3.566V/pc

Lossy dielectric conductivity σeff=0.6(s/m)

Dielectric constant εr=15, Within 45ns

Total Energy Generated by Beam (J) 3.65e-5

Energy stored in cavity (J) 3.25e-5 (FM mode energy=2.06e-5J)

Energy leaked out HOM coupler ports (J) 4.05e-7

Energy propagated into beam pipe (J) 2.11e-6

Energy dissipated in the absorber (J) 2.4e-6

Energy loss on the copper absorber beampipe wall (J) 6.6e-10 (cold copper conductivity=350ms/m )

Energy stored in cavity

3.0E-05

3.5E-05

4.0E-05

4.5E-05

5.0E-05

5.5E-05

6.0E-05

0 10 20 30 40 50

t (ns)

Ene

rgy

(J)

@6.6ns, exciting bunch just left the structure

Absorption

Left beampipe

Right beampipe

Up stream HOM

Down stream HOM

Monopole Single Passage Losses

K-loss=6.86v/pc for 10mm, Total Energy=7.02e-5 (J)absorber (εr=15, σeff=0.6s/m)

Energy absorption/Total HOM energy

Energy leaked into Left beampipe/Total HOM energy

Energy leaked into right beampipe /Total HOM energy

Large System Simulation

Talks:

• Multi-cavity trapped mode simulation – Cho Ng, SLAC

• Globalised scattering matrix simulation in ILC cavities and modules – Ian Shinton, Manchester University

• Large scale 3D wakefield simulations with PBCI – Sascha Schnepp, T.U. Darmstadt

3rd Dipole-Band Trapped Modes in Cryomodule

C.N.

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

1.0E+11

1.0E+12

1.0E+13

1.0E+14

2.5770 2.5772 2.5774 2.5776 2.5778 2.5780 2.5782 2.5784F (GHz)

Kic

k F

act

or

(V/C

/m/m

od

ule

)

K_X

K_Y

Kx[y]=Ky[x]_amp

Kx[0]_amp

Ky[0]_amp

• Modes above cutoff frequency are coupled throughout 8 cavities• Modes are generally x/y-tilted & twisted due to 3D end-group geometry• Both tilted and twisted modes cause x-y coupling

TDR 8-Cavity Module

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

2.577 2.5775 2.578 2.5785F (GHz)

Qex

t

Cryomodule 3rd Dipole-Band Mode - Qext and Kick

C.N.

Trapped Mode using Omega3P

Trapped mode

• TM-like mode localized in beampipe between 2 cavities

• Frequency = 2.948 GHz, slightly higher than TM cutoff at 2.943 GHz

• R/Q = 0.392 ; Q = 6320• Mode power = 0.6 mW (averaged)

Electric field

TM mode

C.N.

Introduction

1. Wake field simulations in arbitrary 3D-geometry

3D-codes3D-codes

2. Accurate numerical solutions for high frequency fields

(quasi-) dispersionless codes(quasi-) dispersionless codes

3. Utilizing large computational resources for ultra-short bunches

parallelized codesparallelized codes

4. Specialized algorithms for long accelerator structures

moving window codesmoving window codes

There is an actual demand for:

S.S.

ILC-ESA collimatorILC-ESA collimator #8

s /

Wz

/ [V

/ p

C]

-4 -3 -2 -1 0 1 2 3 4-100

-80

-60

-40

-20

0

20

40

60 / z = 2.5 / z = 5 / z = 10 / z = 15

Convergence vs. grid step

bunch size 300μm

no. of grid points ~450M

no. of processors 24

simulation time 85hrs

Moving window:3 mm length

S.S.

TESLA / HOM couplerTESLA 9-cell cavity

-15 -10 -5 0 5 10 15 Ez / [kV /m]

bunch length 1mm

bunch charge 1nC

cavity length 1.5m

no. of grid points ~760M

no. of processor cores

408

simulation time ~40hrs

bunch

-5 -4 -3 -2 -1 0 1 2 3 4 5-15

-10

-5

0

5 PBCI ECHO2D

Wz /

(V

/ p

C)

s /

S.S.

Multipacting Simulation

Talks:

• Multipacting simulation for the SNS cavity using Analyst – I. Gonin, FNAL

• Multipacting simulation using parallel code Track3P – Lixin Ge, SLAC

• TTF-III coupler processing and multipacting simulation using MAGIC – Faya Wang, SLAC

Enhanced Counter Function (20 impacts)

1,E+02

1,E+03

1,E+04

1,E+05

0 5 10 15 20 25 30 35

Eacc(MV/m)

1069

MP IN SNS CAVITY I.G & L.G.

Analyst Track3P

Models simulated using Analyst and Track3P are different!

Egap ~ 3.8 V/m;

MP in ILC HOM coupler: notch gap

Enhanced Counter Function (20 impacts)

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

0 0.5 1 1.5 2 2.5 3

Eacc (MV/m)

FE mesh, ~1 million second order elements

Emax ~ 48.1 V/m

= Egap / Eacc ~ 0.16

Enhanced Counter Function (20 impacts)

0

500

1000

1500

2000

2500

0 50 100 150 200 250 300 350

kV/m1.73mm

Enhanced Counter Function for gap 1.73mm and 1.3GHz

EMP_analytical ~ 200 kV/m Eacc= EMP / ~ 1.25 MV/m

Analyst results

Emax ~ 3.8 V/m

HOM coupler

1-D

3-D

Z94_02_Aug24_06

1.0E+10

1.2E+10

1.4E+10

1.6E+10

1.8E+10

2.0E+10

0.0E+00 5.0E+06 1.0E+07 1.5E+07 2.0E+07 2.5E+07 3.0E+07

Test: Q vs. Eacc

MP in the gap

Good correlation between 1D, 3D&measurements

I.G.

6.25mm

20mm

Delta as a function of RF input power and Multipacting order

Cold Coax in TTF-III Coupler

300mm

L.G.

Ion Pump

Ion Pump

e-pickup

PM

PM

F.W.

0 200 400 600 800 1000 1200 14000

50

100

150

Cu

rre

nt o

f Io

n P

um

e: u

A

Forward power: kW

0 200 400 600 800 1000 1200 1400

-100

-50

0

Ele

ctro

n s

ign

al:m

V

electron signal amplitude

vacuum level

600mm long straight SS coax section test results

600mm long straight Copper plated SS coax section test results

Strong MP around 300kW!

F.W.TTF-III coupler processing

0 200 400 600 800 1000 1200-100

-80

-60

-40

-20

0

Forward RF power to coax: kW

ele

ctro

n s

ign

al a

mp

litud

e fr

om

e-p

icku

p: m

V

20us50us100us200us400us

0 200 400 600 800 1000 1200 1400

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Power forward into straight coax section: kW

Ele

ctro

n s

ign

al:

-V

RF processing result of ramping RF from 20us pulse to 400us

20us RF pulse

50us

100us200us

400us

Stainless Steel Coax Tube Copper plated S/S Coax Tube

100 200 300 400 500 6000

1

2

3

4

5

6

7

RF power: kW

Sp

ace

Ch

arg

e a

t Sa

tura

tion

: arb

.u.

sey

max=1.5

seymax

=1.3

Magic Simulation

200 400 600 800 10000

0.2

0.4

0.6

0.8

1

RF Input Power (kW)

Re

lativ

e A

mp

litu

de

of E

lect

ron

Sig

na

l: a

rb.u

.

Track3P simulation (indicated by dashed lines) is independent of SEY.

F.W.