Coupler Multipactor Studies

F. Wang, B. Rusnak, C. Adolphsen, C. Nantista, G. Bowden, Lixin Ge

SLAC Coupler Test Stand - Evaluate Parts Individually

• An SRF coupler is a complex integration of many components

• When a coupler is conditioned, it necessarily responds in aggregate

• This makes it difficult to know which components dominate the behavior and limit performance

• As multipacting and RF conditioning are largely surface phenomena, modeling effectiveness is limited

• We developed a test stand to evaluate individual components of the TTF coupler design

Schematic Layout of Test Stand with Parts to be Evaluated

RF load

window

window WG to coax

Device Under Test

RF in

More Detailed Cross Section of the Test Fixture

RF in RF out

Parts to be Tests

– straight 40 mm diam. coaxial line

– SST and copper plating by different recipes

– straight 40 mm coax with bellows

– cold window assembly with window

– cold window assembly without window

Specialized Vacuum WG-to-Coax Transitions

Designs accommodated existing high-power vacuum windows at SLAC

UCRL-CONF-2319326

Detachable Center Conductor

Significant effort went into ensuring the center conductor could easily disconnect upon opening the test stand, but still maintained solid electrical and thermal contacts with the anchors on each side. This was done by screw-attaching the center-conductor length on the right, and having the center slide into a 4-jaw BeCu receiver on the left.

Test Stand Set UpWR650 vacuum window

custom waveguide-to-coaxial transitions

coaxial step transitions from 62 to 40 mm

straight coax Device Under Test

electron pickup probe

The coupler component test stand at SLAC can test components up to 5 MW at 1.3 GHz at pulse lengths up to 1 msec.

Artist View of our Setup (Viewed Through Plastic Screen)

Plot Showing Vacuum Activity Consistent with Expected Multipacting Bands for a Straight Coax

0 200 400 600 800 1000 1200 14000

0.5

1

1.5

2

Forward RF Power (kW)S

econ

dary

yie

ld

third order

fourth order

fifth ordersixth order

seven order

While observed bands generally agreed with simulations, there were difference in the power level and width. The variations are being further investigated, but are suspected to be related to either onset delays in MP emission relative to the RF wave or space charge effects, neither of which are present in the simulations.

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!

Magic Simulations

3/

4

~ss

sU

UUf

Secondary electron energy:

is the work function of material (4.31 for Fe and 4.98 for Cu)

The peak of secondary electron is at

24 cm coax line, OD 40mm, ID 12.5mm, 1.3 GHz RF, TW

0 500 1000 1500 2000 25000

0.5

1

1.5

2

Primary Electron Energy: eVS

eco

nd

ary

Yie

ld

[a]

[b]

[c]

[d]

[a,d] SEY based on the measurements[b c] scaled to better match data

Simulation results depends on the Secondary electron distribution

Red for Copper and blue for SS

0 200 400 600 800 1000 12000

0.2

0.4

0.6

0.8

1

1.2

RF Input Power (kW)

Ave

rage

Del

ta

third order

fourth order

fifth ordersixth order

seven order

SS(1.3@310)

Cu(1.3@410)

MAGIC Multipacting Simulation and ‘Resonant Finder’ Results for a 40 mm Coax Line

The electron signal turn-on time is unstable after initial processing, likely due to insufficient electrons available to seed the onset of multipacting.[1]

[1] D. Raboso, A. Woode, “A new method of electron seeding used for accurate testing of multipactor transients”, Vol.1, Oct. 1995.

Behavior of the Electron Pickup Signal

E-signal

RF

.

[a] The RF pulse with a 1us high power (3MW) spike;[b] The multipacting electron signal.

0 200 400 600 800 1000 1200 1400 1600 1800 2000

0

0.2

0.4

0.6

[a]

1

Time: ns

Forward RF power level normalized to 300kW

Electron signal amplitude normalized to 30mV

[b]

[a] 200ns RF pulse Spike[b] Multipacting electron signal.

Multipactor Seeding and Time Response

0 50 100 150 200

10-6

10-5

10-4

10-3

10-2

10-1

Time: us

Am

plit

ud

e: a

rb.u

.

DC1st-1.3GHz W/RF2nd-2.6GHz3rd-3.9GHz4th-5.2GHz1st-1.3GHzWO/RF

0 1 2 3 4 5-1.6

-1.4

-1.2

-1

-0.8

-0.6

Time: Ry cycle

Am

plitu

de (

Nor

mal

ized

to

DC

com

pone

t)

klystron power 7001

Harmonics of the Electron Probe Signal

Reconstructed electron signal

Signal Recorded with 5 GHz Scope

TTF3 Coupler Multipacting Simulation Using Parallel Code

Track3P

Lixin Ge

Advanced Computations Department

SLAC

Warm Window cold

TTF3 Multipacting Simulation Components

Taper Region

Input Power: 0-2MW, Scan interval: 50KW.

Cold Coax

300mm 6.25mm

20mm

Simulation Cases:1. with/without reflection

2. with/without external magnetic field

3. with variable center conductor DC biases

Conclusions:Our multipacting simulation results are in excellent agreement with

theoretical calculations and experiment measurements at SLAC.

40 mm Diameter Coax Simulations

Delta as a function of RF input power and Multipacting order

Samples of particle’s resonant trajectory

Reflection: 0.4

Input power level: 160KW

Order: 5th order

Impact Energy region: 542-544 eV

Reflection: 0.8

Input power level: 580KW

Order: 2nd order

Impact Energy region: 1100-1200 eV

Cold Bellows

Model Mesh

Cold Coax with Bellows

NO multipacting activities in the bellows region

Particles Distribution after 20 impactsParticles Distribution at 2nd Period

Particles Distribution at 100th Period

400 kW input power

5mm

5mm

1mm

0.4mm

Ceramic Window Region

Distribution of two points resonant particles’ impact energies, orders versus Input Power between the ceramic window and cold cavity

Model and mesh Field

One resonant particle’s trajectory at 0.68 MW input power

2000eV

3800eV

0 2 4 6 8 10 12 140

0.2

0.4

0.6

0.8

1

1.2

Time: Hrs

RF

Po

we

r in

to C

ou

ple

r P

air

: MW

0 2 4 6 8 10 12 14

10-7

10-6

10-5

10-4

10-3

Time: Hrs

Ion

Pu

mp

Cu

rre

nt:

A

RF-In 10BCPC 10CRF-Out 10D

200s

800s

1100s

100s

400s

50 s

vacuumdebug andRF leak checkRF Leak

Check

Example of RF Processing History – See Smooth Ramp-Up in Power (Slight Dwell at 300 kW)

Similar Results at DESY/Orsay

- D. Kostin, XFEL EIFast XFEL mtg 2006

Overall Summary• Predicted MP is not always observed due to

smearing effects, especially for high order resonances

• Due to absorbed gases on WG surfaces, SEY initally high, producing noticeable MP – however it diminishes as gas is removed from electron/rf interactions

• TTF3 coupler shows no noticeable MP ‘barriers’ during processing.