NLC - The Next Linear Collider Project

Breakdown Studies NLCTA & TTF

Marc Ross

• Diagnostics• Aggressive Vacuum processing

• Conditioning Protocol

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2Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

RF Breakdown Diagnostics

• Goals:– Location within mm

– Quantify energy deposition

• Comprehensive recording

– Observe emitted light

• Provide feedback to manufacturing & fabrication process

• Optimize conditioning protocol

• Observations:– Multi-breakdown events caused by reflection

– Breakdown grouping in time

– Structure damage is not explained by material removed by arc pits themselves

– Many (most) structures show enhanced concentration of breakdown in WG coupler

SRF –’emitter locating’- Diagnostics– Probably the most important is the resistive-thermal mapping

– 0.2 s response time

– 0.15 mDeg resolution

– ~ 100’s of monitors/cavity

Provide details of breakdown/emitter source locations (~mm resolution)

– for post-mortem analysis / feedback to manufacturing

Warm equivalent thermal pulse microphones

Acoustic Emission (AE)Easy for L band structures –

TTF

AE used for industrial structure monitoring (e.g. planes, bridges)

Complementary to “macrosopic microwave” diagnostics

10 mm

TTF FNAL RF Gun Breakdown studies

(most breakdowns from coupler iris)

TTF beam direction

Nov 2001

• 1.5 cell L band • Copper 350 us RF

power in

TTF operation affected by RF gun breakdownDifficult to reliably pinpoint source from RF diagnostics

K. Floettmann J. Nelson D. Ramert

Raw signals triggered by RE protection circuit(35 Mev/m; 300 s)

shows estimate of start time

Vol

ts

sec

TTF RF Gun Breakdown

Zoom showing relative arrival time & pattern recognition error

Cplr iris (wall)

Cplr cell

Inpt WG

Inpt WGCplr iris Cplr iris

Cplr cell (wall)cathode

The downstream side of the coupler iris is always the earliest signal

TTF RF Gun Breakdown

• 3 dimensional geometry:• Group sensors along 3 projections:

input waveguide

circumference of coupler cell

circumference of coupler iris

AE sensor (8 each)

(looking from aisle)(looking from above)

(looking down stream)(Aisle side)

(wall side)

TTF RF Gun Breakdown

(looking from aisle)

input waveguide

5 5

4

4

3

3

speed = 3 mm/us

TTF RF Gun Breakdown

Best guess at breakdown location

circumference of coupler iris

(looking from above)

6

6

55

1

1

(Aisle side)

TTF RF Gun Breakdown

circumference of coupler cell(looking down stream)

22

6

6

7

7

(wall side)

TTF RF Gun Breakdown

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12Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

X-band (NLCTA) acoustic emission

• Clearly audible sound from breakdown – heard from n-1 generation transport components (e.g. flower petal mode converter, bends)

• Small, 1MHz bandwidth industrial or homemade sensors

• 10 MHz bandwidth recorders (3 samples/mm)– Look for start time (TTF) of ‘ballistic phonons’

– or Amplitude (NLCTA)

• Broadband mechanical impulse – (2001- limited by sensor performance)

– Typ. ~ 7 mm

AE sensor results:

• Multi-breakdown pulses• Multi-pulse breakdowns

• Azimuthal breakdown locations• Structure energy deposition

• But: SRF Nb is sheet and Cu is 3D

70 MeV/m TW structure breakdown AE raw signals:(48 10 MHz scope traces)

z

t

normal pulse n-2 n-1bkdn n

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14Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

Input coupler problem:

• Breakdowns concentrated

• Attempt to reduce input WG group velocity appear not to affect breakdown rate

• Forward/reflected RF diagnostics do not localize breakdown beyond indicating which cell

• Fields are a bit higher in the input coupler – but an electrically similar coupler made at KEK shows very different breakdown performance

Acoustic sensor studies of input coupler breakdown

Plan views of two input coupler assemblies

T53 VG3 F (KEK; diffusion bonded cell) T53 VG3 RA (SLAC; H2 braze)

SLAC-built input coupler exactly where are breakdown events?

Cutaway perspective view of VG3RA input coupler

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17Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

1 2 3 4 5 6 7 8 90

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1 2 3 4 5 6 7 8 90

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2 56

78

9

AE sensor response (int. ampl) vs sensor #

All coupler breakdowns come from one side or the other

1 10

2 23 4 5 6 7 8 9 2 3 4 5 6 7 8 9

40 mm

Sensor signals from ~ 600 coupler breakdowns

LeftRight

Data: 1/24-1/30830 bkdns289 R 259 L270 F (30 bulk RA)

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20Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

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21Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

Time evolution of rms amplitude vs azimuth

Right Left

azimuthtime

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22Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

0 5 10 15 20 25 30 35 400

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23Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

• Diffusive vs ballistic

• Plot vs distance

• Source id

• 3 girdles– beam-axial (prev. plots)

– WG axial

– drop – line axial

Vacuum processing (in-situ bake) 2/01

Missing Energy interlock installed 11/00Narrow pulse width fault recovery 3/01EPICs installed 8/01

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25Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

structure imagerImager for standing wave structures

Mirror in profile monitor body

Frame grabber system triggered on breakdowns

Focus on central input coupler

Averaging 07/20/01, 500 images

Averaging from 13pm to 23 pm, 07/19/01, 500 images

Averaging 07/31/01 to 08/02/01, 1000 images

Breakdown in standing wave structures

Average of many images;Spots of light are on accel. iris

close to coupling iris

Up is up

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27Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

Oct Nov Dec Jan Feb0

50

100Oct Nov Dec Jan Feb

10-9

10-8

10-7

Vacuum performance of NLCTA test structures

• Pump current a poor substitute for gauges

DS2S T53T105

Standing wave structure bakeout

In-situ bakeout history

• Showing difference between gassy bake (T20/T105) and clean bake (T53)

RGA –RF 240 ns

I2= 5.5 10-13 A

CO

CO2

CH3O - CH4

C

RF ON 65MV/m 240 ns

I2= 5 10-13 ACO

CO2

CH3

O - CH4

C

RF ON 65MV/m240 ns

TRIP

1e-13

No TRIP

1e-14

1e-12

1e-15

2e-14

1e-14

0

Ion Current

T105 RGA during breakdown

EPICs Control Panel – 8/01

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32Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

EPICs

• Operates synchronously– Digitize RF signals at full 60 Hz rate– Low latency expansion and 120 Hz operation

• Compute missing energy & respond accordingly– Ramp power and pulse width for smooth recovery– Log each event – energy and location

• Skeleton legacy hardware system used for backup only

• EPICs is used throughout the world (except CERN/FNAL)

• (not really designed for high repetition rate pulsed machines)

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33Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

NLCTA RF Breakdown Studies:

• J. Frisch

• K. Jobe

• F. Le Pimpec

• D. McCormick

• T. Naito

• J. Nelson

• T. Smith

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34Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

DS2S Operation – ½ Day close up

• RF vs time – 12 hr period

• Structure damage: 10/00 & 2/01

• Low fault voltage

• Reset time – 2 minutes

• Gaps – logger sampling

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35Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

RF breakdown

1. What are the precursors?Time correlations – multi-breakdown pulses / multi-pulse breakdowns

2. breakdown damage as a run-away phenomenaHow is the parent fault initiated?

How many faults are required to achieve high gradient operation?

3. LocalizationSpatial distribution of faults / Energy distribution / damage distribution

4. Acoustic sensors to understand multiple breakdowns and breakdown sequencesParallel vs serial sensing

5. Vacuum processingproduction & installation

Changing character with new structures

Adsorbed gas, surface defect, surface contaminant, subsurface contaminant

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36Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

Physics of Breakdown• Grouping of events (only possible during ~stable operation)

– Soft events < 10% missing energy• Even after SLED2 high power pulse is fully absorbed in load!

– Hard events -- missing energy

• Multiple arc breakdowns– 30% of trigger sample during ‘processing’ steady increase of RF power– Initiators of prolonged breakdown sequence (spitfest)– moving arc locations

• Breakdown sequence model:– Start with (contaminant?) at random location– multiple arcs upstream of original - large missing energy– large ‘collateral’ damage upstream of initiator

• caused by large VSWR • many sites

– subsequent events eventually ‘heal’?

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37Breakdown Studies – NLCTA & TTF - Marc Ross – LC022/7/02

Structure Processing Protocol

• Reflections are not a reliable method to capture breakdown

• Use missing energy (inputf – loadf)/inputf & compare with nominal (better term is ‘lost energy’)– Typical trip threshold is 10%

• Response:– Ramp short pulse power first, then pulse width

– May use many minutes following vacuum event (mostly in transport)

– Drop target power during extended group of breakdown events