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Breakdown Studies NLCTA & TTF. Diagnostics Aggressive Vacuum processing Conditioning Protocol. Marc Ross. RF Breakdown Diagnostics. Goals: Location within mm Quantify energy deposition Comprehensive recording Observe emitted light

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breakdown studies nlcta ttf

Breakdown Studies NLCTA & TTF

  • Diagnostics
  • Aggressive Vacuum processing
  • Conditioning Protocol

Marc Ross

rf breakdown diagnostics
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
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
Warm equivalent thermal pulse microphones Acoustic Emission (AE)

10 mm

Easy for L band structures – TTF

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

Complementary to “macrosopic microwave” diagnostics

ttf fnal rf gun breakdown studies
TTF FNAL RF Gun Breakdown studies

Nov 2001

  • 1.5 cell L band
  • Copper

350 us RF power in

TTF operation affected by RF gun breakdown

Difficult to reliably pinpoint source from RF diagnostics

TTF beam direction

(most breakdowns from coupler iris)

K. Floettmann J. Nelson D. Ramert

slide6

Volts

Raw signals triggered by RE protection circuit

(35 Mev/m; 300 ms)

shows estimate of start time

msec

TTF RF Gun Breakdown

slide7

TTF RF Gun Breakdown

Cplr cell ¯

Inpt WG ­

Cplr iris (wall)

Inpt WG

Cplr iris ¬

Cplr iris

Zoom showing relative arrival time & pattern recognition error

cathode

Cplr cell (wall)

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

slide8

3 dimensional geometry:

    • Group sensors along 3 projections:

TTF RF Gun Breakdown

circumference of coupler iris

input waveguide

(looking from above)

(looking from aisle)

(wall side)

circumference of coupler cell

(Aisle side)

(looking down stream)

AE sensor (8 each)

slide9

TTF RF Gun Breakdown

input waveguide

Best guess at breakdown location

(looking from aisle)

speed = 3 mm/us

3

···

··

·

·

4

···

··

3

4

5

5

···

··

slide10

TTF RF Gun Breakdown

circumference of coupler iris

(looking from above)

·

···

1

1

6

·

·

···

··

6

5

5

···

··

(Aisle side)

slide11

TTF RF Gun Breakdown

circumference of coupler cell

(looking down stream)

···

··

··

7

6

7

6

···

2

2

···

·

(wall side)

x band nlcta acoustic emission
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. l ~ 7 mm
ae sensor results

Multi-breakdown pulses

  • Multi-pulse breakdowns
  • Azimuthal breakdown locations
  • Structure energy deposition
AE sensor results:
  • But: SRF Nb is sheet and Cu is 3D

70 MeV/m TW structure breakdown AE raw signals:

(48 10 MHz scope traces)

t 

bkdn n

z 

normal pulse n-2

n-1

input coupler problem
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
Acoustic sensor studies of input coupler breakdown

T53 VG3 F (KEK; diffusion bonded cell)

T53 VG3 RA (SLAC; H2 braze)

Plan views of two input coupler assemblies

slac built input coupler exactly where are breakdown events
SLAC-built input coupler  exactly where are breakdown events?

Cutaway perspective view of VG3RA input coupler

slide19

Sensor signals from ~ 600 coupler breakdowns

AE sensor response (int. ampl) vs sensor #

Left

Right

2

3

4

5

6

7

8

9

2

2

3

4

5

6

7

8

9

6

2

5

9

All coupler breakdowns come from one side or the other

Data: 1/24-1/30

830 bkdns

289 R 259 L

270 F

(30 bulk RA)

4

7

1

3

8

10

40 mm

slide23
Diffusive vs ballistic
  • Plot vs distance
  • Source id
  • 3 girdles
    • beam-axial (prev. plots)
    • WG axial
    • drop – line axial
slide24

Vacuum processing (in-situ bake)  2/01

Missing Energy interlock  installed 11/00

Narrow pulse width fault recovery  3/01

EPICs  installed 8/01

structure imager
structure imager

Imager for standing wave structures

Mirror in profile monitor body

Frame grabber system triggered on breakdowns

Focus on central input coupler

slide26

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

Breakdown in standing wave structures

Average of many images;

Spots of light are on accel. iris close to coupling iris

Averaging 07/20/01, 500 images

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

Up is up

vacuum performance of nlcta test structures
Vacuum performance of NLCTA test structures

DS2S

T105

T53

  • Pump current a poor substitute for gauges
in situ bakeout history
In-situ bakeout history
  • Showing difference between gassy bake (T20/T105) and clean bake (T53)
rga rf 240 ns
RGA –RF 240 ns

T105 RGA during breakdown

1e-12

RF ON 65MV/m 240 ns

I2= 5.5 10-13 A

C

CO

1e-13

CH3

TRIP

O - CH4

Ion Current

1e-14

CO2

1e-15

RF ON 65MV/m240 ns

2e-14

I2= 5 10-13 A

CO

O - CH4

No TRIP

CH3

1e-14

C

CO2

0

epics
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)
nlcta rf breakdown studies
NLCTA RF Breakdown Studies:
  • J. Frisch
  • K. Jobe
  • F. Le Pimpec
  • D. McCormick
  • T. Naito
  • J. Nelson
  • T. Smith
ds2s operation day close up
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
rf breakdown
RF breakdown

Changing character with new structures

  • What are the precursors?

Time correlations – multi-breakdown pulses / multi-pulse breakdowns

  • breakdown damage as a run-away phenomena

How is the parent fault initiated?

How many faults are required to achieve high gradient operation?

  • Localization

Spatial distribution of faults / Energy distribution / damage distribution

  • Acoustic sensors to understand multiple breakdowns and breakdown sequences

Parallel vs serial sensing

  • Vacuum processing

production & installation

Adsorbed gas, surface defect, surface contaminant, subsurface contaminant

physics of breakdown
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’?
structure processing protocol
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
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