Breakdown Studies NLCTA

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Breakdown Studies NLCTA TTF

• Diagnostics
• Aggressive Vacuum processing
• Conditioning Protocol

Marc Ross
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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

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SRF emitter locating- Diagnostics

• Probably the most important is the
  resistive-thermal mapping
• 0.2 s response time
• 0.15 mDeg resolution
• 100s of monitors/cavity
• Provide details of breakdown/emitter source
  locations (mm resolution)
• for post-mortem analysis / feedback to
  manufacturing

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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

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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
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Volts
Raw signals triggered by RE protection
circuit (35 Mev/m 300 ms) shows estimate of
start time
msec
TTF RF Gun Breakdown
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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
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• 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)
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TTF RF Gun Breakdown
input waveguide
Best guess at breakdown location
(looking from aisle)
speed 3 mm/us
3




4


3
4
5
5


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TTF RF Gun Breakdown
circumference of coupler iris
(looking from above)


1
1
6




6
5
5


(Aisle side)
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TTF RF Gun Breakdown
circumference of coupler cell
(looking down stream)
7



6
7
6

2
2


(wall side)
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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

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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)
t ?
bkdn n
z ?
normal pulse n-2
n-1
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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

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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
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SLAC-built input coupler ? exactly where are
breakdown events?
Cutaway perspective view of VG3RA input coupler
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Sensor signals from 600 coupler breakdowns
AE sensor response (int. ampl) vs sensor
Left
Right
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2
3
4
5
6
7
8
9
2
3
4
5
6
7
8
9
6
2
5
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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
3
8
1
10
40 mm
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Time evolution of rms amplitude vs azimuth
Right
Left
azimuth
time
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• Diffusive vs ballistic
• Plot vs distance
• Source id
• 3 girdles
• beam-axial (prev. plots)
• WG axial
• drop line axial

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Vacuum processing (in-situ bake) ? 2/01 Missing
Energy interlock ? installed 11/00 Narrow pulse
width fault recovery ? 3/01 EPICs ? installed 8/01
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structure imager
Imager for standing wave structures Mirror in
profile monitor body Frame grabber system
triggered on breakdowns Focus on central input
coupler
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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
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Vacuum performance of NLCTA test structures
DS2S
T53
T105

• Pump current a poor substitute for gauges

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Standing wave structure bakeout
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In-situ bakeout history

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

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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
1e-14
Ion Current
CO2
1e-15
RF ON 65MV/m240 ns
I2 5 10-13 A
2e-14
CO
O - CH4
No TRIP
CH3
1e-14
CO2
C
0
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EPICs Control Panel 8/01
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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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NLCTA RF Breakdown Studies

• J. Frisch
• K. Jobe
• F. Le Pimpec
• D. McCormick
• T. Naito
• J. Nelson
• T. Smith

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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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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
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Physics of Breakdown

• Grouping of events (only possible during stable
  operation)
• Soft events lt 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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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