Breakdown Studies NLCTA & TTF

1. Breakdown Studies NLCTA & TTF • Diagnostics • Aggressive Vacuum processing • Conditioning Protocol Marc Ross

2. 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

3. 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

4. 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

5. 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

6. Volts Raw signals triggered by RE protection circuit (35 Mev/m; 300 ms) shows estimate of start time msec TTF RF Gun Breakdown

7. 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

8. 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)

9. TTF RF Gun Breakdown input waveguide Best guess at breakdown location (looking from aisle) speed = 3 mm/us 3 ··· ·· · · 4 ··· ·· 3 4 5 5 ··· ··

10. TTF RF Gun Breakdown circumference of coupler iris (looking from above) · ··· 1 1 6 · · ··· ·· 6 5 5 ··· ·· (Aisle side)

11. TTF RF Gun Breakdown circumference of coupler cell (looking down stream) ··· ·· ·· 7 6 7 6 ··· 2 2 ··· · (wall side)

12. 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

13. 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

14. 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

15. 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

16. SLAC-built input coupler  exactly where are breakdown events? Cutaway perspective view of VG3RA input coupler

19. 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

21. Time evolution of rms amplitude vs azimuth Right Left azimuth time

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

24. Vacuum processing (in-situ bake)  2/01 Missing Energy interlock  installed 11/00 Narrow pulse width fault recovery  3/01 EPICs  installed 8/01

25. structure imager Imager for standing wave structures Mirror in profile monitor body Frame grabber system triggered on breakdowns Focus on central input coupler

26. 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

27. Vacuum performance of NLCTA test structures DS2S T105 T53 • Pump current a poor substitute for gauges

28. Standing wave structure bakeout

29. In-situ bakeout history • Showing difference between gassy bake (T20/T105) and clean bake (T53)

30. 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

31. EPICs Control Panel – 8/01

32. 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)

33. NLCTA RF Breakdown Studies: • J. Frisch • K. Jobe • F. Le Pimpec • D. McCormick • T. Naito • J. Nelson • T. Smith

34. 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

35. 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

36. 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’?

37. 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