RF Breakdown Study
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Transcript of RF Breakdown Study
RF Breakdown Study
Arash ZarrebiniMuCool RF Workshop 15th October 2008
U.K Cavity Development Consortium
OLD BUT ATTRACTIVETwo common problems in Normal and Superconducting
accelerating structures RF breakdown – W. D. Kilpatrick (1953) Multipactor – P. T. Farnsworth (1934)
A large number of mechanisms can initiate breakdown. However, this occurs Randomly and Rapidly
It is believed surface impurities and defects are dominant cause of breakdown (must be verified)
No matter what mechanisms are involved, the end results are similar:
Fracture/Field evaporation High local ohmic heating
MuCool Button Test
Much of the effort has gone towards evaluating various material and coatings
MTA Testing Area805 MHz Cavity
Button Test Results: 2007 – 2008
– LBNL TiN_Cu2LBNL TiN_Cu2
D. Huang – MUTAC 08No Button
40 MV/m no field
16 MV/m @ 2.8 T
Stronger material and better coating improve performance considerably
A number of questions exist: o Reliability of Existing Results o Reproducibilityo Effects of manufacturing on surface and operation
RF BREAKDOWN
J. Norem, 2003, 2006Jens Knobloch1997
Breakdown is initiated locally while its effects are global
Proposed Research Program To examine the effects of
manufacturing on surface quality, hence the performance of the RF structure
Surface is characterised by: Interferometer (Physical) XPS (Chemical)
Cap Forming Surface Characterisation Holder
Cap Material Selection Surface Characterisation
Final Cap Surface Characterisation High Power Testing
Cap Surface Treatment Surface Characterisation
A Typical Surface After Mechanical Polishing of OFHC Copper
Up to 1500 Angsrom Evidence of re-crystallisation due
to plastic strain and /or local temperature increases
Lower Slab shaped cells with sharp
boundaries
Deeper still More defuse boundaries
Virgin CopperMatthew Stable - 2008
New Button Design MuCool New Design
Cap
Holder
INTERFEROMETR RESULTS
Matthew Stable - 2008
Mechanical polish and chemical etch remove deep scratches while EP reduces the average roughness
XPS RESULTS
Matthew Stable - 2008
FUTURE MTA BUTTON TESTS
More material such as Ta (Robert Rimmer) Different coatings (Jim Norem) Copper button manufactured and processed
differently (UK Cavity Consortium)
In all cases, there can be several factors causing problems to obtain realistic data Limited Stored Energy Inadequate Field Enhancement
POSSIBLE APPROACHES It has been suggested to conduct simultaneous double
button tests, which can in turn: (Robert Rimmer)
Increase in the number of possible tests and results Provide higher surface field enhancement Produce more realistic results Lead to longer testing time
Magnetic insulation (Bob Palmer)
New cavity and button design to address current issues
Diktys Stratakis, 2008
Numerical Studies
Proposed Research Program
Investigating the relations between surface features and RF breakdown which restrict the performance of RF cavities
A series of simulations to study: Electric Field Profile Electron Behaviour and SE Emissions Local heating and tensile stress (due to particle impact)
Model Setup
11365 Quadratic elements
(One explanation for the odd shape of Asperity is the scale of the object compared to the cavity)
805 MHz Cavity with Asperity
Preliminarily Results
Plain Cavity Cavity with Asperity Asperity Overall field profile is similar in both models. Local field enhancements are observed around the Asperity
FUTURE PLANStage 1: Performing particle tracking on 805 MHz cavity using
g4beamline Developing a home-grown Particle racking code Using Several Emission sites and external B field
Stage 2: Re-running simulations for various Cavity and Asperity
shapes and positions Perform Heat transfer and FEA analysis