d eveloping a n u nderstanding o f b reakdown i n nf c u c avities arash zarrebini uknf meeting– 8...
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RF B REAKDOWN J. Norem, 2003, 2006, Jens Knobloch, 1997 Breakdown is initiated locally while its effects are globalTRANSCRIPT
DEVELOPING AN UNDERSTANDING OF
BREAKDOWN IN NF CU CAVITIES
Arash ZarrebiniUKNF Meeting– 8th January 2010
U.K Cavity Development Consortium
OLD BUT ATTRACTIVEThe most common problem encountered in both
Normal and Superconducting accelerating structures is:
RF breakdown – W. D. Kilpatrick (1953)
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 Hence, the loss of operational efficiency
RF BREAKDOWN
J. Norem, 2003, 2006 , Jens Knobloch, 1997
Breakdown is initiated locally while its effects are global
MuCool Button Test
Much of the effort has gone towards evaluating various material and coatings
MTA Testing Area
805 MHz Cavity
A. Hassanein , 2006
Button Test Results: 2007 – 2008
– LBNL TiN_Cu2LBNL TiN_Cu2
D. Huang – MUTAC 08
No Button 40 MV/m no
field 16 MV/m @ 2.8
T
Performance is considerably improved by usingstronger material and better coatings
A number of questions exist: o Reliability of Existing Results o Reproducibility
Experiment
To examine the effects of manufacturing on surface quality, hence the performance of the RF structure
Simulation
Investigate the relations between Surface defects and RF breakdown in RF accelerating Structures
Examine the effects of Surface features on field profile
Track free electrons in RF cavities
Investigate various phenomena such as secondary electron emission, Heat and stress deposition on RF surface due to particle impact
Proposed Research Program
EXPERIMENT (Button Test) MuCool
Single part
New Design
• 2 Individual Parts
Cap Holder
Surface is characterised by: Interferometer (Physical) XPS (Chemical)
Experimental Procedure
Cap Forming Surface Characterisation Holder Forming
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
INTERFEROMETER RESULTS
Matthew Stable - 2008
Mechanical polish and chemical etch remove deep scratches while EP reduces the average roughness
EXPERIMENTAL SETUP AND EP RESULTS
Characteristic Plot for Electro Polishing
XPS RESULTS
Matthew Stable - 2008
BUTTON POLISHING
Current Technique
U shaped cathode inserted within electrolyte
Rotated on Axis until bright finish observed
Changing electric field, Constant I/V
Proposed Technique
Dual `bobbin` 180˚ out of phase
Twin formed cathodes
Changing electric field, constant I/V
PARALLEL RESEARCH In collaboration with BNL (Diktys Stratakis , Harold Kirk, Juan Gallardo,
Robert Palmer)
0.07 cm
0.06
cm
CAVEL
201.23 MHz
Diktys Stratakis, 2008
Model Setup
On-Axis Defect Off-Axis DefectModel 1
805 MHz cavity with no defect (top view)Models 2 & 3
805 MHz cavity with a single defect (bottom view)
700 μm
600 μm
ELECTRIC FIELD PROFILE (MODEL 1 )
The colour bar is a good representation of the field. However, it needs to be scaled in order to represent the
actual field values
803.45 MHzMaximum E Field at the Centre of
Cavity
ELECTRIC FIELD PROFILE (MODEL 2 – OFF AXIS )
803.46 MHz Maximum E Field at the Tip of the Asperity
The overall Field profile is similar to model1, as the Asperity enhances the field locally. This is due to the small defect size compared to the
actual RF cavity
COMSOL IN BUILT TRACKER
Model 2 – Particles emitted from a distance of 0.00071m away from the RF surface (tip of the Asperity)
Model 1
Particle Tracking Procedure
Obtain Cavity’s Field Profile in Comsol
Contact with wall ?
Extract E & B Field Parameters at particle’s position (primary & new)
Obtain new particle position using 4th & 5th order
Runge Kutta Integration
Does Particle go through the Surface ?
Measure the amount of energydeposited onto the Impact surface
Dead Particle
Yes
No
Yes
Measure the number of SEs and their Orientation
Stage 1
Define a new set ofcoordinates for each particles
Investigate surface deformation and heating
No
Stage 2
Stage 3
SO WHERE WE ARE? New Batch manufactured EP and Scanning underway New Acid Mixture is being studied High power RF test (date depending on MTA
refurbishing and above work)
Validating stage 1 results (code almost finished)
Identifying the requirements for stage 2 and 3
Both studies will be ready to be presented at IPAC10