rf breakdown study arash zarrebini uknf meeting– 22 nd april 2009 u.k cavity development...
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RF Breakdown Study
Arash ZarrebiniUKNF Meeting– 22nd April 2009
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, 2006Jens Knobloch1997
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 Area805 MHz Cavity
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
Proposed Research Program
WHY THE NEED FOR BOTH EXPERIMENT AND SIMULATION ?
The majority of Models, assume Asperities are the only source of Electron Emission in an RF structure
Although they are a major contributor, others sources can play an important role.
For Example:
External magnetic fields RF surface band structure
R. Seviour, 2008
Dependence of SEY on Material’s Band Structure
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
INTERFEROMETR RESULTS
Matthew Stable - 2008
Mechanical polish and chemical etch remove deep scratches while EP reduces the average roughness
EXPERIMENTAL SETUP AND EP RESULTS
XPS RESULTS
Matthew Stable - 2008
Effects of Impurities on Band Structure
DFT simulations of Cu surface with P impurity
R. Seviour, 2008
Simulation (Objectives)
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
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
RADIAL FIELDS AND SC EFFECTS ON BEAM SIZE
c
b2b
Rc
Model each individual emitter (asperity) as a prolate spheroid. Then, the field enhancement at the tip is: /
(ln(2 ) 1)
surfTIP e surf
E c rE β E
br
With SC
Without SC
Eyring et al. PR (1928)
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 MHz
Maximum 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)
The local field enhancement due to the presence of Asperity, clearly effects the behaviour of the electron emitted from the tip of the Asperity
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 1 manufactured (spotted problems with the first batch)
EP and Scanning of batch 1 underway (having problems accessing XPS machine at Liverpool )
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