Transcript RF Breakdown Study - Science and Technology Facilities

RF Breakdown Study
U.K Cavity Development Consortium
Arash Zarrebini
UKNF Meeting– 22nd April 2009
OLD
BUT
ATTRACTIVE
The 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
Breakdown is initiated locally while its effects are global
J. Norem, 2003, 2006
Jens Knobloch1997
MuCool Button Test
MTA Testing Area
805 MHz Cavity
Much of the effort has gone towards evaluating various
material and coatings
Button Test Results: 2007 – 2008
–
No Button
LBNL TiN_Cu2
40 MV/m no field
16 MV/m @ 2.8 T
D. Huang – MUTAC 08
Performance is considerably improved by using
stronger material and better coatings
A number of questions exist:
o Reliability of Existing Results
o Reproducibility
Proposed Research Program
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
WHY THE NEED
SIMULATION ?
FOR BOTH
EXPERIMENT
AND
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

Dependence of SEY on Material’s Band Structure
R. Seviour, 2008
EXPERIMENT
(Button Test)
MuCool

New Design
Single part
•
Cap
2 Individual Parts
Holder
Experimental Procedure
Surface is characterised by:

Interferometer

XPS
(Chemical)
(Physical)
Cap Material Selection
Surface Characterisation
Cap Forming
Surface Characterisation
Holder Forming
Cap Surface Treatment
Surface Characterisation
Final Cap
Surface Characterisation
High Power Testing
A Typical Surface After Mechanical
Polishing of OFHC Copper
Up to 1500 Angsrom


Lower


Slab shaped cells with sharp
boundaries
Deeper still


Evidence of re-crystallisation due to
plastic strain and /or local
temperature increases
More defuse boundaries
Virgin Copper
Matthew 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
Characteristic Plot for Electro Polishing
1
0.9
Current (I)
0.8
Sample 7
0.7
Sample 8
0.6
0.5
Sample 9
0.4
Sample 10
0.3
Sample 11
0.2
Sample 12
0.1
Sample 13
0
0
0.5
1
1.5
2
Voltage (V)
2.5
3
3.5
Sample 14
XPS RESULTS
Matthew Stable - 2008
Effects of Impurities on Band Structure
R. Seviour, 2008
DFT simulations of Cu surface with P impurity
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
201.23 MHz
CAVEL
Diktys Stratakis, 2008
RADIAL FIELDS AND SC EFFECTS ON BEAM
SIZE
b
b2
R
c
c

Model each individual emitter (asperity) as a prolate spheroid. Then,
the field enhancement at the tip is:
Esurf  c / r 
ETIP 
Eyring et al. PR (1928)
With SC
Diktys Stratakis, 2008
b
(ln(2 )  1)
r
Without SC
 βe Esurf
Model Setup
Model 1
805 MHz cavity with no defect (top view)
700 μm
600 μm
On-Axis Defect
Off-Axis Defect
Models 2 & 3
805 MHz cavity with a single defect (bottom view)
ELECTRIC FIELD PROFILE (MODEL 1 )
803.45 MHz
Maximum E Field at the Centre of Cavity
The colour bar is a good representation of the field. However, it
needs to be scaled in order to represent the actual field values
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
Extract E & B Field
Parameters at particle’s
position (primary & new)
Obtain Cavity’s Field
Profile in Comsol
Obtain new particle position
using 4th & 5th order
Runge Kutta Integration
Stage 1
Define a new set of
coordinates for each particles
Stage 3
No
Yes
Does Particle go
through the Surface ?
Yes
Dead Particle
Measure the number of SEs
and their Orientation
Contact with wall ?
Stage 2
No
Measure the amount of energy
deposited onto the Impact surface
Investigate surface
deformation and heating
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