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