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