RF Breakdown Study Arash Zarrebini MuCool RF Workshop 15 th October 2008 U.K Cavity Development Consortium

O LD BUT A TTRACTIVE Two 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 Area 805 MHz Cavity

Button Test Results: 2007 – 2008 LBNL TiN_Cu2 – LBNL TiN_Cu2 D. Huang – MUTAC 08 No Button 40 MV/m no field T Stronger material and better coating improve performance considerably A number of questions exist: o Reliability of Existing Results o Reproducibility o Effects of manufacturing on surface and operation

RF B REAKDOWN J. Norem, 2003, 2006 Jens 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 Copper Matthew Stable

New Button Design MuCoolNew Design CapHolder

I NTERFEROMETR R ESULTS Matthew Stable Mechanical polish and chemical etch remove deep scratches while EP reduces the average roughness

XPS R ESULTS Matthew Stable

F UTURE MTA B UTTON T ESTS 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

P OSSIBLE A PPROACHES 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 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

F UTURE P LAN Stage 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