RF Breakdown Study Arash Zarrebini UKNF Meeting– 22 nd April 2009 U.K Cavity Development Consortium

O LD BUT A TTRACTIVE 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 B REAKDOWN J. Norem, 2003, 2006 Jens 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 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 Performance is considerably improved by using stronger 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

W HY THE NEED FOR BOTH E XPERIMENT AND S IMULATION ? 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 Copper Matthew Stable

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

E XPERIMENTAL S ETUP AND EP RESULTS

XPS R ESULTS Matthew Stable

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

P ARALLEL R ESEARCH In collaboration with BNL (Diktys Stratakis, Harold Kirk, Juan Gallardo, Robert Palmer) 0.07 cm 0.06 cm CAVEL MHz Diktys Stratakis, 2008

RADIAL FIELDS AND SC EFFECTS ON BEAM SIZE Model each individual emitter (asperity) as a prolate spheroid. Then, the field enhancement at the tip is: With SC Without SC Eyring et al. PR (1928) Diktys Stratakis, 2008

Model Setup On-Axis DefectOff-Axis Defect Model MHz cavity with no defect (top view) Models 2 & MHz cavity with a single defect (bottom view) 700 μm 600 μm

E LECTRIC F IELD P ROFILE (M ODEL 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 MHz Maximum E Field at the Centre of Cavity

E LECTRIC F IELD P ROFILE (M ODEL 2 – OFF AXIS ) 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

C OMSOL IN BUILT TRACKER Model 2 – Particles emitted from a distance of m 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 4 th & 5 th order Runge Kutta Integration Does Particle go through the Surface ? Measure the amount of energy deposited onto the Impact surface Dead Particle Yes No Yes Measure the number of SEs and their Orientation Stage 1 Define a new set of coordinates for each particles Investigate surface deformation and heating No Stage 2 Stage 3

S O 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