Vacuum RF R&D in UK Arash Zarrebini MuCool RF Workshop – 8 th July 2009 U.K Cavity Development Consortium

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 Why not use a single material and evaluate the manufacturing procedure instead ?

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

Dependence of SEY on Material’s Band Structure

Simulation (Objectives) Investigate the relations between Surface defects and RF breakdown 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

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 Stage 1 No Yes Does Particle go through the Surface ? Measure the amount of energy deposited on the Impact surface Dead Particle No Yes Investigate surface deformation and heating Stage 2 Measure the number of SEs and their Orientation Define a new set of coordinates for each particles Stage 3

S O WHERE WE ARE ? New batch of 20 buttons manufactured (spotted problems with the first batch) EP and Scanning of batch 1 underway (access to XPS machine at Liverpool has been granted ) High power RF test (date depending on MTA timetable) Validating stage 1 results Identifying the requirements for stage 2 and 3