Vacuum RF R&D in UK

Arash ZarrebiniMuCool RF Workshop – 8th July 2009

U.K Cavity Development Consortium

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

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 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

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 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

Stage 1

No

Yes

Does Particle go through the Surface ?

Measure the amount of energydeposited 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 ofcoordinates for each particles

Stage 3

SO 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