1. BCP Analysis Update
Thomas Jones
22/7/16

2. Fluid analysis of BCP processing
Buffered chemical polishing is an acid etching process required to smooth the internal surfaces of the cavity and therefore improve performance.
The operation frequency of the cavity is altered in the process due to the amount of material removal (typically ~250μm).
For elliptical cavities the removal rates are well known via experimentation/experience and the detuning therefore fairly straight forward to predict.
For the novel and complex geometry of the crab cavities this is more difficult.
The UK team are currently in the process of using Computational Fluid Dynamics (CFD) techniques to identify the best technique of etching the cavities to give uniform material removal, and then to predict detuning.
It is planned that an experiment will performed by the UK/CERN team to identify removal rate vs flow speed on more simple geometry. 1

3. Process at CERN Fluid passes through cavity ports.
We need to optimise the process for the most uniform material removal.
In addition following the experiment it would be useful to predict frequency shift.

4. CFD Modelling – Flow through Ports3 1 2 6 4 5
= Inlet
All other ports outlets
Gravity acts down as shown in images
Data taken for 21 points throughout the cavity for each orientation
Analysis 1 2 3 4 5 6
Range (mm/s)
12.7
20.1
6.7
6.5
11.7
14.8
Standard Deviation (mm/s)
3.5
4.4
2.0
1.8
2.6
3.6
Av. Velocity (mm/s)
6.0
2.9
3.0
2.7
4.1
4.0

5. CFD Modelling – Flow through Ports
The results on the previous slide were based upon uniform heating of 1200W/m2 over the cavity surface caused by the etching process.
This heat has a significant impact on the flow. The flow is so low that convection currents have an effect.
Therefore, it is not only crucial to measure etch rate vs flow rate but the corresponding temperature rise in the fluid and subsequently we can calculate the exothermic reaction heating.
The idea of this work is to calculate the flow speed at each point in the cavity, then by using this, we should be able to correlate this with material removal rate then compare this to RF parameter studies and ultimately predict detuning of the cavity.

6. Planned experiment
Clamp 170mm long ID35mm Nb tube into PVC adapter piece which also acts as a clamp.
Measure material removal rate vs. flow rate from 1l/min to 12l/min
Removal rate can be measured both by ultrasonic thickness measurement and change in mass of tube.
Monitor fluid temperature during experiment to ensure no rise above 5°C
Monitor tube temperature for qualification of heat flux due to etch rate.
We can then use the flow rate at each monitor point combined with the experimental data to predict material removal at these points.

7. Flow in BCP tube experiment
Temperature rise
Flow in BCP tube experiment
Reference
Length of ipe =
0.17 m
CERN provided
Pipe inner diameter =
35.7 mm
Acid ensity =
1458
kg/m3
Acid flow velocity = 1
l/min
Acid dynamic viscosity =
0.022
Ns/m2
Heat flux due to BCP =
1200
W/m2
Acid Specific heat capacity =
2090
J/kg/K 2
Flow speed =
0.02
m/s
Reynolds number = 39
Total heat in = 23 W
Mass flow rate =
kg/s
Estimated fluid ΔT =
0.451 K
1. Optimisation of BCP Processing of elliptical Nb SRF Cavities. Cooper et.al
2. Non-temperature controlled etching of niobium in BCP. C Preston et. Al (Niowave paper)
3. 'Niobium Reaction Kinetics: An investigation into the reactions between Buffered Chemical Polish and Niobium and the impact on SRF cavity etching. Malloch et.al.
The system has a large 2kW cooling capacity therefore temperature effects can be ignored for the experiment.

8. CFD Modelling – Experimental setup
Inlet water 1L/min at 18°C
Additional length is to give fully developed flow
ΔT in water 0.467
Agrees within 5% to hand calculation
Flow remains laminar even up to 12L/min and with heat load.

9. Note on temperature at surface
Surface temperature 1L/min
Surface temperature 12L/min
Note that within the boundary layer there are local high temperature rises.
This is caused by a lack of fluid mixing at the surface due to the laminar flow condition, which itself is caused by low flow and high viscosity.
This is what will happen in the cavity also, therefore correlation of results is still valid.

10. CFD Modelling – Flow with rotation
Initially investigated rotating cavity, i.e. analised JLAB BCP method.
Work now superseded as BCP will now be performed at CERN using flow through ports.
However, completed analysis using same 21 monitor points for a comparison of rotation vs port flow to assess best practise.
Multiphase transient analysis
Velocity range: 17.3mm/s
Standard deviation: 5.8mm/s
Average velocity: 6.9mm/s

11. HPR simulation
Investigating a cap to redirect flow out of ports so that water does not bounce off walls of HPR facility and back into the cavity.

13. Further work
Complete experiment at CERN as soon as possible, hopefully within next 2 months.
Correlate experiment with CFD results and predict cavity detuning.
Continue HPR simulation work when required.
Work with CERN term to develop cleanroom tooling for DQW, with review in mid-November of full cavity string assembly procedure.
I am available to perform similar BCP/HPR analysis for RFD if required.