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RFQ Development for FETS

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RFQ Development for FETS. Simon Jolly Imperial College 7 th January 2010. FETS RFQ Development. FETS will utilise a 4m-long, 324 MHz four-vane RFQ channel, consisting of four resonantly coupled sections. RFQ focuses beam from LEBT and accelerates it to 3MeV, ready for Chopper/MEBT. - PowerPoint PPT Presentation
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RFQ Development for FETS Simon Jolly Imperial College 7 th January 2010
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Page 1: RFQ Development for FETS

RFQ Development for FETS

Simon Jolly

Imperial College

7th January 2010

Page 2: RFQ Development for FETS

FETS RFQ Development

• FETS will utilise a 4m-long, 324 MHz four-vane RFQ channel, consisting of four resonantly coupled sections.

• RFQ focuses beam from LEBT and accelerates it to 3MeV, ready for Chopper/MEBT.

• 4-vane cold model showed agreement between CST simulations and bulk RF properties.

• Previous beam dynamics simulations, based on field maps produced with a field approximation code, provide a baseline for the new design.

• Novel design method currently under development to combine CAD and electromagnetic modeling with beam dynamics simulations in GPT.

7/1/10 2Simon Jolly, Imperial College

Page 3: RFQ Development for FETS

FETS Integrated RFQ Design

• Would like to have a method of designing RFQ where all steps are integrated:– Engineering design.– EM modelling.– Beam dynamics simulations.

• Integrating design steps allows us to characterise effects of:– Fringe fields and higher order modes.– Particular CNC machining techniques and

options on beam dynamics.

7/1/10 Simon Jolly, Imperial College 3

Page 4: RFQ Development for FETS

CST Mesh Density

• A lot of effort on optimising meshing in CST.• Need a field map that gives transmission results

similar to RFQSIM.• Important to quantify whether we can model

Electrostatic field of vanes with enough accuracy in CST to measure beam dynamics.

• Non-trivial: modelling 30mm x 30mm x 4m volume with micron accuracy.

• Changes in beam dynamics MUST be unaffected by coarseness of CST field meshing so we can compare to optimised field.

7/1/10 Simon Jolly, Imperial College 4

Page 5: RFQ Development for FETS

First CST Field Map

• Produced with CST, tracked with GPT:– Transmission = 99%

– Mean energy = 1.31 MeV

– Energy rms = 261 keV

7/1/10 Simon Jolly, Imperial College 5

Page 6: RFQ Development for FETS

Increase Vane Radius

7/1/10 Simon Jolly, Imperial College 6

An educated guess: increasing tip radius should improve quadrupole field close to beam axis. Try field mesh with larger tip radius…

Page 7: RFQ Development for FETS

Vane Tip Field Maps

7/1/10 Simon Jolly, Imperial College 7

3.24 mm 7 mm

Page 8: RFQ Development for FETS

Second CST Field Map

• Increase vane radius to 7mm “by hand”:– Transmission = 99.6%

– Mean energy = 3.02 MeV

– Energy rms = 14 keV

7/1/10 Simon Jolly, Imperial College 8

Page 9: RFQ Development for FETS

7 mm Vane Tips

• The Good News: we now have a vane shape that produces comparable transmission results to RFQSIM field!

• The Bad News: we can’t actually build it…

• 7 mm vane tip radius exceeds Kilpatrick Limit…

7/1/10 Simon Jolly, Imperial College 9

Page 10: RFQ Development for FETS

22/4/09 State Of Play (Previous UKNF)• CAD modelling process now pretty mature: can model vane, rod

and “vod” with parameter adjustment on-the-fly (everything except no. of cells).

• Models import into CST and output to GPT: beam dynamics simulations well understood.

• Definite issues in CST modelling:– We think we see “plateau” at 4,100 points and 0.5mm point

spacing of output file.– Clear discrepancies with Alan’s field map: good transmission

and energy with = 7mm vane, = 3.2mm rod but NOT = 3.2mm vane! It should match…

• Are the discrepancies real or down to faulty method?• Need to ensure we’re not re-inventing the wheel: RFQ’s have been

designed before without this process. • Need to ensure CAM systems will understand our CAD models so

we can manufacture what we’re designing (this is the point…).

7/1/10 Simon Jolly, Imperial College 10

Page 11: RFQ Development for FETS

Corrected Input Values

• Couldn’t understand why increasing the radius of curvature made the results closer to Alan’s field map.

• The solution: – Inconsistencies in the units of the RFQSIM

input file.– Reported radius was incorrect, so RFQSIM

actually modelling a larger radius.• Reran RFQSIM with corrected values to get

new modulation parameters.7/1/10 Simon Jolly, Imperial College 11

Page 12: RFQ Development for FETS

Third CST Field Map

• Using corrected input values:– Transmission = 93.8%

– Mean energy = 2.77 MeV

– Energy rms = 470 keV

7/1/10 Simon Jolly, Imperial College 12

Page 13: RFQ Development for FETS

Incoherent Acceleration

• Better results:– Some particles accelerated to the full 3 MeV.– Still not achieving coherent acceleration, even with

new modulation parameters.– This result published at PAC’09.

• Most obvious way to improve results is to increase mesh density:– Prior studies with 7mm vanes showed current mesh

density was sufficient.– Smaller radius of curvature (3.24 mm) needs tighter

mesh to model vane tip region.– CST at computational limit: split RFQ into 5

sections and mesh separately.7/1/10 Simon Jolly, Imperial College 13

Page 14: RFQ Development for FETS

Fourth CST Field Map

• Modelling RFQ in 5 sections:– Transmission = 99.8%

– Mean energy = 1.98 MeV

– Energy rms = 378 keV

7/1/10 Simon Jolly, Imperial College 14

Page 15: RFQ Development for FETS

High Mesh Particle Losses• Many possible causes of

problems in fourth CST field map:– Try hexahedral rather than

tetrahedral meshing.– Try open and tangential

boundary conditions.• Better meshing with

tetrahedral mesh and tangential boundary.

• Eventually found the problem was in the reconstruction of the RFQ from the five sections:– one of the sections was using

the parameters from a different section, and so the modulation pattern was incorrect.

– reran with correct reconstruction

7/1/10 Simon Jolly, Imperial College 15

Page 16: RFQ Development for FETS

Fifth CST Field Map• Five sections reconstructed

into whole RFQ, high mesh density, tangential boundary:– Transmission = 100%– Mean energy = 3.03 MeV– Energy rms = 12 keV

7/1/10 Simon Jolly, Imperial College 16

Page 17: RFQ Development for FETS

Varying CST Mesh (640 points)

7/1/10 Simon Jolly, Imperial College 17

Page 18: RFQ Development for FETS

Varying CST Mesh (4700 points)

7/1/10 Simon Jolly, Imperial College 18

Page 19: RFQ Development for FETS

Particle Tracking For High/Low Mesh

Page 20: RFQ Development for FETS

Conclusions• Finally seeing match between RFQSIM optimised field map and

CST field map from CAD modelling.• Each step has required a lot of effort for small progress, but that

progress has proved extremely valuable.• Advantages of this process already starting to become apparent: for

example, very easy to modify CAD model based on thermal/stress simulations (Scott Lawrie) and measure effect on beam dynamics.

• Next steps:– Output CAD model to Comsol, repeat process from CST to

produce more easily adaptable field map (tighter integration with Inventor and Matlab).

– Compare CST coarse, fine, Comsol and RFQSIM field maps point-by-point to determine whether discrepancies are a result of poor field mapping or more accurate modelling of vane tips.

7/1/10 Simon Jolly, Imperial College 20

Page 21: RFQ Development for FETS

Transverse Field Map Comparison

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Page 22: RFQ Development for FETS

Transverse Field Map Comparison

Page 23: RFQ Development for FETS

RFQ Design Parameters• RFQ parameterised by 3 (+ 1)

parameters:– a and m parameters define

modulation depth.– r0 defines the mean vane

distance from the beam axis and is derived from a and m.

gives the radius of curvature (vane) or mean radius (rod).

– L defines the length of each cell (half sinusoidal period).

• For field approximation method, these values generated for idealised RFQ field.

7/1/10 Simon Jolly, Imperial College 23

L

a

r0 (mm)ma

0 (mm)

L/2

rod axis

beam axis

Page 24: RFQ Development for FETS

RFQ Parameters (from TUP066, LINAC06)

7/1/10 Simon Jolly, Imperial College 24

Page 25: RFQ Development for FETS

RFQ Design Stages

7/1/10 25Simon Jolly, Imperial College

Page 26: RFQ Development for FETS

RFQ CAD Modelling

• Autodesk Inventor CAD package used to model RFQ cold model (and a lot more besides…).

• Inventor can dynamically link to parameters in Excel spreadsheet:– Change spreadsheet

parameters and model updates automatically.

– Use spline to approximate sinusoidal vane shape: only 2% difference.

7/1/10 Simon Jolly, Imperial College 26

Page 27: RFQ Development for FETS

CST MicroWave Studio E-field Modelling

• Four vanes from inventor imported by a macro.

• Model cut into 6 sections (5 plus matching section) for ease of modelling and to increase CST mesh density.

• Potentials and boundary conditions defined in the macro.

• Run solver to produce electrostatic field map.

7/1/10 Simon Jolly, Imperial College 27

Page 28: RFQ Development for FETS

Beam Dynamics Simulations

• GPT used for beam dynamics simulations.

• Import electrostatic field map from text file produced by CST.

• Integration algorithm traces particle movements through time-varying field.

• Compare results to field map from optimised RFQ field expansion.

7/1/10 Simon Jolly, Imperial College 28


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