RFQ CAD Model Tolerance Studies

Simon Jolly14th December 2011

Extending Comsol Modelling

• Comsol/Matlab/GPT process now pretty stable:– Create quadrant model in Comsol (use

symmetries).– Modelled many different CAD models (from SAT-

files): checked machining tolerances.– Using 3D removal map rather than rmax

statementto give more accurate particle losses.• However…with this method we can’t test

asymmetries!– Single quadrant only allows models that are

symmetric in x and y.– These are okay for machining tolerances but not

for alignment tolerances: need to be able to shift single vane tip in both x and y.

• Rewrite Comsol/Matlab code to build 4-quadrant models…

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CAD Models: Matching Sections

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CAD Models: Lead Out/End Flanges

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Comsol 1-Quadrant Meshing• Import CAD model and

select single quadrant: take advantage of RFQ symmetry.

• Optimum meshes different for different regions:– Vane tips: triangular

(extremely fine auto).– “Inner Beam Box”: 2mm x

2mm, swept rectangular (0.25mm x 0.25mm x 32 slices).

– “Outer Beam Box”: 10mm x 10mm, tetrahedral (extremely fine auto).

– “Air Bag”: 15mm x 15mm, tetrahedral (normal auto).

• Model vanes as “terminals”: only interested in surface fields.

• If end flange is present, model as ground plane.

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Vane tips

Air Bag

Inner Beam Box

Outer Beam Box

Comsol 4-Quadrant Meshing

• Import CAD model but select all 4 quadrants.• Adjust meshes accordingly:

– Vane tips: triangular (extremely fine auto).– “Inner Beam Box”: 4mm x 4mm, swept rectangular

(0.25mm x 0.25mm x 32 slices).– “Outer Beam Box”: 20mm x 20mm, tetrahedral

(extremely fine auto).– “Air Bag”: 30mm x 30mm, tetrahedral (normal

auto).• Adjust Selections code to find domains properly.• Add auto-adjustment of mesh density:

– Sometimes model won’t mesh: normally outer beam box.

– Decreasing mesh density of inner beam box solves problems: match mesh to vane tip surfaces.

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Comsol 4-Quad: Last Cell Geometry

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Matching out cell

End Flange

Last cell

Vane/End Flange Mesh Inner Beam Box Mesh

Comsol 4-Quad: Last Cell Meshes (1)

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Outer Beam Box Mesh Air Bag Mesh

Comsol 4-Quad: Last Cell Meshes (2)

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Comsol 4-Quad: Potential

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Comsol 4-Quad: Transverse Potential

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Comsol 4-Quad: Longitudinal Potential

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Comsol 4-Quad: Longitudinal Field

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Transmission & Alignment Tests• Rebuilt CAD model to allow offsetting of all components:

all vanes can move in X and Y by arbitrary amounts.• Generate 4-quadrant field maps for 7 different models:

– “Standard” FETS model ie. should be identical to previous simulations.

– Alignment tolerance tests: move top vane across in steps of 10, 20, 50, 100, 200 and 500 microns.

– Only standard and 10 micron models finished in time for this meeting…

• Simulation parameters the same as before:– Still starting 10.9 mm long bunch at start of matching

section.– 0.25 pi mm mrad waterbag emittance.– Finely grained loss map takes care of losses.

• Run beam through GPT and measure transmission as a function of current; also check losses.

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500 micron Top Vane Offset: Geometry

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500 micron Top Vane Offset: Potential

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500 micron Top Vane Offset: E-field

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Scott Matcher, End Flanges, LossMap

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Scott Matcher, End Flanges, 4-quadrant

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Scott Matcher, 10 micron Top Offset

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Scott Matcher, End Flanges, LossMap

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Scott Matcher, End Flanges, 4-quadrant

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Scott Matcher, 10 micron Top Offset

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Results• 4-quadrant model in Comsol very successful:

– Very similar results for 1-quadrant and 4-quadrant models using otherwise identical conditions: this is good!.

– Some difficulties building 4-quadrant models: sometimes have to reduce inner beam box mesh density a lot to match vane tip mesh.

– Comsol/Matlab code will now build 4-quadrant model alongside 1-quadrant model: just specify at the start.

– Field map file is the same time but takes 9 hours rather than 6 to generate.

• Beam transmission gives interesting results:– No problems as yet using 10 micron offset: other

results still generating…– Power losses are LOWER for 10 micron offset than

standard 4-quadrant model: • I suspect this is a statistical fluctuation in the field map or

GPT simulation, not reality…• Probably gives some idea of the error on the simulations:

power loss looks particularly sensitive.14/12/11 Simon Jolly, University College

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Conclusions• Nice to be able to make 4-quadrant simulations with as much

ease as 1-quadrant:– Gives many more options for testing errors.– Full simulation requires only creating CAD model and writing a

Matlab script: the rest is automated.– Last big simulation step required for paper writing…

• Looks like 10 micron offset doesn’t affect transmission: a good thing! Still waiting on larger offsets.

• Not yet sure what other “errors” would be meaningful:– Pulling X-vanes in and out?– Single vane okay or multiple vanes?– Can produce CAD models relatively quickly, but the rest takes

time: 9 hours for field mapping, 20 hours for simulations.– If we want to try “random” errors on a/ma, assume it will take

36 hours per model: is it worth it to generate stacks of models?

• Is there anything else that needs testing before RFQ installation? Otherwise I will keep writing papers…

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For Next Time…

• Jürgen’s results show that the field leaks out into the end flange: need to start beam 1-2cm back from matching section to include these effects (should be small).

• Run beam backwards from matching section using 2D space charge and 60mA current, calculate trajectories and produce 3D bunch with correct longitudinal distribution that can be started at any point (use Matlab interpolation).

• Check acceptance for all models using zero beam current: not perfect but gives upper limit.

• Check “map3D_remove” GPT element and particle removal map using CAD model.

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