Discussion Forum

Hi

you need to reload your file, the server must have been saturated yesterday, for me it says "0" bytes when I try to read it ;)

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Good luck
Ivar

Hello, thanks for taking the time to help me with this. I really appreciate it :)

I have re-uploaded the file, I think it should be ok now.

Thanks,
Anthony

Does anyone have any suggestions for this problem? I still cannot get the B-field force to act. Any ideas would be greatly welcomed :)

Thanks.

Hi

indeed, so far I havent managed neither ;)
but I have not been using particle tracking many times so far, so I still need to learn it better.

My only though is that the particle velocity is so high that the deformation is negligible, but that one should be able to check by a handcalculation, what I havnt found time to test

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Good luck
Ivar

I still cannot get the B-field force to act.

I didn't quite understand what results you expected and why.

You kind of skipped over that part in your original post:

Given the large magnetic fields, the particle should track the B field streamlines - I have confirmed this using the post-processing particle tracing.

That doesn't make sense to me. Why would charged particles ever follow the B-field streamlines? If you have confirmed this, how exactly?

For the record, the result you get makes total sense to me. Since B and v are both in-plane, the Lorentz force is purely out-of-plane, so you must see no effect. But maybe I'm missing something...

Thanks for having a look, COMSOL is a great program but it still boggles my mind sometimes.

Sorry if I have been slightly unclear. I have attached a image of the same setup using the post-processing particle tracer instead. I've messed around with the colours slighty, the black to white background indicates B-field magnitude, the grey lines are 'streamlines' for the B-field and the coloured tracks are the particle tracing, green indicates high speed, red low. This is the behaviour I am expecting from the CPT module but am not getting; it is not taking the B-field into account. The particles released from the upper surface should roughly follow the field lines - this would be more accurate a statement if the E-field were turned off.

The Lorentz force due to such a large magnetic field should cause the particle to follow the field lines whilst executing a helical orbit. The component of velocity along the field line, as you said, gives no force. The component of velocity perpendicular to the field line results in a force which causes the particle to orbit around the field line. In the image I have attached, the particle tracks are really small helical tracks when one zooms in.

This was news to me having never taken a plasma physics course, but apparently this is widely known - 'Magnetic confinement rests upon the property that charged particles, like those in a plasma, will travel along the lines of a magnetic field'.

I completely agree with your final sentence, however it is precisely the functionality I am asking about. The Lorentz-force is purely out of plane initially and so for all time as far as the CPT module is concerned. I need the CPT module to take this out-of-plane force into account. I believe the 'Include out of plane degrees of freedom' check-box in the CPT settings should allow it to correctly account for this but it throws errors when I try this. If the somewhat 'simpler' post-processing particle tracer can include the out-of-plane forces, as shown by the attached image, I assume the more complicated CPT module also can.

If the CPT module cannot, can someone point me to documentation on coupling my 2D field solution to a 3D CPT solver? Extrusion model coupling seems to be something like this.

Thanks for all the replies.

Anthony

I've never taken a plasma physics course either, but thanks to your elaborate reply I've finally understood how magnetic confinement works. I get it now: as soon as the particle acquires some out-of-plane velocity after release, the Lorentz force will also have some in-plane component. Intuitively, at least, I can see how the particle will end up swiveling along the magnetic field line.

You're absolutely right, activating the out-of-plane degree of freedom should do the trick. And like you, I kept getting some weird error messages. It seems that the particle tracing module is quite picky about the time-stepping algorithm. For example, it won't let me use BDF at all.

Eventually I figured out that one way to get the time-depedent solver up and running is to set the generalized-alpha time stepping to "Manual".

PS: For some reason I couldn't attach the modified model file – I keep getting a "File extension error", whatever that's supposed to mean. But it's really easy to reproduce anyway.

Is there any chance you could send it to me via e-mail? I have tried enabling the 'out-of-plane degrees of freedom' and using a 'Manual' stepping but it is still throwing errors under the 'Dependent Variables 1' sub-section. To be exact:

'Failed to evaluate variable.
- Variable: mod1.qphi
- Geometry 2
- Boundary: 1
Failed to evaluate expression.
- Expression: mod1.qphi
- Feature: Dependent Variables 1 (sol2/v1)
- Error: Failed to evaluate expression'

This was using 'Generalized alpha', 'Manual', '1e-11' time step, '0.75' amplification, 'Linear' predictor. Not sure what else I can play around with.

Thanks very much!

Hi

I'm not by my WS so I cannot check, but was'nt your model in 2D-axi ?
Are we sure we can see correctly the twisting of the e particles without going to a full 3D model, at least rolled out for the particle analysis ?

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Good luck
Ivar

Starting with your original model and doing exactly what you did (i.e. "Include out-of-plane degrees of freedom" to "on", "Steps taken by solver" to "manual", "Time step" to "1e-11"), then re-running the particle tracing study, I got an error message saying "Solution cannot be updated. Clear Solution to expunge. Failed to evaluate variable Jacobian."

I then cleared the solution, re-ran both studies, and voilà, it worked, see figure attached or the modified model file. (Turns out, "File extension error" is the forum software's way of saying, "You need to activate Firefox's Flash plugin." Obviously...)

Activating that check box adds the third degree of freedom to the particle trajectories. Otherwise, the φ-component of the particle's position and velocity is simply set to zero (as one can see in the "equation view" somewhere). However, the geometry for the field variables remains 2d-axisymmetric, i.e. Bφ = 0 throughout. So that provides exactly the functionality Anthony was looking for. The field's rotational symmetry isn't broken until the particles start interacting with it.

Thanks very much! That is exactly what I needed.