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About calculation of the electron trajectory in magnetic lens(2D Axial Symmetry)

Posted Mar 21, 2016, 5:34 a.m. EDT Low-Frequency Electromagnetics Version 4.2 3 Replies

Yudai Kubo
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Hello,
I am a beginner of COMSOL.

I want to calculate the electron trajectory in magnetic lens(2D Axial Symmetry).
I can calculate the magnetic flux density.
Then I try to calculate the trajectory by using Time Dependent calculation.
However, electrons do not be affected by magnetic field and just go straight.

Could you check the attached file?
Any help would be greatly appreciated.

Best Regards,

Attachments:

3 Replies Last Post Mar 24, 2016, 9:15 a.m. EDT
Christopher Boucher COMSOL Employee

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Posted: 9 years ago Mar 21, 2016, 2:20 p.m. EDT
Hi Yudai,

When applying forces to particles, always make sure that the force feature (in this case the Magnetic Force 1 node) is given an appropriate selection. Currently no domains are selected. In addition, if the force in a 2D axisymmetric model has a nonzero phi-component, make sure that "Include out-of-plane degrees of freedom" is selected in the settings window for the Charged Particle Tracing interface. Otherwise, only motion in the r- and z-directions will be modeled.

Chris
Hi Yudai, When applying forces to particles, always make sure that the force feature (in this case the Magnetic Force 1 node) is given an appropriate selection. Currently no domains are selected. In addition, if the force in a 2D axisymmetric model has a nonzero phi-component, make sure that "Include out-of-plane degrees of freedom" is selected in the settings window for the Charged Particle Tracing interface. Otherwise, only motion in the r- and z-directions will be modeled. Chris
Yudai Kubo
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Posted: 9 years ago Mar 22, 2016, 10:03 a.m. EDT
Dear Christopher Boucher,

Thank you so much for your advice. I was able to take one step forward!!
However, I reached a deadlock about other problems.

the error occurs when the setting of "Steps taken by solver" is "Free" and "Strict" and "Intermediate" (refer to "time step setting.png" and "error.png").

If the setting is "manual", the calculation works. However, the result is strange.
the time step is 1e-10, the electron trajectory is converged.
the time step is 1e-11, the electron trajectory is diverged!

I do not understand this result because the smaller time step is set, the better result is obtained.

Could you check the attached file?
Any help would be greatly appreciated.

Yudai
Dear Christopher Boucher, Thank you so much for your advice. I was able to take one step forward!! However, I reached a deadlock about other problems. the error occurs when the setting of "Steps taken by solver" is "Free" and "Strict" and "Intermediate" (refer to "time step setting.png" and "error.png"). If the setting is "manual", the calculation works. However, the result is strange. the time step is 1e-10, the electron trajectory is converged. the time step is 1e-11, the electron trajectory is diverged! I do not understand this result because the smaller time step is set, the better result is obtained. Could you check the attached file? Any help would be greatly appreciated. Yudai
Christopher Boucher COMSOL Employee

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Posted: 9 years ago Mar 24, 2016, 9:15 a.m. EDT
Hi Yudai,

For relativistic particles in 2D axisymmetric geometries with out-of-plane degrees of freedom, typically "Strict" or "Manual" time stepping is best. For "Strict" you may need to specify an initial or maximum time step.

One way to notably improve the time step convergence of the solution is to specify a lower absolute tolerance for the out-of-plane degrees of freedom. You can control the tolerances on different degrees of freedom from the "Absolute Tolerance" section of the settings window for the "Time-Dependent Solver" feature. In this model, the field for the out-of-plane degrees of freedom is denoted "mod1.oqcpt".

If the particles are moving at relativistic speeds, also remember to select the "Relativistic correction" check box in the settings window for the "Charged Particle Tracing" physics interface.

Chris
Hi Yudai, For relativistic particles in 2D axisymmetric geometries with out-of-plane degrees of freedom, typically "Strict" or "Manual" time stepping is best. For "Strict" you may need to specify an initial or maximum time step. One way to notably improve the time step convergence of the solution is to specify a lower absolute tolerance for the out-of-plane degrees of freedom. You can control the tolerances on different degrees of freedom from the "Absolute Tolerance" section of the settings window for the "Time-Dependent Solver" feature. In this model, the field for the out-of-plane degrees of freedom is denoted "mod1.oqcpt". If the particles are moving at relativistic speeds, also remember to select the "Relativistic correction" check box in the settings window for the "Charged Particle Tracing" physics interface. Chris

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