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Note Re: Electrons bouncing on the volume boundary in pure magnetic field 

Keywords: Magnetic field, electrons, ions, tracking
Forum: Fields: Magnetic and Otherwise
Re: Question Electrons bouncing on the volume boundary in pure magnetic field (Ilya)
Date: 07 Mar, 2006
From: John Apostolakis <John Apostolakis>

Please find below a few suggestions for your reported issue/problem. I have also a few questions regarding your report.

1. You do not mention what direction the field is in. From the printout it appears to be in the z direction -- and the particles appearing to bounce or loop have no momentum in the z direction. In this case for a strong field it is physical for tracks to loop ( and in a constant field in a closed circular trajectory when neglecting interactions), and only energy loss or multiple scattering causing a substantial change in the track.

The track potentially feels the effect of scattering (the track length in the world, vacuum volume varies). For clearer information about the 'orbit' of the particle, I suggest limiting the maximum step size in the world volume to 10 cm or, and to see other points on the particle's trajectory. A related point: I am intrigued at the closeness between your value for the maximum step ( 50 cm ) and the step size in vacuum ( ~ 49 cm ). Did you choose your simulation run (or its initial values) out of several runs or are you reproducing a problem you discovered ?

If further investigation is needed of the field tracking, it will be good to revise the SteppingVerbose to print also the momentum direction and the position with higher precisions. But first I suggest investigating the physics and materials:

2. I notice that your electron does not lose energy even in the foil - in which it seems to step several microns:

 Step#  X        Y       Z      KineE   dEStep  StepLeng  TrakLeng  Volume Process
 1  -611 mum -1.18 mm   10 mum  55.4 keV  0 eV   9.28 mum  9.28 mum  Foil  Transportation
 3  -668 mum -1.15 mm  -10 mum  55.4 keV  0 eV  61.2 mum   49.3 cm   Foil  Transportation

For me this means that there is a problem with either the physics (the list or a process), with the material of the foil in your setup. A problem in an energy-loss process implementation is possible, but not certain.

a. Physics lists: Could you please let me know whether your physics list is a copy (exact or revised) of one from an example -- or if it is your own creation ? In any case, especially if it is revised or created by you, I think that sharing it would help check whether there are any issues related to this causing your difficulties.

b. Foil material: Could you check it and let us know ?

c. If neither a) nor b) is the cause, then probably there is a problem in a process. I will be glad to forward it to our responsibles -- but I will request your assistance to document it in our problem reporting system to help us track its progress and more easily inform you.

3. Other In a first reading your field parameters look ok. One potential exception is your line

 > fStepperType = 4 ;      // ClassicalRK4 is default stepper

which requires will change the type of stepper only when used together with other code as in our field examples.

I hope these suggestions help. Best regards, John Apostolakis

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