There are many options to terminate orbits in the 3D charged-particle code OmniTrak. We have had several user reports that particles appeared to stop mysteriously part way through the solution volume. This article discusses how to diagnose and correct the problem.
The first and most critical step is to find out why the particles are stopping. Add the ORBINFO command to the PARTICLES section. In this case, the program makes an entry of termination parameters in the OLS listing file that looks like this:
Final parameters for Particle No. 4
X: 5.50000E+01, Y: 3.17605E+01, Z: 1.19334E-01
Px: 7.27768E-01, Py: 4.41095E-01, Pz: 6.19750E-03
Kinetic energy Tp: 1.60000E+05
Time: 3.53654E-09, Distance: 6.87136E+01
Stop condition: Orbit outside the solution boundary X2Bound: 5.50000E+01
Here is list of the categories of stop conditions:
- Number of integration steps exceeds NStepMax
- Elapsed time exceeds ETimeMax
- Kinetic energy exceeds KELimMax
- Kinetic energy less than KELimMin
- Total distance exceeds DsMax
- Orbit outside the solution boundary
- Orbit crossed stop plane
- Entered material Region
- Invalid electric field calculation
- Invalid magnetic field calculation
Conditions 1-5 are straightforward. The orbit calculation exceeded a default program limit set to prevent infinite calculations. For example, if you have a long recirculating orbit in a magnetic field, it is generally necessary to increase the maximum number of time steps with the NSTEPMAX command.
The application of Condition 6 (leaving the solution volume) depends on whether the file contains a BOUNDARY command. In the absence of the command, OmniTrak sets the dimensions of the solution volume equal to those of the smallest field solution. Including the BOUNDARY command sets the ballistic flag. In this case, the orbit continues if it leaves the electric and/or magnetic field solution. It is assumed that the fields outside have zero values.
Condition 7 applies if you set a stop plane. Condition 8 occurs when a particle crosses into a material region (e.g., an electrode). If you don't want the particle to stop, you can change the status of the region with the VACUUM command.
Conditions 9 and 10 may be the most challenging. Field interpolation errors may occur if the program cannot locate the element occupied by the particle. In this case, try using a smaller time step. Another possible source of error is that OmniTrak cannot collect enough data points to make a valid interpolation (e.g., point inside a small hole). Either eliminate the detail or use a finer mesh to resolve it. Finally, I observed an interesting problem in a recent consulting job. The electric field solution covered a large volume with non-zero field values only in a small portion. At remote points, all potential values that the program collected for an interpolation had exactly the same value. The least-squares fit crashed because of a matrix inversion degeneracy. The resolution was to use a reduced electric field solution that covered only the significant volume and include the BOUNDARY command so that particles continued in the field-free region outside.
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