Variable maximum step length or time.
An ‘advanced option’ is available (on /setting up rays/space-charge options/advanced option for tube diameter/) for changing the maximum step length or time during ray tracing (that is, trajectory integration).
For example you might want the maximum step length to be 0.1mm most of the time but to be reduced to 0.01mm for the part of the trajectory that is between z = .95 and 1.05mm. You might also want to specify a second region of changed dlmax, for example to 0.02mm between z = 1.5 and 1.6mm.
Or you might want to control the step length or time by the ray time or energy. For example you might be using step time intervals instead of step lengths and you might want the maximum step time to be 5e-7ms for most of the ray but to be reduced to 5e-8ms for the part of the trajectory that is between time = 1e-6 and 2e-6ms. You might also want to specify a second region.
Or you might be modelling a field emission source, where a variable step length is almost obligatory.
Or it might be important to start a trajectory with a small ‘initial step length’. The program sets this at
(initial step length) = 0.25*(maximum step length).
which might not be short enough.
An example is given in xmpl3d13.
A maximum of 9 regions can be specified.
If more than one region is specified then the program treats the nth region after the (n-1)th region, which means that the later regions can be superimposed inside the earlier regions. For example if the main value of dlmax is 0.1 and there are two regions that have the following z limits and dlmax:
0.95 1.05 0.01
0.99 1.01 0.005
then dlmax is 0.1 for z < 0.95, 0.01 for z from 0.95 to 0.99, 0.005 for z from 0.99 to 1.01, 0.01 for z from 1.01 to 1.05, 0.1 for z > 1.05.
If a region is given a negative value of dlmax then it is ignored.
There is also an option for forcing the step length or time to be constant.
There is also an advanced option for changing the radius of space-charge tubes and another for changing the averaging radius for evaluating space-charge density for the stochastics option.
Return to general note on step lengths or times