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). Use of this option can usually give a considerable reduction in the computing time.
For example you might be dealing with a cathode and so want short steps near the cathode (typically comparable in length with the size of the cathode segment) but larger step lengths away from the cathode. Or you might be investigating space-charge effects at a waist in a beam, and so want to restrict the step lengths in the vicinity of the waist.
Or you might want to control the step length or time by the ray time or energy.
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’ for some other reason. The program automaticallysets the initial length at (maximum step length)/4, but this might not be short enough.
An example is given in xmpl3d13.
When this option is used in space-charge simulations then it is usually accompanied by the advanced option for changing the radius of space-charge tubes
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.
.Return to general note on step lengths or times