Section 3.4 of the User's Guide for CPO2D and CPO3D

(or proceed to section3.5)

Choosing the subdivision numbers.

The accuracy of the field and ray calculations can depend critically on the way in which the individual electrodes are subdivided into segments, within the limitation of a fixed total number of segments.

Important: It is generally true that the best accuracy is obtained when all the segments carry approximately the same charge. This is particularly important for the critical parts of a system, such as a cathode and its surrounding electrodes. It is less important for electrodes that are far from a beam or are not ‘seen’ by the beam. Please look at general advice on segmentation.

For many simulations a straightforward way of choosing the subdivisions is to let the program do it! This is achieved by starting with a smallish number of subdivisions per electrode and then letting the program increase the number in 3 or more stages, a process called iterative subdivision or adaptive segmentation. Using this option it is possible to subdivide so that all the resulting segments carry approximately the same charge. The result will be that the segments will be smallest and most dense in those parts of the electrodes where the electric field strengths are highest. This is often the best choice of subdivision (for a detailed explanation see paper on Improved Extrapolation Technique... in list of papers).

A more controlled choice of subdivisions is sometimes required. (as with the aperture lens dealt with in file xmpl2d02.dat, or when it is important to apply the inscribing correction in CPO3D). In such cases the user should aim to concentrate the segments in the regions that are most likely to have the greatest effect on the parameters of interest in the simulation (whatever they might be, such as potentials, fields or ray end points). A way to do this is:

(1) Set up fairly large numbers of segments for the important electrodes but small numbers for the unimportant ones.

(2) Let the program subdivide in a few stages using the iterative subdivision option.

(3) Inspect the results of doing this, either in the information box on the screen or in the output data file.

(4) Empirically adjust the segment numbers.

Further steps to obtain the greatest accuracy (in a given computing time) for the parameters of interest (whatever they might be, such as potentials, fields or ray end points) are:

(5) With the present set of segment numbers, calculate the parameter of interest and use this value as a temporary standard.

(6) Decrease the number of segments in a selected region to test the sensitivity to the parameter of interest.

(7) Repeat this for other regions, so building up information on where large numbers of segments are needed and conversely where small numbers can be used.

In 3D systems it is important that the segments should not be too long and thin -see the note on choosing shapes of 3D triangles and rectangles.