It is very easy to specify a cathode in CPO2DS and CPO3DS. The N segments that comprise the cathode are simply the first N segments in the list of segments. One ray starts from the centre of each cathode segment. The cathode can therefore have any shape, size or potential, and the parameters can be easily changed.

Cathodes are easy to specify because the Boundary Element Method is ideally suited to cathode and space-charge problems.

The types of cathode available are:

CPO2DS and CPO3DS: thermionic, field emission and extended Schottky emission, all including non-zero temperature.

A special 'User-defined cathode' version is also available

This version of the 2D and 3D space-charge programs allows the User to define the emission properties of new and unconventional cathodes.

The user can for example define a thermionic temperature that depends on position, or new forms of cold-field or Schottky emitters, such as insulators.

As in the regular versions, the cathode is subdivided into segments and one ray starts from the centre of each segment. At the start of each ray the program sends information to the User-supplied routine on the positions of the cathode segments and on the strengths and directions of the electric fields at their surfaces. The routine then returns information on the emitted current densities. The rays are traced in the usual way, automatically taking care of any space-charge.

The User-defined cathodes fall into two types, depending on whether the space-charge in front of the cathode is significant or insignificant. In the first type (called ‘type 1’ in the databuilder) the User can for example define a thermionic temperature that depends on position. The program will then automatically take care of the space-charge. In the second type (‘type 2’) the User can deal for example with new forms of cold-field or Schottky emitters, such as insulators. The choice of type is made in the databuilder.

The CPO package includes 2D and 3D examples (in C++) that can provide a convenient starting point for further editing by the User. In brief:

1. For a ‘type 1’ cathode the example routine deals with a thermionic cathode which is flat and for which Childs Law is valid.
2. For a ‘type 2’ cathode the example routine deals with conventional cold field emission.

A User-supplied data file could be used to define parameters such as the maximum current density of a thermionic cathode or the work function of a field-emission cathode, although this is not done in the present examples (where the values entered in the databuilder are used instead).

In the 2D programs the User can select the colours of rays and contours at the interactive stage by clicking on Pallet and following the instructions, while in the 3D programs the colours of the electrodes can also be selected.