Test planes that are needed for a cathode iteration
At least two 'test planes' must be defined. The first is used to test the cathode (see below) while the second is a normal test plane, situated for example at a focal plane.
The first test plane is used to reduce an instability that sometimes arises in the iteration process, particularly for a thermionic cathode. When a space-charge is built up that is too high it can cause rays to slow down too much or to be reflected, leaving an unrealistic space-charge at the slowing-down position and thus causing the total space-charge to be built up even more. This instability is avoided by ignoring the space-charge (that is, by resetting the current to zero) of any ray that does not reach the first test plane. The first test plane must therefore be carefully positioned.
The user can, if necessary, effectively disable this restriction by placing the test plane very near to the cathode, so that all rays cross it, whatever they might do later.
Another way to disable the restriction when the cathode is curved is to make the last of the test plane parameters 1,000,000 or larger (which the program will interpret as the trigger to remove the restriction).
The restriction is automatically disabled for the ‘special’ stochastic option.
The second test plane will be used for the beam section, the summary output information, the focus iterations, etc. The first and second test planes can of course be chosen to be the same.
There is an option to limit the number of crossing points that are displayed on the screen. This option is sometimes useful when a limited part of a cathode is of interest. The option is labelled ‘Maximum number of beam section points’ and it can be found in the data-builder under /tracing the rays/field of view. If the number N that is entered is less than the total number of rays then the crossing points of the first N rays will be shown. The pre-set value of N is the maximum number of rays.
Return to general note on iterating cathode systems.
Return to general note on cathodes.
Return to test planes.