Troubleshooting for cathode simulations


Most of these remarks concern the thermionic cathode, for which there can be problems in iterating.

The currents and other parameters oscillate:

This is the most usual problem for cathodes. The CPO programs provide a set of different methods of damping, which are controlled by the user through a single global damping parameter. See note on damping of cathode iterations.


The cathode is not as specified:

The user can specify the number N of segments that form the cathode, and these are always the first N segments in the list of segments (see note on specifying a cathode). But the program might change the number of segments of an electrode. So the user must look at the screen plots or at the processed data file to see the total number of segments into which the cathode has been subdivided.


The rays will not start:

(1) A common reason is that the specified initial temperature or current per unit area (see note on specifying a cathode) is too large. It is advisable to use values not more than a factor of 4 higher than the expected current given by Child’s Law. A second maximum current density is also specified later, in connection with the cathode iterations, which enables the user to start the iterations gently.

(2) Another reason is that the rays do start but they do not cross the first 'test plane' and so are not registered as cathode rays (see note on cathode test plane).


The initial conditions seem to be wrong:

(1) The usual reason is that 'ray fractional inaccuracy' is not small enough to deal satisfactorily with the potentials and fields within the cathode region (although the program usually uses a smaller inaccuracy for this region than the user has asked for). See note on accuracy for ray tracing with cathodes.

(2) Another reason, when the anode voltage is very large compared with the potential differences near the cathode, is that the 'zero total charge' option has not been disabled.

(3) Or the cathode depth 'd' might be too small (see notes on region near cathode). A suitable value for 'd' is in the region of the average width of the cathode segments. The program will not allow 'd' to be smaller than 0.2 times the largest width of segment.


The ray tracing is slow:

The usual reason is that too many rays with too many steps are being used, see the relevant note.  Seealso the notes on troubleshooting.



Finally, remember that it is very important to experiment with parameters for cathode iterations


Special care is needed when using a cathode with an anode at high voltage.