Xmpl3d45, 45th 'example' data file for CPO3D

Photocathode, illustrating the lambertian distribution of directions.


The file was written when the memory and speed of PC's was much more limited than at present, so the available number of segments was small and the requested inaccuracies were fairly high to give a quick demonstration.


See also xmpl2d25.

Photocathode, illustrating the option of initial ray conditions of fixed energy and lambertian directions.


The photocathode is at 0V and the screen is at 10000V, 300mm away. A middle electrode from z = 30 to 100mm is at 500V. The insulators from z = 25 to 30mm and z = 100 to 110mm are simulated by linearly varying voltages.


The segments are chosen to be shorter along the z axis (that is, in the direction along which the potentials vary) than around the axis. The segments before and after the two insulators, and of the insulators themselves, are shorter than the other segments of the system.


The photoelectrons have energy 0.5eV and a lambertian spread of directions, achieved by entering the thermal energy as -0.5eV (see Help). Although 12 segments are specified for the photocathode the program subdivides it into 14 segments (as can be seen from the screen plots), and so the number of rays that is specified above is 14.


Warning: No attempt has been made to optimise this design. It is included only to point out the precautions that should be taken when setting up a photocathode. The screen is not placed at the image plane.


Precautions to be taken:

(1) The photocathode should be divided into the largest practical large number of segments (in fact only 14 are used in the present example, for a quick demonstration).

(2) The direct method of ray tracing should be used (because with the mesh method the mesh points might extend behind the cathode, giving rise to inaccurate interpolated fields near the cathode).

(3) The initial step length should be small.

(4) The ray tracing inaccuracy should be the smallest allowed value (although here it is not, for a quick demonstration).

(5) The starting distance of the rays from the photocathode should be as small as the program will allow.

(6) The 'zero total charge' option should be disabled, to avoid the potentials near the photocathode being affected by existence of the large anode potential.