Xmpl2d41, 41st 'example' data file for CPO2D
Photocathode
The following data were obtained 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.
25th example for CPO2D (see also xmpl3d45).
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 length of most of the segments is 2. The exceptions are (1) the segments of the photocathode (length 0.33), (2) the 5 segments before and after the two insulators, which vary from length 2 away from the insulators to 0.2 in contact with them, and (3) the segments of the insulators (length 0.5).
The output data file temp25a.dat gives the following end results at a test plane near the screen:
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.
Precautions taken:
(1) The photocathode is divided into a large number of segments.
(2) The direct method of ray tracing is used (because with the mesh method the mesh points would extend behind the cathode, giving rise to inaccurate interpolated fields near the cathode).
(3) The initial step length is small.
(4) The ray tracing inaccuracy is the smallest allowed value.
(5) The rays are started at a distance s from the photocathode, where s is greater than about a quarter of the photocathode segment lengths.
(6) The ray starting points are opposite the mid-point of a photocathode segment.
To help with point (5) the inscribing correction is disabled to give more control over the positions of the photocathode segments.
To satisfy point (6) the program given as program 17 used to produce the ray initial conditions, and the ratio (number of photocathode segments)/(number of rays) is made an odd integer (in the present case 150/10, although only the first 7 of the 10 rays are included above).