xmpl3d12, 12th 'example' data file for CPO3D

Automatic optimisation of an electron gun with a hairpin cathode.


To give a quick demonstration, the geometry is crude and only 6 rays are used (but with 2 planes of reflection symmetry, giving a total of 24 rays).

The program might change the number of subdivisions of the cathode, but the program automatically takes account of this because the cathode has been defined by the number of electrodes from which it is formed.

The wire from which the hairpin cathode is formed has a radius of 0.1 mm, and the tip of the hairpin is a hemisphere of radius 0.1 mm. The cathode is placed 0.4 mm behind a grid that has a hole of radius 0.5 mm. The anode has a hole of the same size and is 1 mm in front of the grid. 6 rays start on the front end cap of the hemisphere (from the front 0.25 of the area of the hemisphere). The cathode is at 0 volts and the anode at 100 volts, while the grid voltage is adjustable, to give the smallest focus on a screen 8 mm in front of the anode.

Only 3 space-charge iterations are used at each voltage, for a quick demonstration, and so the current obtained here is not accurate. The final current is approximately 0.0095 mA before reflections, 0.076 mA after reflections, giving a current density at the cathode of approximately 4.8 mA/square mm.

The tolerance level for the iterations has been set low, 0.001, because with only 1 adjustable voltage, as in the present example, there is a tendency for the program to satisfy the tolerance condition prematurely. The minimum 'mean modulus' option is called, for all 6 rays.