The RCA Selectron -- Memory by means of Secondary Electrons

This is a very simple simulation of Secondary Electron Emission Bistability.
(150 kb java applet)

The electron beam from the cathode deposits electrons (charge) on the eyelet...under certain conditions. The dynamic blue region represents the accumulated charge on the eyelet. This applet allows the user to "pulse" the eyelet high and low which represents the capacitive coupling of an external control voltage. The eyelet itself "floats" with respect to the external control voltage: it is capacitively coupled. When the pulse is removed, the eyelet will slowly return to its nominal potential.

(The user can pulse the eyelet, positively or negatively, by moving the mouse cursor over the black buttons.)

Pulsing the eyelet negatively (upwards) lowers the energy of the incoming electrons. If the energy is too low (the potential difference between electron and target too small) the electrons will be deflected.

Pulsing the eyelet positively (larger potential difference) far enough, the electrons become so energetic that they will "kick out" extra secondary electrons, and the eyelet then loses net charge. This increases the potential differences between the eyelet and the incoming electrons. If sufficient secondaries are ejected, the potential of the eyelet becomes so large that electrons are accelerated through the hole in the center and impinge upon a phosphor grain.

Note that this model demonstrates bi-stability: it has two stable operating points, charged and discharged. It has memory. By applying an appropriate pulse, the eyelet will, when released, "float" to a stable potential and either pass or reject electrons. In the physical device the mode can be set or reset by application of the appropriate voltage waveform to an electrode capacitively coupled to the eyelet.