Okay, intelligent physics folk, can you help me understand this phenomenon? The green spot on the phosphor does not appear unless the beam is swept from the glass upwards. When swept downwards, there is no spot. There appears to be some sort of hysteresis effect, but I cannot explain it at all. For information, the anode voltage is around 50V. When it is 200V, this effect is not observed (the spot is always visible on the phosphor). I'm not currently able to (easily) test voltages between 50V and 200V. You can vaguely make out a diffraction pattern on the dark spot when sweeping down.