High Pressure Sodium and Metal Halide lamps near plasma ball

[BetaZeta2016]

I bought one of these plasma globes, decided to do some experiments with it.
Since a fluorescent bulb will light up if It's put near the plasma ball, i tried the same with two HID lamps.

The Metal Halide shows a very pale white arc, which disappears as soon as the plasma ball is turned off...

But the HPS is different, it emits a purple glow, which stays for several seconds after the plasma ball is shut off.

1. Why is this?

2. Also, since the only thing powering those bulbs is the plasma ball, is there any risk of they exploding due to the lack of a ballast?

 

[Tom.G]

It's phosphoresence and normal for Sodium lamps, also visible in fluorescent lamps to a lesser extent. See the video where the Sodium lamp is turned off around 4:25 and fades to a purple afterglow.

 

[BetaZeta2016]

Thanks, understood :)