Non shorting toroíd?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Wim Bosma" <wbosma-at-stud.tue.nl> 

Hi all,
After having made a toroíd by taping Al repair tape around a big ring of 
expanded polystyrene, I was disturbed by the e-mail about the heating up by 
hf current when the toroíd is close to the upper winding of the secondary 
and acting as a shorting ring. I have completely overlooked this phenomenon 
and now expect the toroíd's core will melt-down and/or the Al strips to 
loosen from the core (their glue becoming soft).
First thing to do: lifting the toroíd a certain distance.
If I get corona around the last winding of the secondary, an anticorona 
ring around it or a small toroïd over it. This one should not act as a 
shorting ring, so I imagined to split it up in two halves, e.g. remove the 
conducting layer around the toroïd in two places over a distance of let's 
say 3 mm.
My question is: what can I expect about break-out in these places?
My second question is: is there a rule of thumb for the greatest distance 
between upper winding and heart of the toroïd?
Regards, Willem