The terrible torroid

It was Halloween.  I was getting ready to take my small Tesla Coil to a 
friends to scare some kids and I had just got a brand new torroid (20"). 
I happily placed it on the top of the secondary, turned on the power, and...
and...lots of noise, but no spark! At all!

What happened? If I put the old top back (an upside down metal lamp 
cover), it works just fine (18" sparks).  When the new torroid is on, 
nothing at all, not even little sparks. (The noise was from the spark gap)

This coil is powered by 15KV -at- .06A (neon sign trans). The secondary is 
8" diameter, 3 feet tall, (500KC resonant freq). The primary is 5 turns 
1/4" copper tubing around a 16" diam form. The primary cap is .007uf.

I know that this coil is not matched and has a low Q, but I still did not 
understand the response from placing on the torroid.

Does anyone have any ideas?

Dan Krones