flashover

["Herwig Roscher" <herwig.roscher-at-gmx.de> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi coilers,

I am about tuning my CW, DC driven VTTC = Vacuum Tube Tesla 
Coil. V plate = 10 kV, primary coil = 36 turns of AWG 7 stranded 
wire on 6" form, secondary coil = 1300 turns of AWG 26 on 4" 
form, toroid 1" x 5"

Today the secondary was hit for the 3rd time by a flash from the 
primary, starting under an angle of about 45 deg from the 
uppermost turn. Luckily the secondary was not fried, but in order to 
avoid this, I'm about to rewind the primary to a 8" form.

Now my question to the experienced coilers:
To protect the secondary furthermore, I'm intending to add a PVC 
or acrylic (appearance) tube between the two coils. I could use a 5" 
tube (wall 0.15" thick), which is closer to the secondary or a 6" 
tube (wall 0.18" thick), which is closer to the primary. Which 
solution would protect my secondary best taking into consideration 
the E-field distribution?

Thank you for your advice,

Herwig