arc-overs!

To: tesla-at-poodle.pupman-dot-com
Subject: arc-overs!
From: Tesla List <tesla-at-stic-dot-net>
Date: Tue, 15 Apr 1997 23:31:47 -0500
Approved: tesla-at-stic-dot-net

Subject: 
        arc-overs!
  Date: 
        Sat, 12 Apr 1997 17:32:42 -0400
  From: 
        chuck seguin <seguinc-at-algonquinc.on.ca>
    To: 
        tesla-at-poodle.pupman-dot-com

Good afternoon everyone,

Here is a frustrating event I have a challenge with.

While I was tuning my last coil design, and pushing the envelope a
little a witnessed a nasty arc-over from 
the bottom of my secondary to the ground of my primary.  Here is the set
up.

Cp=75.7 nF
Lp=45 uH, flat spiral
Ls=125.7 mH, 113mm x 667mm

static spark gap
DC supply 15kV, operated variably using a 120V variac

The system is designed to operate at 75 kHz.  My primary starts at 1/4"
away from the secondary coil form, 
1/4" interwind spacing, 11.5 turns, 1/4" soft copper tubing.  The ground
start nearest the secondary coil 
form.  Since I operate at DC I have an actual ground for the power
supply and a seperate RF ground to the same 
AC mains ground.

When I operated the system to its best, I had lifted the primary to
roughly 1.5" away from the bottom of the 
first winding on the secondary; in the hopes of improving the
primary-secondary coupling.  This is where the 
arc-over happened.  The arc started about 1.5 turns into the secondary
and smacked into the ground connection 
on the primary.  This persisted if I kept the voltage high enough.

What surprises me is that there would even be enough potential 1.5 turns
into the secondary to cause arcs to 
ground on the primary.  This was a high current arc as well; very
blue-green because of the copper vapor and 
very thick and bright.

What am I missing here?

Chuck