Re: the cure for racing sparks

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>




  Now my problem was that the very top winding of
> the coil arced the entire lenght down the coil to strike the ground
> connection of the secondary.  Is this understood to also be known as racing
> sparks??  It seems that this problem is simply lack of insulation (secondary
> not long enough to handle voltage) and perhaps the lack of field formation
> desired by a toroid(since i use a sphere).  


Precisely true... If you had a toroid instead of a sphere, two factors
would come into play that would reduce the flashover:

1) The toroid has a smaller radius of curvature than your sphere (esp if
the toroid is made of corrugated flex tubing, etc. as opposed to a polished
sphere).  Therefore, breakout will start earlier, at a lower voltage.  This
reduces the field along the secondary (i.e. the V/meter).

2) The toroid tends to shape the field in a more uniform way along the
secondary.  A bare secondary (or one with a sphere sitting on top) has a
real field concentration at the top turns, because the radius of curvature
(i.e. half the diameter of the secondary) is small.  with a 6" diameter
secondary, the roc is 3", and the breakdown voltage is going to be around
200 kV...


Coupling also comes into play...  High coupling puts a lot of energy into
the secondary quickly; raising its voltage so that breakdown occurs.  It
also raises it fast enough that a big streamer doesn't have time to develop
and suck up the energy, so the voltage actually probably goes higher, and
you get the characteristic brushy "lots of small sparks" effect.  If the
voltage gradient along the secondary gets high enough (say >20 kV/inch)
then surface/creeping sparks are a real likely result.

Taking a typical coil that's 4" in diameter and 20" long, with a 6" by 20"
toroid on top...  The 6" dimension of the toroid is going to set the max
voltage at the end of the coil to around 200 kV (=3 inches * 70 kV/inch)..
at that voltage, the gradient along the secondary is going to be 200/20 or
10kV/inch, which is fairly low.

But, the factor that limits the voltage due to ROC is the formation of
sparks, which suck charge and energy from the topload, keeping the voltage
down (i.e. Q = CV... ).  If you put energy into the topload really fast
(high coupling), then the sparks can't get enough charge out.  Also, they
do have a fairly high resistance, also raising the voltage of the topload.