Re: On sparks

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

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>

John-

No, you didn't misunderstand me--and I accede to your better knowledge on
spark-gap coils.  What you say makes sense as regards the residual heat
in the air; I wouldn't have thought it would have that much effect over
the time period of 1/120 to 1/240 sec but it seems as if it does.  But
now that I think on it, I do recall that my sparks tend to stay put at 1
spot on the toroid when I up the rate from 1's to 10's per second.

I think that my experience of shorter sparks with higher rep.-rates is
wholly caused by the decline in my storage-capacitor charge with
increasing current-drain.  I re-charge them via current-source power
supplies & so they will not fully recharge between "bangs". 
Incorporating some 43,000 uF to be recharged from the 115 V mains, I need
some sort of control to avoid unwanted welding of this and that.

I'm about to re-configure my primary to incorporate 3 MOSFET modules, 3
capacitor modules and just the 1 equivalent turn again.  If I get X
ampere-turns with my present set-up of 4M + 4M + 3T, then I will get
upwards of [(3/4 for the reduced loop-voltage) x ((3/1)^2 for the reduced
primary impedance) x (1/3 for the reduced quantity of turns)] = 2.25X
ampere-turns.  That, at the cost of ((3/1)^2 x 3/4) or 6.75 x the present
MOSFET current, which they should stand since I calculate the present
peak half-cycle current at ~8 A and they have an 85 A Idm rating.  Of
course, I will experience more drop in the MOSFETs which will nullify a
fraction of that.  So we shall see...

If I were to go to 6M + 6M + 2T, which I just have room for in my 24"-sq
"footprint" except for my big switching chokes, then I might see [(6/4
for the increased voltage) x ((3/2)^2 for the reduced primary impedance)
x (2/3 for the reduced quantity of turns) = (also) 2.25X; except that the
MOSFET current would then be (3/2)^2 x 6/3 = 4.5 x the present 8 A
instead of 6.75 x that.  Less MOSFET drop at the expense of doubling the
quantity of assemblies--maybe not a good trade-off.

Raising the peak current so much will necessitate reducing the on-time
proportionally, no doubt to ~1 ms or so, making for a thinner spark.

Ken

On Sat, 31 Mar 2001 22:02:47 -0700 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" 
> <FutureT-at-aol-dot-com>
> 
> In a message dated 3/30/01 11:50:42 PM Eastern Standard Time, 
> tesla-at-pupman-dot-com writes:
> 
> >  > I should say here that I define a low break
> >  > rate as something around 120 bps.  I define a high break-rate 
> as
> >  > something around 240 bps and higher.  The effectiveness of the
> >  > input power to produce long sparks seems to decline as the
> >  > break-rate is increased beyond 120 bps or so, at least in the
> >  > work I've done.  Of course if the break-rate is too slow, then
> >  > the benefits of the growth of sparks due to partial ionization 
> of
> >  > the air, is lost, or severely diminished, and the spark length
> >  > is reduced.
> >  
> >  I've had no experience with spark-gap coils but what you say 
> makes me
> >  think this:  At the higher break-rates, I wonder if you're not
> >  experiencing reduction in the voltage at which the gap(s) break 
> down, due
> >  to the residual temperature of the air, in the area of the gap, 
> remaining
> >  higher.  Absent that effect, I'd think that each break would 
> produce
> >  exactly the same spark regardless of the rate--because the 
> impulse of
> >  energy applied to the primary would be the same since the gap's 
> breakdown
> >  voltage would be the same.
> >  
> >  Ken
> 
> Ken,
> 
> In the example I gave, the input power remained the same, because
> I deliberately turned down the variac to maintain a constant
> input power at various break-rates.  Yes, the gap was firing at
> a lower voltage since I wanted to compare high and
> low break-rates at a constant input power. 
> 
> In other tests, I kept
> the bang size the same and increased the break-rate, and of
> course the input power increased and the sparks got longer.
> I measured the bang size (voltage) on a scope, so I know it was
> remaining constant.
> 
> If I understand your comment, you're suggesting that for a given
> bang size (same impulse size of energy), the spark length should
> remain the same regardless of the break-rate.  I can assure you
> that it doesn't behave that way in a disruptive TC.  A higher 
> break-rate
> causes longer sparks for a fixed bang size.  For instance, when I 
> kept
> a fixed bang size (verified by a scope) and doubled the break-rate 
> from 120 to 240 bps,
> the input power doubled, and the spark reached a steady state 
> which was 23% longer.  This greater length is due to the 
> re-ignition
> of previous hot ionized streamer paths.  Please let me know if
> I am misunderstanding what you're saying.
> 
> In contrast, when I kept the break-rate constant, but doubled the
> bang size (and the power input), the sparks got 40% longer.  This
> is why I like low break-rates.
> 
> John
> 
> 
> 
> 
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