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Re: [TCML] Spark gap Resistance



In a message dated 11/19/2007 10:14:29 P.M. US Eastern Standard Time,  
bartb@xxxxxxxxxxxxxxxx writes:

I think  Chris brings up a "very" valid point! The problem is, it is 
experience  that drives the idea of a higher surge impedance. There's a 
reason it's  termed "surge" impedance. There is a difficulty at quenching 
after the  first notch. Those who have done this have reported in the 
past (from my  memory) that they had better sparks lengths on 2nd or 3rd 
notch quenching  as compared to 1st notch quenching.
Those experiments which obtained 1st notch quenching did so by  increasing
losses by using multiple gaps.  It was a bad trade off.  I think  the reason
it's better to let a coil quench at 2nd or 3rd notch if it "wants" to, is  
because
most of the spark growth occurs during the first transfer.  So  the extra 
transfers
(during a single bang)
don't do much except waste some power and steal some cap charging  time.
If one forces the 1st notch quenching by using lossy gaps, then even that  
first
important transfer is weakened.  This reduces the spark length.   There is
evidence that large powerful coils more easily quench on the first  notch.
For example Ed Wingate's magnifier quenches on the first notch at full  power.
But at low power I think it quenches on the 3rd notch.  I seem to  remember
that the "effective" coupling for his magnifier is about k = 0.2 maybe  less. 
 It's more
or less the same as a classic coil.  The 12 point series rotary may be  
helping
the quenching.  It would be interesting to monitor the quenching using  a more
typical 4 point series rotary.  In any case the lower frequency of a  larger 
coil
makes the notches wider, so it's easier for the ions to de-ionize during  the 
notch
and let the gap quench.  



Chris, several years ago, 1st notch quenching was the assumed  ideal and 
we tried to do that for all the reason's you stated. What was  found is 
that 1st notch quenching was not easy. Then, we found that when  it 
occurred, it wasn't "wonderful". How could that be? Well, losses of  course.
Chris is trying to quench faster by using a fast rotary.  That won't  happen
because the quench is not controlled by the mechanical dwell time.
The spark at the gap will arc before the electrodes line up, and  stretch
out if needed after the electrodes pull apart.  To really stretch the  spark
and force the quench would require a rotary gap construction well  beyond
what anyone has tried.  Generally rotaries are more for timing when  the
spark occurs rather than for quenching purposes.  Quenching is more 
about draining the energy out of the system quickly.  For the most  part,
air streamers don't drain the energy that quickly, and that's the  main
problem preventing a fast quench.  In any case, as you said Bart, a  fast
quench is not really very important.  It's interesting to consider  the
various DRSSTC designs and SISG coils which have lower switching
losses.  They may be more efficient because of that, but the  improvement
is not huge compared to a spark gap coil.  This shows that the gap  losses
(at least the part of the gap losses that matter) are not that large.  



It's difficult to figure out and in my mind, it's still "not"  figured 
out. We do know that when the surge impedance is increased due to  higher 
inductance, we can get better spark output. But, there of course is  a 
limit. It is counter-intuitive to physics when all the pieces of the  
puzzle are not accounted for. The only way "I" personally can explain it  
is that the losses incurred during energy transfer in a single notch  
arena are huge. 
If the single notch quenching is "forced" by deliberately increasing  the
gap losses, then yes the gap losses will be high.  It's best not  to do that.
It's best to accept a 2nd or 3rd notch quench, but keep the gap losses as 
low as possible.
Forcing the quench in that way is really just wasting the energy in  the
gap so there's mostly none left by the time the 1st notch arrives.  
 

Now, are  those losses in the gap? 
If first notch quench is forced by using a lossy gap, then yes the  losses
are in the gap.  But in all coils the losses are shared between the  gap,
the primary and the secondary, in some proportion.
 

Are they  also shared 
in the secondary or primary to a large degree? The question is  "where 
are the losses and what is their distribution" in this 1st notch  quench 
situation?
Again, if 1st notch quench is forced by making a lossy gap, then
the extra losses are in the gap, or mostly in the gap.  If however the 
quench could be forced
by an air blast or magnetic field or something similar, then there  would
be a benefit because the caps would have more time to charge.   But
the spark might not get longer, because energy is not being
transferred faster.  We can transfer the energy faster by increasing  the
coupling, but then the sparks won't be able to  drain out the  energy
fast enough.  It will again become more difficult to quench on the 1st  notch,
and an even stronger air blast or magnets, etc., will be needed.  
 
The reason that tighter coupling increases spark lengths is because
the energy is transferred faster, before too much of it is wasted in  gap 
losses.
It's all a matter of getting as much energy to the secondary during  the
first transfer as far as I can tell.  This energy will produce the  spark 
length,
then if some energy reflects back to the primary, the spark length has  
already
been produced.



Were talking about high energy pulse currents. If there is an  escape 
route, high energy pulse currents will find it.

It seems to  me that what we are doing is increasing energy transfer time 
to a degree  in which the secondary and spark gap can "handle" the energy 
as a combined  system. I believe that when we attain first notch 
quenching, we are simply  releasing energy that is not being accounted 
for. It's not getting to the  sparks, so it's a loss somewhere else.
Yes, when the quench is forced by building a lossy gap, then the gap  wastes
more energy.  Since the energy is now used up more quickly, it lets  the
gap quench more quickly.  But if first notch quenching can be  obtained
by using air blasts or something similar but without making the gap
lossy, (and without reducing the coupling) then a benefit may be  seen.  
Reducing the coupling has a similar effect on spark length, as using a  more 
lossy gap.  Both waste energy before it can get to the  secondary.  
 
John



Take care,
Bart

FutureT@xxxxxxx wrote:
> In a  message dated 11/19/2007 6:54:37 P.M. US Eastern Standard Time,  
>  list@xxxxxxxxxxxxxxxxxxxxxxxxx writes:
>  
>  Chris,
>  
> If the current is less overall, then the gap  losses are  lower.  Using a 
high 
> impedance
> primary  results in less overall current and less overall losses.  When  
more  
> inductance
> (more turns) are used in the primary, the  inductance increases more than  
the
> resistance increases, thus  the primary losses are reduced.  The Q is  
higher.
> The  result is that
> both the gap losses and the primary losses are  reduced.  Of course  this 
only
> works up to a point.  At  some point the secondary wire will be  too
> thin and will show  high losses.
>  
> Generally low frequencies are believed to  be more efficient in  producing 
> long sparks.
> Maybe  something in the range of 30kHz to 150hHz.  Also at higher   
frequencies,
> it's harder to achieve a first notch quench.  The  sparks themselves  may 
grow
> better at low frequencies.   
>  
> Large coils are generally more efficient than small  ones.
>  
> Tank caps generally are able to provide their  current fast enough for
> TC operations.  
>  
>  Generally high breakrate coils need more input power to produce a   given
> spark length.  It's not known exactly what breakrate is  best.  It  may vary
> somewhat among coils.    Somewhere between 100bps to 200bps  usually
> works well.   
>  
> John
>
>    
>>   Sorry for the amount of "ponders" in  this  mail.  It is just my 2cents 
>>     
>  worth 
>   
>>     that a higher  frequency with less primary turns and a faster RSG 
would   
>>    overall reduce losses far more than anything   else.
>>     
>
>  Chris
>
>




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