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Modelling a Magnifier


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From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Sunday, March 08, 1998 4:37 PM
To:  tesla-at-pupman-dot-com
Subject:  Modelling a Magnifier

Hi all,
        Here is an excerpt of an offlist post to Jim Fosse. I thought
it might be of interest to the SPICE modellers. 

<include from other post>
> Believe it or not, I am experimenting with a magnifier right now. I 
> am working up to repetitive firing with a couple of neons but am 
> taking full advantage of the opportunity to double check my thoughts 
> on tuning requirements (they were 100% correct), comparing Vout with
> Ep and Csec+Cterm etc. etc. and a host of measurements. I have to do 
> some photographing and storage scoping very soon. I had the resonator 
> throwing bolts the length of the windings and slightly greater at 
> 1BPS.
> Until I popped a corona control shield at the bottom of the extra 
> coil, I was getting 
> the odd bolt up the winding (encouraged) by corona streaking off it. 
> In fact at one point there was a graceful arc of corona from the top 
> of the winding stretching down to the bottom portion of the resonator.
> Tuning is far touchier than the old 2-coil. I'm sold on small 
> maggeys. For one thing, you can get most of the capacitance in the 
> topload. This one is about 2 1/2"  diameter and winding length is 
> about 11" with 0.45mm wire closewound. Lowest frequency I can take it 
> according to my 5 skin depths rule is around 550kHz which allows 
> around 35pF of total capacitance in the extra coil system. Using Vout
> = sshot inches x 35kV seems like a good guide. The first experiment 
> was to get it tuned and running with as much Vout as the winding 
> could stand. Expt #2 was to calculate Flowest and the required 
> amount of topload and then to build another primary coil to tune with 
> the same cap and up primary voltage to get Ep sufficient to match the 
> winding length Vout. So far, everything is going according to plan. 
> It is a touchy beast to fine tune though. Tuning the primary to the 
> extra coil is definitely the way to go for tight-coupled drivers. 
> BTW, I expect this system to work very well with a static gap.
<snip> 

     I might add that the brilliance of the output discharge is 
heavily dependent on capacitance in the resonator system and seems to 
have little to do with the comparatively high running frequency.

    Important notes: in changing the primary, I used a smaller 
diameter (fits inside the driver secondary) and attempted to keep
k up by making the primary longer to compensate. The previous primary
also went inside the secondary but fitted snuggly into the tube on 
which the secondary is wound. Ns/Np has dropped from around 10 to 
around 4. Inductance went up from about 4.5uH to around 9uH. I won't 
know how significant the turns ratio is until the system is fine 
tuned with the larger topload but so far system ouput voltage appears 
to be obeying the lumped rules pretty closely which suggests that 
driver turns ratio is not all that important, at least for this mode 
of operation.

     Final plans for this are to run it with the larger topload from 
two 15kV 60mA transformers with the gap set to around 15kV. This 
evening, I plan to put the smaller topload back on and up Vp until 
the corona shields flashover (see if it gets past the winding length).

     To facilitate tuning, I have made up a number of terminals fo 
differing sizes from styrofoam blocks that can be placed on top of 
the resonator in various combinations. Although it causes Vout to 
vary it is easier to tune than trying to tap a buried primary.

Malcolm