Magnetic pulse Tesla Coil

From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Sunday, April 05, 1998 7:11 PM
To:  Tesla List
Subject:  Re: Magnetic pulse Tesla Coil

Hi Scott:

> From:  Scott Stephens [SMTP:stephens-at-enteract-dot-com]
> Sent:  Sunday, April 05, 1998 3:33 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Re: Magnetic pulse Tesla Coil
> At 09:46 PM 3/29/98 -0700, you wrote:
> >From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
> >To: Tesla List <tesla-at-pupman-dot-com>
> >Subject: Re: Magnetic pulse Tesla Coil needs megawatt directional co
> >
> >Hi Scott,
> >           I printed this post off the HV List and carefully read it 
> >during the weekend. I have a few comments, the first of which that it 
> >is a little difficult to tell exactly where your 10mH inductor fits 
> >into the finished item (apart from being an energy store).
> >
> Fet switch has source - ground, drain - inductor 1, inductor 2 - +200V

That I gathered.
> >> From:  Scott Stephens [SMTP:stephens-at-enteract-dot-com]
> >> Sent:  Thursday, March 26, 1998 7:52 PM
> >> To:  tesla-at-pupman-dot-com
> >> Subject:  Magnetic pulse Tesla Coil needs megawatt directional coupler
> >> 
> >
> >> A FET switches a 200 volts across an inductor of 10mH for 2.25 milliseconds
> >> (ms); 200V for 2.25ms will charge up to (200V=10mH di/dt) or 45A, and store
> >> E=.5LI^2=10J. This 10mH inductor will take (if I'm guesstimating correctly)
> >> a slug of volume around 2 cubic inches. Far more compact than the cap.
> >
> >The slug is quite incapable of storing that amoung of energy. Most of 
> >the energy is stored in the air around the slug. The gap is so large 
> >that you would be better off with no core (and non-linearities) at 
> >all IMHO.
> Realy? Ferrite, with mu-eff=1000? I'll find out.

Try this reality check: what happens if you complete the magnetic 
circuit by *adding more* magnetic material to form a closed loop? 

> >> But how do you get the energy out in a form that can 1) drive a tesla
> >> primary and 2) not destroy a wimpy FET?
> >
> >I am not at all clear on the total setup. Could you post a simple 
> >ASCII diagram please? Forget jpgs unless they are uuencoded. My 
> >mailer can't handle the various arcane formats that come my way. 
> >
>           +200V
>             |
>           __|
>           B
>           B 10mH
>           B||
>     |-----B||B-----BBBBB----------------B
> ---||<-    ||B   Hard ferrite           B 10ohm Tesla primary
>     |--    ||B   magnetic saturating    B
>       |    ||B   switch                 |
>       |  core|    80 ohms off           |
>      GND     |    2  ohms on           GND
>             GND
> >You can do an awful lot better than 1/3. A HV diode is typically used 
> >as the steering element in flyback designs.
> Where can I find a HV diode that can handle around 2KV at 2KA!

Westcode amongst others make them. However, recovery times are not 
stunning. In fact, I have plans on the backburner to use some of 
these plus some Westcode thyristors to replace the gap in a lo-F 

> >> 3) (extra credit) Save or conserve any reflected energy from the Tesla Coil,
> >> via the reflected port on the directional coupler. Another directional
> >> coupler? A circulator?
> >
> >Try an energy recovery winding. An additional benefit is that it can 
> >be used to clamp Vds to a specific voltage above Vsupply if the 
> >seconday ever becomes unloaded. 
> Energy recover? Where & how?

It is an additional winding on the transformer core. It is connected 
between the gnd and 200V rails with a diode in series and winding 
sense set such that it passes current during the flyback period. 
Flyback voltage appearing across the FET is determined by the turns 
ratio between the primary and this extra winding. 

> >Perhaps I'm misreading what you are trying to do. A diagram would be 
> I've been reading about things like transductors and saturating switches.
> Because the magnetic diode's core saturates, at high currents it acts as a
> low impedance. After 1/3 to 1/2 the energy get absorbed by the coil, the
> reflected  energy returning to the magnetic diode won't have enough energy
> to saturate it. Kind of like a high ocean wave can wash over a wall, and
> then the wave ripples in a holding pool.

Interesting idea. I think a system using the ordinary old primary 
capacitor would be a lot easier to implement.