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Magnetic pulse Tesla Coil needs megawatt directional coupler


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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

I have been researching methods and striving to build a compact, high power
and even solid state tesla coil. So far I've discovered numerous reasons why
this is not a trivial task.

I hope to avoid re-inventing the wheel, loose money, damage myself and my
environment with hot plasma, high voltage, ionizing and non-ionizing
destructive radiation, chemical pollution and violent neighbors enraged over
another lab accident, (so far I've survived all the above).

Capacitive discharge TC's may store 1 to 20 Joules of energy per pulse or
discharge. Since E=.5CV^2, a typical cap might be 50nF at 20KV
(10J=.5(5*10^-8F)(20,000V)^2). A 50nF 20KV poly-oil cap could require 30mil
poly, of area C(nF) = .013A(sq. inch) (derived from
Cpf=.223EeffArea/thknessMils) so the cap needs around 5000^2", which is a
roll around 1 cubic foot (a roll 10" high and 2" thick with 20 layers of
50mil/layer -at- 10" avg. diameter). Filled with oil, it will probably weigh
30lbs. A big, heavy, ugly, gooy mess.

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.

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?

The FET's ON time is not too significant, since the inductor will naturaly
take time to charge up - its rate of change (dV/dt) is limited by its
inductance. But the FET's OFF time determines the rate energy will flow out
of the inductor. I want to open the FET in 5us (microseconds) so it will
drive a 100KHz TC 2ndary.

Now the inductor absorbed 10J in 2.25ms resulting in around 5000 Watts
power. It will release 10J in 5us or 2 megawatts of peak power. After 5us of
the FET open, the inductor which 5us prior had 200V across and 45A through
now has (V=.01H 45A/5us=90,000V) or 90KV! Poor FET went POOF! Stopping that
current was like trying to stop a freight train.

One solution would be to place a spark gap on the inductor, and let the gap
fire to deliver its charge to a TC primary. All the energy would need to be
absorbed in the first cycle, as the next oscillation a nasty voltage level
would appear at the FET drain (FET is connected common source). So a high K
value, like .999 would be necessary with a hot spark mandator.

Along with mega-watt TVS or transorbs on the FET. They eat power. Yuk.

What is realy needed is a directional coupler. A directional coupler between
the FET-Inductor and TC primary-load would allow energy to flow from the
inductor to the load, and redirect any unabsorbed energy from the TC load to
a resistor. With a K of .5 such as with a magnifier coil arangment (the
switched inductor would serve as the magnifier coil) maybe 1/3 of the energy
would make it to the 2ndary.

I know how Tesla coils are built. And microstrip hybrid directional
couplers, but how do I:

1) make a directional coupler to tranfer energy from a flyback inductor, to
bottom-feed a Tesla Coil? Guanella hybrid transformer? What material &
construction issues happen with kilo-amp currents?

2) make a coupling network between the flyback inductor and directional
couple that rejects/reflects 2.5ms charging voltage/field and transmits 5us
flyback pulse? 'Simple' LC network?

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?

Surely this has been done before. I've seen a neat circuit called a magnetic
pulse compressor/magnetic switch. Uses inductor saturation as a switch for
high power. A good example of one is at Allied Signal's web page, the one
that features that ass-kicking Metglas core material.