TC as pulse transformer

From:  Greg Leyh [SMTP:lod-at-pacbell-dot-net]
Sent:  Thursday, June 04, 1998 5:58 AM
To:  Tesla List
Subject:  Re: TC as pulse transformer

Jim Lux wrote:

> And, finally, the energy in the secondary could be dissipated in
> creating a spark, which we would consider desirable.  The trick, then,
> is to set up the secondary LCR circuit such that it and the spark
> together are critically damped, because this will provide the maximum
> energy transfer into the the loss (i.e. the spark).

Indeed, although relying on a non-linear resistance to critically
damp an LC ckt might be tricky.  I haven't as yet seen a TC whose
output current waveform was critically damped; way underdamped seems
to be the rule, hinting towards higher Z secondaries.

> Returning to making big sparks. We know from laboratory research that
> making a big spark requires a slow rise time voltage pulse (many, many
> microseconds, if not milliseconds). We also know that we want to get
> energy from our storage reservoir (the primary cap) into the
> transformer, and then open the switch. This is starting to look like the
> desired switch is something that is unidirectional, reasonably fast and
> low loss. Perhaps a thyratron or an SCR?

An active switch will be the ultimate way to get the most out 
of a TC -- I've noticed that on Electrum if I can get the gap 
to quench on the first beat, the arcs are about 25% longer and 
capable of gnd strikes.  
It would be nice to avoid the hazards of high speed armatures
as well...The Electrum rotors store 211kJ of rotational energy
at full speed, enough to lift a 150lb adult 1000ft in altitude.

Thyratrons can handle the kV and the di/dt, but fall far short
on the Iavg rating (typ a few amps -- a TC carries 15-20A avg).
The plasma quench times are tens of uSecs typ, and even longer
for the higher current deuterium tubes.

Optimised GTO's can take the di/dt and Iavg, but only come with
Vdrm (off-state) ratings up to 4500V.  Stacking is therefore
required, with isolated gate drive circuits.  Just the gate drive
alone requires 200A per device, with 1uS risetimes. Dv/dt ratings 
of 200V/uS also limit the practical quenching times.

Ironically, active switching for disruptive TC's is likely to 
be practical only on larger scale coils, due to the quenching
limitations of the switch.