RE: SSTC modelling- any math geniuses?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Steve,

On 25 May 2004, at 17:03, Tesla list wrote:

 > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >
 >  >That is the real problem and gets much worse when the streamers
 >  >connect with a target.
 >
 > Ordinary untuned primary SSTCs that I and others have built, seem
 > quite happy when the arc connects to ground. The current draw
 > decreases, and in the limit when the toroid is shorted to ground, the
 > current draw from the DC bus is nearly zero.
 >
 > OTOH, the new DR and ISSTCs are capable of going "nuclear" when
 > shorted out by an arc to ground. The low impedance at the resonator
 > top transforms to a near infinite impedance at the base, and the tuned
 > primary sees no load, allowing the primary current to ring up without
 > limit.
 >
 > We get round this one by running them in pulsed mode, ie the inverter
 > operates for say a 200uS burst every 10ms. Even if the secondary was
 > totally shorted, 200uS is not enough time for the current to ring up
 > to _really_ scary levels. But it can still cause a higher than normal
 > current, hence not a good idea to totally short one of these coils out
 > with an arc to a very close by target. (A long arc to ground has more
 > resistance and can dissipate power safely.)
 >
 > The primary current can also increase without limit if breakout does
 > not happen, but this is less of a problem, it usually just ends in a
 > pri-sec flashover rather than IGBT-Ma-Geddon. After all, the secondary
 > voltage increases without limit in this case too :)))
 >
 > Anyway, somewhere in between these extremes of "totally short" and
 > "totally open" there is a streamer load impedance that leads to
 > optimum power transfer. Richie and I are working on a design procedure
 > that will let you start by specifying what length of streamers you
 > want, then work back to find the primary L/C/k you need to match your
 > inverter to that particular streamer load.
 >
 > It will also tell you the peak primary current, hence what size of
 > IGBTs you need. And if that turns out bigger than the ones you've got,
 > you start again with a smaller streamer length :)

When I spoke of difficulties, I was referring to the difficulty of
defining a streamer impedance (which as we all know depends on
current), rather than ensuring MOSFET survival etc. It may be that
for air streamers, there is a reasonably well defined value (Terry
and Greg came up with values some time ago). Perhaps the streamer
tends to adhere to such values by lengthening, broadening into
branches etc., a property of the discharge medium (i.e. air at about
1 atm).

      As far as defining the characteristic impedance of the resonator
goes, a better option is to consider the topload as a termination
rather a distributed part of the line. The Corums did all this a long
time ago and produced Smith Charts to go with their writings. A
problem which occurred with their analysis when considering
disruptive coils was that their output voltage calculations were
applicable only to CW coils and invalid when applied to disruptive
discharge operation with the coil being fed with a charged capacitor.

Malcolm