1-turn coil inductance nomogram

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

Original poster: "K. C. Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

I don't know if this will be new to the List or not but I'll prevail on
Terry to put up http://hot-streamer-dot-com/temp/Loop-L.gif.  It returns
inductance of a 1-turn loop using the loop- and conductor-diameters.  I
found it in one of my ancient textbooks.

It prompts me to notice that the inductance of a 12"-dia. x 1"-tube
1-turn primary is just about 0.5 uH (and no doubt Terry reported
something like that for his OLTC).  That yields ~0.44 ohms reactance at
140 KHz and if I were to put 640 V across that, I'd need to switch 1450 A
in a reconfigured s.s. primary, thru a bunch of MOSFETs (assuming much
less resistance in the circuit).  So there's why I've gone to 3 turns at
present, suffering the consequences.

1/4" dia. tubing only increases the inductance to ~0.75 uH, resulting in
967 A.  

IRFP460A MOSFETs are rated at 85 A Idm -at- 25 C.  IXYS VMO 60-05F's, on the
other hand, are rated at 240 A Idm -at- 25 C.  The 460As are $2.80 each/25
(from Fortune) or 3.3¢ per Idm Amp; and I recall the IXYS being quoted
recently at about $40 a pop (unfortunate word but there it is) or 17¢ per
Amp.

Idm is the maximum for a 10 us pulse width, being limited by the
junction-temperature rise during that interval, so it's probably a safe
rating to use in choosing the quantity of transistors in this
application, perhaps holding the spark duty-cycle to a modest value.  

Hmmm...using the 1" tubing, 1450/85 x 2 x 4 = 136, 460As are needed vs.
1450/240 x 2 x 4 = 48, 60-05Fs.  

Hmmm...did I hear someone mention IGBTs?  Which ones are going to turn on
& off nicely in no more than 1 us, I wonder?

Ken Herrick