Wednesday, August 26, 2009

"The authors report an estimated pulse energy of greater than 15 J in a 100 ns pulse with a peak power of about 100 MW. They state: 'Accurate measurement of pulse energy was difficult since the laser energy produced a plasma in the thermopile used.' " 

What that translates to is that the "thermopile" (a device routinely used to measure laser power) got "blowed up good" -- sustained physical damage, destroying the measuring head.  Ironically, blowing up sensor heads during the development of a new laser design is a common sign of great success.


I have a knack for finding/saving useful parts for which documentation is at best hard to find and at worst, flat-out nonexistent.  For instance: I have a little trigger / igniter circuit board out of a very old American Laser Corporation model 60 argon laser head I used to have.  That was an ex-laser even before I got my mitts on it, and long ago I cannibalized bits out of it and gave the rest away as a collection of laser head parts.

I saved the little igniter board because it was a self-contained trigger pulse generator that "might come in handy some day", with all parts including trigger transformer.  As it happens, I now find myself in need of a small trigger pulse generator.  I pulled it out and began wondering what voltage to feed it to see whether it still works or whether I need to replace one of the parts on it. (all except for the unknown trigger transformer are dirt cheap, readily available parts.  The trigger transformer is very unlikely to have failed.)

Fortunately, the thing is simple enough (12 parts total) that I can trace and redraw the schematic without much difficulty.  Armed with that and the component values (thankfully, all but the trigger transformer have readable numbers or color codes, and I can guesstimate most of what I need to know about that transformer by a few simple measurements).  Naturally, one of the parts was discontinued 20 or 30 years ago, but it's a general purpose UJT, so if by chance that one turns out to be bad, I can probably find a replacement.  For all I know, the whole board may be perfectly good.  I just need to know how much DC to feed it.  It will be in the neighborhood of a few hundred volts.

Sometimes saving parts for ten years pays off.  I have been wondering how I was going to lay hands on a decent trigger transformer.

Oh yeah: I had been half-hoping it would turn out to be practical to trigger my new spark gap switch directly from a trigger transformer, if I could just find one with enough voltage and a fast enough rise time.  After tracking down almost every manufacturer of such devices in existence and reading all the spec sheets, it turned out that there is zero chance of multi-channeling such a switch from a trigger transformer.  They are all too slow (due to the inductance of the ferrite core) and the highest voltage unit I could find was about 40kV.  That might be enough, but I'd really rather have about 60. (sounds like a lot, but the current and energy for these trigger pulses is really small.)

Thus, last night's mention of making a small, fast Marx generator to trigger the switch.  But the Marx itself needs a triggering device for its very first stage, and that will be a cute (tiny) little trigatron I'm gonna build, which will in turn need a high voltage trigger pulse of its own (which can be fairly slow rise time, ie; a trigger transformer will be just fine).

This is the nature of high energy pulsed power systems.  You start with whatever whacking huge exotic switching device is required to handle whatever it is you're doing with the main energy store.  But such things nearly always require very fast, very high voltage trigger pulses, so then you have to have another, somewhat less exotic device to create THAT, and so on, backwards, with each preceding triggering or switching device being less exotic, slower, and of a lower voltage than the latter, until you arrive at a simple push button, relay, or logic input that starts the whole chain reaction.  Of course, the entire chain reaction is typically over in a small fraction of a second.

For instance, the original design for my big pulser, as I originally received it, worked like this:

1. Operator presses the "go" button, which starts the main capacitor bank charging, along with several other high voltage capacitors used in the various triggering stages.

2. When the desired voltage is reached, a Simpson Relay Meter closes a pair of light duty contacts.

3. The meter relay contacts close an intermediate relay having contacts rated for higher voltage and current.

4. That relay turns on a small pentode (a type of vacuum tube often used for light duty switching).

5. The pentode switches a small capacitor (charged to a few hundred volts) to a pulse transformer, the output of which is a pulse of a few thousand volts which is delivered via HV cables to a hydrogen thyratron inside the capacitor cabinet.

6. The thyratron fires, delivering energy stored in each of six capacitors, to each of six mercury ignitrons.  There was one ignitron on each main capacitor.  The ignitrons were the final, big switches in the original design.  They are obsolete, lower-performing, fragile, and very expensive devices, and all of them were inoperative before I got my hands on the thing.  Since those particular tubes cost $1500 each, I'm replacing them with a single railgap switch, but that's another story.

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