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thyratrons for long pulses

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saturatEd

Thu Nov 13 2008, 03:16PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
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Posts: 14

Greetings to all

I'm currently working on a hypothetical system (playing with math at the moment, and circuit diagrams), that has a requirement
of high current switching devices, conducting long pulses (by long I mean about 0.5 ms)
Now, the system would possibly be in the presence of gamma radiation, so I limited myself to electronic tubes

The most obvious solution is a thyratron, but they are normally switching much shorter pulses. Most current ratings in datasheets of
thyratrons are given for maybe 2-3 microseconds.
And there is no derating method given for prolonging pulses.

The current requirement is 250A (but could be made 125 with two parallel thyratrons) , and the pulse width is
0.5 ms.

The pulses are not constantly repeated. There are two pulses followed by a longer pause.
The time between these two pulses is 1.5ms, and then there is a longer pause of minimum 2 seconds (but would probably be much longer than that)

I think this makes it a bit easier, because the tube has time to cool down.

But I still can't get any quote or practical example of any stock thyratron being used for such a thing, though I assume
many of them could do the job.

Now I have two questions about this.

first one is, can someone confirm that an existing thyratron (past or present model) could pass 250 (or 125) amps of current
for 0.5 ms, and not significantly reduce its typical life.

and second question is. Is there a reliable way to calculate maximum current for a given pulse length , using data provided by the manufacturer.

For example, I thought about using the rating of amount of charge passed in a single pulse. Is that a reliable way to calculate current vs. pulse width?

thanks

rp181

Thu Nov 13 2008, 10:42PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

I dont think gamma radiation will affect semiconductor's, maybey SCR's will work?
You could alway make your own, its been done for large capacitor banks.

What exactly is the project?

saturatEd

Fri Nov 14 2008, 12:03AM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
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Posts: 14

The project is a kind of a magnetic accelerator and transport tube for diamagnetic or superconducting particles.
It involves a series of magnets which fire in sequence, making a stable moving magnetic trap.
But the basic idea is transporting antimatter, to some remote annihilation point (a reactor), and this is where the
bursts of gamma rays come from.

So, all that stuff about tube and EMP , is that all a myth?

...

Fri Nov 14 2008, 12:38AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

It all depends on how much gamma rays you are planning on having... It probably takes a few orders of magnitude more to affect a thyratrons than a mosfet, but by the time you have reached either of those points you are also killing any computer processor in the neighborhood...

In any case, thyratrons are nice to use once in a while, and the nice thing about tubes is that the only think that limits their power dissipation is keeping the electrodes from melting, so as long as your duty cycle is reasonably low any moderately large thyratron would probably do fine. You just need to find one that is big enough so that you don't physically blow up the tube.

saturatEd

Fri Nov 14 2008, 04:22PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
Location:
Posts: 14

Yes, well, it has two sections. One is the accelerator, which is kept far away from the annihilation point, which is driven by IGBT's, and the other part which simply
focuses the "projectile" path and keeps it at a steady path all the way to the annihilation point.
This second part is suppose to be ran entirely by tubes because parts of the tunnel come within a couple of meters from the actual reactor. Due to the very potent nature of
antimatter "fuel", making continuous fuel supply difficult (dosage of small particles is difficult), the reactor is operated in pulses (sort of like throwing chunks of coal into a furnace if you will), which means you get bursts of megawatts worth of gamma radiation. Most of it is absorbed by the heat exchange tungsten walls, of the chamber, but even if you get 0.1% to come out, it will be enough to fry electronics.
All the computers are kept on the far end of the transport tunnel , where the IGBT accelerator is.

saturatEd

Sat Nov 15 2008, 01:27AM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
Location:
Posts: 14

Oh, I just thought of something...

I gnitrons

I looked at some specs, and it seems they can handle about 100ms pulses and currents up to 700kA ( lightning would be ashamed).
The only problem is at their current ratings they last about 1000 shots. the lifetime is increased by derating the current by a factor of 3.
At required 200-300A this gives millions of shots.

I'll have to do a lot more research, as I know very little about ignitrons, but I think this might be it.

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