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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
Mary: The problem with thyratrons is that they take a relatively long time to de-ionize, so they can only be used in a disruptive mode, unlike a vacuum tube that can directly generate RF power at the resonant frequency. Richard Hull built a hydrogen thyratron Tesla coil years ago, but the performance he got was pretty poor compared to even a VTTC.
I can't say I'm familiar with the use of thyratrons in the kind of topology you seem to be suggesting here, Steve.
I had imagined a conventional Rayleigh line PFN where Zout = Zintcp but had clearly misunderstood the question, which I thought was to solicit information about disruptive discharge technologies.
I don't see the dionization time of a typical H2 thyratron - say 50us - as being problematic, as this will allow, say 1000 - 5000 1us pps, (depending on heat dissipation). Surely anode current rise times of 1000 - 2500A per microsecond (typical of small and medium H2 thyratrons) would suggest a role for the hydrogen thyratron in disruptive discharge devices.
Cold cathode thyratrons like the pseudo-spark switch are all the rage in ultra fast power switching technology if your pockets are deep enough.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Steve McConner, if a disruptive coil using a thyratron and a PFN with a proper pulse width were used to exite a parallel primary that it may work. it may be a bitch to set up but it may be pausibe to get a 1 to 1khz interupion may be achieved. they used to use that type of setup in radar units of old.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You can't drive a Tesla coil off a PFN. The reason is that a PFN is designed to produce a single flat-topped pulse, the Fourier transform of which has the energy spread over a broad spectrum of frequencies.
A Tesla coil on the other hand, needs the energy concentrated into two spot frequencies. If we take the inverse Fourier transform of that, we get the familiar sine wave oscillation with envelope.
The two waveforms look nothing like each other, either in the time or the frequency domain, therefore most of the energy supplied by a PFN would be wasted. This is just hand-waving, but I think you can prove it formally with the Fourier transform and Parseval's theorem.
The Tesla coils that we call disruptive use topologies very similar to the old radar modulators. My Tesla-2 spark-gap coil was heavily inspired by the chapter on spark-gap modulators in Craggs and Meek.
But it's important to understand that the Tesla coil primary circuit itself is the "PFN", and in order to generate the right waveforms to excite the secondary, the switch current needs to reverse, which it never does in radar modulators.
The VTTC is something completely different, nothing to do with a radar modulator at all. It's more like a transmitter than a modulator. The unsmoothed HV supply or staccato circuit is the "modulator".
If you made a "disruptive" VTTC with a GMI-90, it would be like the transmitter from an old Chain Home radar. Maybe 50 kilowatts peak power, less than 1kW average, because of the GMI-90's modest plate dissipation rating. And it would need a big storage cap on the HV DC supply, and some kind of pulser to modulate the oscillations.
In an attempt to answer a previous question: You could probably have taken the spark gap out of my old Tesla-2 and fitted a hydrogen thyratron or GMI-90 in its place, with a string of diodes across it to handle the reverse current, and it would have worked about the same.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Steve wouldn't you agree that any tesla can be excited with energy at the proper frequency. Even a VTTC operates in a class C configuration and is a device that is on in the positive cycle. A DRSSTC uses a squarwave to ecite a seriese resonant circuit. and a regular SGTC uses a nonlinear pulse for excitation. From my experience if the frequency and a pulse width that is equal to or less than 180 deg of the cycle can be used to excite any resonant circuit system. cirtanly harmonics can be genererated but if the pulse width is syncronis with the fundimental or first and second harmonic energy can be sloshed in a resonant circuit and be transformed by the secondary. what do you think
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
teravolt wrote ...
From my experience if the frequency and a pulse width that is equal to or less than 180 deg of the cycle can be used to excite any resonant circuit system.
Yes, that's true. But as was already pointed out, if it takes 50us for a hydrogen thyratron to deionize, then it can't efficiently generate energy at the coil's resonant frequency. It might be able to do 1000pps, but it can't do 100,000pps to excite a 100kHz coil, because then there are only 5us available for it to go back to the non-conducting state.
Derek Woodroffe once showed an interesting variation on the OLTC, where the switch was run at a very high breakrate that was a subharmonic of the coil's resonant frequency. So, f/3, f/4, f/5 etc., instead of just 100 or 400pps. Maybe you could do something like this with a thyratron too.
Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
Dr H: It would be something like that:
I would use G2 as feedback grid and G1 for pulsing the tube. The pulser would be something like a miniSSTC with Ton < 3 microsec (as specified in datasheet). The variable spark gap controls the voltage on the G1 (to not overvolt the grid).
It is just a first idea of course. New modifications can be done.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Steve McConner wrote ...
teravolt wrote ...
From my experience if the frequency and a pulse width that is equal to or less than 180 deg of the cycle can be used to excite any resonant circuit system.
Yes, that's true. But as was already pointed out, if it takes 50us for a hydrogen thyratron to deionize, then it can't efficiently generate energy at the coil's resonant frequency. It might be able to do 1000pps, but it can't do 100,000pps to excite a 100kHz coil, because then there are only 5us available for it to go back to the non-conducting state.
Derek Woodroffe once showed an interesting variation on the OLTC, where the switch was run at a very high breakrate that was a subharmonic of the coil's resonant frequency. So, f/3, f/4, f/5 etc., instead of just 100 or 400pps. Maybe you could do something like this with a thyratron too.
Well this does surprise me, Steve, as many thyratron data sheets cite shock excitation of tuned circuits as being one of their major applications. Nor is there any shortage of literature:
E. M. M. Costa RESONANCE ON TRANSFORMERS EXCITED BY SQUARE WAVES AND EXPLANATION OF THE HIGH VOLTAGE ON TESLA TRANSFORMER Progress In Electromagnetics Research B, Vol. 18, 205{224, 2009
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VTTC question
