High Voltage sub nanosecond switching

[Marco]

I think it would be cool to build a 3D scanner that can scan entire mountain. Artillery range meters can easily handle distances like 80km, so it should be possible to make 3D scanner that capable.

A diode ain't going to do that ... you're going to need mega or gigawatts of peak optical power.

[WarSim]

I’m looking for a part that can switch 4000 volts into a 50 ohm load within a nanosecond. The pulse duty cycle should not exceed 1 microsecond (80 amps for at maximum 1us). I was thinking of a mercury relay or a hydrogen thyratron, but the ones I have seen seem to slow or lack the specifications. Has someone used, or know of a part that is capable of this? Maybe a high power VHF/UHF transmit tube?

If I am reading your requirements correctly, there is no solution for your <1ns response time.  It is possible to make such a pulse but the time from stimulus to pulse is normally 2-4ns, twice your requirement. 
An old rule of thumb is under 2ns requires a pre-trigger. 
Turning on 4kv with ~200ps rise time is definitely possible but I believe you are talking about clamping a non characterized transient. 
I believe you will need to use a suppression method to extend the response time.  Hopefully your application can compensate for such colouration. 


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[Alexei.Polkhanov]

A diode ain't going to do that ... you're going to need mega or gigawatts of peak optical power.

Range finders on laser diodes exist that can measure distances up to 10km. YAG lasers used for range meters that capable of 20km. I am talking about off the shelf models. Here is one example: http://www.laseroptronix.se/dispu/dm10000e.html. Longer distance is possible but it requires favourable weather conditions to use of course. It is not all about power. Since diode laser used to pump the crystal inside YAG laser it is technically a hybrid.

[Marco]

Okay, I guess low single digit MW is enough on a clear day ... it's not all about power, but the peak power of laser diodes is enough orders of magnitude too low for power to be the limiting factor. You need a rod or fiber laser (preferably near 1.5u, much smaller wavelength and it reaches the retina, much longer and it dumps it all on the outer surface of the cornea). Pump sources can be lasers diodes, but they don't need to be driven with ns pulses.

[Sander]

If I am reading your requirements correctly, there is no solution for your <1ns response time.  It is possible to make such a pulse but the time from stimulus to pulse is normally 2-4ns, twice your requirement. 
An old rule of thumb is under 2ns requires a pre-trigger. 
Turning on 4kv with ~200ps rise time is definitely possible but I believe you are talking about clamping a non characterized transient. 
I believe you will need to use a suppression method to extend the response time.  Hopefully your application can compensate for such colouration. 


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The switching element needs to go from non-conducting to conducting within a nanosecond, the switch delay may as well be in the millisecond range. I will probably use a mercury wetted reed switch, embedded in a coil. So the search continues for a mercury wetted read switch rated 4kV, 80 amps for a microsecond and preferably 50 ohm impedance…

Today I tried the read switch setup with a normal, non mercury wetted, reed switch. Just at 40 volts and with a 22nF cap instead of 200 meter coax. Measurements with a 500 MHz scoop got me under 2.5ns, getting to the end of the scoops bandwidth. The first bounce was about 1 us from first contact. Next test will be 100 meter coax and some test gear with an higher bandwidth (spectrum/network analyzer and some matlab/excel reverse fft?).

[WarSim]

If I am reading your requirements correctly, there is no solution for your <1ns response time.  It is possible to make such a pulse but the time from stimulus to pulse is normally 2-4ns, twice your requirement. 
An old rule of thumb is under 2ns requires a pre-trigger. 
Turning on 4kv with ~200ps rise time is definitely possible but I believe you are talking about clamping a non characterized transient. 
I believe you will need to use a suppression method to extend the response time.  Hopefully your application can compensate for such colouration. 


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The switching element needs to go from non-conducting to conducting within a nanosecond, the switch delay may as well be in the millisecond range. I will probably use a mercury wetted reed switch, embedded in a coil. So the search continues for a mercury wetted read switch rated 4kV, 80 amps for a microsecond and preferably 50 ohm impedance…

Today I tried the read switch setup with a normal, non mercury wetted, reed switch. Just at 40 volts and with a 22nF cap instead of 200 meter coax. Measurements with a 500 MHz scoop got me under 2.5ns, getting to the end of the scoops bandwidth. The first bounce was about 1 us from first contact. Next test will be 100 meter coax and some test gear with an higher bandwidth (spectrum/network analyzer and some matlab/excel reverse fft?).

Oh that allows many more possibilities. 
I am skeptical about a reed being fast enough.  I suspect the physical restraints will cause a Ton well over 8ns and a much longer Toff.  Unless there has been great strides in reed design to overcome the inertia of the contractor to move through the self conducting range.  To move the contractor beyond the breaking point would be much longer.  So long that you will need to calculate contact life biased on arc over and in motion transitions.  One thing that may help is there has been some new gasses developed in the last while that I don't have the details on.  So please keep testing my assumption may be out of date now.  I am just skeptical. 


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[David Hess]

The first fast reference level pulse generators used mercury wetted reed relays to good effect but I do not see how a reed relay would work at high voltages.

http://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_other/109.html
http://www.linear.com/docs/29168

[Alexei.Polkhanov]

I think even if it was possible to get fast and clean rising edge out of mercury switch falling edge will be nowhere near desired result. If I understand right that is important requirement for most if not all pulse generators. Sander mentioned that he wants switching rate in KHzs in original question as well.

[Marco]

I think even if it was possible to get fast and clean rising edge out of mercury switch falling edge will be nowhere near desired result.

That's not really a problem with a transmission line pulser and a matched load.

[tboy]

A number of years ago  I built acircuit for switching a Pockels cell.  I don't recall the exact specs but it was a few kV switched in a few pS.  this was before electronic record keeping so I don't have access to the schematic now that I am retired.  I think by Googling "Pockels Cell Driver Circuits" you can come up with something close to what you want.

[calexanian]

A friend of mine who used to work at United Electronics in the 50's figured this one out for us. The 4PR60 tube was made for this sort of duty. I would use 3 in parallel.

[Marco]

A number of years ago  I built acircuit for switching a Pockels cell.  I don't recall the exact specs but it was a few kV switched in a few pS.  this was before electronic record keeping so I don't have access to the schematic now that I am retired.  I think by Googling "Pockels Cell Driver Circuits" you can come up with something close to what you want.

I'm pretty sure I'm not going to find anything in the single ps range That's on the level of the fastest electronic pulse generators period.

Can you remember if you used a stack of avalanche transistor? That is mostly what google turns up for the really fast Pockel Cell drivers.

[Alexei.Polkhanov]

I Google is right 4PR60 is a "TERODE" tube, designed for RF not for fast pulse switching. It could be that this tube was used for pulser due to it accessibility?

In description of modern solid state designs I often see Thyratron mentioned as the default core of tube-based designs. 4PR60  is not a Thyratron .

[calexanian]

I Google is right 4PR60 is a "TERODE" tube, designed for RF not for fast pulse switching. It could be that this tube was used for pulser due to it accessibility?

In description of modern solid state designs I often see Thyratron mentioned as the default core of tube-based designs. 4PR60  is not a Thyratron .

I don't want to be a jerk, but..... My recently departed friend Howard Brady designed that tube in the 50's at United electronics. It was used as a pulse modulator in radar systems and is related to the 5D21. the name is 4PR60

4 electrode, "PR" pulse requirement 60 watt anode dissipation.

typically they are limited to 18 amps pulse. but if you have a 1 microsecond pulse and a low enough duty cycle that the anode dissipation is not an issue, they can provide considerably more anode current that rated.

Here is a later data sheet from eimac.
http://frank.pocnet.net/sheets/140/8/8252W.pdf

Not trying to be rude, but I just happen to have a little, personal,experience lets say with that design.

[calexanian]

Oh yeah.. Thyratrons.... They have a practical limit of very slow speeds due to the ionization characteristics of the gas used. Typically hydrogen. In practice the limit is 1khz and in heavy pulse it can take well over a ms to turn on all the way. For precision switching you need a HEAVY DUTY pulse forming network because once they ionize they will conduct until there is no longer any real plate current because the anode cathode voltage will only be a few tens of volts to sustain ionization.  Ignitrons are even slower, but when they conduct, BANG!

[T3sl4co1l]

Oh yeah.. Thyratrons.... They have a practical limit of very slow speeds due to the ionization characteristics of the gas used. Typically hydrogen. In practice the limit is 1khz and in heavy pulse it can take well over a ms to turn on all the way. For precision switching you need a HEAVY DUTY pulse forming network because once they ionize they will conduct until there is no longer any real plate current because the anode cathode voltage will only be a few tens of volts to sustain ionization.  Ignitrons are even slower, but when they conduct, BANG!

No, those are the common (for power applications) xenon and mercury thyratrons -- typical turn-on time is comparable to a fluorescent light (the gas part, not the filament part) or neon light, or... thyratron, and is in the 10us range.  Compare to semiconductor thyristors, which deliver a cascade of charge carriers within a few microseconds, latching on (similar physics, really).  The deionization time is the hard part, taking upwards of milliseconds (I guess one way to think of it, the ionization potential of atomic mercury vapor or xenon gas is low, so there is less incentive to suck electrons back up).

Hydrogen thyratrons are extremely fast, in the 10s of ns range I think (turning on), presumably due to the much smaller size of the ion (about two orders of magnitude), and the higher ionization potential (so the voltage drop will be higher).  Deionization time I think is microseconds?

As for the 4PR60 and related devices, they were specifically made to handle extraordinary currents and voltages, but are otherwise absolutely plain thermionic vacuum devices.  Notice the heater power (26V x 2.1A = 55W) is almost greater than the total plate dissipation alone -- these are massively disproportionate tubes compared to their continuous-wave brethren!  They are made for extreme peak current (space charge, backed up by a massive filament) and high voltage (unusually large plate-grid spacing, though not at too much expense to operating characteristics, a nice achievement).

In peak power versus weight, you probably can't match these devices with semiconductors -- nah, that's not fair, if you count die weight versus whole envelope.  But if you look at active cathode area and compare that, you'll probably come out ahead.  300kW is nothing to sneeze at from something three inches across!  (Thinking about this further, a 4" puck SCR might have a 3" diameter die inside, which can switch 40kA peak at 4kV = 160MVA.  The die itself might be 20 mil or so, just guessing; maybe it's not so easy to compete, despite the higher voltage.  The speed-power tradeoff might still win, though you'd then have to compare against IGBTs or MOSFETs to be fair.  Dunno.)

Tim

[calexanian]

In my experience thyratrons are just not that fast. they begin conducting, then go into a limbo state for a bit, then by about .1 ms the arc begins, and that has a "Bloom" as the old timers put it. What i work with mostly dates to the late 70's at most recent, but the the rule of thumb I was given was over 1khz, look for something else. Also as they get hotter you get to a point particularly with mercury ones that they loose grid sensitivity and will just arc when sufficient anode voltage is present varying by temperature.  We have had this happen actually in some of our welders.

[Alexei.Polkhanov]

Thanks for the datasheet calexanian. I would love to play with one of those and I can see that they are still available at affordable prices - "New Old Stock". I have a concern however, datasheet that you linked noted that these are pumped out to hard vacuum. After so many years lying on shelves somewhere could it be that some air can get in? One I found on Ebay also look kinda rusty  I have purchased old photomultiplier tube in the past just to find that it has some air that got into it somehow.

[T3sl4co1l]

Well, only way to know for sure is to test it.

There's usually a getter flash (a thin film of barium or other reactive metal) deposited on the glass, that makes it apparent what condition it's in.  Though, high power transmitter tubes were often made of adsorbent materials (graphite, tantalum, etc.) which don't indicate gas presence -- and also must be operated at elevated temperatures (the plate usually glows red hot) to reach useful gettering activity.

It doesn't look like there's a flash on that one, so you probably need an electrical test to tell.

Bring it up slowly, and check things as you go.  Start by applying filament power, run it at somewhat reduced heat and no electrode voltage for some time, then bring it up to full filament voltage, and check that the current draw is within spec.  If it's too low, it may be worn (filament evaporation, excess filament voltage in a previous lifetime..?!); if too high, it may be running cold due to convection cooling (i.e., lots of gas).

Then, test the electrodes at low power to see if the characteristics match up with the graphs, then operate it at rated power (60W at DC should be easy to achieve on that sucker!) for a while.  You probably don't want to go straight to full power, because some gas will have diffused in over the years (and out from the materials), which could cause internal arcing and damage.  Allowing everything to reach normal operating temperatures gives time for the getter to do its job and actually improve the vacuum.

As for the vacuum itself, glass is an incredibly good container, and the glass-metal seals are also top quality.  The most likely invisible failure is an extremely small leak at a poorly made glass-metal seal -- but one so slow that it wasn't caught during manufacturing burn-in test.  So, these are very rare indeed.  Otherwise, if it doesn't have obvious visible damage (smashed envelope, cracking around seals), it's probably fine.

Tarnish on outside electrodes is normal from old age, not really a good sign, but nothing fatal; it might be worthwhile brightening the higher amperage connections, just to keep contact resistance low.  But don't be hasty; if you use chemicals, you may cause more corrosion later on, or if you use abrasives, you might cut right through a low resistance or [somewhat-]corrosion-resistant layer.

Do make sure to operate it with the specified hardware -- cooling ventilation as needed, heat sinking plate connector, etc.  Overheated seals are a prime cause of early (operational) failure!

Tim

[calexanian]

Thanks for the datasheet calexanian. I would love to play with one of those and I can see that they are still available at affordable prices - "New Old Stock". I have a concern however, datasheet that you linked noted that these are pumped out to hard vacuum. After so many years lying on shelves somewhere could it be that some air can get in? One I found on Ebay also look kinda rusty  I have purchased old photomultiplier tube in the past just to find that it has some air that got into it somehow.

The zirconium coating on the anode will begin sorbtioning any evolved gas at just about cherry temperature. I can tell you this though. that tube was in the 1X10-7 scale and screaming hot when it was tipped off. The chances of a good tube having gone gassy, but not just outright bad is highly unlikely. In the exhaust process by about 200C the gas film on the inner wall of the glass has been driven off and by 300C where tubes are typically baked out at any gas occluded or diffused into the glass is mostly gone. At that point glass derived glass is pretty much just from decomposition of the glass itself at that temp. The metal parts will have already been degased by RF bombardment up to orange heat. The zirconium coating on the anode will give off most of its gas at that heat, and will actually re adsorb most gases, excluding noble gases, from cherry to red heat. This is a convenient means of refreshing transmitting tubes to a certain degree.  Also the tungsten filament will combine with any remaining oxygen or nitrogen at red heat range, and give everything back of that was not converted to a compound on its surface at incandescence. The seals will be of tungsten/nonex glass at the bottom and the top will either be of tungsten nonex as well, or a kovar to intermediate glass, to nonex graded seal and thats going to be pretty good. I am reasonably sure if you pick one up NOS its still going to be pretty good. Please bear in mind those cathode current numbers are in pulse condition. Trying to draw amps from it DC will just strip it, or cause ionic bombardment and kill it that way. Bonded thoria (Distinctly different from thoriated tungsten) really is the preferred type of filament for this sort of affair.

[Marco]

I see one circuit on line for the 4PR60 which hits the grid with a 1 kV pulse from a variant avalanche transistor Marx ... is that the kinda circuit you would need to actually get it to turn on in the ns range?

http://www3.telus.net/schmaus2/elect/ftron.html

[T3sl4co1l]

Check out the operating point, drive specs, curves... you'll need a hearty bit of drive, yes.  A cathode follower from a smaller transmitter tube might do, or if you can get enough from something like that, sure.

Uh, worth noting by the way... this particular tube boats 50pF max Cin (Cg1k + Cg1g2), and looks to have about 2" leads into the electrodes, so easily 40nH per lead (not quite half for the cathode since it has two pins, but both grid and screen need consideration here!).  Which is a cutoff around 112MHz.  You'll be lucky to get nanosecond edges off this particular one.

More specifically, for pulses, you'll have a hard time terminating that stub properly.  A planar triode is better for this, since you can just lay it in a stripline and call it a day.  And even then, you'll probably only get so many dB per stage, so you might expect to use one 100W (average) device to drive two in parallel (for the final), and so on back to your signal generator.

Wonder if a shock line might be better.  See what #61 or higher frequency ferrites do?  Advantage is, it can be switched with an IGBT (or stack), or something like that.

Tim

[calexanian]

that may have too much loss for switching that fast now that i think about it.

[TerraHertz]

Now I remember where I saw that FBI rubbish about triggered spark gaps and terrorists with nukes. This is pretty funny:

http://www.fbi.gov/news/stories/2007/december/counterproliferation_121307
http://www.rell.com/products/High-Energy-Transfer-Products.html
http://www.excelitas.com/downloads/DTS_Triggered_Spark_Gap.pdf

Considering that those things are way slower than OP needs.

[Marco]

Slower but able to supply far more energy ... that said, I doubt Iran would have much trouble making a triggered sparkgap or Marx.