spark plugs?

[Henning]

Found something interesting on Wikipedia: Nitrogen Laser (Sub-Section: Spark Gap)

Connection between spark gap and laser channel based on traveling wave theory:
* The low inductance spark gap may be inserted into a strip transmission line
* biconical spark gap
* biconical spark gap
* biconical spark gap

The breakdown voltage is low for helium, medium for nitrogen and high for SF6 (Patent 4237404), though nothing is said about the spark thickness variations.

8E10A/s are possible with a spark gap (see here), this nicely matches the typical rise times of 1E-8s and typical currents of 1E3A occurring in nitrogen lasers.

I’ve modified some links, so they reference better.

So after all it is patentable.

[mchargue]

Henning wrote: So it would be something like this:

Nice picture, and yes, that’s the idea.

As far as residue build-up inside the switch body, or on the transmission media, (maybe glass) you should be able to control that. Pick a LASER that can excite the working gas in the switch. Pick a transmission media that is ‘clear’ for the frequency of interest, and that is stable in operation, (little/no out-gassing) and doesn’t react with the gas used in the switch. (a noble gas?) Maybe a HeNe LASER to excite a HeNe gas switch?

The switches could be built/tested offline. Older parts could be removed from the FF reactor for service, and new/rebuilt ones swapped in as needed.

You can secure a LASER with sufficient power to generate a plasma. I mean, have you read up on the NIF experiment of late?

It is important to note that the switch can be designed independent of the FF reactor vessel. That means that the gases used in the switch, and their pressure, aren’t limited/controlled by the gas/pressure requirements of the FF reactor vessel. They’re just fast, high-current, electrical switches. Their gas/LASER requirements will be unrelated to the FF rector vessel, and determined solely by their desired electrical characteristics.

Finally, while symmetry seems to demand a hole through the center of the electrodes to conduct the LASER pulse, there is no such requirement. In order to decrease complexity, the LASER beam routing can be made independent of the electrical wire routing. Further, the LASER need only be fired across the gap, not just is center.

Pat

[mchargue]

OK, I went hunting with google for a bit, and we turned up some interesting papers…

M. J. Kushner, W. D. Kimura and S. R. Byron, “Arc Resistance of Laser Triggered Spark Gaps,” J. Appl. Phys. 58, 1744 (1985).
http://uigelz.eecs.umich.edu/pub/articles/jap_58_1744_1985.pdf

M. J. Kushner, R. D. Milroy and W. D. Kimura, “A Laser Triggered Spark Gap Model,” J. Appl. Phys. 58, 2988 (1985).
http://uigelz.eecs.umich.edu/pub/articles/jap_58_2988_1985.pdf

M. J. Kushner, W. D. Kimura, D. H. Ford and S. R. Byron, “Dual Arc Formation in a Laser Triggered Spark Gap,” J. Appl. Phys. 58, 4015 (1985).
http://uigelz.eecs.umich.edu/pub/articles/jap_58_4015_1985.pdf

W. D. Kimura, M. J. Kushner, and J. Seamans, “Characteristics of a Laser Triggered Spark Gap Using Air, Ar, CH4, H2, He, N2, SF6, and Xe”, J. Appl. Phys. 63, 1882 (1988).
http://uigelz.eecs.umich.edu/pub/articles/jap_63_1882_1988.pdf

H. Pak and M. J. Kushner, “Simulation of the Switching Performance of an Optically Triggered Psuedo-Spark Thyratron”, J. Appl. Phys. 66, 2325 (1989).
http://uigelz.eecs.umich.edu/pub/articles/jap_66_2325_1989.pdf

*pffft*

And here, I thought I was original in this. Ah well, schooled by giants ain’t too bad.

The idea seems to already be in the literature, and likely supported by experimentation. It’s likely that a template may already exist that covers FF needs, or enough information to dispense with this avenue.

Comments?

Pat

[Aeronaut]

Thanx for the research, Pat. I’m impressed that it wasn’t behind a paywall. Being a State fan, this could help me rethink my attitude about U of M 😉

I have two questions concerning the laser power requirements: How much power, size, and unit cost per switch, (very rough guesstimate, of course) and
Might it be more cost-effective to use a single large laser?

[mchargue]

Aeronaut wrote: Thanx for the research, Pat. I’m impressed that it wasn’t behind a paywall. Being a State fan, this could help me rethink my attitude about U of M 😉

I have two questions concerning the laser power requirements: How much power, size, and unit cost per switch, (very rough guesstimate, of course) and
Might it be more cost-effective to use a single large laser?

Cost… Um… Bigger than a bread-box?

Not helpful, I know, but I haven’t even done a back-of-the-napkin estimate. As well, there’s the added complication that I don’t work in manufacturing, so it’s hard to know what materials’ cost would be. My thinking is that switches now represent the biggest issue to date – both for experimentation, and for robust operation of a fielded system – and that the problem must be solved.

Whatever solution comes from this is going to be more expensive than plunking down $50 for a set of NGK spark-plugs, but the solution should resolve the switch-to-switch jitter issues, and guarantee that all switches fire.

LASER selection & power.

Based on what I’ve read, the best choice for a switch gas is a noble gas with a low atomic weight (i.e. He) in order to make the switch rise-time fast. I would think that this will also make the LASER more effective, as there will be a single, dominant, frequency for gas excitation.

That LASER power – power/unit area. (W/cm^2) – that would needed to pre-ionize the switch gas should be available from the paper(s). Multiply that by the number of switches that you want to fire, and you that will give you a LASER power value to shoot for.

I would run all switches off of the same LASER, as that should help ease switch-to-switch jitter. Doing this, however, removes the ability to dynamically ‘tune’ the firing pattern of the switches through any other means than by lengthening the LASER light-path distance relative to other switches.

Although…

One neat possibility, one that would decrease complexity by removing the LASER path, would be the use of individual LASER diodes (solid-state LASERs) on each switch. Operated in pulse mode, you can get several watts out of ’em, but frequency selection may be an issue.
http://www.repairfaq.org/sam/laserdio.htm#diocss7b

This would, though, put the ability to tune the timing of the switches (relative to one another) back in the realm of electronics. (or maybe lengths of fiber-optic lines) Care would have to be taken, though, to try to minimize the differences between LASER diodes. (rise-time, mostly)

Here are a few high-power LASER diodes.
http://www.sony.net/Products/SC-HP/datasheet/90216/data/a6804813.pdf (20W)
http://www.sony.net/Products/SC-HP/datasheet/90216/data/a6804814.pdf (40W)
http://www.sony.net/Products/SC-HP/datasheet/90216/data/a6810228.pdf (60W)

http://www.lumics.com/Multimode-Diode-Laser.361.0.html

Anyway, the point is that there seem to be high-power, solid-state lasers available (you can get a 2W version on ebay!) that should be able to drive pre-ionization in a gas switch using pulse-mode operation. If I were engineering this I would:

Select working gases.
Determine excitation frequencies for each.
Determine pre-ionization power requirements.
Talk to a solid-state LASER diode manufacturer, and design the system with them. (use expert help)
Make & test one.
Clone the working version.

Tougher than using spark-plugs, true, but it should carry you through experiments – hopefully to a working system.

Pat

[mchargue]

More google searches:

—–
We present a simple method of switching a nitrogen laser with three parallel, self-breakdown spark gaps by incorporating them into a two-stage Blumlein circuit. These spark gaps are preionized by ultraviolet radiation from an auxiliary spark which suppresses their breakdown jitters and improves their temporal sychronization. The breakdown time delay of these parallel spark gaps enables strong ultraviolet preionization of the laser channel. As a result of these improvements, the laser output is doubled and is more reproducible than that obtained using the one-stage Blumlein circuit.

http://iopscience.iop.org/0957-0233/8/7/019
—–

—–
A laser-triggering scheme for air spark gap switches was conceived and investigated for its potential to reduce shot-to-short time jitter. The scheme utilizes a pulsed ultraviolet laser of relatively low energy to generate resonant enhanced multi-photon ionization (REMPI) within the atmospheric air medium of the spark gap switch. With an applied voltage below the self-breakdown level, the laser-induced pre-ionization initiated avalanche breakdown within the gap and the subsequent triggering of the switch. This laser induced pre-ionization process relied solely on gas phase ionization and not surface effects, since the laser does not strike either electrode. This triggering scheme produced sub-nanosecond jitter with low enough laser power that it could be transmitted through fiber optics, which would be advantageous for multi-switch triggering of a high current pulse. The laser pre-ionization effects of space charge, electric field distribution, and active species within the gap were analyzed for their role in driving electron multiplication leading to avalanche breakdown below the self-breakdown voltage. Experimental results will be presented, including arc timing and statistical jitter measurements, as well as optical images and spectral analysis of the arc emission.

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4743641
—–

—–
A KrF laser (248 nm) is used to volume preionization trigger a 40-100-kV, > 10-kA, 100-ns spark gap switch. This method of triggering creates reproducible and axisymmetric spark columns having low temporal and spatial jitter. A short pulse (< 5 ns) tunable dye laser and a Mach-Zehnder interferometer are used to obtain spatial and temporal measurements of the spark column. The spatial resolution of the interferograms is better than 5 ¿m. The fringe shifts of the interferograms are used to calculate the electron and heavy particle density distributions within the spark column as a function of time during the spark. Results are presented for sparks in 5-percent SF6/ 20-percent N2/75-percent He and 1-percent Xe/99-percent H2 gas mixtures. Dc and pulsed self-breakdown voltages are also measured in order to provide a reference for the laser-triggered results. Data on laser-triggering reliability and spark breakdown delay time are also presented.

http://ieeexplore.ieee.org/Xplore/login.jsp?url=http://ieeexplore.ieee.org/iel5/27/4316527/04316536.pdf?arnumber=4316536&authDecision=-203
—–

—–
A circuit for the preionization and main discharge of a pulsed gas laser provides that the sparks intended for the preionization of the working gas have an independent switch function. Capacitive energy storage means 7, 8, 9, 10 are connected low-inductively to the spark electrodes 3, 4, 5, 6. The current flowing in the spark discharge is used selectively for the preionization by means of the spark or for generating a compressed high-voltage pulse. The compressed high-voltage pulse is applied as prepulse to the main electrodes to momentarily greatly increase the voltage obtaining between the main electrodes and thus initiate and supply a homogeneous main discharge.

http://www.freepatentsonline.com/4797888.pdf
—–

[Henning]

The manufacturer of LPP’s switches R.E. Beverley (see here) also offers laser spark gap switches (see specs).

I think Eric and team have a good reason not to use them, be it costs or complicated laser beam setup. Another reason could be, that with every shot the optical system gets misaligned, because of the whole construct moving with the applied force.

[Brian H]

Henning wrote: The manufacturer of LPP’s switches R.E. Beverley (see here) also offers laser spark gap switches (see specs).

I think Eric and team have a good reason not to use them, be it costs or complicated laser beam setup. Another reason could be, that with every shot the optical system gets misaligned, because of the whole construct moving with the applied force.

On a quick skim thru the specs, I note that they have a max firing rate around 100 Hz, and an expected life from 5-20,000 shots. This would be a minute at most of FF operation.

[Henning]

Brian H wrote:

The manufacturer of LPP’s switches R.E. Beverley (see here) also offers laser spark gap switches (see specs).

I think Eric and team have a good reason not to use them, be it costs or complicated laser beam setup. Another reason could be, that with every shot the optical system gets misaligned, because of the whole construct moving with the applied force.

On a quick skim thru the specs, I note that they have a max firing rate around 100 Hz, and an expected life from 5-20,000 shots. This would be a minute at most of FF operation.

Given life time is for maximum voltage/amperage. If you stay well beneath it, a million shots are possible. Not very much either as it’s maybe a day of full operation.

[Brian H]

Henning wrote:

The manufacturer of LPP’s switches R.E. Beverley (see here) also offers laser spark gap switches (see specs).

I think Eric and team have a good reason not to use them, be it costs or complicated laser beam setup. Another reason could be, that with every shot the optical system gets misaligned, because of the whole construct moving with the applied force.

On a quick skim thru the specs, I note that they have a max firing rate around 100 Hz, and an expected life from 5-20,000 shots. This would be a minute at most of FF operation.

Given life time is for maximum voltage/amperage. If you stay well beneath it, a million shots are possible. Not very much either as it’s maybe a day of full operation.!
Not even! Assuming 330 Hz is possible, then 1,000,000 shots is 3030 seconds, or less than 1 hr. !

[mchargue]

Brian H wrote:

The manufacturer of LPP’s switches R.E. Beverley (see here) also offers laser spark gap switches (see specs).

I think Eric and team have a good reason not to use them, be it costs or complicated laser beam setup. Another reason could be, that with every shot the optical system gets misaligned, because of the whole construct moving with the applied force.

On a quick skim thru the specs, I note that they have a max firing rate around 100 Hz, and an expected life from 5-20,000 shots. This would be a minute at most of FF operation.

Given life time is for maximum voltage/amperage. If you stay well beneath it, a million shots are possible. Not very much either as it’s maybe a day of full operation.!
Not even! Assuming 330 Hz is possible, then 1,000,000 shots is 3030 seconds, or less than 1 hr. !

I think that the ‘rated maximum number of shots’ using a switch being <1e6 means that you're going to have to use a noble gas as the switch gas. SF6, (the switch gas that's now being used that came from research on switches that not suited to the FF application) while it has advantages, has the significant disadvantage that it breaks down, and coats the interior of the switch with sulphur. That would tend to alter the way each switch works – differently for different switches – and would degrade the switches for use with the FF device.

That’s why I think that you’ll have to come up with a better long-term solution for the switches. Something that allows you to build encapsulated switches that can be swapped out, rebuilt, and re-used. A switch gas that doesn’t break down, and with electrodes that can be large enough to withstand the thermal cycling.

I have to think that, using the spark-gap setup described here, you’re already experiencing switch-to-switch drift due to electrode break-down, and sulphur deposits.

Experimenters: Based on the results thus far, do you see these kinds of switches working for the final design, or for repeatable experimentation?

[mchargue]

And here’s the patent:
—–
Light initiated high power electronic switch
United States Patent 4771168
The invention disclosed herein includes a low pressure, light initiated, glow discharge switch for high power application. The switch is comprised of an insulating envelope formed into a cylindrical shape having conductive plates at each end. Contained within the envelope is a cathode cup and an anode cup. Each of the cups has a plate at one end which defines two central apertures. The central apertures are positioned opposite one another a short distance apart and centrally axially aligned. A quartz window at the lower end of the cathode cup defines a visual opening to allow unfocused high energy electromagnetic radiation (UV light) to be shined upon the back side of the cathode plate. When UV light is presented to the back of the cathode plate, a photoemissive mechanism is initiated which causes an avalanche effect in the gas-filled chamber of the switch which leads to the discharge of current across the gap between the anode and cathode allowing the switch to close. A system includes the electronic switch is also disclosed which may be used to trigger a high energy flash lamp or excimer laser, as well as other high power applications. A system for controlling the flow of gas into the chamber defined by the envelope of the electronic switch is also disclosed.
—–

—–
Inventors:
Gundersen, Martin (San Gabriel, CA)
Kirkman, George (Los Angeles, CA)

Application Number:
07/046405

Publication Date:
09/13/1988

Filing Date:
05/04/1987

Assignee:
The University of Southern California (Los Angeles, CA)
—–

Someboody get these guys on the phone!

Pat

[Brian H]

If a patent is for 17 yrs(?), that one’s expired!
Here’s the document: http://www.freepatentsonline.com/4771168.html

I can’t quite decipher if the amperage capacity is high enough, or the response is fast enough, though.

[mchargue]

Some products…
—–
http://www.perkinelmer.com/Category/Category/KeyName/IND_DEF_CAT_Spark Gaps_051
http://www.perkinelmer.com/Category/Category/KeyName/IND_DEF_CAT_Thyratrons_054
http://www.reb3.com/pdf/sg_specs.pdf
http://pulsesciences.com/html/AdvPSwitching.htm

Some discussion
http://home.earthlink.net/~jimlux/hv/hvtrigsg.htm
—–
Though made for a different market, designs may be applicable for experimental purposes.

And, yeah, I know. Not suited to shoe-string operations, either. Who knows, though. Maybe one of ’em wants a lock on design-in?

I’d certainly talk to the folks. They do this bread & butter, so they might be able to steer you in a good direction on this… And if they’re trying to sell you something, what boots it? Explain the problem, listen politely, and maybe they’ll connect you to an engineer who just breathes this stuff. HE may have an interest, and a bench to crank out some test articles.

These folks can smell a paper, too. A by-line on a ‘how I did it’ article appearing in an industry-specific rag wuld be good for everyone.

Pat

Pat

[mchargue]

Brian H wrote: If a patent is for 17 yrs(?), that one’s expired!
Here’s the document: http://www.freepatentsonline.com/4771168.html

I can’t quite decipher if the amperage capacity is high enough, or the response is fast enough, though.

Brian;

I have the feeling that the originators of the patent didn’t push their switch very hard. They’re application was for pumping LASERs at 12-15Kv. I would expect that, with a different switch gas, and higher voltages, the current would climb up there too. Not sure if it would make the 250KA that is claimed as a limit for the spark-plug spark gap-switches, though. (I don’t know if the spark-plug spark-gap switches will do that either)

Based on other reading, control of the hold-off voltage is done through gas selection, gas pressure, and switch electrode distance. (with an eye toward: higher pressure is better)

What I think would be a winner for this type of switch is that it would be more immune from switch-to-switch manufacturing (spec) differences. So long as each switch can hold-off the potential difference between electrodes, and initiate an arc when the LASER fires, minor differences in hold-off voltage between switches would, I think, make little difference.

I think that this contrasts well with the spark-plug based spark-gap design. The spark-plug switch would, to my thinking, be very sensitive to hold-off voltage differences (controlled by electrode distance, gas pressure, and gas mixture differences between switches). This is because changes to the values would change the hold-off voltage. Differences in the hold-off voltage, coupled with the rise-time of the triggering voltage, would mean that each switch would fire at a different time. (a different time along the trigger voltage ramp)

Varying the distance is a way to tune things, but then somebody melts, or the gas mix degrades, and the switches have to be re-tuned.

I think that a LASER initiated gap would render these complications moot.

Pat

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