[mchargue]

More google searches:

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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
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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
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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
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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
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[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]

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

[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]

Mr Lerner;

From what I’m reading, you’re using a spark-plug, without the grounding arm, to initiate a plasma within the switch. The plasma then conducts, via the spark-plug’s center electrode, the current pulse over/along the plasma. Now you’re looking for a spark-plug with a larger center electrode to act both as a better heat-sink, and to lower the current density in the electrode. (with the current density used, I can see why they are beating themselves to death)

The real issue seems to be how do you start a plasma in the plasma switch without overloading the small electrodes that you need to assure that they fire together. Larger electrodes will conduct heat better, but they will not fire together in time as well as the smaller electrodes. In both cases, switch-to-switch timing is affected by the switch-to-switch gap mis-match due to the initial firing pulse ramping-up, as different gap distances will arc at different voltages. (times along the ramp)

If that’s the case, have you considered dispensing with the spark-plug altogether?

It may be easier to initiate the plasma using another means. One that would absolutely insure that all the plugs fired simultaneously, and could give you a way to use a larger diameter electrode. I had suggested this earlier, but the suggestion either withered, or was left off for good reason. Anyway, back to how I would do this.

How about using a LASER to initiate a plasma in the plasma switch? No issues with ramp-time as with the initial electrical pulse used to start the switch, and no issue with the switch-to-switch gap mis-match between the conducting electrodes that will arc at different voltages (times) as the initial electrical pulse ramps-up.

One LASER pulse can be routed to all switches simultaneously, (or tuned by varying the distance between source & destination) insuring that they all initiate a conductive plasma at the same time. Because the main conductive electrodes no longer need to be used to initiate the plasma, they can be made larger in diameter, with better attendant thermal characteristics, and a lower current density.

The issues would be the gas that’s excited by the LASER, and used to conduct the main capacitor bank’s current; finding an off-the-shelf LASER that can do the job; the selection of materials for the construction of the new conductors; and making the switch so that a LASER can be fired across it to start a plasma.

I think it likely that some of this research has already been done in the past, probably connected with lightning strike dampers, or some other current-dump system associated with power generation/switching systems.

From what I read, this is a critical issue; both so that experimentation can proceed, as well as for fielding a robust system in the future. I’m thinking that this means that the electrode heating/current density problem has to be solved, and that the switch-to-switch gap mis-match must be rendered moot as a maintenance issue.

What say you?
Patrick

[mchargue]

I hope that the engineering folks talk to you, as you seem to have a solid grasp on the uses & handling of the substance, and the alternatives that exist. My hat’s off to you, sir.

[mchargue]

Aeronaut wrote: My understanding is that we’re validating McNally’s 1975 observation that bremstrahlung can be reduced by raising field strength, which has a lot to do with why high input currents are critical to reaching unity. This was mentioned in the GoogleTalks and in the patent, if I remember correctly. At this point it’s a theory which seems to be proving out experimentally, but I may be wrong on that- the relative motion of ions and electrons are beyond my direct understanding of the process.

That’s kinda’ what I was thinking, which was why I mentioned it. It seemed to be ‘supporting evidence’ for the contentions made in the patent, and in the talk. I didn’t describe as well, though.

[mchargue]

jamesr wrote: The strong magnetic field effect you refer to has nothing to do with the spacecraft problem. This effect only becomes important in very strong fields, in many orders of magnitude stronger than you have in the solar wind out where we are, they are typically measured in nT = 10^-14Gauss, in FF we have fields of 1^9 Gauss, ie 23 orders of magnitude higher.

The point of the article seems to be that a relatively weak magnetic field was responsible for blocking the solar wind on the moon, due to magnetic fields ‘frozen’ into ferrous deposits. It also talked about how a similarly weak magnetic field was used in an experiment to qualify the effect, and how the result of the experiment led some scientists to believe that a magnetic shield was back on the table for manned spaceflight.

I drew attention to the article because it seems to catalog an affect that might help explain why electron heating was minimized in the presence of a magnetic field.

[mchargue]

So here’s some of the relevant FF hypothesis re the major heat generation cycle, from the patent:

The higher atomic charge, Z, of B11 greatly increases the x-ray emission rate, which is proportional to Z.sup.2 making it difficult to achieve ignition, e.g., the point at which the thermonuclear power exceeds the x-ray emission. The present invention overcomes these difficulties using a detailed quantitative theory of the plasma focus, described below, and the high magnetic field effect (HMFE). This effect, first pointed out by McNally, involves the reduction of energy transfer from the ions to the electrons in the presence of a strong magnetic field. This in turn reduces the electron temperature and thus the bremsstrahlung emission.

It sounds as if the energy transfer between electrons and ions is being impeded by the magnetic field. (and that is, apparently, a good thing) In an earlier post I made, I linked to an article that described the same mechanism w.r.t shielding a spacecraft from the ‘solar wind’. (a plasma) The article describes the disparate effect that a magnetic field has on electrons, and (their formerly associated) ions.

For spacecraft engineers, this was an unexpected phenomenon, but it seems in line with the idea that a magnetic field can lessen the heat transfer to electrons, and decrease the bremsstrahlung radiative losses.

https://focusfusion.org/index.php/forums/viewthread/576/

What say, elders?

[mchargue]

‘ello, ‘ello, ‘ello!

I’m hoping that it’s time for a monthly update. Maybe soon, before the month runs out?

I’d like to hear how things are progressing on the ‘how to get there from here’ curve.

TIA;
Pat

[mchargue]

Even if it were true, they’d never be able to commercialize it.

It’s seems that the fearless leader of a starving populace – who bowls perfect 300 games, can win all Olympic events, and can cause fusion by compressing the plasma with his mind – can only do this with one device at a time. I guess even He has limits…

Pat

[mchargue]

QuantumDot wrote: Polywell’s are supposed to increase to the fifth power of the radius and the fourth power of the magnetic field.

As in w = r^5 * B^4?

That’s a good bit!

[mchargue]

Henning wrote: DPFs scale linearly with size, as opposed to Polywell or tokomaks, which get better the bigger they get.

How are these two descriptions different?

Also, doesn’t the Polywell energy output scale as the square of the radius? (a function of ‘falling’ from a greater distance)

As for the tokomacs, from what I can see from the example of ITER, the only thing that scales on the tomomak is the price. Probably price as a cube of the radius. 😉

Pat

[mchargue]

Breakable wrote: I almost missed this PR:
http://www.lawrencevilleplasmaphysics.com/index.php?option=com_lyftenbloggie&view=entry&year=2010&month=04&day=02&id=13:a-great-month-for-focus-fusion&Itemid=90
It is because its not linked from focusfusion.org
I would suggest to show the news feed from http://www.lawrencevilleplasmaphysics.com somewhere in the main page. I have registered to the news feed, so its not a problem for me, but maybe some other fan.

I second that notion.

Pat

[mchargue]

Well, not so much from the forums, but here is some ready news about LP and DPF,

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At the beginning of March, good shots (those without pre-firing and with pinches) were a bit under 50% of the shots we fired. Since mid-month, we have increased that to 90% good shots. The two time-of-flight neutron detectors have produced more evidence that we are already duplicating the high ion energies achieved with higher currents in the Texas experiments. In our best shots, ion energies were measured in the range of 40-60 keV (the equivalent of 0.4-0.6 billion degrees K).
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More on,

Dense Plasma Physics Update – A Great Month for Focus Fusion

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