First off, congrats to these people for publishing and providing hard numbers for a real device relative to electron and ion (?) confinement. Also, while not in any way earth shaking relative to economical plasma confined fusion, achieving Beta = 1 or there about is a nice milestone for the Polywell device. Relative to any “cusp” confinement system, yes, achieving a Beta =1 is a big deal but such a plasma confinement configuration is a lost cause if, as I understand some current research on this particular topic, the Bremsstrahlung radiation loss issue isn’t somehow suppressed (while their electron "shielding well" may suppress some issues of Bremsstrahlung production, frankly, I do not see this effect stopping/reversing deuterium ions with no net energy cost/loss - for real fusion producing economic power, the number of required ions will be huge and creating a suppressing "electron well" would still appear far beyond what this Polywell has demonstrated.) Has this key problem ever been addressed by these people or other Polywell research or proven wrong/irrelevant?
When they state this in the conclusion –
“If the deep potential well can be formed and the scaling of the electron beam confinement is found to be favourable, as conjectured by Grad and others, it may be possible to construct a compact, low cost, high b fusion power reactor based on the Polywell concept.”
What!? This statement appears rather unsupported considering that their data does not address most energy issues for any such device. Besides, placing all responsibility for this statement on Grad and others for "proof" of this argument/conclusion, relative to their work this conclusion appears rather out of place. Second, they haven’t produced any neutrons at all much less produced any significant energy so the energy statement makes little sense. More critically, relative to anextremely energetic plasma, it does not appear to me that they have shown any results at all for a stable ion based plasma that is under going significant fusion producing energy - (i.e. any measurable energy) much less anything approaching a more realistic energy density of at least getting within four or five orders of magnitude of break-even (rather critical for this closing statement, wouldn't you think?) Finally, and I’d think most critically, what of energy loss by the ions that are forced by the Polywell field/confinement configuration to reverse and create the afore mentioned Bremsstrahlung radiation problem or are we somehow good on that?
The US Navy Publishes on Polywell.
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Dennis P Brown
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Dan Tibbets
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Re: The US Navy Publishes on Polywell.
The purpose of this machine was primarily to overcome one barrier. That is the creation and very importantly the demonstration of the high Beta state. This seems to have been accomplished. Also demonstrated is densities of over 10^21 charged particles during the admittedly breif test runs. This density is adequate for useful fusion in moderate sized machines.Even full up, much stronger (magnetic field) machines have a target density of ~ 10^22 particles per cubic meter. This is ~ 100-1000 times greater than Tokamak projections which translates simply in machines with 100-1000 times less volume than a tokamak for the same fusion output. Actually Nebel gave a conservative estimate of a fusion output intensity ~ 60,000 greater than a tokamak. This is in a production working Polywell (the key work here is Working).
As mentioned in the paper, the virtual cathode/ potential well formation is not the issue. They seem to be satisfied with this. Remember this is only one machine and it is the smallest machine created only for a specific purpose. There does seem to be a question about the depth of the potential well and how much input energy is needed to achieve it. This in a pessimistic assessment might limit the machine to D-T fusion.
The bremsstruhlung issue is of course critical, but it applies mostly to advanced fuels like boron or helium 3. For deuterium or deuterium- tritium the issue is much less significant. Even Rider conceded that D-D might work. And at least those physicists working with the Polywell think Rider (and Nevens) got it wrong. Not their math but the assumptions and model that math was based on. The often mentioned counter argument comes partially from work by Chacon.
L. Chacón, G. H. Miley, D. C. Barnes and D. A. Knoll, "Energy gain calculations in Penning fusion systems using a bounce-averaged Fokker–Planck model" (Physics of Plasmas, Dec. 2000)
Why didn't they do fusion? Probably because they couldn't and even if they did get feeble D-D fusion it would be meaningless. Look at the details of this machine. They had a relatively feeble high energy electron gun, and the vast majority of the plasma came from an explosive injection of plastic by the two plasma guns. A powerful blast of electrons at perhaps ~ 1500 V and ~ 400 thousand amps, vaporized and ionized several thin layers of a plastic in only a few microseconds. Lots of hydrogen and carbon and even some tungsten was injected. It was an extremely dirty and diluted plasma from a fusion standpoint. What they wanted was a lot of ions and electrons. That a fair portion of the ions was high Z ions (multiply ionized) was a benefit as their primary demonstrative measurement was the bremsstruhlung output. They wanted to show the Wiffleball effect, and to do so within the constraints of budget and time they chose this smaller machine armed with cleverly designed diagnostics and a cheap shortcut to quickly filling the machine with enough density and energy to push out the magnetic field and create the high Beta, high confinement conditions that is THE ABSOLUTE REQUIREMENT for the machine to work. The explosive plasma guns is not necessarily the only way to accomplish this, but it was the easiest way that matched their budget and target.
The useful and profitable fusion question is based on the Triple product- confinement, temperature/ energy, and density.
This machine and paper addresses the confinement issue sucessfully. The other issues have been studied (successfully, or at least partially?) in the rest of their non reported body of work.
I will say though, that this machine achieved impressive densities, and demonstrated in these limited conditions that thermalization is not a rampant issue. How so? Well. the blast of plasma was at below 2000 Volts, and the x-ray filters stopped any bremsstruhlung or electron- wall x-ray emissions originating from this portion of the plasma electrons from reaching the detectors. Only the E-gun injected electrons at up to ~ 7,000 Volts contributed. If they thermalized with the extremely higher numbers of plasma gun charged particles (electrons ), the temperature would only have been changed a few volts, eg: 1500 eV to 1510 eV with only a small high energy thermal tail. I suspect this would have resulted in bremsstruhlung readings completely buried in the baseline noise. The measured signal implies that the high energy injected electrons maintained their "monoenergetic " distribution long enough for the measurements. How this extrapolates to other conditions is complex, but at least it does give a minimum baseline.
The measured bremsstruhlung was due to the density of the e-gun provided high energy electrons (that maintained their KE above the 2000 eV cutoff limit). First the e-guns were turned on, and an equilibrium was established based on the injection rate/ loss rate. This took a period of time (~ a few microseconds?). Once the plasma guns blasted in a high density lower energy plasma, this was added to the resident e-gun electrons. The high Beta/ Wiffleball condition was established about as fast as the plasma blast (~ 7 micro seconds). Once this condition was established, the confinement was improved ~ 40 times, so the input rate of high energy electrons was relatively higher than the loss rate. A new equilibrium was established, and this took a few micro seconds to occur. That is why there is a delay in the bremsstruhlung signal. The plasma injection was a pulse and once finished, the density and thus Beta would begin to drop due to losses from escape and radiative losses. So the duration of the Beta =1 (or near there) was limited, there was no makeup for the electron losses (and to a lesser degree the ion losses) so the confinement soon returned to the baseline. The e- gun high energy electrons was only a tiny portion of the total input represented, so they contributed trivially to the Wiffleball condition. What they very importantly did provide though, is the method for demonstrating the Wiffleball though their improved confinement- resultant increased equilibrium density (~40X?)- resultant increased bremmstruhlung output.
This is a good demonstration of how a good experimental physicist can do more with less.
Dan Tibbets
As mentioned in the paper, the virtual cathode/ potential well formation is not the issue. They seem to be satisfied with this. Remember this is only one machine and it is the smallest machine created only for a specific purpose. There does seem to be a question about the depth of the potential well and how much input energy is needed to achieve it. This in a pessimistic assessment might limit the machine to D-T fusion.
The bremsstruhlung issue is of course critical, but it applies mostly to advanced fuels like boron or helium 3. For deuterium or deuterium- tritium the issue is much less significant. Even Rider conceded that D-D might work. And at least those physicists working with the Polywell think Rider (and Nevens) got it wrong. Not their math but the assumptions and model that math was based on. The often mentioned counter argument comes partially from work by Chacon.
L. Chacón, G. H. Miley, D. C. Barnes and D. A. Knoll, "Energy gain calculations in Penning fusion systems using a bounce-averaged Fokker–Planck model" (Physics of Plasmas, Dec. 2000)
Why didn't they do fusion? Probably because they couldn't and even if they did get feeble D-D fusion it would be meaningless. Look at the details of this machine. They had a relatively feeble high energy electron gun, and the vast majority of the plasma came from an explosive injection of plastic by the two plasma guns. A powerful blast of electrons at perhaps ~ 1500 V and ~ 400 thousand amps, vaporized and ionized several thin layers of a plastic in only a few microseconds. Lots of hydrogen and carbon and even some tungsten was injected. It was an extremely dirty and diluted plasma from a fusion standpoint. What they wanted was a lot of ions and electrons. That a fair portion of the ions was high Z ions (multiply ionized) was a benefit as their primary demonstrative measurement was the bremsstruhlung output. They wanted to show the Wiffleball effect, and to do so within the constraints of budget and time they chose this smaller machine armed with cleverly designed diagnostics and a cheap shortcut to quickly filling the machine with enough density and energy to push out the magnetic field and create the high Beta, high confinement conditions that is THE ABSOLUTE REQUIREMENT for the machine to work. The explosive plasma guns is not necessarily the only way to accomplish this, but it was the easiest way that matched their budget and target.
The useful and profitable fusion question is based on the Triple product- confinement, temperature/ energy, and density.
This machine and paper addresses the confinement issue sucessfully. The other issues have been studied (successfully, or at least partially?) in the rest of their non reported body of work.
I will say though, that this machine achieved impressive densities, and demonstrated in these limited conditions that thermalization is not a rampant issue. How so? Well. the blast of plasma was at below 2000 Volts, and the x-ray filters stopped any bremsstruhlung or electron- wall x-ray emissions originating from this portion of the plasma electrons from reaching the detectors. Only the E-gun injected electrons at up to ~ 7,000 Volts contributed. If they thermalized with the extremely higher numbers of plasma gun charged particles (electrons ), the temperature would only have been changed a few volts, eg: 1500 eV to 1510 eV with only a small high energy thermal tail. I suspect this would have resulted in bremsstruhlung readings completely buried in the baseline noise. The measured signal implies that the high energy injected electrons maintained their "monoenergetic " distribution long enough for the measurements. How this extrapolates to other conditions is complex, but at least it does give a minimum baseline.
The measured bremsstruhlung was due to the density of the e-gun provided high energy electrons (that maintained their KE above the 2000 eV cutoff limit). First the e-guns were turned on, and an equilibrium was established based on the injection rate/ loss rate. This took a period of time (~ a few microseconds?). Once the plasma guns blasted in a high density lower energy plasma, this was added to the resident e-gun electrons. The high Beta/ Wiffleball condition was established about as fast as the plasma blast (~ 7 micro seconds). Once this condition was established, the confinement was improved ~ 40 times, so the input rate of high energy electrons was relatively higher than the loss rate. A new equilibrium was established, and this took a few micro seconds to occur. That is why there is a delay in the bremsstruhlung signal. The plasma injection was a pulse and once finished, the density and thus Beta would begin to drop due to losses from escape and radiative losses. So the duration of the Beta =1 (or near there) was limited, there was no makeup for the electron losses (and to a lesser degree the ion losses) so the confinement soon returned to the baseline. The e- gun high energy electrons was only a tiny portion of the total input represented, so they contributed trivially to the Wiffleball condition. What they very importantly did provide though, is the method for demonstrating the Wiffleball though their improved confinement- resultant increased equilibrium density (~40X?)- resultant increased bremmstruhlung output.
This is a good demonstration of how a good experimental physicist can do more with less.
Dan Tibbets
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Dan Tibbets
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Re: The US Navy Publishes on Polywell.
R. Hulls comment about the fuzziness of the plasma glow in the cusps is perhaps telling. I was dismayed by the seemingly huge spacing between the magnets. This had to harm cusp confinement. I know that the magnets have to have separation to limit ExB losses and keep these losses miner compared to the cusp losses. I was just amazed by the magnitude of the separation. I have wondered if this was based on the density and energy and B fields anticipated in the machine. WB6 had much smaller separations. The separation is and seems greater. It seems even greater because this is a smaller machine and the sizes of the magnets themselves are smaller, so the gaps are relatively larger.
The image of the machine at high Beta shows the cusp loss channels. It also shows that the glow also drops off gradually as you move from the mid line of the cusp. It is not a sharp demarcation as would be expected by the claimed sharp B field border on the Wiffleball edge. I suspect the fuzziness is thus a visual manifestation of the electron ExB drift going on. And since this visible glow is a result of recombination of free electrons with ions or neutral excitation/ relaxation processes it also shows that there are ions/ neutrals present. It doesn't say much about the relative loss rates. Id does though suggest that there were perhaps significant neutrals present, they are not contained and provide a convenient background for the electrons to interact with and create glow discharge.
I wonder about the efficiency of Plasma guns. They would certainly splatter and vaporize the vast majority of the plastic, but what percentage is ionized? Then there is the issue of recombination...
Again, a demonstration that this machine was far from ideal, but despite the inherent limitations, it got the job done. It was a bull in the china shop approach. A clever and trained bull, but still...
ExB drift or diffusion, for those interested, is an unavoidable loss through magnetic fields. It is based on an random walk process where a charged particle jumps across a magnetic field due to collisions with other similar charged particles. Diffusion is dependent on each jump, which is dependent on the gyro radius of the charged particle; and the density driven collision rate. This is one thing that drives tokamaks to such large sizes. The ion gyro radius is the limiting factor. In the Polywell, where only the electron gyro radius applies, has much slower ExB drift. This is one factor that allows Polywells to be smaller despite increased densities.
Dan Tibbets
The image of the machine at high Beta shows the cusp loss channels. It also shows that the glow also drops off gradually as you move from the mid line of the cusp. It is not a sharp demarcation as would be expected by the claimed sharp B field border on the Wiffleball edge. I suspect the fuzziness is thus a visual manifestation of the electron ExB drift going on. And since this visible glow is a result of recombination of free electrons with ions or neutral excitation/ relaxation processes it also shows that there are ions/ neutrals present. It doesn't say much about the relative loss rates. Id does though suggest that there were perhaps significant neutrals present, they are not contained and provide a convenient background for the electrons to interact with and create glow discharge.
I wonder about the efficiency of Plasma guns. They would certainly splatter and vaporize the vast majority of the plastic, but what percentage is ionized? Then there is the issue of recombination...
Again, a demonstration that this machine was far from ideal, but despite the inherent limitations, it got the job done. It was a bull in the china shop approach. A clever and trained bull, but still...
ExB drift or diffusion, for those interested, is an unavoidable loss through magnetic fields. It is based on an random walk process where a charged particle jumps across a magnetic field due to collisions with other similar charged particles. Diffusion is dependent on each jump, which is dependent on the gyro radius of the charged particle; and the density driven collision rate. This is one thing that drives tokamaks to such large sizes. The ion gyro radius is the limiting factor. In the Polywell, where only the electron gyro radius applies, has much slower ExB drift. This is one factor that allows Polywells to be smaller despite increased densities.
Dan Tibbets
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Dan Tibbets
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Re: The US Navy Publishes on Polywell.
Dr Parks gave a talk today at U. Cal Irving. And is giving a talk Monday at U Wisconsin Madison Monday 6/16. He is making the rounds. I hope more information is revealed.
http://www.physics.uci.edu/seminar/spec ... -high-beta
http://www.talk-polywell.org/bb/viewtop ... =10&t=5433
"Hey folks, got this email today from John Santarius. Dr. Park will be speaking at Univ. of Wisc. next week, and I will be in the audience
. Any questions (scientific ones) you'd like me to ask?
"Colleagues,
Dr. Jaeyoung Park, President of EMC2, will give a talk on recent
cusp-confinement experiments in one of their Polywell devices. A
flyer is attached.
Date: Monday, June 16
Time: 2:30 PM
Location: 106 ERB"
Dan Tibbets
http://www.physics.uci.edu/seminar/spec ... -high-beta
http://www.talk-polywell.org/bb/viewtop ... =10&t=5433
"Hey folks, got this email today from John Santarius. Dr. Park will be speaking at Univ. of Wisc. next week, and I will be in the audience
. Any questions (scientific ones) you'd like me to ask?"Colleagues,
Dr. Jaeyoung Park, President of EMC2, will give a talk on recent
cusp-confinement experiments in one of their Polywell devices. A
flyer is attached.
Date: Monday, June 16
Time: 2:30 PM
Location: 106 ERB"
Dan Tibbets