[jamesr]

Brian H wrote: Will the alpha waves rip the containment vessel to smithereenies?
😉

No – if anything it is the opposite. Showing a mechanism by which the alpha energy is deposited in the core plasma rather than escaping.

Plasmas are full of different waves at a vast range of wavelengths and timescales. It is the interaction of these various oscillations mechanisms with each other and with individual ions/electrons that is the main focus of plasma physics. In this case it is the collisionless coupling of the alpha motion with the Lower Hybrid wave that was being studied. The lower hybrid wave is just one particular resonant oscillation at a particular frequency combination of the ion and electron cyclotron frequencies. See http://en.wikipedia.org/wiki/Lower_hybrid_oscillation

[jamesr]

I am in the same group as the author James Cook. He started a couple of years before me and is just finishing his PhD and this short PRL (Physics Review Letters) paper should be followed up by a longer paper going into more detail fairly soon.

For reference; the final published version is here http://prl.aps.org/abstract/PRL/v105/i25/e255003

[jamesr]

MTd2 wrote:

Basically this give a limit on the current a filament can carry where the radiation pressure outwards balances the pinch force inwards. For deuterium the figure is around 1.6MA.

According to Sing Lee, this limit is due the impedance of the contracting plasma not due radiation pressure.

It can be overcome by increasing the voltage between the cathodes to high values, like above 90KV:

http://www.plasmafocus.net/IPFS/2010 Papers/2010 Pp2 IJER.doc

The Las Vegas DPF can do that:

https://focusfusion.org/index.php/site/article/july_switch_update/

Unfortunately, LPPX cannot…

The 1.6MA figure is a generic limit for a single current carrying filament of plasma (a Z-pinch) regardless of its size or origin.
The particular geometrical & electrical arrangement of a DPF is the topic of Sing Lee’s paper. The Pease-Braginskii current is not a hard limit, in that is cannot be exceeded. It is just that if the current is higher than the critical value for that fill gas, the filament will be unstable and begin to collapse because the pressure is no longer balanced.

[jamesr]

Actually I think I got that last bit back to front. You need the radiation to stop the filament collapsing. What we want is to maintain several filaments for longer and not let them merge until the last moment. This may be where the injection of angular momentum helps, by getting the filaments spin & twist around each other before coming together. Each one able to carry its full quota of current.

[jamesr]

MTd2 wrote:
Summing up, not even with Poseidon, which has banks of 750KJ and a voltage of 80KV, were capable of achieving more than 1.5MA of current in the pinch. This is real bad since it means a neutron yield one magnitude lower than what is desired for LPPX for an equipment with much higher capacity.

The figure of 1.5MA seems close to the Pease-Braginskii limit.
From Robson’s 1989 paper: link.aps.org/doi/10.1103/PhysRevLett.63.2816

The concept of the Pease-Braginskii current in a Z pinch is reexamined in the light of the anomalous resistivity that arises in a plasma when the electron drift velocity is greater than the ion sound speed. Radial profiles of density and current are derived on the assumption that anomalous resistivity will prevent the drift velocity from exceeding the sound speed. The value of the Pease-Braginskii current then depends upon the line density, and may be significantly greater than its classical value.

Basically this give a limit on the current a filament can carry where the radiation pressure outwards balances the pinch force inwards. For deuterium the figure is around 1.6MA. Normally it is assumed other instabilities will break up a filament well before it reaches this limit, but if you can delay their onset, or be fast enough that they haven’t had time to develop, it gives an upper limit due to the radiative effects.

I was wondering, since this figure though is related to the collisionality parameter know as the Coulomb Logarithm, which would be different for pB11. The peak attainable could be lower rather than higher. Although this limit for a single filament – we start with a pair of filaments per cathode. These merge together to eventually form one at the focus. So the total peak current can be higher when there are still many filaments, but as they merge the total current they can carry goes down. When the plasmoid forms, the strong magnetic field effect hypothesized to reduce the bremstrahlung in pB11 could increase the Pease-Braginskii current limit, but this may be too late to get any more energy into the plasmoid.

I guess the question is – Is the magnetic field strong enough by the time the last few filaments merge, to suppress the radiation enough to allow more current? Or is this completely irrelevant and I’m just extrapolating a bit too far.

[jamesr]

delt0r wrote: Unfortunately anything that’s going to burn B11 and H can also burn plain old DD.

I don’t think its quite that simple. If the size of the device is optimised for pB11 it won’t necessarily be able to reach breakeven for DD

[jamesr]

Brian H wrote:
That’s a very interesting image; the knife-edge is tapered down to a rounded one for about half the circumference. What’s the benefit? Is that for research/investigation only, or an actual `working’ design?

I think it was just one of various profiles experimented with. The full presentation is at
http://www.physicsessays.com/doc/s2007/PF50-7-_03_07-CurrentTrends-bk.pdf

[jamesr]

MTd2 wrote: Would mind then explaining what is the nature of a plasma propagating inside the LPPX device?

I think the plasma in this medium should have a shockwave component. As the plasma progresses, it compresses the gas ahead. But if the gas has a polar nature, it will tend align its charges to perpendicular slightly ahead the shock wavet. Such alignment should make the gas ahead of it work a lightining rods elongating the wave front. So, the plasma would ride behind a hot wave front. The resistance should be smaller.

Here is a description of the model by Sing Lee http://www.plasmafocus.net/IPFS/modelpackage/File2Theory.pdf

It’s a bit maths heavy, but you can skip most of the equations and just read the explanatory text.

[jamesr]

MTd2 wrote:

I’m not sure why you think this is an issue, but the polarity or lack of in the fill gas is really completely irrelevant. At the voltages involved any atom/molecule coming close to the edge at the base of the cathode where the breakdown first occurs is basically ripped apart by the strength of the E field.

The point is not where it first occurs but that there is a slight bias in the orientation of the polar molecules that follows the potential lines of the field. This molecules will be like small wires that guide the plasma and diminishes the resistance. According to the Nov. 11th report, resistance was one of the causes for the slower formation of the pinch.

This is the resistance of the plasma sheath as it travels down the length of the electrodes. Once it is ionised its properties have no relation to the structure of the fill gas molecule.

As I mentioned before the conductivity of the plasma is related to the temperature (as T^(3/2)). The current heats the plasma more by normal Joule heating (=resistance*current^2). As the temperature rises the conductivity goes up (ie resistance goes down) and so more current can flow. This continues until the resistance is so low, the joule heating component is negligible.

The speed the sheath accelerates down to the end is related to the JxB force, ie more current = faster acceleration. The sheath will quickly reaches the sound speed of the fill gas it is plowing its way through. I suspect it is the interplay between the shock formation and the temperature of the sheath that could retard the plasma slightly and so increase the time to the pinch. ie the heating due to the compression of the shock front is needed to supplement the joule heating in order to get the temperature high enough, to bring the resistance down quickly enough for the current to accelerate the sheath further.

I would guess the way to test this would not be to change the fill gas, but to pre-heat it to as high as you can before the pulse – so for a given pressure p=nT, the temperature is higher but the density is slightly lower. Then the sound speed in the gas will be a little higher (sound speed is proportional to sqrt(T)). Or vice-versa – cool the fill gas to have a slow sound speed, and sooner shock formation

So rather than just experimenting with the fill pressure, the independent contributions due to density and temperature could be investigated.

[jamesr]

MTd2 wrote: The idea is using polar molecules to shorten the time of ionization. Molecules would align faster and the density of the plasma would be higher since electrons would have less time to scatter from the hot zone. Why not 6Li2H? http://en.wikipedia.org/wiki/Lithium_hydride

I wasn’t thinking about using heavy water to be an end of the research. Just to see if the idea is feasible, this is why I didn’t consider the Bremsstrahlung.

I’m not sure why you think this is an issue, but the polarity or lack of in the fill gas is really completely irrelevant. At the voltages involved any atom/molecule coming close to the edge at the base of the cathode where the breakdown first occurs is basically ripped apart by the strength of the E field.

I’m not sure exactly what the profile of the base of the cathode is on FF-1 but attached is a slide with an image from a different DPF showing the area where the breakdown (ionization) first happens. It is by changing the profile of this that you can adjust the nature of the breakdown.

Attached files

[jamesr]

Once a gas is ionized to a plasma it conducts very well. Above a few thousand degrees the atoms of any molecular gas are fully dissociated, and ionized. Adding anything like oxygen into the mix would just radiate heat away and pollute the fusion reaction. (not to mention that the oxygen would react with everything as it cooled at the end of the shot)

One difference from most normal materials though is that plasmas become better conductors the hotter they get (conductivity rises as T^(3/2) ). So at the 100,000K of a filament the conductivity is similar to a metal like copper. As it gets hotter the conductivity is essentially infinite (ie it is not the limiting factor in the circuit).

[jamesr]

zapkitty wrote:

Here’s an image of what I reckon it should look like (photo is actually of a lightbulb filament)

Cool! Hadn’t realized the coil itself was coiled. I take it this is going to be very noisy to cool with pressurized helium?

Why would you waste helium on the coil? You’re not limited to the confined volume of the FF core so you can run a (much cheaper) oil or water loop through there.

… and I was thinking of the open-ended style of Rogowski coil…

Good image – just what I had in my mind.

[jamesr]

tim Lardenoit wrote: james, have you ever seen a lighting? its a non confined plasma to, so don’t just say it cannot work. You simply don’t know…

And a glowing neon lamp contains a plasma, but this is a world away from fusion capable plasma conditions.

Ignoring for a minute the impossibility of having a spherically symmetric pressure on the walls – your diagram shows the outer shell containing water at ~250bar. To contain this the wall would have to be many inches of steel, or similar. You then propose heating the core using microwaves. How are the microwaves supposed to travel through the containing wall and water?? In order to couple and heat a plasma an RF antenna must be within a few ion skin depths of the plasma, otherwise it is just reflected back.

[jamesr]

No, it is more the fact that the centrifuge concept is so fundamentally flawed that it should not be along side concepts like RFP, polywell etc. which can at least confine a plasma and get some fusion, even if they have little real chance of producing nett energy gain.

[jamesr]

I don’t mean to be harsh, but can an admin move this thread to the Noise & ZPE category.

[Author]

Posts