Magnetic field expulsion from a spinning brass wheel

Discuss how polywell fusion works; share theoretical questions and answers.

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Magnetic field expulsion from a spinning brass wheel

Post by DeltaV »

This reminded me of Polywell's wiffleball effect, even though the mechanism is very different.
Spin the plasma?

An Experimental Investigation To Determine Interaction Between Rotating Bodies ... 020760.pdf
A brass (copper+zinc) wheel, with a 4-in diameter and I .4 in thick, was used for this investigation.
Ceramic ball bearings were used to safely spin the wheel up to 40,000 rpm. The wheel was also
electrically insulated from the rest of the armature. For spinning, an air turbine was used. The rotational
velocity was measured by two methods: (1) A simple strobe light and (2) a photodiode that detected
laser beam pulses as they passed through a slot in the rotating shaft. The magnetic sensor is based on a
giant magnetoresistivity, and consists of a balanced bridge circuitry. The position of the sensor was as
close as possible to the rim of the wheel. The linear dimension of the sensor is =8 mm so that the offset
from the surface is on the order of 15 percent. We did not use any goniometer system, so the accuracy of
the angular position is not high, being estimated within a few degrees, with the main uncertainty being
the direction of Earth's magnetic field. We attempted to fit the experimental data with the presented
theory by selecting the best value for the electrical conductivity of the wheel. The results of this procedure
are displayed in figure 11, where the black dots represent experimental values. A slight misfit on
the right shoulder can be due to slight angular misalignment from a 90° position. The obtained value for
the resistivity is 43 nΩm, which compares well with those listed in table 1 . We can conclude, based on
these measurements, that the proposed theory satisfactorily explains our experiments.
SpinningBrassWheelDiamagnetism_NASA_TM-2003-212286.png (27.06 KiB) Viewed 7295 times

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

Re: Magnetic field expulsion from a spinning brass wheel

Post by D Tibbets »

I'm uncertain of the applicability of this. It looks like they are trying to address some theories far buried in basic physics perceived reality, like Super string theory and some adaptations of general relativity.
Irregardless, a spinning plasma does expel an external B field. This is a basic tenet of Tokamaks. The problem with a spinning plasma though is that it creates it's own pervasive magnetic field due to the predominate flow of charged particles in one direction around a ring or ball. This introduces B fields that are not always convex towards the bulk plasma, edge instabilities become much more problematic. The plasma is magnatized, not by being embedded in an external penetrating field, but by it's own charged particle motions.

A plasma generated gross magnetic field can only be prevented by totally random particle motions within the confined space, or by matched motions that cancel each other out. The best example of this is in a Polywell that has idealized radial motions of charged particles only on radial paths, the inward motions and outward motions cancel each other out, while still producing an outward pressure against the confining magnetic field. There can be scattering to non radial directions, but these must be random so that no dominate angular momentum flow can become dominate on the gross scale. Of course on microscopic scales local interactions are mixing magnetic effects between the particles local magnetic fields in a more chaotic manner, but these short range variations must (I think) remain on scales much smaller than the gyroradius associated with the particle energies within the confining B field- not excluded. IE: the local collisional effects must totally destroy any magnetic memory of the charged particles for the bulk of the plasma. Obviously this is a more mixed picture right at the Wiffle Ball border or at lower Beta conditions.

I think the Polywell may have no confluence/ central ion focus and still work with D-D fusion, but P-B11 requires some central confluence. Without the central confluence, the charged particle motions are more chaotic, and may have considerable angular momentum, but the angular momentum motions are random, no preferred direction about some axis. Introducing such intended characteristics, deviates from the core Poolywell concepts. Such efforts might be best seen in Stellarators or perhaps FRC . Efforts to create alternate flow patterns in Polywell, are mostly concerned with bunching charged particles in time at certain radial distances. This may allow for slamming a bunch of ions in the center all at once, with net improvements in fusion over an average radial flow distribution. POPS is an example of this.

There have been efforts to spin the plasma in two magnet opposed mirror machines and in cylindrical multiple magnet mirror machines, but any gains in cusp confinement were offset by proportionately worse consequences. I think most of the introduced negatives were associated with edge instabilities worsening more than any gain in cusp confinement. How this edge instability is addressed in FRC is a mystery for me, though I suspect it has something to do with a scrape off layer.

A torus of magnatized spinning plasma can be contained very much better than in a cusp geometry, even with a Wiffleball condition. This is done in FRC (?), Tokamaks, Stellarators , Spheromaks, etc. The very important consideration though is that this spinning plasma confinement approach suffers from edge instabilities that is proportional to ~ the denssity squared. And as the fusion ideally scales as the density squared, the Tokamaks, at least, require large volumes of plasma to get the desired fusion output because of the density limits. ExB issues also contribute. A Polywell with it's non magnatized plasma, and EXTREAMLY important confining B field convex geometry tolerates higher densities. The Edge instabilities are not the problem, the ion ExB losses are not the problem, the cusp leakage is . It is a trade off. The proportionate cusp leakage can be much worse- at correspondingly increased densities, the density gains outweigh the poorer cusp confinement limit provided you can push Beta to near one and establish Wiffleball conditions that significantly improves the cusp confinement. It is still poor, well under a second, versus perhaps a thousand seconds for a Tokamak, but the operating pressure can be pushed to more than make up the difference from a triple product standpoint.

If a Tokamak or Stellarator can be made to limit the edge instabilities through various interventions and ExB ion issues can somehow be addressed, then they may support higher Beta/ densities and eclipse the advertized Polywell advantages. I've touched on ExB issues, but I will resist launching on a further long discussion, except to say that the Polywell avoids this issue to a large degree because it does not have a spinning magnatized plasma. Ideally ion losses are dominated by collisional up scattering to escape the potential well and hit a cusp or alternatly be leaked through the B field by ExB diffusion. Here the ExB problem is much less because the proportion of the ion population that reaches this magnetic domain is a small percentage of the total. The ExB losses is like having a plasma at lower density without actually doing so..

Tokamaks with it's spinning plasma, is limited by ExB diffusion (of ions) and edge instabilities. Polywells are resistant to these concerns and are limited by cusp losses, primarily of electrons. You choose your poison, and go from there. The designs are mutually opposed to each other. One avoids cusps, the other embraces them. One embraces plasma flow induced magnetic fields, the other abhors them. Trying to incorporate the two approaches into some compromise hybrid is probably doomed to failure.... But, if the experts had a good comprehensive handle on plasma behavior we would already have high Q reactors. They might not be economical, but the physics would be solved or abandoned.

Dan Tibbets
To error is human... and I'm very human.

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