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Art Technica - Magnetic Mirror Holds Promise for Fusion

Posted: Tue Jun 23, 2015 5:28 am
by CelticWarrior72 ... or-fusion/

"The Polywell reactor arranges magnetic fields differently to that of a standard, traditional tokamak. In a tokamak, the fields set up a plasma that has a donut shape. The plasma is free to circulate around the donut but is squeezed to a thin, high pressure stream.

The Polywell concept, instead, tries to create a magnetic box to confine the plasma in place, which reduces the turbulence and solves many control problems. However, a true box is simply not possible. This is because the force applied by a magnetic field depends on the direction of motion of a charged particle, which causes the electrons and ions to travel in a corkscrew motion around magnetic field lines. At each corner of the magnetic field box, the magnetic field lines point outward away from the center of the box, so the plasma can spiral out of the box. The upshot is that the harder you squeeze, the quicker the plasma leaks out, leaving you with a low beta plasma.

To help overcome this, additional high energy electrons are injected into the plasma. The electrons create a large negative potential that draws the ions to the center of the box, slowing their escape. Nevertheless, even with the electrostatic draw slowing ion escape, the magnetic field still wins in the end, because the electrons are also driven to spiral along magnetic field lines.

But researchers quickly realized that if the plasma was dense enough—in other words, if it had a high beta—it would exclude the magnetic field lines, creating a sharp boundary between the plasma and the magnetic field. The sharp boundary acts like a mirror for charged particles, vastly slowing their rate of ion escape. This unfortunately creates a chicken and egg scenario: if you have a high beta plasma, a Polywell design will keep it confined at high beta. But first, you must have a high beta plasma."

Re: Art Technica - Magnetic Mirror Holds Promise for Fusion

Posted: Tue Jun 23, 2015 7:00 am
by D Tibbets
He is close in many regards but off in perhaps fundamental ways also.
Beta is the ratio of plasma pressure relative to magnetic pressure. You can have Beta=1 very easily. Just have a single charged particle in a stupendously weak confining magnetic field. It doesn't even need to be very hot. The key is to establish a Beta very near to 1 at temperatures and densities that are useful. That is an entirely different story.

There is some ambiguity due to the terms used in plasma physics. Mirroring might be used to describe a particle bouncing off of an effective hard surface - billard ball rebounds, which is also close to what happens with Grad's description of high Beta B field particle interactions at the sharp magnetic domain boundary . It might also refer to charged particle motions in a magnetic field line , and/ or the "bouncing"- most common term for charged particles trapped on a field line and oscillating back and forth without entering a cusp loss cone- a stable orbit or oscillation is also used to describe this condition. This handicaps any discussion as they are mediated by the same physics, but the resultant behavior is far different, but the descriptive terms may not reflect this.

As for not demonstrating the Beta state, I actually think the Magnetic probes that measures a drop in the magnetic field strength is a direct measure of this. The measurement of resultant hot electron confinement conditions through Bremmstruhlung measuremants is admittedly not a direct measure of Beta but a consequence of the improved confinement expected of high Beta. Again the magnetic measurements is a direct measure of the B Field exclusion.

I am uncertain of the modeling that derived a Beta of ~ 0.7 from the magnetic field exclusion measurement, but I think it is probably reasonable.

The comments about plasma injection are also close, but then he makes a stupid extrapolation. The plasma was created by passing a large current through a thin plastic sheet. This atomized and ionionized much of the former solid plastic and a significant portion of this exploded into the interior volume where it was cusp confined and due to its density and modest temperature, it pushed out the confining B field to the measured extent. When he makes the statement that 10 times greater pressure would be needed, I am not sure what he is saying. At similar temperatures and B field strength, I estimate that to go from Beta of 0.7 to 1.0 would require the square of the density increase, or a factor of ~ 2 (0.7 * 0.7 = 0.49).

The next statement is the stupid one. He says the plastic used for the plasma injection was a solid so that any increased density was unreasonable. Simplistically you can not compress a solid or liquid much, at least in comparison to a gas. This is of course true, but has nothing to do with the conditions inside the magrid volume. Here the plastic has been converted into a gas/ plasma, and compression of this gas is independent of the solid condition it was in earlier. You might say that the gas/ plasma could not be compressed to a larger extent than the original solid, but you are conveniently ignoring the volume considerations. The volume inside the magrid might have been around a litter (very rough guess). The volume of the original two plastic sheets were only a tiny fraction of a mm thick and perhaps a cm in diameter. The volume difference between the two states of matter in the system was perhaps 1 liter versus 0.01 milliliters. Throw in some tungsten, etc contaminates, and still the difference is still at least 10,000 X. This implies by his own arguments that the maximum limit might be ~ 10,000 times greater than the conditions in in the test article. Plenty of room for pushing Beta.

Another flaw in this statement is the ignoring of temperature. The injected plasma was cool. Without checking, I think the temperature was ~ a few tens to a few hundred eV. Maintaining the density while increasing the temperature would increase the corresponding Beta proportionately. Using the Sun as an example of obtainable plasma density may be reveling. At ~ 100,000 atmospheres gas/ plasma density in the Sun's core gives a density of ~ 6 * 10^ 28 particles per 22.4 liters or ~ 3* 10^27/ liter. Some hydrogen bombs may exceed even this density, as may the Dense Plasma Focus albeit in a tiny volume. A plastic with a density similar to water may have a one liter weight of ~ 1 kg. As a 1 liter volume of hydrogen plasma this may represent a particle number of ~ 500 moles of deuterium or about 3 * 10^26 particles in a liter. This assumes the plastic was composed of only hydrogen, considering the carbon would push the numbers down some, but for ball park purposes... These rambling comparisons serve to demonstrate the absurdity of his final claim. It was late at night or he was desperate to present a presumed show stopper against the Polywell.

PS: I persist in my opinion that the Mini-B test magrid was a poor example for low beta cusp confinement. This is because the separation between the small magnets was relatively massive (the same separation may be required in a larger machine for ExB considerations, but with a magnet diameter of one or more meters, the ratio of the loss holes in the corners is smaller relative to the total surface area of containment volume. Secondly, the two coaxial cables sticking into the interior of the machine which were used to measure the magnetic field had to be large targets for charged particles that were otherwise contained. It is sort of like the WB4 example. Edges/ structures sticking further into the interior relative to the magnetic coils are harmful. Because of this the baseline low cusp confinement of this machine was reported as being about 7 passes. I suspect that a better baseline would be the claimed WB6 performance of ~ 60 passes, or what ever they achieved with WB8, which is unknown by me. Calculations for the necessary blast currents/ material required to generate high Beta conditions would be adjusted accordingly. This is not a criticism of the design. With the time, budget and goals of measuring increased hot electron confinement and measuring B field expulsion, the experiment was successful. But, as a baseline for extrapolating scaling, it may be very conservative. I don't know if Dr Park's made adjustments based on WB7 and WB8 results or only used the Mini- B data, when he made his projections.

Dan Tibbets

Re: Art Technica - Magnetic Mirror Holds Promise for Fusion

Posted: Tue Jun 23, 2015 7:13 am
by D Tibbets
He also confuses the issue by saying the electron density was rather high during the time the magnetic field was on. For the duration of the experiment the Magnetic fields were steady state, they were turned on before the plasma injection, and I think before the E-gun was turned on, and not turned off till well after the pertinent measurements were made. The increased confinement of hot electrons was not terminated by the magnets being turned off, but by the decay of the high confinement state due presumably to decay of the high Beta state, caused by the injected plasma cooling and escaping. The confinement of the hot electrons increased with a constant input current, so that the ratio of injection rate over confinement time increased . This led to the increased hot electron accumulation which also resulted in the increased Bremsstruhlung emission. I do not know of any alternative explanation for these results, especially as the experiment accounted for X-rays from contaminates such as the tungsten.

Dan Tibbets

Re: Art Technica - Magnetic Mirror Holds Promise for Fusion

Posted: Wed Jun 24, 2015 8:24 am
by CelticWarrior72
OK. I'm not a technical expert in this stuff.

Perhaps it might be useful to point out some of your concerns to Ars Technica author?

Re: Art Technica - Magnetic Mirror Holds Promise for Fusion

Posted: Thu Jun 25, 2015 4:57 am
by bennmann
Dan has a great history of exposition here, nothing personally directed at the choice of article or the author that I can see.

I enjoy lurking here and reading Dan's awesome exposition. It is also a good point, Celtic, about him possibly cross-posting here and there though. But I still like to see his posts here anyways.