Van Allen Belts

[ohiovr]

Hello,

have a look at the video on this page:

http://www.space.com/18756-van-allen-ra ... dings.html

Graphically depicted and shown in demonstrations here on earth show that the larger Van Allen belt touches the earth at the poles. But in the video I watched, the inner belt, which has a lot of high energy protons; does not touch the earth at all. In the video it is mentioned that protons can stay in this inner belt for years.

Now the earth is a weak but enormous magnet. I have had difficulty understanding how magnetic fields contain plasma. But what I am told, charged particles in a magnetic field travel around in loopty loop patterns. I used to think that the particles just get repelled by the magnetic field. Am I still wrong?

Anyway you have this perfect containment field about 1000 miles above the earth. The field generator is obviously not a torus. But the belt sure is. Why does this happen?

Is it that the gyro radius of charged particles around a magnet that is ginormous (the earth) just so happens to be the right size for perfect containment. When I mean perfect, sure, it isn't. But its billions of times better than what we've done on earth.

Why can't we make an artificial inner Van Allen belt in the lab? Seems like if we could, we got the perfect solution to the containment problem.

[ohiovr]

ps sorry for blowing up the FAQ long ago. I didn't have the necessary skill to keep it from being messed with by hackers. I wanted to send someone the database but no one took me up on it.

[ohiovr]

Wouldn't ya know someone had already wrote this up elsewhere?

http://farside.ph.utexas.edu/teaching/p ... ode22.html

The proton and electron gyroradii, expressed as fractions of the Earth's radius, take the form

Darn it.

Well it was slightly fun to think about at least.

[ohiovr]

Maybe if we had this large wooden badger....

[asdfuogh]

It basically just comes down to a magnetic mirror here.. The difference in the strength of the magnetic field at the surface vs as far as possible (ie. the equator) is quite large. The more interesting thing is when you have solar flares or sudden larger influxes of solar wind hitting the magnetic fields, causing (maybe) a reconnection event on the night side fields.

[D Tibbets]

That link shows several important features. First, that the magnetic field strength change gradient is very much slower or larger than the gyroradius of the charged particle at typical energies. This is a common consideration of plasmas as it is more simple to describe. From the gyroradius perspective the field is almost constant, so gyro orbit is almost circular. This is much different where the magnetic field strength is varying at rates similar to or shorter than the gyroradius at the given test point within the field. In this case the gyro orbit is prolonged, mildly to extremely elliptical. In the Polywell, especially at the Wiffleball border this applies. The gyroradius is short on the outside of the orbit but long on the inside. This allows the charged particle to complete ~ 1/2 of the highly elliptical orbit and then travel towards the center of the machine. Most of the gyro orbit is so long that the width of the machine is exceeded before the single orbit is completed. This results in this inner plasma being effectively non magnetic. It is not trapped on a field line. That is - it is turned/ reversed by the magnetic field, but subsequent behavior is highly dominated by other processes,such as collisions. There are U tube videos that show electrons mirroring (bouncing) back and forth along field lines. This is misleading inside the machine because this behavior would only be seen by particles that have penetrated well past the Wiffleball border (Magnetic field strength is not changing as fast as at the border). This is why an analogy like the "Wiffle Ball " toy is more descriptive of what is going on inside the machine rather than charged particles mirroring back and forth on field lines. Getting electrons into and leaking out of the cusps is somewhat different. Here more classical mirroring perspectives may be more useful.

http://farside.ph.utexas.edu/teaching/p ... mlLikewise

... the gyroradii of such particles are much smaller than the typical variation length-scale of the magnetospheric magnetic field. Under these circumstances, we expect the magnetic moment to be a conserved quantity: i.e., we expect the magnetic moment to be a good adiabatic invariant. It immediately follows that any MeV energy protons and electrons in the inner magnetosphere which have a sufficiently large magnetic moment are trapped on the dipolar field-lines of the Earth's magnetic field, bouncing back and forth between mirror points located just above the Earth's poles.

Magnetic fields do not stop charged particles, they turn them, and this behavior is dependent on the field strength locally, which has to be considered at every position of the charged particle as it moves. For the Earth, and to a lesser extent even a Tokamak, the plasma charged particles will have more circular orbits. That and the dominate magnetic field created by the moving charged particles themselves determines a lot of the observed behavior. The Polywell plasma is considered as non magnetic for the most part, while the Tokamak plasma is magnetic for the most part. They are different beasts.

Secondly, ExB diffusion or collision driven movement of charged particles within a dominate magnetic field results in particles traveling through a magnetic field. This limits containment based on the particle gyro radius in the magnetic field (here assumed to be constant for convenience) and the total distance the particle has to travel to transverse the field. The Earth's magnetic field is extremely wide relative to the gyroradius, so it takes an extremely large number of collisions before the trapped particle penetrates to the atmosphere. This allows for good containment and is one of the things that drives Tokamaks to such large sizes. A small narrow magnetic field might contain charged particles very well also, but this is because the density of charged particles is very low, so the ExB driving collisions are very rare. Penning traps that are used to contain antiparticles, etc. fall into this category.

The loss of particles to the atmosphere at the poles is a different process, and is essentially what happens at the cusps in the Polywell. A Tokamak does not have cusps so it does not suffer this problem. Ignoring extremely important instabilities in the Tokamak, the magnetic containment is due solely to ExB diffusion/ drift issues. In the Polywell, mostly due to the decoupling of ion containment from magnetic electron containment, the picture is changed. ExB drift becomes much more manageable at smaller scales, mostly because onye electron ExB needs to be considered and this is a much slower process than it would be for ions. This allows for temperature and density inputs to be greater at a given B field strenth without the need to grow the machine size as much. This allows Polywells to have a fusion energy density much greater than Tokamaks, ie smaller machines at the same fusion output.

The plasma containment in a Polywell is much worse than in a Tokamak (perhaps 10s of milliseconds vs 100s of seconds) but this has to be compared to the relative density , temperatures and resultant fusion rates. A few milliseconds of containment in the Polywell is sufficient.
Very importantly, the fusion rate versus the loss rate still the same (Lawson criterion met), but the fate of the fusion products is different. At higher temperatures (MeVs) the fusion products have much slower collision rates than the fuel ions. In the Tokamak, the long confinement times at the relative densities still allows for these energetic particles to reach thermodynamic equalibrium with the rest of the plasma. The fusion products heat the plasma, often called ignition once the fusion ions contributes more heat to the plasma than that input needed to make up for losses . The Polywell with it's much worse losses through cusps allows for these fusion ions to leave before reaching thermodynamic equalibrium with the fuel ions. In an ignition machine, once this condition is reached, no additional power needs to be input and it will continue to run until the fuel is depleted (or other things terminates the burn), It the Polywell, which is not an ignition machine, the fusion continues until the input power is shut off, then, the fusion stops very quickly. Bussard described the Polywell as an amplifier for this reason. This is a fundamental difference in the machine design. It also allows for the fusion ions with most of their KE intact to deposit their energy well outside the reaction space. This allows for direct conversion of much of the output energy, In an ignition machine this is not possible because most, if not almost all of the fusion energy is needed to continue to directly heat the plasma. The maintained heat eventually is transferred into the walls and this drives a thermal conversion plant. This can be done with a Polywell (or DPF, and perhaps other schemes) but direct conversion can enhance the yields in terms of final useful energy (electricity).

Dan Tibbets

[ohiovr]

Thanks Dan,

What if we shot a torodial plasma ring at a spherical neodymium magnet?

http://spectrum.ieee.org/tech-talk/ener ... sion-power

Would the plasma ring want to hang around the magnet? Is this what the spheromak is all about?

[ohiovr]

Thanks Dan,

What if we shot a torodial plasma ring at a spherical neodymium magnet?

http://spectrum.ieee.org/tech-talk/ener ... sion-power

Would the plasma ring want to hang around the magnet? Is this what the spheromak is all about?

"The analysis also showed that plasma confinement improved as the temperature increased."

https://www.llnl.gov/str/Hill.html

[D Tibbets]

Thanks Dan,

What if we shot a torodial plasma ring at a spherical neodymium magnet?

http://spectrum.ieee.org/tech-talk/ener ... sion-power

Would the plasma ring want to hang around the magnet? Is this what the spheromak is all about?

I don't think so, but I am not an expert by any stretch of the imagination. It looks more like the torus that has its own confining magnetic properties as does the current torus in a Tokamak. Important to note that they quote a temperature equivalent to the Suns surface. That is a trivial- 7,000 K ~ equal to 0.7 eV. It is at the Sun's core temperature of ~ 1.4 KeV that things start to get interesting for fusion, at least for extremely high densities and astounding confinement times as in the Sun's core. Gravity works very well for confinement and heating of plasmas both, but only with huge masses contributing. No mention is made of the confined density or the rate of loss in the link. Only the time (at relatively tiny temperatures) it takes the identifiable ring structure to completely fall apart.

Does it represent a useful property? I don't know. Is it different than widely studied torus systems? I doubt it. Does it have to be extrapolated to extreme proportions before it reaches significant fusion temperatures and densities? Probably...

If the ring held together longer at increased initial temperature, that may or may not be significant. What is really important is energy confinement. If the initial temperature increase merely means that it starts at a higher point and loses energy at a constant rate, it will take longer to dissipate. The energy loss rate could be the same or even worse. Much more information is needed to appreciate the results.

My understanding of the Tokamak is that the basic torus morphology is straight forward and extremely favorable for useful fusion. The problem is those pesky edge instabilities, huge sizes needed, diverter challenges, heating challenges, tritium production needs, and ....

Dan Tibbets

[ohiovr]

Thanks Dan,

What if we shot a torodial plasma ring at a spherical neodymium magnet?

http://spectrum.ieee.org/tech-talk/ener ... sion-power

Would the plasma ring want to hang around the magnet? Is this what the spheromak is all about?

I don't think so, but I am not an expert by any stretch of the imagination. It looks more like the torus that has its own confining magnetic properties as does the current torus in a Tokamak. Important to note that they quote a temperature equivalent to the Suns surface. That is a trivial- 7,000 K ~ equal to 0.7 eV. It is at the Sun's core temperature of ~ 1.4 KeV that things start to get interesting for fusion, at least for extremely high densities and astounding confinement times as in the Sun's core. Gravity works very well for confinement and heating of plasmas both, but only with huge masses contributing. No mention is made of the confined density or the rate of loss in the link. Only the time (at relatively tiny temperatures) it takes the identifiable ring structure to completely fall apart.

Does it represent a useful property? I don't know. Is it different than widely studied torus systems? I doubt it. Does it have to be extrapolated to extreme proportions before it reaches significant fusion temperatures and densities? Probably...

If the ring held together longer at increased initial temperature, that may or may not be significant. What is really important is energy confinement. If the initial temperature increase merely means that it starts at a higher point and loses energy at a constant rate, it will take longer to dissipate. The energy loss rate could be the same or even worse. Much more information is needed to appreciate the results.

My understanding of the Tokamak is that the basic torus morphology is straight forward and extremely favorable for useful fusion. The problem is those pesky edge instabilities, huge sizes needed, diverter challenges, heating challenges, tritium production needs, and ....

Dan Tibbets

Making your 13 KeV plasma is the kind of the easy part, even kids armed with table top fusors can do this. Preventing the plasma from draining all its energy before any productivity seems to be the hard part. I guess with my line of reasoning, you can easily pump energy into this vortex using gyrotrons or magnetrons although this might not be the most useful way. As the plasma is twisting in a vortex the charged particles naturally make magnetic fields. Its natural field configuration makes for good confinement. If the energy you pump into the vortex could also make it twist faster, you get a better field automatically and your heating energy also.

[D Tibbets]

An interesting (and very expensive) experiment trying to inject charged particles into the Earth's exo atmospheric magnetic field and measuring persistence and distribution was undertaken with nuclear weapons in 1958. This Argus experiment is described in this declassified video.

https://www.youtube.com/watch?v=2kMLdwbaIj8

Dan Tibbets

[choff]

Re: Stating the obvious thread & Van Allen Belts, thought I'd repeat this link from the End of the World thread, since it talks about how long humans have been observing plasmas plus nuke bomb generated auroras.

http://www.theplasmaverse.com/verse/squ ... lyphs.html

[D Tibbets]

Thanks Dan,

What if we shot a torodial plasma ring at a spherical neodymium magnet?

http://spectrum.ieee.org/tech-talk/ener ... sion-power

Would the plasma ring want to hang around the magnet? Is this what the spheromak is all about?

I don't think so, but I am not an expert by any stretch of the imagination. It looks more like the torus that has its own confining magnetic properties as does the current torus in a Tokamak. Important to note that they quote a temperature equivalent to the Suns surface. That is a trivial- 7,000 K ~ equal to 0.7 eV. It is at the Sun's core temperature of ~ 1.4 KeV that things start to get interesting for fusion, at least for extremely high densities and astounding confinement times as in the Sun's core. Gravity works very well for confinement and heating of plasmas both, but only with huge masses contributing. No mention is made of the confined density or the rate of loss in the link. Only the time (at relatively tiny temperatures) it takes the identifiable ring structure to completely fall apart.

Does it represent a useful property? I don't know. Is it different than widely studied torus systems? I doubt it. Does it have to be extrapolated to extreme proportions before it reaches significant fusion temperatures and densities? Probably...

If the ring held together longer at increased initial temperature, that may or may not be significant. What is really important is energy confinement. If the initial temperature increase merely means that it starts at a higher point and loses energy at a constant rate, it will take longer to dissipate. The energy loss rate could be the same or even worse. Much more information is needed to appreciate the results.

My understanding of the Tokamak is that the basic torus morphology is straight forward and extremely favorable for useful fusion. The problem is those pesky edge instabilities, huge sizes needed, diverter challenges, heating challenges, tritium production needs, and ....

Dan Tibbets

Making your 13 KeV plasma is the kind of the easy part, even kids armed with table top fusors can do this. Preventing the plasma from draining all its energy before any productivity seems to be the hard part. I guess with my line of reasoning, you can easily pump energy into this vortex using gyrotrons or magnetrons although this might not be the most useful way. As the plasma is twisting in a vortex the charged particles naturally make magnetic fields. Its natural field configuration makes for good confinement. If the energy you pump into the vortex could also make it twist faster, you get a better field automatically and your heating energy also.

Yes it is easy to accelerate charged particles. and moving charged particles create their own magnetic field, and under certain conditions this plasma may hold together for long times. But, the very important problem is useful fusion, not some fusion. For significantly useful fusion yields, you need densities that results in collisions. Collisions tend to disrupt magnetic confined architecture. It is a competitive battle between confinement time versus density and temperature. Any one parameter can easily be addressed, even two may not be too challenging. But getting all three to coexist harmoniously is the challenge. It is easy to heat a gas to very high temperatures. Even millions of eV are easy for table top accelerators. It is easy to get a high density- any gas phase changed to a liquid demonstrates this. It is somewhat easy to confine a non collisional plasma at cold temperatures. Non collisional plasma is another way of saying a very low density plasma. Storage rings for Muons, penning traps for antimatter plasma are examples of this. Confinement times in days or even weeks is not unreasonable. But these are at very low densities.

You could conceivably build a fusion reactor with a low density plasma, and with good energy recovery systems, perhaps have a net positive energy output. You spend a 100 million dollars, and you have a machine that produces 1 milliwatt of net output power. This is not useful. You need much higher densities to drive fusion output to useful levels and that leads to all of the problems.

It's like being in a fight with yourself. You can't win, unless ...

Dan Tibbets

[ohiovr]

Thanks Dan, very easy to understand you. Did you read my fusion idea in the general forum?

[D Tibbets]

I just briefly looked at it. I agree that the General Fusion approach is perhaps a good example of this idea. Blasting a shock wave to compress and heat a target volume is a good idea, but the devil is again in the details. The Laser inertial compression scheme is another example. Remember that the Lawson criterion of triple product is the issue though. Not only compression, but the stability/ confinement time of the compressed plasma is critical. Implosions are apparently very finicky beasts . If there is any inconsistency/ turbulence in the compression shockwave the plasma will quickly spurt out of the weak spots. The confinement time is thus compromised. Efforts to minimize this shock wave inconsistency seems to be the critical issue, not how much overall compression you might get in the absence of instabilities/ conformational irregularities of the compression shock wave. Thus the complex efforts in laser inertial confinement for fusion, along with atom bombs, etc.

Apparently General Fusions efforts , as well as NIF efforts, to stabilize and prolong the confinement time is where the most gains can be made.

Dan Tibbets