But, wouldn't raising the B-field while keeping the same E-field decrease beta, thus impairing confinement?MSimon wrote:
What actually happens is the density gets higher in the core. The e-field can stay the same.
New Superconductor Found "Immune To Magnetism"
If I'm not wrong (again 
), on a neutral plasma in equilibrium with magnetic confinement, pressure against the b-field depends mainly on the kinetic energy and density of its particles. But we are talking about a non-neutral plasma. Doesn't that change things a bit?
A little mental experiment. We start with a wiffle-ball already formed but "empty" of ions (plasma of only electrons and low density D). It has a (mean) radius R and a net charge Q.
If we begin to inject neutrals (that very quickly become ions and electrons) to ramp up density, lets say 100 fold, will R grow proportionaly?
If after that we switch from injecting neutral to injecting electrons to double Q, will then R grow?
Maybe I'm just thick
), on a neutral plasma in equilibrium with magnetic confinement, pressure against the b-field depends mainly on the kinetic energy and density of its particles. But we are talking about a non-neutral plasma. Doesn't that change things a bit?A little mental experiment. We start with a wiffle-ball already formed but "empty" of ions (plasma of only electrons and low density D). It has a (mean) radius R and a net charge Q.
If we begin to inject neutrals (that very quickly become ions and electrons) to ramp up density, lets say 100 fold, will R grow proportionaly?
If after that we switch from injecting neutral to injecting electrons to double Q, will then R grow?
Maybe I'm just thick
It's just really complicated. The concept of "pressure" is a thermodynamic one based on neutrals colliding with each other at random. Ions and electrons collide with each other over large ranges because they have an electric field that extends over all space. We assume the Debye length mitigates that a little, and try to estimate what pressure means in a plasma.
If you only have one species, life is a little less complicated in terms of math, but it is way more complicated in terms of long range effects. A blob of electrons tries to run away from itself - it blows up really fast. A blob of plasma oscillates around its center. It can still blow up, but it is a lot slower than a single species of charged particles.
Yes - if you add particles things want to get bigger and it takes more effort to contain them. It's like holding down a balloon. As long as your hand is larger than the balloon, no problem. Once it gets a little bit bigger - it squirms out.
If you only have one species, life is a little less complicated in terms of math, but it is way more complicated in terms of long range effects. A blob of electrons tries to run away from itself - it blows up really fast. A blob of plasma oscillates around its center. It can still blow up, but it is a lot slower than a single species of charged particles.
Yes - if you add particles things want to get bigger and it takes more effort to contain them. It's like holding down a balloon. As long as your hand is larger than the balloon, no problem. Once it gets a little bit bigger - it squirms out.
Thank you drmike.
I hope I'm not taking advantage of you, but can you tell me if the next statements are right? (last questions in this threath, I promise
)
1)In a [fully ionized] neutral thermal plasma magnetically contained the "pressure" it exerts agains the field is mediated by all its charged constituents (+ions and electrons). In this situation the ions are the most difficult to retain due to their much higher mass.
2)Plasma pressure in this case is mainly dependent on density and average kinetic energy/temperature.
3)In a Polywell, where there is a slight negative net charge and the kinetic energy of the +ions near the magrid is low, the plasma pressure is mostly mediated by the electrons.
4)Point 3 is what (supposedly) makes possible ion confinement with such low b-fields in contrast with tokamaks.
5)In this case plasma pressure is mainly dependent of density, kinetic energies of electrons and ions, and net charge.
P.D. Wouldn't be a good idea to create a forum of frecuent Q&A, so rookies like myself dont bother too much by asking the same questions once and again? I've searched and read a lot of material about fusion with PW from papers and forums, but much of it is too technical or too simplified, or just obscure. Could be a cooperative efort, someone writes a short article on one aspect or other of the Polywell, and it get reviewed on line by the other members to prevent mistakes.
I hope I'm not taking advantage of you, but can you tell me if the next statements are right? (last questions in this threath, I promise
)1)In a [fully ionized] neutral thermal plasma magnetically contained the "pressure" it exerts agains the field is mediated by all its charged constituents (+ions and electrons). In this situation the ions are the most difficult to retain due to their much higher mass.
2)Plasma pressure in this case is mainly dependent on density and average kinetic energy/temperature.
3)In a Polywell, where there is a slight negative net charge and the kinetic energy of the +ions near the magrid is low, the plasma pressure is mostly mediated by the electrons.
4)Point 3 is what (supposedly) makes possible ion confinement with such low b-fields in contrast with tokamaks.
5)In this case plasma pressure is mainly dependent of density, kinetic energies of electrons and ions, and net charge.
P.D. Wouldn't be a good idea to create a forum of frecuent Q&A, so rookies like myself dont bother too much by asking the same questions once and again? I've searched and read a lot of material about fusion with PW from papers and forums, but much of it is too technical or too simplified, or just obscure. Could be a cooperative efort, someone writes a short article on one aspect or other of the Polywell, and it get reviewed on line by the other members to prevent mistakes.
No problem! The process of answering wrong is where I learn thingscharliem wrote:Thank you drmike.
I hope I'm not taking advantage of you, but can you tell me if the next statements are right? (last questions in this threath, I promise
Yes. The pressure is the same thermodynamic concept as with neutral
1)In a [fully ionized] neutral thermal plasma magnetically contained the "pressure" it exerts agains the field is mediated by all its charged constituents (+ions and electrons). In this situation the ions are the most difficult to retain due to their much higher mass.
atoms - how many times atoms hit a surface in a given unit of time. With
a magnetic field, the surface is a uniform magnetic field strength.
Yes.2)Plasma pressure in this case is mainly dependent on density and average kinetic energy/temperature.
There is a negative net charge in the center, I'm not sure about near the grid. I think the kinetic energy of the ions near the grid should be low, but in the center of the grid I'm not so sure (cusp regions). The equation of state for a non thermal plasma is not going to be pretty, so I'm going to stick with "I don't know" (which really means "I don't have a clue because I haven't got my particle distribution code to work yet").
3)In a Polywell, where there is a slight negative net charge and the kinetic energy of the +ions near the magrid is low, the plasma pressure is mostly mediated by the electrons.
No. The confinement is a combination of electrostatic forces (the ions see the negative center and positive grid so they want to go to the center) and magnetic mirror forces (the ions reflect at a cusp) along with the fact that plasmas are stable when the curvature of the magnetic field is negative relative to the plasma (i.e. the plasma is outside the coil).
4)Point 3 is what (supposedly) makes possible ion confinement with such low b-fields in contrast with tokamaks.
Charge has nothing to do with pressure. It is all the other properties that define pressure. The charges determine where the pressure gets applied, and that's what makes the whole thing so complicated.
5)In this case plasma pressure is mainly dependent of density, kinetic energies of electrons and ions, and net charge.
P.D. Wouldn't be a good idea to create a forum of frecuent Q&A, so rookies like myself dont bother too much by asking the same questions once and again? I've searched and read a lot of material about fusion with PW from papers and forums, but much of it is too technical or too simplified, or just obscure. Could be a cooperative efort, someone writes a short article on one aspect or other of the Polywell, and it get reviewed on line by the other members to prevent mistakes.
It's a matter of taste. Some people like wiki's for that. I think repetition is good because different people say the same facts in different ways, so it is easier for more people to "get it". Sometimes too many facts get in the way.
But I also think we need to make sure this can really work. Evangelical optimism is great, but it needs to be backed up with reality. A few months waiting will be well worth it - if we find out it can't work because of some major flaw nobody thought of before but is discovered by experiment then it's much better to stay as a back water of research. If we find out it can work, we can go gung ho full blast knowing we're backed up by reality.
Just my 2 cents.
I think you are right about ions reflecting at a cusp. I don't think it is significant (at least until we can get fields in the 20 to 80T range).No. The confinement is a combination of electrostatic forces (the ions see the negative center and positive grid so they want to go to the center) and magnetic mirror forces (the ions reflect at a cusp) along with the fact that plasmas are stable when the curvature of the magnetic field is negative relative to the plasma (i.e. the plasma is outside the coil).
The LHC has some humongous 20T coils. So we should start seeing an effect in that range. i.e alphas bent away from the center of metal to where the metal slopes more - it will make cooling/higher power operation easier.
Engineering is the art of making what you want from what you can get at a profit.
Great. One of the things I find confusing is trying to picture the values and signs of potentials in the 3-D world of a polywell. Like, the plasma's going to have a small negative net charge, but what's the potential? What's the potential and sign at the surface of the coils? The guns? The vacuum tank?It's a matter of taste. Some people like wiki's for that. I think repetition is good because different people say the same facts in different ways, so it is easier for more people to "get it".
And for extra credit, how 'bout the same thing where the polywell has the famous decelerating grids for alphas?
TBDJohnP wrote:Great. One of the things I find confusing is trying to picture the values and signs of potentials in the 3-D world of a polywell. Like, the plasma's going to have a small negative net charge, but what's the potential? What's the potential and sign at the surface of the coils? The guns? The vacuum tank?It's a matter of taste. Some people like wiki's for that. I think repetition is good because different people say the same facts in different ways, so it is easier for more people to "get it".
And for extra credit, how 'bout the same thing where the polywell has the famous decelerating grids for alphas?
Engineering is the art of making what you want from what you can get at a profit.
That's what I meant when I asked if in Polywells plasma pressure [near the magrid] was mostly mediated by electrons, that the force they exert against the magnetic field is orders of magnitud higher than that of the ions.MSimon wrote:I think you are right about ions reflecting at a cusp. I don't think it is significant (at least until we can get fields in the 20 to 80T range).
But I promised, no more questions from me in this thread.
And by the way, I'm no engineer nor physicist, just a teacher, but I think I'm good finding ways to explain math and science to students and laymen (once I understand them myself, of course
). For what is worth sign me in if the FAQ/Wiki thing goes on.Yes Simon, I did.
I've been reading all the info I could find on Polywells (and fusion in general) for the last 3 or 4 months. I'm not afraid of maths.
I found Tom's article most interesting and clear (thanks Tom).
My point is that during my search I've found many [very tough] technical papers about physics related aspects, and I've found some more texts (like Tom's) aimed to the layman, but what I have not found is texts in the middle (for those who are not physicists but even so want to know a bit more), and at last, about a few I have found almost nothing. For instance there is very little about the wiffle-ball efect:
-How is a wiffle-ball formed?
-Does the plasma created b-field nullifies or just displace the coils field?, and how does it do it?.
-What is exactly the geometry of the Polywell's cusps (with some numbers)?, are the corner's line or point and why?
-How does that the expansion of the plasma b-field close the cusps?
So, my point is that about some matters there is little info, and about others there is, but only too advanced or too simplified.
I've been reading all the info I could find on Polywells (and fusion in general) for the last 3 or 4 months. I'm not afraid of maths.
I found Tom's article most interesting and clear (thanks Tom).
My point is that during my search I've found many [very tough] technical papers about physics related aspects, and I've found some more texts (like Tom's) aimed to the layman, but what I have not found is texts in the middle (for those who are not physicists but even so want to know a bit more), and at last, about a few I have found almost nothing. For instance there is very little about the wiffle-ball efect:
-How is a wiffle-ball formed?
-Does the plasma created b-field nullifies or just displace the coils field?, and how does it do it?.
-What is exactly the geometry of the Polywell's cusps (with some numbers)?, are the corner's line or point and why?
-How does that the expansion of the plasma b-field close the cusps?
So, my point is that about some matters there is little info, and about others there is, but only too advanced or too simplified.
Actually, I think the 6 in 62500 was a typo. Based on his numbers in that post, it looks like he meant 2500 (unless I'm missing something). Still very impressive, though.A calculated 62,000 times increase in density vs ITER with 10 T magnets. ITER is using 20 T magnets.
As he points out, that's even without ion focussing.
I would recommend the Valencia paper. p9.-How is a wiffle-ball formed?
http://www.askmar.com/ConferenceNotes/2 ... 0Paper.pdf
Unlike a tokamak, there isn't a plasma-generated B field.Does the plasma created b-field nullifies or just displace the coils field?,
To the extent we know, Indrek's drawings are probably the best.What is exactly the geometry of the Polywell's cusps (with some numbers)?, are the corner's line or point and why?
The electron pressure tends to flatten the field lines facing inward. As the fields flatten the gaps between them smaller. At B=1, they are very nearly closed.-How does that the expansion of the plasma b-field close the cusps?
I did a quick in my head and Nebel's number seemed OK. But even a 2,500X increase in density is excellent.TallDave wrote:Actually, I think the 6 in 62500 was a typo. Based on his numbers in that post, it looks like he meant 2500 (unless I'm missing something). Still very impressive, though.A calculated 62,000 times increase in density vs ITER with 10 T magnets. ITER is using 20 T magnets.
As he points out, that's even without ion focussing.
Engineering is the art of making what you want from what you can get at a profit.