Hi, all
I got an idea about fusor design, it's basically a tokamak with some modification. Here is it:
Adjust the magnetic field of a tokamak device so that the magnetic field is strong enough to confine electrons but too weak to does serious impact on ions. Since ions are thousands of times massive than electrons, this should be achievable.
Inject electrons into this kinds of tokamak we should be able to get a torus shaped "electron cloud"(the virtual cathode). Then inject ions into this device, the virtual cathode _might_ be able to confine the ions and get fusion reactions. From this point of view, this device is like a polywell.
Because the magnetic field of the tokamak is weakened, the virtual cathode may not be able to accelerate the ions fast enough to fuse. Even if the ions can fuse, the fusion rate should be lower than a standard polywell. Because the virtual cathode in this device is torus shaped, the ion density will be lower than a standard polywell.
Any ideas about this?
Thanks
torshie
tokamak-polywell hybrid fusor
mmm i dont think this is going to work. you need magnetic coils circling around the toroid (as opposed to the rings in a polywell, where they are arranged on the face of the taurus). and like you said, the electrons will be in a ring shape so if you imagine a circular cross section of a portion the tokamak, ions will converge to the center of that (instead of the center of the device), meaning the fusion area has been spread out a lot more than with a traditional polywell.
tokamaks were designed to achieve fusion via thermal movement, whereas polywells are more inertial by means of charge attraction. what would be the purpose of combining polywell and tokamak designs? with the descriptions given, it seems that it would require more energy to coax such a machine to produce fusion
tokamaks were designed to achieve fusion via thermal movement, whereas polywells are more inertial by means of charge attraction. what would be the purpose of combining polywell and tokamak designs? with the descriptions given, it seems that it would require more energy to coax such a machine to produce fusion
IMO, by combining of the two designs we can get the strong points of both design (of course, we may just get the shortcomings instead of strong points of the two designs).
The strong point of tokamak its good electron confinement ability. The strong point of polywell is the "charge attraction" like you said.
I do agree with you that this machine is more difficult to coax to produce fusion than a standard polywell, if not impossible. But it should be easier than a standard tokamak, since in this design the ions are confined by electrostatic field instead of magnetic field.
What is the density of "electron cloud" we can get in this design? I think this is the question should be answered first other than the question "whether this design will work". In this design, if the magnetic field is too strong the ions will also be impacted, thus reduce the effect of the "charge attraction". If the magnetic field is too weak the density of the "electron cloud" will be very low, and the "charge attraction" will be too weak to fuse ions.
The strong point of tokamak its good electron confinement ability. The strong point of polywell is the "charge attraction" like you said.
I do agree with you that this machine is more difficult to coax to produce fusion than a standard polywell, if not impossible. But it should be easier than a standard tokamak, since in this design the ions are confined by electrostatic field instead of magnetic field.
What is the density of "electron cloud" we can get in this design? I think this is the question should be answered first other than the question "whether this design will work". In this design, if the magnetic field is too strong the ions will also be impacted, thus reduce the effect of the "charge attraction". If the magnetic field is too weak the density of the "electron cloud" will be very low, and the "charge attraction" will be too weak to fuse ions.
The purpose of the polywell design is to keep a bunch of electrons confined with magnetic fields, then get ions to accelerate towards them.
It is far far more successful to confine ions in a toroidal field with no cups at all than with a multi-cusp field. Ions could then be injected, just like polywell intent, so that they trace back-and-forth across poloidal diameters of the device such that, in poloidal cross-section, a toroidal polywell and a multi-cusp wiffle-balled polywell would look the same diagram.
So, yes, you are exactly correct. I have mentioned it a couple of times here in other contexts, but no-one here has commented that they understand this.
It is far far more successful to confine ions in a toroidal field with no cups at all than with a multi-cusp field. Ions could then be injected, just like polywell intent, so that they trace back-and-forth across poloidal diameters of the device such that, in poloidal cross-section, a toroidal polywell and a multi-cusp wiffle-balled polywell would look the same diagram.
So, yes, you are exactly correct. I have mentioned it a couple of times here in other contexts, but no-one here has commented that they understand this.
If you can't confine electrons in a toroidal field sufficiently to cause ions to reciprocate through it, there's damned-all chance of being able to do it with one with cusps in! At the very very least, it seems like a 'step 1' experiment to me, to prove out that ions can be contained by the space-charge of an electron field. I do not recognise a single experiment that yet demonstrates this. Even if the toroidal experiment doesn't do fusion because it doesn't bring ions to a unique point, it would still hold up as 'polywell relevant', and I tend to think you should, still, get good fusion along the main toroidal circumference (the 'central point' extruded toroidally) so I would expect to see some beam-beam fusion going on, which is more than I recognise for polywell so far. And, actually, I'd be kinda surprised if it does not beat the multi-cusps version due to the efficiency of zero geometric routes for electron loss.torshie wrote:I do agree with you that this machine is more difficult to coax to produce fusion than a standard polywell, if not impossible.
I heard about a small tokamak that was made available to people who wanted to run their own experiments on it. You could change the parameters via the Internet. Maybe you could test out your ideas that way.
http://tokamakglobal.com/
http://tokamakglobal.com/
Checked the link, but I think we have to add an e-gun to a standard tokamak in order to experiment the ideas talked above.TimTruett wrote:I heard about a small tokamak that was made available to people who wanted to run their own experiments on it. You could change the parameters via the Internet. Maybe you could test out your ideas that way.
http://tokamakglobal.com/
I missed that one. What an interesting project and idea. I love this way of spreading knowledge and interest into science by letting people actively participate.TimTruett wrote:I heard about a small tokamak that was made available to people who wanted to run their own experiments on it. You could change the parameters via the Internet. Maybe you could test out your ideas that way.
http://tokamakglobal.com/
Double thumbs up!
I can't say I am as warm to this as one might hope to be. So there is a specific experiment with specific variables, and they're asking for someone to set a few of the conditions? It's like running an experiment to prove that drugs are harmless, and you ask 'the world' to participate in this global experiment by determining the temperature of the drugs before the subject takes them.Giorgio wrote:I missed that one. What an interesting project and idea. I love this way of spreading knowledge and interest into science by letting people actively participate.TimTruett wrote:I heard about a small tokamak that was made available to people who wanted to run their own experiments on it. You could change the parameters via the Internet. Maybe you could test out your ideas that way.
http://tokamakglobal.com/
Double thumbs up!
One might claim that the experiment is flawed in the first place, and picking a few variables within the method, already set, does nothing much for scientific benefit or 'global participation'.
Seems a bit weird to me... I don't buy it as a 'global experiment' in which people can participate.
I do not see this as addressed for field scientists, I see it more a way to bring interest in students about this type of research.
Even amateur students if you want.
I also agree that from the scientific point of view the results are near to nil, but from the PR point of view this is what the Fusion scientific community (especially the non ITER community) desperately needs.
Of course these are just personal opinions and everyone is free to have his own.
Even amateur students if you want.
I also agree that from the scientific point of view the results are near to nil, but from the PR point of view this is what the Fusion scientific community (especially the non ITER community) desperately needs.
Of course these are just personal opinions and everyone is free to have his own.
fair play. I suppose as a 'calling card' for tokamaks, it probably works at the high school level!
I am still interested in why the experiment is still hammering on the same old, one-and-only, first proposed solution, magnetic confinement. 60 years after the original suggestion by Spitzer to confine thermonuclear plasmas, there is still no specific evidence that it is possible to do it continuously. You'd think 60 years of an experiment would tend folks towards other solutions. OK, so we have a few 'fringe' groups/individuals at it seriously, but it'll seem odd in the future that the mainstream still promoted this to the geek-younglings, even though it didn't work out.
I am still interested in why the experiment is still hammering on the same old, one-and-only, first proposed solution, magnetic confinement. 60 years after the original suggestion by Spitzer to confine thermonuclear plasmas, there is still no specific evidence that it is possible to do it continuously. You'd think 60 years of an experiment would tend folks towards other solutions. OK, so we have a few 'fringe' groups/individuals at it seriously, but it'll seem odd in the future that the mainstream still promoted this to the geek-younglings, even though it didn't work out.
I believe this has been touched on before. I believe the key points may be the torus shape and the lack of cusps. The charged particles are free to travel along the axis, and so intentionally or otherwise a dominate direction will develop along the torus axis. This would result in a magnitized plasma. No Wiffleball could form. Though with the much better confinement it may not be needed. Of course in this case the density could not be improved over the general Tokamak, so very large machines would be needed to generate the same amount of power. This power density is independent of general confinement issues (sort of!). The torus shape also means there is an inside curve and an outside curve, centrifical forces , etc would need to be considered. There would be no near spherical geometry. A spheromak might be a mildly better candidate.
Also, keep in mind, that to prevent ambipolar flow the difference between the sphere and torus needs to be considered. I'm not sure what this would entail.
The question boils down to: has anyone tried to work with non neutral plasmas in cuspless machines? I have heard hints that TriAlpha's FRC may incorporate this to some degree (or at least polarizing the plasma) .
Keep in mind Rider's, etc. criticisms, mostly concerning the thermalizing of the electrons to the degree that at least P-B11 fusion is impractical . The cusps provide a relatively cheep mechanism for removing the high energy tail of the electrons. Cuspless machines do not have this. In the Polywell, the electrons that are contained/ recirculated have a high radial velocity component over their lifetime. Longer lifetimes would negate this as the electrons assumed spiraling paths along field lines. Even with a excess of electrons I don't know if a potential well could form. ie, Goldilocks analogy again. Life time too short and it costs too much energy, lifetime too long and the electron cloud characteristics lose their ability to form a potential well. The ions and electrons would be randomly mixed- no potential well. Even if this was still acceptable from a fusion reaction rate, the inhibitions against bipolar flow would be greatly minimized- which means you would not have any, or much reduced, ion confinement advantages. This, I believe, was part of the basis for A. Carlson's argument that bipolar flow could not be avoided. There was no polarization of the plasma. Or better stated, the Polywell allows for polarized charged particle energy distribution, not just population distribution.
Another point. Ignoring the density question, the potential well is critical for ion containment. If there is a thermalized ion distribution (and electron distribution. you would not have the advantage of the electrostatic driven turnaround for the ions. They could enter the magnetic field domain more frequently, and more importantly, do so at high energy with the corresponding high gyroradii. ExB drift could then proceed is a similar manner to neutral plasma Tokamaks. There may be some retarding of this process as the excess electrons are left behind due to their smaller gyroradii. Bipolar flow restraint considerations might result in some benefits- the same ion confinement time in marginally smaller machines, and this might be worthwhile (?), but would fall far short of what occurs in a Polywell. I think for even this miner effect to occur, the electron lifetimes would need to be shorter than the ion lifetimes, otherwise once the initial distribution was established, the system would equilibrate at some mild polarization that would not have any further drift inhibiting effect. The ion density might be increased slightly towards the electron density, bu as there is only a few parts per million that can be achieved, the effect would be meaningless. Enter here all sorts of plasma parameters like Debye length, speed of sound, various waves, etc that would probably give a plasma physicist a headache.
Basically, you may not be able to achieve the density advantage (and remember the triple product- confinement time needed, density, and temperature are all dependent on each other), so the increased confinement inherent in the Tokamak is no advantage, if you cannot increase the critical density or to a lesser degree the temperature. Shrinking the machine would save money, but without increasing the density, the fusion output also drops at r^3 scaling.
The cusps, while often described as a disadvantage, are actually of vital importance for electron energy management, ash removal, and for any consideration for direct conversion.
Dan Tibbets
Also, keep in mind, that to prevent ambipolar flow the difference between the sphere and torus needs to be considered. I'm not sure what this would entail.
The question boils down to: has anyone tried to work with non neutral plasmas in cuspless machines? I have heard hints that TriAlpha's FRC may incorporate this to some degree (or at least polarizing the plasma) .
Keep in mind Rider's, etc. criticisms, mostly concerning the thermalizing of the electrons to the degree that at least P-B11 fusion is impractical . The cusps provide a relatively cheep mechanism for removing the high energy tail of the electrons. Cuspless machines do not have this. In the Polywell, the electrons that are contained/ recirculated have a high radial velocity component over their lifetime. Longer lifetimes would negate this as the electrons assumed spiraling paths along field lines. Even with a excess of electrons I don't know if a potential well could form. ie, Goldilocks analogy again. Life time too short and it costs too much energy, lifetime too long and the electron cloud characteristics lose their ability to form a potential well. The ions and electrons would be randomly mixed- no potential well. Even if this was still acceptable from a fusion reaction rate, the inhibitions against bipolar flow would be greatly minimized- which means you would not have any, or much reduced, ion confinement advantages. This, I believe, was part of the basis for A. Carlson's argument that bipolar flow could not be avoided. There was no polarization of the plasma. Or better stated, the Polywell allows for polarized charged particle energy distribution, not just population distribution.
Another point. Ignoring the density question, the potential well is critical for ion containment. If there is a thermalized ion distribution (and electron distribution. you would not have the advantage of the electrostatic driven turnaround for the ions. They could enter the magnetic field domain more frequently, and more importantly, do so at high energy with the corresponding high gyroradii. ExB drift could then proceed is a similar manner to neutral plasma Tokamaks. There may be some retarding of this process as the excess electrons are left behind due to their smaller gyroradii. Bipolar flow restraint considerations might result in some benefits- the same ion confinement time in marginally smaller machines, and this might be worthwhile (?), but would fall far short of what occurs in a Polywell. I think for even this miner effect to occur, the electron lifetimes would need to be shorter than the ion lifetimes, otherwise once the initial distribution was established, the system would equilibrate at some mild polarization that would not have any further drift inhibiting effect. The ion density might be increased slightly towards the electron density, bu as there is only a few parts per million that can be achieved, the effect would be meaningless. Enter here all sorts of plasma parameters like Debye length, speed of sound, various waves, etc that would probably give a plasma physicist a headache.
Basically, you may not be able to achieve the density advantage (and remember the triple product- confinement time needed, density, and temperature are all dependent on each other), so the increased confinement inherent in the Tokamak is no advantage, if you cannot increase the critical density or to a lesser degree the temperature. Shrinking the machine would save money, but without increasing the density, the fusion output also drops at r^3 scaling.
The cusps, while often described as a disadvantage, are actually of vital importance for electron energy management, ash removal, and for any consideration for direct conversion.
Dan Tibbets
To error is human... and I'm very human.
This exposes what sounds like a huge bias you have in your thinking, Dan.D Tibbets wrote:I believe this has been touched on before. I believe the key points may be the torus shape and the lack of cusps. The charged particles are free to travel along the axis, and so intentionally or otherwise a dominate direction will develop along the torus axis. This would result in a magnitized plasma. No Wiffleball could form.
You say it won't work because charged particles are free to take up a toriodal path - and I am inclined to agree with you because you may expect a system to take up the entropy maximum.
..BUT in a wiffleball ions are equally free to circulate around the centre in circumferential orbits and, indeed, in many other paths (more than the tokamak, in fact, so it can take up an even higher entropy state, thus even harder to maintain non-Maxwellian distributions).
So if this argument holds as an argument that a 'tokamak' polywell will thermalise, then it will also be an argument that cuspy polywell will thermalise. It is, in fact, one of the arguments that Art and I have said shows that polywell will thermalise because there must be some collisions not at the centre of polywell, and when they do, those ions may then orbit around inside the wiffleball.
I'm uncertain of the detail of chaos theory or lowest entropy states, but I'm basing my statement mostly on others comments. The Tokamak is like a transformer with the plasma acting as the secondary windings. I suppose it is possible to have a centrally convergent (towards the center of the toroidal crossection), and it is possible to have a purely random motion of the plasma particles, but due to the Lorentz force, the particles will tend to turn at right angles to the magnetic field, so some linear motion along the toroid length will tend to form and this will establish a direction which will cascade till most of the plasma has this behavior. It would be difficult and probably expensive to prevent this.
In a Polywell, with it's near spherical shape I suspect this spinning would be inhibited as this would not be the lowest energy state(?). If it was closed ended cylinder (as opposed to en equivalent open ended cylindrical torus) then a dominate linear (or circular in a torus) motion may develop to a significant degree. This is not necessarily bad. It depends on the interactions. It may help with the DPF for instance.
I suspect the most critical aspect though is the initial near radial vectors of the electrons towards the center and the short lifetime of the electrons (on the order of ~ 100 microseconds without recirculation (which resets the electrons to virgin conditions). If the electrons survived long enough they would tend to assume magnetic field line orientations and become a circulating plasma that would thus be magnetized. The cusps play a role also as they represent interruptions to this magnetic surface transport. The electrons would be more likely to bounce back, or exit the cusps, thus interrupting the building angular momentum. This process will keep the radial motion of the electrons dominate. I doubt anyone would argue this point directly. The details though are critical. How short does the lifetime need to be? How much does the input radial current have to dominate, and how much does a given recirculation ease this energy input requirement? ie: recirculation is not only important for maintaining the electron density at a given input power, but also important for decreasing the energy cost of maintaining a dominate radial motion component for the electrons (it permits shorter electron lifetimes and thus less time for energy thermalization AND for lorentz force deflections leading to a dominate circular direction and thus magnatized plasma. Also, keep in mind that the ions are introduced without any angular momentum, so they may impede this process in the lighter electrons, thus easing the time limits involved. In, other words the time limits may depend mostly on the ion momentum.
Using some numbers:
Electron lifetime 0.1 ms (energy input costs 10-100 times less due to recirculation).
Some fudge fact for the cusps disturbing this angular momentum direction dominance if it actually develops in a spherical geometry.
Ion lifetime to assume similar angular momentum characteristics (the plasma is weakly coupled). Ion life time may be ~ 10-20 ms before fusion or loss.
Now, develop some formula to incorporate the electron and ion lifetimes, their weak coupling interactions, the anticipated lifetime limit for a simple cuspless sphere before a significant single direction angular momentum develops (if it does at all), multiplied by some cusp modifying constant. The answer is ?
Dan Tibbets
In a Polywell, with it's near spherical shape I suspect this spinning would be inhibited as this would not be the lowest energy state(?). If it was closed ended cylinder (as opposed to en equivalent open ended cylindrical torus) then a dominate linear (or circular in a torus) motion may develop to a significant degree. This is not necessarily bad. It depends on the interactions. It may help with the DPF for instance.
I suspect the most critical aspect though is the initial near radial vectors of the electrons towards the center and the short lifetime of the electrons (on the order of ~ 100 microseconds without recirculation (which resets the electrons to virgin conditions). If the electrons survived long enough they would tend to assume magnetic field line orientations and become a circulating plasma that would thus be magnetized. The cusps play a role also as they represent interruptions to this magnetic surface transport. The electrons would be more likely to bounce back, or exit the cusps, thus interrupting the building angular momentum. This process will keep the radial motion of the electrons dominate. I doubt anyone would argue this point directly. The details though are critical. How short does the lifetime need to be? How much does the input radial current have to dominate, and how much does a given recirculation ease this energy input requirement? ie: recirculation is not only important for maintaining the electron density at a given input power, but also important for decreasing the energy cost of maintaining a dominate radial motion component for the electrons (it permits shorter electron lifetimes and thus less time for energy thermalization AND for lorentz force deflections leading to a dominate circular direction and thus magnatized plasma. Also, keep in mind that the ions are introduced without any angular momentum, so they may impede this process in the lighter electrons, thus easing the time limits involved. In, other words the time limits may depend mostly on the ion momentum.
Using some numbers:
Electron lifetime 0.1 ms (energy input costs 10-100 times less due to recirculation).
Some fudge fact for the cusps disturbing this angular momentum direction dominance if it actually develops in a spherical geometry.
Ion lifetime to assume similar angular momentum characteristics (the plasma is weakly coupled). Ion life time may be ~ 10-20 ms before fusion or loss.
Now, develop some formula to incorporate the electron and ion lifetimes, their weak coupling interactions, the anticipated lifetime limit for a simple cuspless sphere before a significant single direction angular momentum develops (if it does at all), multiplied by some cusp modifying constant. The answer is ?
Dan Tibbets
To error is human... and I'm very human.
Without the ability to accelerate ions to collision speeds, with paths that actually intersect.... Isn't the result going be ions moving in a circle?
And thats the best part about a polywell or even a fusor, is the quasi spherical accelerator cause ions to collide......
I do understand the basics of toridial and Polywell fusion, but not much more. I just dont see it though.
And thats the best part about a polywell or even a fusor, is the quasi spherical accelerator cause ions to collide......
I do understand the basics of toridial and Polywell fusion, but not much more. I just dont see it though.
I like the p-B11 resonance peak at 50 KV acceleration. In2 years we'll know.
The ions bounce around, orbit, curve turn around, converge towards the center, glance off of magnetic surfaces, get captured by magnetic surfaces, field lines. And all of this motion creates magnetic fields around the individual particles. If the ions (or electrons) develop a dominate motion- like in a Tokamak there will be a net magnetic field intensity and direction established by this gang of charged particles. There can still be scattering collisions and mixing on individual scales- and that is what possibly leads to most of the fusion in a Tokamak. It is the average linier flow around the torus that leads to the magnatized plasma and secondary transformer like properties.
In a Polywell or IEC device the charged particles move towards and past an electrode. This motion tends to be radial (depending on the container shape and the electrode placement). With plenty of other charged and possibly uncharged particles, mixing (thermalizing), but any motion that results from externally applied power is towards an electrode. Because of the short lifetimes of the charged particles, they are not left on their own long enough to develop much organization. That said, it is a very simplified view. Various plasma effects, instabilities, etc can very quickly form. Also, other forces can be applied- magnetic, microwaves, etc that can modify things. So, it gets very complex very fast.
You might wonder how a Wiffleball can form if the charged particles are traveling in all directions, and do not generate their own macroscopic magnetic field. I wonder also. My guess is that since the electrons and then the ions are almost totally radial, at least on their first few passes, they are moving back an forth past each other and this radial motion cancels out the magnetic fields on a macroscopic scale ( 1 + minus 1 = 0). Yet the kinetic energy of the charged particles traveling outward push against the electromagnet fields and thus inflate the Wiffleball- much as a gas inflates a balloon. Even as the radial 'purity' relaxes, the effect would persist, up until some time when plasma becomes magnetized- assumes a dominate direction. In a spiky sphere this is more difficult than in a nice long ( infinitely long in a sense- torus) tube. The dynamics of the initial conditions, leakage rate, restoring forces, thermalizing times (both in the sense of energy, and direction) are all important.
Some may think that injecting a bunch of electrons one time creates a persisting potential well. Experiments show that this well is actually a very transient effect. There are tricks that can prolong its time to a limited degree (this is one of the things that was measured in some early POPS work. Having good adiabatic recoil from a near spherical magnetic surface can help, perhaps to a considerable extent, especially if the holes in this surface are small enough (Wiffleball effect). But, the root mechanism for creating and maintaining the potential well is to accelerate a bunch of electrons into the machine at a rate faster than they escape or lose all of their radially dominate motions.
The next to last mechanism was a show stopper in EMC's earlier work up to WB5. It was not just an energy balance question. They could not pump in electrons fast enough (darn the efficiency) to build and maintain a deep potential well (WB 4 was presumably intermadiate). They improved on this with WB5, but at the cost of unacceptable ion losses. WB6 also did it but through optimizing recirculation. Recirculation not only increases effective confinement efficiency, it is also a very cheap power amplifier. The ~ 40 Amps of electron gun current needed to establish (?) the Wiffleball effect and maintain a deep potential well was magnified/ amplified by a factor of ~ 10X by recirculation, so the effective new and reconditioned used electron current was ~ 400 Amps. Presumably it was this current that was needed to keep the machine in optimal steady state. The cusp leakage is a cost but also a critical necessity for the maintainance of the optimal dynamics.
Dan Tibbets
In a Polywell or IEC device the charged particles move towards and past an electrode. This motion tends to be radial (depending on the container shape and the electrode placement). With plenty of other charged and possibly uncharged particles, mixing (thermalizing), but any motion that results from externally applied power is towards an electrode. Because of the short lifetimes of the charged particles, they are not left on their own long enough to develop much organization. That said, it is a very simplified view. Various plasma effects, instabilities, etc can very quickly form. Also, other forces can be applied- magnetic, microwaves, etc that can modify things. So, it gets very complex very fast.
You might wonder how a Wiffleball can form if the charged particles are traveling in all directions, and do not generate their own macroscopic magnetic field. I wonder also. My guess is that since the electrons and then the ions are almost totally radial, at least on their first few passes, they are moving back an forth past each other and this radial motion cancels out the magnetic fields on a macroscopic scale ( 1 + minus 1 = 0). Yet the kinetic energy of the charged particles traveling outward push against the electromagnet fields and thus inflate the Wiffleball- much as a gas inflates a balloon. Even as the radial 'purity' relaxes, the effect would persist, up until some time when plasma becomes magnetized- assumes a dominate direction. In a spiky sphere this is more difficult than in a nice long ( infinitely long in a sense- torus) tube. The dynamics of the initial conditions, leakage rate, restoring forces, thermalizing times (both in the sense of energy, and direction) are all important.
Some may think that injecting a bunch of electrons one time creates a persisting potential well. Experiments show that this well is actually a very transient effect. There are tricks that can prolong its time to a limited degree (this is one of the things that was measured in some early POPS work. Having good adiabatic recoil from a near spherical magnetic surface can help, perhaps to a considerable extent, especially if the holes in this surface are small enough (Wiffleball effect). But, the root mechanism for creating and maintaining the potential well is to accelerate a bunch of electrons into the machine at a rate faster than they escape or lose all of their radially dominate motions.
The next to last mechanism was a show stopper in EMC's earlier work up to WB5. It was not just an energy balance question. They could not pump in electrons fast enough (darn the efficiency) to build and maintain a deep potential well (WB 4 was presumably intermadiate). They improved on this with WB5, but at the cost of unacceptable ion losses. WB6 also did it but through optimizing recirculation. Recirculation not only increases effective confinement efficiency, it is also a very cheap power amplifier. The ~ 40 Amps of electron gun current needed to establish (?) the Wiffleball effect and maintain a deep potential well was magnified/ amplified by a factor of ~ 10X by recirculation, so the effective new and reconditioned used electron current was ~ 400 Amps. Presumably it was this current that was needed to keep the machine in optimal steady state. The cusp leakage is a cost but also a critical necessity for the maintainance of the optimal dynamics.
Dan Tibbets
To error is human... and I'm very human.