looking for an equation, where is the main FAQ for polywell?

[ohiovr]

Polywell FAQ part I

Well that's the beginning of a FAQ at least... I'm sure there are a lot more questions than that.

[Art Carlson]

I'd leave the question open until some detailed engineering is done. For reasons of both power balance and power density, contrary to widespread fantasies, this thing will produce neutrons. To keep them out of your superconductor you will need at least 50 cm of shielding. If you have the space, then go for it.

My admittedly BOE calculation says that 5 cm of H2O is enough to thermalize the neutrons and a few mm of B10 is enough to absorb them. Assuming X-Rays are not a problem for the coils. Of course you have the added heat load from B10 neutron absorption - but as Arts says. That is just engineering.

My figure was something I sort of remember from tight aspect ratio tokamak designs, that put a premium on getting the plasma as close as possible to the central column. I wouldn't bet my life it is right. One reason for the difference may be that absorption of neutrons is relatively easy, but not a good idea. In a D-T tokamak, the neutrons are needed to breed tritium. It looks like it is possible, for various reasons, to have a breeding ratio a few percent above unity, but you don't really have neutrons to burn (or absorb). Of course if you don't need tritium (because you burn D-D, or mine He-3 on the moon), that is a different game.

[TallDave]

TallDave wrote:
Electron losses are separate from what's necessary to have a non-Maxwellian distribution of electrons.
---
Art Carlson wrote: Not according to Rick Nebel. His plan to maintain a non-Maxwellian distribution is to extract electrons before they have a chance to (fully) equilibrate.

Right, they are related that way, I'm just making the point they are diferent issues to contend with. You need to limit electron losses AND maintain a non-Maxwellian distribution. If anything those goals work against each other.

But here again is Bussard.

If electrons live sufficiently long in the machine they could
become Maxwellianized (thermalized) and develop high
energy loss distributions. However, this has been found not
to be the case. The same arguments have been found for the
ions, as well. Detailed analyses show that Maxwellianization
of the electron population will not occur, during the lifetime
of the electrons within the system. This is because the
collisionality of the electrons varies so greatly across the
system, from edge to center. At the edge the electrons are all
at high energy where the Coulomb cross-sections are small,
while at the center they are at high cross-section but occupy
only a small volume for a short fractional time of their
transit life in the system. Without giving the details, analysis
shows that this variation is sufficient to prevent energy
spreading in the electron population before the electrons are
lost by collisions with walls and structures. Similarly, for
ions, the variation of collisionality between ions across the
machine, before these make fusion reactions, is so great that
the fusion reaction rates dominate the tendency to energy
exchange and spreading.

If you are thinking of losses parallel to the field, then electrons that slow down will fill up the bottom of the potential well (near the magrid).

Right, which is where they would tend to be lost to the Magrid (that's where the unshielded metal is, and the anode). IIRC Rick had mentioned this as an issue to contend with (you don't want a screening sheath).

[KitemanSA]

Without giving the details, analysis shows that this variation is sufficient to prevent energy spreading in the electron population before the electrons are lost by collisions with walls and structures. Similarly, for ions, the variation of collisionality between ions across the machine, before these make fusion reactions, is so great that the fusion reaction rates dominate the tendency to energy exchange and spreading.

Yeah, this is what I said. Thermalization here (center for electrons, edge for ions) prevents (ok, delays) thermalization in general. This is called annealing.

[D Tibbets]

Polywell FAQ part I

What is a maGrid?
The maGrid is short for magnetic grid. It is the circular electromagnets that contain the electrons and ions in the potential well. All polywell magrids so far have had 6 electromagnets. Future designs may employ more than 6 (how many? Why is more better?)

Why is polywell claimed to have a non Maxwellian distribution of ion energies?
According to Dr. Bussard's lecture at google video (22:25) all the ions in the potential well are of the same energy distribution. This is unlike a Maxwellian or thermal distribution of energies which vary widely across a broad spectrum for a given temperature. A Maxwellian distribution looks like an elongated bell curve while polywell's distribution is a spike. Only a fraction of the ions in the most optimal Maxwellian distribution are optimal for fusion. Where as the polywell distribution (correct thing to say?) all ions can fit the best velocity profile for optimal fusion conditions. And this is because of some process called annealing. And the loss time is shorter than the thermalization time. And??

Ok ions in the polywell are of a single energy level theoretically most of the time. What energy level is optimum for T+D reactions, D+D reactions and P+11B reactions and the corresponding coil drive voltages?
...P+11B reaction requires a 100KV coil drive voltage but since the 11B ion has a charge of 5 it will receive an energy of 500KV at the bottom of the potential well (source google video lecture 23:21).

How many nuclei collisions are required for one fusion event in the polywell?

Say boron 11 proton fusion were achieved in polywell. Those ions with several MeV energy levels will zoom out of the well and will be harnessed with deceleration grids or they will hit the apparatus. Many millions of collisions per square centimeter will be occurring every second on the electromagnets sputtering the surface away and contaminating the potential well with metalic ions. How will this be dealt with? What about collisions on the surface of the deceleration grid wires and electron guns?
?

How will fresh fuel be injected and the spent thermonuclear exhaust be removed? Fuel is injected via ion guns. It is exhausted by using? What about the helium nucli flying around inside of the polywell, what will be done with them?
?

What is the ion and electron density of the inside of the polywell? Is it a gradient? What about outside the polywell where ions are decelerated?
?

What is a deceleration grid and is it a polywell only invention? I mean does it have any prior use before polywell?
?

What does the WB in WB 6 mean?
Whiffle Ball

What is the WB-100 I some times hear mentioned?

How are ions accelerated towards the center of the polywell?
The ions are accelerated by the electric forces of the electrons circulating in the center of the polywell.

How are they confined?
They are confined by the maGrid (magnetic grid).

What are cusps and cusp losses?
?

What is a funny cusp?
?

WB-6 produced a .1 miliwatt output for a 7,500,000 watt input. If bigger is better and can lead to net power, just how big does it have to be for x amount of gross power? Is there a way to find out how much power output and input from an equation due to size?
This is still a topic of debate.

Could the polywell have applications besides fusion power (in case this has no use for power generation)? Xray source perhaps?
?

Alot of questions in one thread, but some clearifications on some of the above comments (based on my understanding).

The input power into WB 6 was not anywhere near 7,000,000 watts.
Keep in mind that there are three main and seperate power inputs. The magnetic fields are created by high current, but low voltage input (car/ boat batteries that were providing hundred to thousands of amps, but only at perhaps 12 or 24 volts- eg: 1000 amps at 12 volts = 12,000 watts.
Another major power input was the acellerating voltage on the magrid casing- ~12,000 volts and perhaps a few amps- eg: 5 amps at 12,000 volts = 60,000 watts.
The electron guns also ate power, but these were made from car headlight bulbs/filaments. They drew alot of amps, but at low voltage, again possibly 12-24 volts.- eg: 1,000 amps at 12 volts = 10,000 watts.
I made up these numbers based on vague recollections of what I have read. The sum would be in the ball park of what I recall reading about the input power that the WB6 used- which was ~ 100,000 watts.

The magrid as used in WB6 had two properties. A strong magnetic field to contain the electrons once thay were inserted into the internal volume of the magrid assembly, This magnetic field is strong enough to stop the escape of most of the electrons, but the much heavier ions are not significantly impeaded. It is the electron cloud contained by the magnetic field that traps/ contains the ions and accelerate them towards the center. Apparently, the average position of the electrons are far enough towards the center that they have a net central acellerating force on the ions that must be born (ionized from neutral gas) or injected (via ion guns) into a region on the perifery of the electron cloud.

The second property is the electrostatic potential applied to the surface of the magrid casings. This served to acellerate the electrons that were emitted by the electron guns at low voltage outside the magrid, through a cusp and into the interior at high speeds. I'm guessing that this high speed is nessisary to maintain a tight independant cloud of electrons that bounce around and forms the deep potential well. If the electrons were not high speed they would persumably quickly cluster around the positively charged magrids- getting as close as the grids as the shielding magnetic fields (and mutual electron repulsion) allowed them. Even with a continous supply of new high speed electrons, how the low energy electrons are reenergized or replaced is magic. I suppose that the incoming high speed electrons will bounce off the slow speed electrons that have been acumulating near the magnets and/or created from the ionization of neutral gas molecules (different dynamic if ion guns are used intead of gas puffers) and speed them up. This may be related to the reported limit on the potential well depth versus the input electron energy (~ 80%).

As far as nuclear ash removal, once two ions fuse, thier products fly off at a speed much higher than the fuel ions speed, so that, even those products that are charged cannot be stoped by the electron cloud, so they automatically leave the core region, and fly laterally untill they are stoped by a physical surface, or decellerated to a near stand still by a periferal energy recovering grid. Once the particals have been stoped by heating a surface or generating electricity and picked up electrons from the surface of the grounded vacuum vessel wall (so that they are now neutral gas molecules), they drift around. They would then need to be quickly removed from the system by a robust vacuum pumping system so that they do not lead to arcing. Those fusion ions that hit the magrid on thier way out could be neutralized. They would then be embedded in the magrid wall or floating around inside the magrid, ready to be ionized by the electric field or electron impacts and act like the original fuel molecules, thus polluting the core. Some active removal system would presumably be needed. Dr Nebel has hinted that such concerns have been considered. Pulsating modes, resonate microvave heating , etc. might serve to selectively remove fusion product ions from the core. The neutrons would immediatly leave the system by passing through the walls or embedding in them- but then transmutation and secondary radiation products may be a contaminate concern. And, consideration for all the sputtered contaminates have to be controlled....
Obvously, alot of 'engeenering' concerns have to be worked out

I have read that ~ 10,000 collisions are needed befor there is a likely probability that one of them will result in a fusion.

What is a Funny Cusp? Well, there was this traveling salesman... Well, that provides a perspective as good as any other I have. It needs to be defined consistantly.

Ion energy levels would be optomized to maximize their chances of fusion (highest cross section) while minimizing the costs of obtaining those conditions- such things as x- ray production, arcing, etc. A compromize presumeably would be used. Looking at cross section graphs (look up Fusion on Wikopedia). The resonant peak of ~ 75,000 eV might be better for p-B11 fusion than the higher eV regions where the graph is higher because of x-ray losses or other concerns. In a thermalized plasma the x-ray losses increase with temperature (eV). That is one reason why Tokamaks can only produce net fusion power from the D-T reaction at realitively low temperatures. The Polywell sneaks around this limit because it is not a Maxwellian limited machine (at least, that is the claim) and the electrons that produce the x-rays when they are traveling fast and hit an ion (Bremmstralung) avoid this due their variable speed in different regions within the magrid.


The Polywell seems to be a bunch of physics compromises that add up to a working net energy producer- or not, depending on various assumptions and intrepretations. Even if the system works from a physics perspective, the engeenering challenges will be formidable.


Dan Tibbets

[D Tibbets]

...

As far as nuclear ash removal, once two ions fuse, thier products fly off at a speed much higher than the fuel ions speed, so that, even those products that are charged cannot be stoped by the electron cloud, so they automatically leave the core region, and fly laterally untill they are stoped by a physical surface, or decellerated to a near stand still by a periferal energy recovering grid. Once the particals have been stoped by heating a surface or generating electricity and picked up electrons from the surface of the grounded vacuum vessel wall (so that they are now neutral gas molecules), they drift around. They would then need to be quickly removed from the system by a robust vacuum pumping system so that they do not lead to arcing. Those fusion ions that hit the magrid on thier way out could be neutralized. They would then be embedded in the magrid wall or floating around inside the magrid, ready to be ionized by the electric field or electron impacts and act like the original fuel molecules, thus polluting the core. Some active removal system would presumably be needed. Dr Nebel has hinted that such concerns have been considered. Pulsating modes, resonate microvave heating , etc. might serve to selectively remove fusion product ions from the core. The neutrons would immediatly leave the system by passing through the walls or embedding in them- but then transmutation and secondary radiation products may be a contaminate concern. And, consideration for all the sputtered contaminates have to be controlled....

It occurs to me that a portion of the nuclear ash may actually be reused directly without first needing to be removed from the system, reprocessed, and injected into a different machine as a new fuel. Using the D-D reaction, two of the fusion products can themselfs be burned at conditions only mildly more demanding than the D-D reaction, or much easier. Four Deuterium fuse to produce 1 proton, 1 tritium, 1 Helium3, and 1 neutron.
The neutron leaves the system. The proton, tritium, and He3 are all charged. The proton would need to be removed, but the tritium and the He3 could be reused to considerably boost the power output for each deuterium introduced to the system.
Using the idea of greatly increasing the minor diameter of the torus shaped magrid (making it fatter) sugested by A. Carlson in another thread about increasing internal volume for more wire turns and cooling structures, would also increase the realitive area of the inward facing surface of the magrid that would intercept the outward flying tritium and He3. These would (most?, might need time to saturate the surface layers with these products) bounce off of the positively charged grid- no chance to collect electrons and thus be neutralized, and be essentially new fuel ions injected at that point. So the single Polywell could simutainously be burning the primary deuterium fuel and also be burning a significant portion of the secondary fuel- trituim and He3. Obvously, the magrids would be recieving alot more direct impact heating from the protons, neutrons (especially the high energy neutron from D-T fusion?) , tritiums, He3, and x-ray heating. But, there would also be alot more internal volume aviable for magnetic field boosting wire turns,shielding, and cooling elements. I don't know what tradoffs would be involved with sputtering concerns/ contamination, and surface area heat load limits. Also, removal of the final true ash would be more challenging.

Might it be workable, would it screw up the entire system, is it stupid?


Dan Tibbets

[TallDave]

WB-6 produced a .1 miliwatt output for a 7,500,000 watt input.

LOL That would be failing pretty hard. 7.5MW is actually Bussard's estimate for WB-100.

I think D Tibbets has the right figures above.

[ohiovr]

Thank you Mr Tibbets I am a bit more optimistic now with your information. You spent a good amount of time on that post I appreciate it. We really need that F.A.Q hosted and maintained somewhere so the knowledgeable people can move on to other things while keeping the ignorant people in the loop (like myself). Where would we put the F.A.Q? I see another thread about the (lack of a) F.A.Q.. I would love to make it but I am not an expert on the topic by far. And there is a lot of disputed claims and counter claims. I have a little bit of experience in the software industry. One of the tools I used was a current version system. We need something like that for F.A.Q builders. Maybe WIKI is the way to go?

[MSimon]

Ash shouldn't be much of a problem (if it is neutralized) because the material flow in a reactor will be 10X to 1,000X the rate of fuel consumption.

[Art Carlson]

It occurs to me that a portion of the nuclear ash may actually be reused directly without first needing to be removed from the system, reprocessed, and injected into a different machine as a new fuel. ...

Might it be workable, would it screw up the entire system, is it stupid

It's not stupid, but it is complex. Many scenarios have been proposed for D-D fuel cycles. You can burn the T and He3 in situ, or just the T (which is easier to burn and a bigger headache once it gets outside). You can also use your D-D reactor as a source of fuel for separate D-T and D-He3 reactors. You can store the T for a couple decades until it decays into He3. One important knob to control these things is the plasma temperature. If you ever get so far that you can burn D-D economically, you have a lot to play with.

What worries me more is the He4 ash. Once it gets neutralized on some surface, it seems about as likely that it gets re-ionized to pollute the plasma as it is to hit a pumping port. A tokamak works a bit different because you can use deep fueling, but being able to pump helium gas fast enough is still a (solvable) concern. He4 in the core is bad because it dilutes the fuel, thereby reducing the fusion power, while at the same time increasing the Z_eff, thus increasing radiation losses. I'm not sure how easy it would be to make a generic but quantitative estimate of this effect. (If it were real easy, I'd just do it.)

[ohiovr]

Regarding the magrid power... At what point would the magnetic field be powerful enough to trap the fusion products? That would put an upper limit on the power output of the machine in terms of its size, which is only important for the very speculative rocketry applications of the polywell (which is why I am here). I am considering the D+3He or 3He+3He fuel cycles as almost all of their fusion products are charged kinetic particles (perfect for powering the relativistic electron beam), and the conditions to fuse them seem to be milder compared with fusing p+11b. Super conducting coils seem to max out at 20T. Is this too powerful?

[D Tibbets]

Regarding the magrid power... At what point would the magnetic field be powerful enough to trap the fusion products? That would put an upper limit on the power output of the machine in terms of its size, which is only important for the very speculative rocketry applications of the polywell (which is why I am here). I am considering the D+3He or 3He+3He fuel cycles as almost all of their fusion products are charged kinetic particles (perfect for powering the relativistic electron beam), and the conditions to fuse them seem to be milder compared with fusing p+11b. Super conducting coils seem to max out at 20T. Is this too powerful?

R. Nebel recently revieled that at anticipated fields of 5-10 Tesla, a power producing Polywell (perhaps 3 meters in diameter) will retain alpha particles from P-B11 fusion for ~ 1000 passes befor spitting them out of a cusp at essentially full speed. A. Carlson anticipates fewer passes, but apparently the physics is straight forward. So, it is not so much a question of weather the fusion products will be contained, as how long they will be contained. 20 Tesla would possibly contain them longer (depending on the relative cusp geometries at the higher field strengths?). 1000 passes is two few for similar fusion products from D-D fusion (He3 and tritium) to contribute much additional fusion (though I wonder about the D-T reaction). Also the MeV energies of these particles actually have lower crossections than at lower speeds.

If the scaling laws are acurate a 3 meter D-D fueled Polywell with ~10 Tesla fields will produce ~ 100 MW gross power (that is were the nick name of WB 100 came from- originally by M. Simon I think). At 20 Tesla, the gross output would theoretically be ~ 1.6 GW [Edit, corected number]. Handling the heat loads at the lower field strength is apparently going to be difficult. The higher outputs (at the same size) would become increasingly hard to handle. So, for pratical reasons 5-10 Tesla seems to be a reasonable goal (based on my expert analysis ).

Without again looking up the fusion reactions for D-He3 or He3-He3 I'm guessing the former would be mildly harder than D-D, while the latter would be possibly similar to P- B11. It depends on how well the machine can handle various high KeV drive energies and losses. For space propulsion, due to percentage of neutrons produced along with the need for charged particles, it is my understanding that P- B11 is by far the preferred fuel (if it works), while D- He3 is the easiest. One nice thing about these types of reactors in space is that the challenges of maintaining a hard vacuum outside of the magrid goes away.


Dan Tibbets

[MSimon]

I think the 1,000 passes is not for all alphas. Just for alphas below the energy imparted by the E field. i.e. - (GridV * charge of the particle) in eV. However, that is not what Rick said so I may be in error.

I would expect the higher energy alphas will get focused due to the magnetic fields. Also deflected from the grids.

[MSimon]

One nice thing about these types of reactors in space is that the challenges of maintaining a hard vacuum outside of the magrid goes away.

I think that is a misconception. To maintain a given pressure you have to have a fuel flow far in excess of that needed to replace reacted fuel. Even in space pumps will be required (if you are going to recycle unused fuel). However, space may be useful in establishing the initial vacuum.

[D Tibbets]

One nice thing about these types of reactors in space is that the challenges of maintaining a hard vacuum outside of the magrid goes away.

I think that is a misconception. To maintain a given pressure you have to have a fuel flow far in excess of that needed to replace reacted fuel. Even in space pumps will be required (if you are going to recycle unused fuel). However, space may be useful in establishing the initial vacuum.

Mmm...
A vacuum vessel would presumably be needed to recycle fuel, maybe.
If the escaped fuel remains ionized it might be collected electrostatically- something like a mass spectrometer without a continous 'vacuum shell' Simply have a big hole in the shell exposed to space with electrostatic/ magnetic grids to divert, differentiate, collect and finally recycle the fuel while dumping the ash. I don't see how it would be significantly different from using a pump. You would still need some method to recover the atoms/ions from the very rarified gas.

A significant amount of the kinetic energy of the 'ash' could be left intact if direct thrust is wanted (as opposed to powering an ion or MHD plasma drive.. Additionally, if the reacter is providing high speed alpha particles for thrust, mixing in the escaped fuel ions (at least the hydrogen (protons)) would be no worse than injecting an inert gas into the exaust flow to boost thrust. I think this is reasonable within limits.

Just how fast would fuel ions escape the system anywhey?


Dan Tibbets