Question: How is the electron not getting into the machine?

[Robthebob]

Last we publicly heard from emc2 is that they have problems pushing electrons into the machine operating at high field.

Can someone explain this to me in more detail? And answer some of my questions?

1. does the electrons have too low energy or too high energy?
2. does the difficulty of the electrons getting into the machine a function of how many electrons are already in the machine?
3. If there are ions in the system, does that make it easier for the electrons to get into the machine?

Now on a completely different topic.

How does neutral beam injection work in toks? I know non-neutral beams wont penetrate the plasma in toks, so neutral beam can penetrate the tok plasma and get ionized at the center.

1. Is this a function of the energy of the neutral beam? (higher the energy of the beam, higher the energy of the plasma that ionizes the beam, and at the center, the plasma has higher energy)
2. Would the stripped electron possess similar energies compared to the ion, the one that just had the electron stripped?

Thanks

[krenshala]

The electrons are too low energy (or speed?) to get through the cusps in sufficient quantity to maintain the well due to the higher magnetic field on the magrid. The higher field on the magrid makes the cusps smaller, so the electrons need either to be at just the right angle/path or they need higher energy. Its much easier to just up the electron energy.

I'm sure I'm missing something on this, but its what I understand of it based on what folks have discussed in the News section.

[KitemanSA]

The electrons don't get in because EMC2 didn't include X-Cusps in their design!

[Robthebob]

Hmmm, would not being able to push electrons into the machine necessarily be bad (it probably is bad, I doubt its easy for electrons to bounce off the outside mirror and somehow get into the machine)? the mirror also works on the outside, I think the mirror works better on the inside, and as the electrons build up and warp the field on the inside, the mirror works even better, I think this is part of the reason the WB effect occurs.

If high fields are a problem in this manner, does that mean electron re-circulation isnt very important at high fields? Because the electrons that leaves the machine and attempt to re-enter the machine would have a hard time getting back in.

Lastly, sorry, but what are X-cusps?

[D Tibbets]

We are operatring from a highly ignorant viewpoint, both in terms of expertise and very importantly from a starvation of data.

There are some hints that better electron gun design, geometry, power, distance, etc . some have suggested that this need is driven by trouble injecting the electrons. That is a possibility, though higher expectations may be more important. The refined e- guns, may be in expectation of high voltage P-B11 fusion.

Remember that the baseline WB6 e-guns were auto headlight filaments with pretty much omnidirectional electron emission. At low voltages this may be tolorable because it is only <1% of the accelerating voltage on the magrid. If higher voltages are used on the e-guns, the omnidirectional emission is more significant. This implies that they may be compromising, using a blend of accelerating voltages on both the e-guns and magrid (or it may not).

As the B field is increased the cusp loss area decreases in relation to the total surface area of the boundary layer whether viewed from the internal or external perspective. The difference is that there is no Wiffleball effect from the exterior perspective. This suggest to me that the geometry of electron vectors needs to be more precise in order to remain within the angles that do not mirror back outside of the cusps and that this limitation scales as the B field squared (?), but not with the additional effect of the Wiffleball that only effects the cusp morphology on the inside. Thus the internal containment always exceeds the external by the Wiffleball factor, but this does not imply that the B scaling can be ignored from an electron injection perspective. I'm not certain if there is a greater energy requirement/ penalty for injecting the electrons into greater B fields. It may be mostly a question of accuracy and precision of the e- guns focus and placement.

Recirculation in WB6 demonstrates high efficiency of electron injection at ~ 90%, and most of the remaining inefficiency may be the up scattered electron losses and this is not all bad, along with the nub losses. If WB7 or 7.1 was run at at mildly higher B fields, such as 1200 or 1500 Gauss, they may have data about recirculation with some scaling of the B field. WB8 at perhaps 6,000 to 8,000 Gauss would give more definitive answers. If there is a drop in recirculation efficiency that would be consistent with increasing difficulty in powering electrons into the machine (whether new or recirculated electrons) with increasing B fields. If the recirculation does not suffer, then the new electron injection should not suffer, except for the margins of error that placement and focus of the e- guns introduce.

If it is more energetically difficult to inject or re inject electrons with higher B fields, I think the feasibility of the system suffers greatly. Note that the key that I am addressing is the difficulty from an energy perspective versus the difficulty from an engineering placement and design perspective.

Then there are manipulations with pulsations that might address some difficulties if needed.


As for electrostatic considerations. like real estate, location rules. The potential on the magrid has a higher acceleration effect on the external electrons because it is closer than the internal space charge from the contained electrons. The inverse square law applies. Also due to inefficiencies the potential on the magrid is greater than the potential well (internal space charge). Also, don't forget Gauss's Law. This allows for the electrons to be accelerated inward by the potential on the magrid, but once inside the magrid the electrons motions are governed by the other internal electrons in a cumulative manner (space charge, in addition to local collisions) , but they are not further effected by the magrid charge .They do not experience an outward acceleration force back towards the radius of the magrid due to the charge on the magrid. This magrid charge only effects electrons (or ions) that are at a greater radius than the magrid. This is why recirculation works. Using a mixed high voltage on the magrid, the e- guns, ion guns, repellers (generally considered to be bad influence on the ions), possible pulsation or bunching of the charged particles allows for a huge amount of possible variations. There is probably some compromise that allows for the greatest net benefit.

Dan Tibbets

[KitemanSA]

Lastly, sorry, but what are X-cusps?

When you wind both the square and triangular magnets of a cubeoctahedral MaGrid with a little rounding of the corners, the result looks something like this. The X-Cusp is where the funny cusp would be except there is no metal there.

[happyjack27]

We are operatring from a highly ignorant viewpoint, both in terms of expertise and very importantly from a starvation of data.

There are some hints that better electron gun design, geometry, power, distance, etc . some have suggested that this need is driven by trouble injecting the electrons. That is a possibility, though higher expectations may be more important. The refined e- guns, may be in expectation of high voltage P-B11 fusion.

...

Then there are manipulations with pulsations that might address some difficulties if needed.

....

aye, but there's the thing, if they just need to control the fact that they need a gun that puts out a higher volume, then why are they pulsing it? by any measure you'd et more input per second by running it continuously. so clearly there has to be a different justification for pulsing it. once so overwhelming that they'd sacrifice net power output for it. my guess is to get more diagnostic data.

[KitemanSA]

aye, but there's the thing, if they just need to control the fact that they need a gun that puts out a higher volume, then why are they pulsing it? by any measure you'd et more input per second by running it continuously. so clearly there has to be a different justification for pulsing it. once so overwhelming that they'd sacrifice net power output for it. my guess is to get more diagnostic data.

My guess is to avoid burning out the magnet at such high field strengths.

[D Tibbets]

aye, but there's the thing, if they just need to control the fact that they need a gun that puts out a higher volume, then why are they pulsing it? by any measure you'd et more input per second by running it continuously. so clearly there has to be a different justification for pulsing it. once so overwhelming that they'd sacrifice net power output for it. my guess is to get more diagnostic data.

My guess is to avoid burning out the magnet at such high field strengths.

There is a difference between pulsing the e-gun current and the electromagnet current. Both might be employed in a complex dance of electrons. The problem with pulsing the magnet current is that the electron speeds are 10 million meters per second or greater at the Wiffleball border. Assuming a distance between the Wiffleball border and the magrid casing of 10 cm, the electron would travel this distance in under 10 nanoseconds (10^-8 seconds). That means any significant weakening of the magnetic field would need to be significantly shorter, perhaps 1-2 nano seconds, otherwise magnetic containment of the electrons would go to pot. Then there are the mechanical stresses of the magnet wires with variable current, B field strength- they would vibrate, leading to early fatigue and also quick failure of the varnish insulation. Also, if superconducting, varying the current would be difficult.

As for pulsing the e-gun current or voltage, I don't know if this would ease the focus and transport of electrons through the cusp. I wonder if this functionality may be aimed at bunching the electrons once they are inside the Wiffleball. This could be another way of introducing POPS like effects, without, or in addition to using microwaves.

Concerning neutral beam injection across B fields in Tokamaks, I don't understand it well. My picture is that the gas is ionized, and the resulting ions and electrons are accelerated (heated) through separate paths, then recombined into a dense common beam for injection. The plasma is much too hot (perhaps 10,000 eV)for much recombination to occur so it is not a beam of hot neutral gas, but charged particles. Normally I would expect this to be effected by magnetic fields in the expected way. The only way around this that I see, is that these combined beams of electrons and ions are packed so densely that this plasma is closely coupled. Fusion density plasmas are generally considered weakly coupled, which means space charge effects dominate. At densities of 10^19 to 10^23(?) charged particles / M^3 this applies. I assume the "neutral beam" must be hot plasma at higher densities- perhaps 10^26 to 10^28 charged particles / M^3. It could be considered a neutral gas with extended electron orbits. The challenge of holding this beam together against various instabilities long enough must be a challenge.

Coupling is a plasma term that describes the local interaction vs the distant interaction- space charge or magnetic fields. Neutral molecules/atoms are very closely coupled. Cold dense plasmas are closely coupled, hot and less dense plasmas are loosely coupled. If the electrons are bound to a nucleus it is net neutral, but there are still separations of charge. Even if the electrons are moving semi independently to the nuclei, so long as they are not moving too fast or there is not too much distance between the nuclei, from a distance the mixture would act like a neutral gas. Of course there has to be containment in this case as the situation is not stable like a truely neutral gas. The inverse square law and time effects the behavior, so everything is relative.

At some point a very strong magnetic field can tear apart even cold neutral atoms into a plasma with resultant ions and electrons diverging in the field.

Note also that a plasma can contain both positively charged and negatively charged ions. Playing with these ratios is another way to possibly stabilize a dense"neutral beam Plasma" long enough.

Dan Tibbets

[Robthebob]

When you wind both the square and triangular magnets of a cubeoctahedral MaGrid with a little rounding of the corners, the result looks something like this. The X-Cusp is where the funny cusp would be except there is no metal there.

I see, this is a geometry issue, and we shouldnt blame them for going with the safe route (if such a thing exists).

I'm strongly considering building a polywell for my PhD, I'll start the process of gathering information and details soon (not that I havent been doing that).

Maybe I can just try another geometry and build a small machine, vary the fields, look at the well depth, etc.

[mattman]

Rob,

I was thinking about this the other day.

When Bussard's last machine was only full of electrons, it had about 2E12 electrons. Their average energy was 2,500 eV. Now deuterium gas is puffed towards the rings. When it exchanges energy with the electrons, it ionizes. It ionizes if it is hotter than 16 eV. The ions fall into the center. They may hit, they may fuse.


But what happens to the two electrons created?


1. These electrons feel a Columbic repulsive force from the cloud in the center. So they will not necessarily go in that direction.

2. If they fly outside the rings, they will see a 12,500 volt cage field pushing them back towards the rings.

3. They also feel a magnetic field from the rings themselves.



Given all this, I am not surprised we are having problems with electron injection.

[KitemanSA]

I'm strongly considering building a polywell for my PhD, I'll start the process of gathering information and details soon (not that I havent been doing that).

Dr. B wanted to run two more small scale MaGrids before going to full scale, one being a square planform cubeoctahedron and another being a higher order form. You could do some real research by making two units, a round planform WB6 like machine and a square planform equivalent. Keeping all else the same, Dr. B thought the square would perform ~5 times better than the round. Real data would help a lot.

Personally, I think a bow sided square planform would be slightly better still, but I have no modeling to back that up.

[Robthebob]

did you just say 5 times?!

I dont know if my boss will let me do this, I still have to get my masters first, so it wont be for a little while. If I can, I'll probably build a very small one, probably pulsed too, about the size of the first Sydney machine.

There's a lot of technical details (like spacing, dimensions, etc) I dont know, I dont know if published papers will have them, I might have to ask the folks in Sydney for help, (given this might be way too much of a mess) but I would like to replicate their experiment, use that as the control, then build 2 machines of other geometries. (or maybe just 1, we'll see)

As for the electron problems, I'm just confused about what they're doing.

[KitemanSA]

Comments in RED

did you just say 5 times?! . . . . . . . . . . . . Yes. Seems the line-like cusps between the torii dominate the electron loss equation, and shortening them with square planform magnets should improve the situation a lot.

I dont know if my boss will let me do this, I still have to get my masters first, so it wont be for a little while. If I can, I'll probably build a very small one, probably pulsed too, about the size of the first Sydney machine. Please don't construct it the same way. They hold the Teflon spools together with angle brackets placed in EXACTLY the wrong place! It also seems to me that it would be better and just as easy to build one and dunk it into liquid Teflon rather than use that wierd spool thinggee.

[happyjack27]

i tape electrons to those little magnetic spheres you can buy online -- buckyballs i think they're called -- and i just throw them in. it's been working pretty well.