My understanding of how the Magrid Works

[jmc]

I've still only read up seriuosly on this concept for a few months so please correct me if I'm wrong, but this is my understanding of how the magrid achieves high levels of electron recirculation:

1) electron guns inject electrons axially, parallel to the field lines into the centre of the device.

2) The Magrid supples 80% of the energy accelerating the electrons into the centre the other 20% is supplied by the electron gun itself.

3) The electrons then collide with the plasma deflecting their trajectory causing them to bounce against the magnetic field rather than returning along the path they came.

4)If there was no magnetic field the voltage of the magrid would be insufficient to contain the electrons.

5) Electrons that do happen to pass through the holes and escape the magnetic mirror have to pass through a region of strong magnetic field gradient, due to the conservation of magnetic flux this implies a large radial component of the magnetic field pointing towards the centre of the 'holes' in the magnetic mirror.

6) This radial magnetic field converts a huge portion of the escaping electron's parallel velocity into velocity perpendicular to the field.

7) In order to bring about electron recirculation the voltage in the magrid only has to bestrong enough to suck all the parallel velocity from the escaping electrons before they hit the vacuuum wall, and while it has insufficient voltage to suck all the total velocity out of the escaping electron it does have sufficient voltage to remove their parallel velocity (perhap I should say radial velocity by paralel I mean velocity parallel to the magnetic field lines pointing radially away from the centre of the device)

Thus excluding collisions at the strong field region practically all the electron end up getting recirculated.

Have I missed something? Am I completely wrong?

[drmike]

I'm not sure. I am going to spend some time grinding thru Maxwell's equations to try to find out over the next few days/weeks though!

As a rough guess, the cusp regions will have concentrating fields, so the electrons should mirror there. Basicly, the number of "flux lines" trapped in an electrons orbit is constant, so as the flux density increases, the orbital radius has to decrease. If most of the energy is perpendicular to the field lines the electrons should stay well trapped.

So instead of injection along field lines, I'd guess you'd want to inject perpendicular. But without grinding thru the math, I don't really know.

If there was no mag field, the positive grid would eat all the electrons. Not good!

[windmill]

This raises an issue I've been struggling with for some time, namely, how the well depth is determined by the machine parameters. I was initially under the impression that the electron guns impart the KeV necessary to accelerate the electrons to slightly more than the resulting potential well depth, with the Magrid acting to confine them at sufficient density for maintenance of the well. Now, I'm seeing posts that suggest that the Magrid, and not the guns, is the primary driver of electron well depth.
Bussard's papers seem a bit ambiguous on this as well. The earlier stuff all assumes the guns do all the work, but the "Valencia paper" says that experiments show the "primary emitters" can even be "turned off", after start-up, as the polywell will use Child-Langmuir extraction to suck electrons from souces external to the Magrid. I can't see how you can form a 200kV deep well with, say, only 50KeV electrons. Anyone shed some light on this?

[Roger]

What happens when you combine 50Kev emitters shooting electrons..

with a MaGrid....

The MaGrid is positive to the wells' negative, and the MaGrid is operating at what level charge ?

Once sufficient electron density occurs, the MaGrid and the well push back at one another, equilibrium occurs, the well has formed. So if one should claim the MaGrid shapes the well, confining electrons... should it also follow that the MaGrid should dictate electron behavior inside the well ?

Since the McGrid creates the space known as the "potential well", its MaGrid rules inside the well.

[Roger]

What happens when you combine 50Kev emitters shooting electrons..

with a MaGrid....

The MaGrid is positive to the wells' negative, and the MaGrid is operating at what level charge ?

Once sufficient electron density occurs, the MaGrid and the well push back at one another, equilibrium occurs, the well has formed. So if one should claim the MaGrid shapes the well, confining electrons... should it also follow that the MaGrid should dictate electron behavior inside the well ?

Since the McGrid creates the space known as the "potential well", its MaGrid rules inside the well.

[bcglorf]

This raises an issue I've been struggling with for some time, namely, how the well depth is determined by the machine parameters. I was initially under the impression that the electron guns impart the KeV necessary to accelerate the electrons to slightly more than the resulting potential well depth, with the Magrid acting to confine them at sufficient density for maintenance of the well. Now, I'm seeing posts that suggest that the Magrid, and not the guns, is the primary driver of electron well depth.
Bussard's papers seem a bit ambiguous on this as well. The earlier stuff all assumes the guns do all the work, but the "Valencia paper" says that experiments show the "primary emitters" can even be "turned off", after start-up, as the polywell will use Child-Langmuir extraction to suck electrons from souces external to the Magrid. I can't see how you can form a 200kV deep well with, say, only 50KeV electrons. Anyone shed some light on this?

Bussard's main point in the Valencia paper is that the whole function of the Polywell rests on relative electron densities. The energy of the electrons has an influence on the density distribution, but the potential of the central well only depends on the electron density in the centre. 1E13/cm^3 electrons in the core will create the same potential difference, hot or cold.
The function of the magrid as described by jmc sounds correct from what I understand.
The magnetic field on the magrid serves two primary purposes:
1.Creates an electron trapping effect in the core that results in a higher electron density inside the machine than outside.
2.Shields the positively charged magrid from becoming an electron loss sink.
The positive charge on the magrid is primarily for electron recirculation. The whole idea's based on minimizing electron loses and maximizing core electron density.

[windmill]

So, once the central virtual cathode is stabilised at a particular well depth, the well can be maintained at that depth just by ensuring that the electron losses are balanced by sufficient injected electron current? Does this square with the idea that the polywell system acts as a momentum transformer, i.e., infalling ions are accelerated by the potential energy of the well, which in turn comes from the kinetic energy of the electrons?

[rexxam62]

So, once the central virtual cathode is stabilised at a particular well depth, the well can be maintained at that depth just by ensuring that the electron losses are balanced by sufficient injected electron current? Does this square with the idea that the polywell system acts as a momentum transformer, i.e., infalling ions are accelerated by the potential energy of the well, which in turn comes from the kinetic energy of the electrons?

the whole idea is that every electron is used in the fusion process. ie no losses. This was what Dr. Bussard said created the good results. And if you think about it, if electrons hit the magrid it will heat up plus electrons will be lost. So the basic idea is to create a system were every electron is used. This is my understanding.

[bcglorf]

So, once the central virtual cathode is stabilised at a particular well depth, the well can be maintained at that depth just by ensuring that the electron losses are balanced by sufficient injected electron current? Does this square with the idea that the polywell system acts as a momentum transformer, i.e., infalling ions are accelerated by the potential energy of the well, which in turn comes from the kinetic energy of the electrons?

To your first question, my understanding from the Valencia paper is yes. Maintaining the well after startup only requires injecting electrons equivalent to the rate of loses to walls and other structures.

"ions are accelerated by the potential energy of the well"
Correct, ions are accelerated by the potential difference between themselves and the electrons in the well.
"which in turn comes from the kinetic energy of the electrons"
The potential energy of the well comes from the electron's negative charge. Their kinetic energy just helps in getting their density right to have more electrons in the centre than are re-circulating outside.

When ions are injected near the magrid, the negtive charge of the electrons in the centre accelerates them towards the centre. As they pass through the centre, that same force now slows them for another pass. They orbit through the centre until they fuse or thermalize. Which comes first and/or dominates is a major criticism.

[bcglorf]

What happens when you combine 50Kev emitters shooting electrons..

with a MaGrid....

The MaGrid is positive to the wells' negative, and the MaGrid is operating at what level charge ?

Once sufficient electron density occurs, the MaGrid and the well push back at one another, equilibrium occurs, the well has formed. So if one should claim the MaGrid shapes the well, confining electrons... should it also follow that the MaGrid should dictate electron behavior inside the well ?

Since the McGrid creates the space known as the "potential well", its MaGrid rules inside the well.

The positive charge on the magrid will of course attract electrons. The virtual cathode in the centre will also repel electrons. It is the magnetic field from the magrid that prevents the electrons from ramming straight into the magrid. If the magnetic field is too weak compared to the charge on the magrid or from the potential well then electron loses will be excessive.
Bussard's research for the last decade has been in getting the right balance of magrid charge, B field strength, and electron energy to maximize potential well strength and minimize electron loses. The Valencia paper gives big picture details of their tests in that time.

[rexxam62]

What happens when you combine 50Kev emitters shooting electrons..

with a MaGrid....

The MaGrid is positive to the wells' negative, and the MaGrid is operating at what level charge ?

Once sufficient electron density occurs, the MaGrid and the well push back at one another, equilibrium occurs, the well has formed. So if one should claim the MaGrid shapes the well, confining electrons... should it also follow that the MaGrid should dictate electron behavior inside the well ?

Since the McGrid creates the space known as the "potential well", its MaGrid rules inside the well.

The positive charge on the magrid will of course attract electrons. The virtual cathode in the centre will also repel electrons. It is the magnetic field from the magrid that prevents the electrons from ramming straight into the magrid. If the magnetic field is too weak compared to the charge on the magrid or from the potential well then electron loses will be excessive.
Bussard's research for the last decade has been in getting the right balance of magrid charge, B field strength, and electron energy to maximize potential well strength and minimize electron loses. The Valencia paper gives big picture details of their tests in that time.

So what was Bussards conclusions to minimize electron loses? are the figures known? Id love to get the numbers at witch he found this balance.

//Rexxam62

[bcglorf]

What happens when you combine 50Kev emitters shooting electrons..

with a MaGrid....

The MaGrid is positive to the wells' negative, and the MaGrid is operating at what level charge ?

Once sufficient electron density occurs, the MaGrid and the well push back at one another, equilibrium occurs, the well has formed. So if one should claim the MaGrid shapes the well, confining electrons... should it also follow that the MaGrid should dictate electron behavior inside the well ?

Since the McGrid creates the space known as the "potential well", its MaGrid rules inside the well.

The positive charge on the magrid will of course attract electrons. The virtual cathode in the centre will also repel electrons. It is the magnetic field from the magrid that prevents the electrons from ramming straight into the magrid. If the magnetic field is too weak compared to the charge on the magrid or from the potential well then electron loses will be excessive.
Bussard's research for the last decade has been in getting the right balance of magrid charge, B field strength, and electron energy to maximize potential well strength and minimize electron loses. The Valencia paper gives big picture details of their tests in that time.

So what was Bussards conclusions to minimize electron loses? are the figures known? Id love to get the numbers at witch he found this balance.

//Rexxam62

The biggest revelation for Bussard was after WB5 they realized that the electron losses were still coming through the magrid itself. They redesigned the grid with the specific goal of providing maximum magnetic shielding to the grid(shaping the magrid to the magnetic field).

Required relative densities:
"Typically this requires electron densities at the interior boundary of order 1E13/cm3, or higher. While the exterior densities (of neutrals able to be ionized) must typically be below 1E10/cm3 or less."

On electron loses while trying to achieve the above:
"there must be no more than about 3E-5 fractional metal surface area unprotected by magnetic field insulation. Otherwise, direct field-free electron losses will exceed both WB and MG transport power flows, and system will not be able to yield positive gain. Corollary: No closed box configuration can be made to function as a net power Polywell, with any
conceivable practical magnetic coil surface protection windings. I.e. it is not possible, in a practical, constructable system, to cover all but 1E-5 of a closed box system with protective fields. This means that the ONLY Polywell
systems that can be made to work are those in which there is NO metal surface exposed - this requires open cusp, recirculating electron flow,"[/url]

[rexxam62]

The positive charge on the magrid will of course attract electrons. The virtual cathode in the centre will also repel electrons. It is the magnetic field from the magrid that prevents the electrons from ramming straight into the magrid. If the magnetic field is too weak compared to the charge on the magrid or from the potential well then electron loses will be excessive.
Bussard's research for the last decade has been in getting the right balance of magrid charge, B field strength, and electron energy to maximize potential well strength and minimize electron loses. The Valencia paper gives big picture details of their tests in that time.

So what was Bussards conclusions to minimize electron loses? are the figures known? Id love to get the numbers at witch he found this balance.

//Rexxam62

The biggest revelation for Bussard was after WB5 they realized that the electron losses were still coming through the magrid itself. They redesigned the grid with the specific goal of providing maximum magnetic shielding to the grid(shaping the magrid to the magnetic field).

Required relative densities:
"Typically this requires electron densities at the interior boundary of order 1E13/cm3, or higher. While the exterior densities (of neutrals able to be ionized) must typically be below 1E10/cm3 or less."

On electron loses while trying to achieve the above:
"there must be no more than about 3E-5 fractional metal surface area unprotected by magnetic field insulation. Otherwise, direct field-free electron losses will exceed both WB and MG transport power flows, and system will not be able to yield positive gain. Corollary: No closed box configuration can be made to function as a net power Polywell, with any
conceivable practical magnetic coil surface protection windings. I.e. it is not possible, in a practical, constructable system, to cover all but 1E-5 of a closed box system with protective fields. This means that the ONLY Polywell
systems that can be made to work are those in which there is NO metal surface exposed - this requires open cusp, recirculating electron flow,"[/url]

So to sum up:

1 No closed box machine can ever yield net fusion power (Electrons would smash into the machine itself witch would heat up the machine and cause electron losses making breakeven and net fusion impossible)
2 The inescapable conclusion is that all polyhedral polywell machines must operate as open recirculating devices.
3 Magnet coils must not touch at their adjacent corners, but must be spaced sufficiently far apart to ensure that no B fields internsect their containers.
4 Some means must be found to ensure large electron density within the machine, while maintaining it at small levels outside the machine.
5 The high density relative to the low density outside the machine can be attained by neutral gas injection directly into the machine, followed by subsquent very rapid ionization of this gas, before it can escape into the exterior region.
6 This means that if you have a small system you need to fuel the machine with ions injected from ion guns placed on cusp axes.
7 Large machines does not have these problems, That is why people trying to make small once are looking into POPS.

[Roger]

So the basic idea is to create a system were every electron is used.

Very important theme,

IIRC Bussard talked about WB7 & WB8 being 3 to 5 times better than WB6, I think he may have been infering that the 2 tweaks that the truncated cube and dodec represent, should reduce electron losses further, than WB6.

[jmc]

Firstly, I've realised the function of the cage, its earthed. Electrons are created just inside it at around 5, 10eV or so. And then the Magrid does practically all of the remaining acceleration, I think that implies the only electrons that can escape are those that crash into the Magrid and those which are scattered out towards the vacuum vessel. Another source of loss of electron energy would be ionisation of neutrals and radiation (Bremstrahlung/syncrotron)

If the well is to remain concentrated at the centre then the voltage of injection must be atleast the potential of the well. I think you might possibly be able to increase the overall potential of the well by draping additional layers of lower energy electrons around the outside, but not by much methinks.

The two things that determine the potential of the well are:
1) The net charge of electrons after subtracting the ion charge, this can be calculated by balancing the rate of injection with the rate of loss.

2) The spatial concentration of that charge, after all, a sphere off radius 1cm containing a given charge will be at a far higher potential than a sphere containing that same charge but of radius 1 metre. Ideally, assuming the electrons contain no anglular momentum to make the electron concentrated you want to match the accelerating voltage of the magrid with that of the potential well so that the electrons stop at the centre thus spending the most time there. If the electrons contain angular momentum then I suppose you want to match the potential of the magrid with the potential at their distance of closest approach to the centre.