## Design for preferential cusp field weakening, guiding Alphas

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

Moderators: tonybarry, MSimon

KitemanSA
Posts: 6114
Joined: Sun Sep 28, 2008 3:05 pm
Location: OlyPen WA
zimdlg wrote:
The main problem is that if you "fire" the ions from outside, they arrive at the edge of the well with some residual velocity. This in theory will allow them to fall down the well, climb up the other side and still have that residual velocity. If they have it and are pointed at cusp, they will exit the MaGrid with increasing velocity and impact the vacuum chamber with great energy, ALL of which is lost.
Can't this problem be solved by giving the ions just enough energy and no more to get past the magrid then the -ve charge on the wiffleball will hold the ions inside the magrid?
I am not sure what you mean be "on the wifleball". The electrons are spread throughout the plasma. And unless the ions loose some energy while moving thru the plasma, they will reach the equivalent point on the other side of the well with enough energy to continue on.
zimdlg wrote: Once the +ve ions are inside the +ve magrid they feel equal repulsive forces from all directions so the forces cancel each other out and the ions feel no net repulsion from the magrid, not so?
True, but thus it can't help retain it either. I guess that is what the "develop an ion gun" work was all about this past year. I wish I knew what the results of the work was.

DrB. seems to have thought that puff-gas inside the MaGrid would be much more effective in a large Polywell. We'll see, I hope.

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am
zimdlg wrote:
True, but the Polywell outside the MaGrid is + to attract the electrons into the center to create the wiffle ball in the first place.
I agree, an electron outside the magrid must see the magrid as +ve overall so that it gets pulled towards the magrid. Then the magnetic fields will channel them into the interior of the magrid.
The main problem is that if you "fire" the ions from outside, they arrive at the edge of the well with some residual velocity. This in theory will allow them to fall down the well, climb up the other side and still have that residual velocity. If they have it and are pointed at cusp, they will exit the MaGrid with increasing velocity and impact the vacuum chamber with great energy, ALL of which is lost.
Can't this problem be solved by giving the ions just enough energy and no more to get past the magrid then the -ve charge on the wiffleball will hold the ions inside the magrid?

Once the +ve ions are inside the +ve magrid they feel equal repulsive forces from all directions so the forces cancel each other out and the ions feel no net repulsion from the magrid, not so?
I've been strugling with this . If the ions enter the magrid through a cusp with some residual velocity, wether the magrid has some positive charge on the inside or not (through induction from the excess electrons within the magrid), the ions will accelerate to the center, then decelerate to reach the same height when next it finds a cusp, which in isolation would mean the ion reaches above the magrid and is then repelled to the wall if the magrid has a positive charge. If the magrid is grounded, the ion would not be accelerated to the vessel wall, but would it see the internal negative charge? If it is attracted to the grounded magnetically shielded magrid there may be some recircultion effect on the ions in addition to the electrons. This effect depends on Faraday cage/ Gauss's law conciderations, potential on the magrid, electron guns, ion guns(?), and vessel wall potential, etc, etc. which in my mind degenerates into confusing circular arguments.
I guess based on hear say that the magnetic confinement of the ions is ~ 1000 passes (based on what Dr Nebel said about alpha particles), and the electrostatic confinement increases this to ~ 100 thousand or more for the lower energy fuel ions. I'm guessing that the key may be that the ions have a lower average speed than the injected speed due to inefficiencies- scattering (loss of pure radial motions). ie- some scattering/ thermalization may be good within limits. You would be losing the high end tail (those ions that maintain thier original radial velocity or are upscattered. The recirculating electrons (traveling bothways in the cusps) and sheaths complicate the situation, and I'm having enough problems trying to follow the 'simple' initail conditions. Then throw in the effects of the curved magnetic surfaces, the ion gyrordius inside the cusps, any other number of considerations and my vision dims even more.

Consider an example of my rambling speculations- the space between the magnets. The spacing allows for the electron gyroradius to clear the magnets in the cusps, but the ions have a larger gyroradius, Near the top of the ions potential well, they may actually hit the magnets bordering the cusps (or at least come very close so that the induced positive charge (from the excess internal electrons) repells them) so that they fall back into the Wiffleball (with a minimum of waste heat transferred to the magrid casing). This would serve two purposes. One- decrease the charged particls escaping to the outside and contributing to arcing, and two- reseting the effective potential well peak for those ions so that it is now slightly below the midpoint of the magrid. But, what would that do to the high energy fusion ions trying to get through the cusp...?
[EDIT] For that matter, what would it do to the original ions being injected?

Dan Tibbets
Last edited by D Tibbets on Mon Aug 10, 2009 2:03 am, edited 1 time in total.
To error is human... and I'm very human.

ltgbrown
Posts: 176
Joined: Mon Jun 22, 2009 11:15 am
Location: Belgium

### First post

Hello all. I have been following Polywell for almost 3 years and finally have decided to take the plunge and post my first comment.

I am a Naval Aviator with an 18 year old BS in physics which lets me think I can almost understand what you are talking about and has led me to think I have something constructive to add.

Forgive me if I am missing something fundamental, but I am assuming the cusps are not perfect lines. I tend to imagine them as hourglasses with the concave ends cut off. If that is true, couldn't the ions be "dropped" or "fired" along (or near) the "surafce" of the hour glass shaped cusp? Thus they would enter the plasma at angle that would result in them not heading directly at a cusp on the opposite side. If this is possible/true, then would it not be possible to calculate an optimum angle to maximize the number of "passes" through the center before an ion would find a cusp to exit?

Be gentle, its my first time.
LTGBrown
Famous last words, "Hey, watch this!"

KitemanSA
Posts: 6114
Joined: Sun Sep 28, 2008 3:05 pm
Location: OlyPen WA
Be gentle, its my first time.
LTGBrown
_________________
Do or Do Not, There is No Try!
Welcome Yoda.

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

### Re: First post

ltgbrown wrote:Hello all. I have been following Polywell for almost 3 years and finally have decided to take the plunge and post my first comment.

I am a Naval Aviator with an 18 year old BS in physics which lets me think I can almost understand what you are talking about and has led me to think I have something constructive to add.

Forgive me if I am missing something fundamental, but I am assuming the cusps are not perfect lines. I tend to imagine them as hourglasses with the concave ends cut off. If that is true, couldn't the ions be "dropped" or "fired" along (or near) the "surafce" of the hour glass shaped cusp? Thus they would enter the plasma at angle that would result in them not heading directly at a cusp on the opposite side. If this is possible/true, then would it not be possible to calculate an optimum angle to maximize the number of "passes" through the center before an ion would find a cusp to exit?

Be gentle, its my first time.
LTGBrown
Welcome to the forum.
I have wondered if having the ion guns, and electron guns slightly offset on the diverging field lines near the bore of a cusp might allow them to inject thier particles. But, not because the particles aimed straight down a cusp would end up heading straight for an opposite cusp. As the particles approach the center on thier first and subsequent passes thier trajectory will be bent or even rebounded from near the center*, so any subsequent chance of hitting a cusp and escaping is a random event , with very little if any chance of the opposite cusp being hit due to the initial injected trajectory. My reasoning for having the guns offset is so that they are out of the way of the alphas (or other charged fusion products) that come barreling out of the cusps with less curvature along the diverging magnetic field lines due to thier increased kinetic energy.

* The first few thousand particles may travel fairly straight across the Wiffle ball, but as the numbers build the chaotic mutual repulsions and attractions will randomize them. Since most of these effects will occur near the center thier radial velocities will be maintained , but the trajectories will be in random radial directions.

Dan Tibbets
To error is human... and I'm very human.

zimdlg
Posts: 19
Joined: Wed Jun 10, 2009 8:09 pm
Hi Itgbrown

I think the more people that are involved in this technology the better

Your suggestion ties up with my current understanding of the BFR. The alpha particle collectors have to be directly above the cusps to capture the alphas, so the ion guns will have to be offset and fire at an angle into the cusps. I'm not sure that there would be an optimal angle, maybe EMC2 is working that out as we type.
the future is near.

ltgbrown
Posts: 176
Joined: Mon Jun 22, 2009 11:15 am
Location: Belgium
D Tibbets wrote "My reasoning for having the guns offset is so that they are out of the way of the alphas (or other charged fusion products) that come barreling out of the cusps with less curvature along the diverging magnetic field lines due to thier increased kinetic energy."

I was thinking the same thing but did not want to get too verbose on my first posting!!

LTGBrown
Famous last words, "Hey, watch this!"

MSimon
Posts: 14331
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:
In high magnetic fields the cusps are going to be small. Maybe very small.

The difficulty will be in getting anything into the reaction space in the volumes required. It may require very careful alignment. And then you have to keep your aim correct in the face of mechanical changes as the reaction comes to equilibrium.

When Bussard said that all that remained was "just engineering" he was telling a joke. Maybe a joke that only engineers get.
Engineering is the art of making what you want from what you can get at a profit.