just plain electrostatic confinement

Discuss how polywell fusion works; share theoretical questions and answers.

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KitemanSA
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Re: just plain electrostatic confinement

Post by KitemanSA »

ohiovr wrote:A point at the center of a cage contains a negative charge (without there being any electrons at that point) which attracts ions. Super! If the potential is high enough how could the ions ever gain enough energy to get far enough away from the point charge to collide with the grid?
The static charge on the grid only seems to be a point charge when the ion is OUTSIDE the grid. When the ion is inside, the static charge is evenly distributed around it and thus effectively disappears. The ion acceletates TO the grid and continues at that velocity thru the core. It then exits the grid and the static charge is all on one side of the ion and starts attracting it again.

D Tibbets
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Re: just plain electrostatic confinement

Post by D Tibbets »

KitemanSA wrote:
ohiovr wrote:
KitemanSA wrote:A fusor appears to be a point NEGATIVE charge to an ion which accelerates past the grid, passes THRU the fusor core, exits the grid and is then attracted by that point negative charge again. Thus it slows, accelerates back, passes the grid, thru the fusor core, ad infinitem until it hits another ion and fuses, or hits a grid and is lost, or thermalizes and can't fuse anymore.
If you can have a core of electrons before you start adding ions, why not just start with a group of ions with no electrons?
are you talking "fusor" or "Polywell"? There are no electrons in a Farnsworth Fusor, except those on the grid.
Thelast post by KitemanSA is fairly concise and to the point. You can shootions into the center, or you can place them quietly in the center. The latter will not result in any fusion. Shooting them in at high speeds can result in fusions, but remember fusion resulting collisions are uncommon. If the ions only scatter back out of the center or pass through the center they will continue on till they hit the shell and the input energy is lost.

Using the Polywell as an example, the ions have to travel at high speed at least 10,000 meters before there is a fair chance of one of the collisions resulting in a fusion. If the machine is only 1 meter wide this means you are you are losing the KE of more than 10,000 ions for the chance that one will fuse. Considering that the ions need a speed of up to ~ 100,000 eV, then losing 10,000 of them means you lose ~ 1,000,000 eV [EDIT- that should be 1,000,000,000 eV] of energy. If your single fusion yields ~ 3,000,000 eV it is obvous that you are falling short by a factor of over 100 (and these are conservative estimates). You have to recycle or contain the high energy ions for long times/ passes/ distance. There are two basic ways. Either turn them with magnetic fields while they maintain high speeds/ energy. This is not very good because of heat loss and leakage of the magnetic fields (ExB mostly) , The other way is to alternately accelerate the ions then decelerate them for each pass. Fast in the center and slow and actually stopping before the wall or magnetic surface is reached. This is done with electrostatic charges which can be on a real grid , centrally placed ball or needle electrode, etc. Or the electrode can be virtual due to a local excess of electrons at a smaller radius then the ions when they stop near the edge radius. The electrons act as a single point source while the ion is at a greater radius. When the electron [EDIT- should be ion] passes inside the cloud or shell of electrons (at what ever radius this shell is located, then Gauss Law becomes dominate and the ions continue at the same speed that it has been accelerated to. There are all sorts of qualifies you can add but this is basically the concept. You can accelerate and decelerate the ion back and forth in a limited distance to achieve the required confinement time and speed. Alternately, you can keep the ions at a high speed and race them around a circular racetrack as in the torus of a tokamak The problem with the race track is that the ions are susceptible to ExB diffusion through the confining magnetic field. This limits the density to size ratio. So the tokamak has to be large in order to contain the ions long enough. And because the density has to be relatively small the machine (reaction volume) has to be large. The edge instabilities/ macro instabilities is another factor that limits density in the tokamak like machines.
The Polywell has the advantage in both areas- higher density and no vulnerability to edge instabilities (which would otherwise limit density below what could otherwise be achieved). This is fortunate, as the Polywell needs high densities for adequate magnetic confinement of the electrons (Wiffleball effect).

A real grid , central sphere or needle electrode will intercept a percentage of ions on each pass back and forth. The very best grids, even multiple grid arrangements will intercept ~ 1-10% of the ions each time they complete a pass in and out. This compared to the 10,000 pass minimum that Bussard stipulated means you are falling short by a factor of at least 100. With other considerations actual fusors with a real cathode grid can manage a Q of ~ 0.000000001. You have to have a virtual cathode. Excess electrodes provides for this. Even if an ion collides with an electron, the ions momentum/ KE will not be changed more than a tiny amount, and multiple collisions will tend to cancel out even this effect provided the ion and electron temperatures are about the same, which they are in the Polywell ( if you average the relative energy over the entire volume of the machine). Ions hot/ fast in the center and slow on the edge, while the electrons have a reverse speed distribution, but the average is the same. This inverse speed distribution of the electrons and ions effects the potential well shape and thinfgs like Bremsstruglung, but that is another discussion.

How do you create a virtual cathode (negative local charge) you ask? Well, the same way , except you use a real anode to accelerate the electrons so that they can overcome mutual repulsion and collect in a defined volume. This is the ETW variation of the fusor. But, while this can electrostatically contain electrons, now the ions are lost if they pass outside of the anode. No ground is gained. One arrangement directly opposes the other. The trick is to confine the electons magnetically after accelerating them towards the core from outside of the machine /Magrid. The hot electrons are magnetically contained and form a virtual cathode that contains and centrally accelerates ions. This is now like a typical cathode gridded fusor, except there is no metal grid for the ions to hit. You do not need to contain the ions magnetically at all, at least in the ideal . In actuality things are a lot more messy.

Your concept of a metal ball not having an effect on charged particles is flawed. Outside of the ball the attraction or repulsion is normal . It is inside a conductive shell/ sphere that is applicable to Gauss Law. INSIDE the hollow sphere a point charge (charged particle) sees all of the gazillions of point charges of the shell, but because one side pushes while the other pulls and because of inverse square law considerations, all of the surfaces charges cancel each other out. It is not that there are not charge attracting/ repelling effects, it is that there are no NET effects, which is equivalent to zero effects. The same applies to a cloud of electrons whether arranged in a diffuse cloud or concentrated to a tiny volume in the center of a sphere. So long as the positively charged ion is outside (most of) the cloud it will be attracted to all of the point charges. You can calculate the acceleration vectors for a thousand billion billion electrons, and you will find that you can simplify it to a single centrally located point source doing the work. Again there are qualifies, but I will restrain myself from further disertation :lol:

Dan Tibbets
Last edited by D Tibbets on Fri Nov 07, 2014 4:09 am, edited 1 time in total.
To error is human... and I'm very human.

KitemanSA
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Joined: Sun Sep 28, 2008 3:05 pm
Location: OlyPen WA

Re: just plain electrostatic confinement

Post by KitemanSA »

D Tibbets wrote: Thelast post by KitemanSA is fairly concise and to the point. You can shootions into the center, or you can place them quietly in the center. The latter will not result in any fusion. Shooting them in at high speeds can result in fusions, but remember fusion resulting collisions are uncommon. If the ions only scatter back out of the center or pass through the center they will continue on till they hit the shell and the input energy is lost.
But that is why one puts the electro-static charge on the grid, to keep the ions from simply heading off and hitting the shell.

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

Re: just plain electrostatic confinement

Post by D Tibbets »

KitemanSA wrote:
D Tibbets wrote: Thelast post by KitemanSA is fairly concise and to the point. You can shootions into the center, or you can place them quietly in the center. The latter will not result in any fusion. Shooting them in at high speeds can result in fusions, but remember fusion resulting collisions are uncommon. If the ions only scatter back out of the center or pass through the center they will continue on till they hit the shell and the input energy is lost.
But that is why one puts the electro-static charge on the grid, to keep the ions from simply heading off and hitting the shell.
The point I was trying to make is that you cannot have a closed container of fast ions without some force to control their speed within the closed container- an appropriatly charged grid, virtual grid/ electrode, etc. The tokamak is in effect an open container. The torus has no end walls. So long as you can keep the ions moving along the axis of the torus, it can move/ circulate indefinatly at whatever speed it has.

Of course, centrifugal (or is that centripetal?) effects, collisional effects, etc again complicates the picture. You could argue that a spherical volume with the ions rotating with all of their energy in angular momentum and none in radial energy, but this describes a fat torus- essentially a spheromac.

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

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