Grid Losses

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Tom Dressel
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Grid Losses

Post by Tom Dressel » Sat Jul 14, 2001 7:20 pm

I started viewing Richard Hull's vidio tapes on the fusor. The firse tape includes an interview of Tom Ligon. He mentions that making neutrons is relatively easy in an IEC fusor , the real problem is getting around the "grid losses", and getting the efficiency up. What exactily are "grid losses"?

DaveC
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Re: Grid Losses

Post by DaveC » Sat Jul 21, 2001 7:28 am

It has been my understanding that the term "grid losses" refers collisions by neutrons, and/or ions that collide with the spherical wire cage(s) electrodes that form the cathodes and anodes. These particles deposit their energy as heat in the electrodes, and are thus not available to be gathered outside the fusor.

Some losses will obviously be unavoidable. By the proper shape and size of the wires, the cages can have a minimal, but not zero, reduction in total neutron energy.

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Re: Grid Losses

Post by r_c_edgar » Sat Jul 21, 2001 12:23 pm

It's really more ion collisions than neutron collisions - fusion neutrons, at 2.45 million electron volts, are not that easy to stop! That's one of the reasons detecting them is such an effort.

Really though, grid losses are comparatively minor next to some other design inefficiencies.

First, at pressures we are typically working at (a few microns), the mean free path might only be a few chamber diameters, vastly reducing ion recirculation - one of the greatest advantages of the fusor.

Of course, it we reduce the pressure, the current that is drawn is reduced, and on top of that, the fraction of that current that is due to ions is reduced as well - much more of the current is due to electrons emitted from the inner grid (decreased pressure means decreased ionization). Bring the pressure too low on the simple, two-grid fusor and you will have effectively zero ion current.

And then, above and beyond both of those, a significant number of ions are not created near the edge of the fusor, but instead closer to the center - where they fall through a much smaller part of the potential well, and hence may not reach energies where fusion is likely.

These inefficiencies, however, are addressed in the more advanced Hirsch-Meeks design (three grids), which can operate at lower pressures, and at the same time primarily ionizes gasses at the edge of the potential well, increasing fusion yield accordingly.

-Ryan Edgar

guest

Re: Grid Losses

Post by guest » Thu Oct 18, 2001 5:01 am

Why can't you make the grid in to an electromagnet and keep it at high voltage by hooking each end of the ti wire to a leg of your voltage multiplier.

DaveC
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Re: Grid Losses

Post by DaveC » Thu Oct 18, 2001 6:54 am

The chief difficulty with connecting the grid across the High voltage multiplier is that it would short out the output.. no High Voltage, too low a resistance.

But, I am not sure what you would gain, by making either the inner or the outer grid into an electromagnet. The field lines would seem to misdirect the electron trajectories... Perhaps you could elaborate on what you are thinking...

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Re: Grid Losses

Post by quinnrisch » Mon Oct 22, 2001 12:37 am

If you are speaking of a sphereical fusor: let me just tell you that there is not such thing as sphereically symetrical magnet field lines.

If you are thinking of a cylindrical fusor, and using the inner cathode as a electromagnet, with the field lines directed in the axial direction: All this would do is "unfocus" the circular trajectory of the duetrons and make head on collisions inside the cathode impossible; that is to say, ensure any fusion would have to take place at the cathodes surface or just outside of the cathode. I don't think that this would help at all.

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Re: Grid Losses

Post by guest » Mon Oct 22, 2001 12:59 am

I think what Tom Ligon is really talking about when he mentions grid losses as a problem is grid heating, pure and simple. In a tiny amateur fusor, ion bombardment of the grid is sufficient to turn it red hot or even melt it if you're not careful. Even decent refractory metals like tantalum get eaten away by sputtering given sufficient time. This is for a device that only consumes a few hundred watts of electrical power and with a reaction rate of about 10^5/sec.
Imagine if you will a power procducing fusor with ion fluxes on the order of hundreds of milliamperes or even amperes, and you begin to see the nature of the problem. Bussard's polywell approach uses a virtual cathode of electrons confined in a cusped magnetic field to help get around this problem.

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Re: Grid Losses

Post by guest » Thu Oct 25, 2001 3:49 am

could you not pump liquid hydrocarbons,boronhydrides, or lithium over the grid reclaiming losses to make fuel plasma

use a tube with pores for grid/limiter combo

I THINK FOR FUSION TO WORK WE MUST COMBINE TECH FROM ALL FIELDS
if i'am a pain in the ass tell me

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Richard Hull
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Re: Grid Losses

Post by Richard Hull » Thu Oct 25, 2001 2:30 pm

No, you are not a pain.

All ideas offered in good faith are listened to and hopefully, weighed and considered against the complex mix of tradeoffs.

Actually your idea is an old one. I beleive two years ago someone suggested on the original list, (now archieved), that SS hypodermic tubing might make a cooling scheme that would allow operation beyond where we are currently operating. The flowing fluid then was water or liquid sodium. Regardless, no one has tried it.

For max effect and recovery, I would probably just suggest immersing the enitre device in borated water. This would result in a 100% recovery of all energy spent in the device plus that of the neutrons emitted!! Ideally recovering 100.00000000001% of the incoming energy.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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