methods of controlled ionization

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

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happyjack27
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methods of controlled ionization

Post by happyjack27 »

how about oscillating the mag field strength? that would induce an oscillating electric current in the plasma, which would presumably be stronger closer to the electromagnetic. and as you get further from the electromagnets you get closer to the core i.e. denser which means not only is the current less, its energy is divided up among more atoms. so in two ways this method of ionization would prefer neutrals closer to the magrid, no?

KitemanSA
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Post by KitemanSA »

POPS?

93143
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Post by 93143 »

It's very difficult to rapidly vary the field of a superconducting magnet.

Even with non-superconducting coils, the magnetic forces involved are enormous. Generally, you want to keep enormous forces as static as you can lest structural fatigue become a major failure mode... Also, these magnets probably have too much inductive inertia to be oscillated efficiently at the required frequency...

What frequency range were you thinking of?

There were early proposals to use RF to stimulate ionization via ECR. This would probably be for a neutral feed design; it seems that the research is gravitating towards ion guns (which might use internal ECR anyway...). On the other hand, some level of ECR might turn out to be useful to keep the neutral population low... The POPS stuff, I think, also uses an RF driver, though at a different frequency, and was originally developed for fusor-type devices (ie: no magnetic coils, just an electrostatic grid); there's still no official word on whether it will work in a Polywell...

happyjack27
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Post by happyjack27 »

as i understand it RF is for ICR whereas for ECR you need microwave. due to the difference in charge-to-mass ratio between an ion and electron.

as to the speed i was thinking, well, sufficiently fast enough to create a current with enough power to ionize. which given the considerations you mention, is probably impractical.

93143
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Post by 93143 »

Technically, microwaves are part of the RF spectrum.

On the other hand, depending on where in the edge region you want the ECR to peak, you could actually get quite a large value of B; you could be looking at sub-millimetre wavelengths, which are (just barely) outside the definition of RF (and microwave radiation), though not really small enough to be called infrared...

I wonder how an attempt at ECR would interact with the high field gradient at the wiffleball edge...

Oh yeah, there was also a plan to use something resembling ICR to selectively excite the fusion ash and clear the low-energy tail, or something - anyone remember that?

D Tibbets
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Post by D Tibbets »

Oscillating the magnetic field is possible to a degree. It depends on the magnitude of oscillation desired. A superconducting steady state electromagnet could be used with a piggyback copper wire coil to vary the intensity of the B field, say 10.0 T for the superconductor, and 0.1 T from the copper wire. You would still have alot of stress on the oscillating current copper wire, but it might be manageable.

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

93143
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Post by 93143 »

Maybe. I'm a mechanical engineer by training, at least up until I started my Ph.D., so I'm very dubious about introducing significant oscillating stresses if there are other ways to accomplish the same thing.

The relative smallness of the oscillation in your example doesn't help as much as you'd think, since (assuming you want to keep the fields symmetric at all times, and thus opposing coils would be oscillated in phase) magnetic force is proportional to B^2. In other words, the oscillating 0.1 T field by itself would represent a stress 0.01% as large as the 10 T steady field does, but if you add that same 0.1 T oscillation on top of an existing 10 T field, the oscillating stress is 2.01% of the mean, or 201 times what it would be on its own. Fine for steel, but not every substance has a fatigue limit, and the cycle frequency here is very high... and if there are parts that can move and rub against one another under oscillating forces, well...

It's probably worth a closer look by some engineer somewhere, if Polywell turns out to be viable. But I wouldn't bet on it being a good idea.

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