Talk-Polywell.org

Thanks for that titbit Dr Nebel. Its always good to hear the history of the development of a technology by someone whose worked in the field for a long time.

rnebel wrote:....
There is also another lesson from this worth noting. NIF is very likely (almost certainly) going to operate before ITER does. If NIF hits ignition, it’s going to put a lot of pressure on ITER. Politics is at least as important as technical issues in these decisions.

we had all rather hoped that you might hit ignition first Dr Nebel.

I would have guessed that "ignition" in this context means "fusion," but apparently not, because the BFR* has already had some fusion, right? Does "ignition" then mean break-even? Or is there some other intermediate step that I have missed?

Bill Flint

*BFR - new "official" designation - see poll under "general"

I thought ignition meant a self-heating plasma. An article I read a while back (in IEEE Spectrum, IIRC) referred to ignition as Q=infinity. If this is true, neither NIF nor WB-100 can ever reach ignition, because their operating principles involve external heating.

It's a technicality, of course; all we really want is Q>>1. Being able to shut down the heating equipment after startup is mostly a tokamak thing...

93143 wrote: It's a technicality, of course; all we really want is Q>>1. Being able to shut down the heating equipment after startup is mostly a tokamak thing...

The concept has merit with the NIF (National Ignition Facility); Once Fusion starts, no further heating is required. All lasers can be `off'.

Agreed, the concept of ignition also has merit with the MIF (Monstrosity In France); but is kind of meaningless for Polywell, so it seems.

Okay, I see. Within the context of a single pulse, ignition makes sense. You do, of course, have to turn on the lasers again for the next pulse, so in practice it means a different thing than with ITER...

I think we could define polywell ignition as the point when the alpha collectors put out enough current to run the power supplies & refrigerators etc. and are actually hooked up to do so.
It is a bit beyond break even.
Sort of like when a car finally starts running.

That's "net power". A tokamak can make net power without igniting, so long as the heating power stays below the fusion power.

Polywell is a bit funky because the plasma doesn't really self-heat (in fact any self-heating is counterproductive), and the heating power is applied indirectly via the electron replacement power. This will never go away completely (and due to electron thermalization issues you wouldn't want it to), so technically, ignition cannot be achieved in a Polywell.

I thought 'theoretical' ignition was when Lawson's triple product was exceeded. No?

rcain wrote:I thought 'theoretical' ignition was when Lawson's triple product was exceeded. No?

That does appear to be the case:
http://en.wikipedia.org/wiki/Lawson_criterion
Thank you!

Bill Flint

Ignition is when, in principle, if heating was turned off by fuelling continued, the reaction would go on forever. In practice you still need some heating for non-inductive current drive, but in essence it is when the reaction heats itself.

If the field in a Polywell was strong enough, I'm sure the alphas would be deflected back into the reaction volume, can a Polywell not reach ignition because cusps look alot wider to the alphas that the electrons and thermal ions?

Ignition makes no sense for a Polywell, because it's not thermal. It's an inapplicable term, like asking how many home runs the Raiders need to win the Super Bowl.

Even if you did reflect the alphas, they wouldn't help your reaction. Fusion in an IEC scheme is driven by the electrostatic force, not heat.

The closest thing comparable would be powering the electron drive from the electric output, as 93143 mentions above. At that point, as with an ignited tokamak, there is no net energy input into the system. Of course, in a Polywell this isn't that much different than "net power."

Remember a lot of people aren't even sure whether fusion is possible at all.

One can easily point out to those people that Farnsworth was producing fusion before most of them were born.

I think what people are skeptical of is fusion as a power source. Unfortunately, ITER/DEMO doesn't do much to alleviate that doubt, because it's not only 40-50 years away from producing power but probably can't compete with fission when it gets there.

TallDave wrote:Ignition makes no sense for a Polywell, because it's not thermal. It's an inapplicable term, like asking how many home runs the Raiders need to win the Super Bowl.

Even if you did reflect the alphas, they wouldn't help your reaction. Fusion in an IEC scheme is driven by the electrostatic force, not heat.

The closest thing comparable would be powering the electron drive from the electric output, as 93143 mentions above. At that point, as with an ignited tokamak, there is no net energy input into the system. Of course, in a Polywell this isn't that much different than "net power."

i undertood the Tripple product to be inertial in dimensions - therefore it takes account of thermal and non thermal momentum, velocity, pdf. it doesn't care if its manifestation if gravitational, electrostatic, magnetic or thermal.

therefor 'ignition' in that strict sense would apply to Polywell, surely?

the relationship between Lawson Criteria and overall Q is interesting.

Hmm, I don't think so. Basically, it's only applicable for a thermal plasma.

In nuclear fusion research, the Lawson criterion, first derived[1] by John D. Lawson in 1955 and published[2] in 1957, is an important general measure of a system that defines the conditions needed for a fusion reactor to reach ignition, that is, that the heating of the plasma by the products of the fusion reactions is sufficient to maintain the temperature of the plasma against all losses without external power input.

If you're not heating/accelerating the plasma with fusion products, it can't apply, and of course you can't directly accelerate with fusion products.

Though, you could argue that if you're feeding fusion products through an alpha conversion mechanism or even (D-D/D-T) through a steam turbine to power the electron drive, that might essentially amount to a less direct version of the same thing.