Talk-Polywell.org

102 seconds @ 50 million degrees Celsius

http://www.sciencealert.com/china-s-nuc ... sma-record


i'll just leave this here.

Cool! Thanks for the posting.
I wonder how long it will take the German Stellarator to reply?

This isn't even close to being a fair comparison. One's a Tok and the other is a Stellarator.

I don't know what the target temperature and target confinement time is supposed to be on the top of my head. But I kinda don't think the W7-X was meant to show it can hit those targets. I thought it was just to show, we can make a huge Stellarator, we can get the temperature high, we can do all this with zero instabilities.

Robthebob wrote:This isn't even close to being a fair comparison. One's a Tok and the other is a Stellarator.

I don't know what the target temperature and target confinement time is supposed to be on the top of my head. But I kinda don't think the W7-X was meant to show it can hit those targets. I thought it was just to show, we can make a huge Stellarator, we can get the temperature high, we can do all this with zero instabilities.

Confinement time and temperature are fair comparisons.
Even if they are different type of machines in the end they need to give you same result. You can think it in a way you compare a gas engine with a Diesel Engine. Two different ways to reach the same comparison terms.

I also think that the base comparison for all machines is "fusion conditions sustained".
It would seem that temp & confinement time is a good basic measure.

ok, in that case, how good is the news of such long time at those temperatures?

is that bad news for other fusion technologies competing with Tokamaks?

I would say it pretty good.
That said, there are a couple of other contenders that may perform similarly or better in the near future.
To get a good sense, we would need to put together a little table to compare the various efforts.

Confinement time and temperature are measures of performance, but it doesn't suggest anything close to achieving goals unless density is also included. It is the Triple product that is the criteria that has meaning.

My recollection is that Tokamaks or the ITER goals, I don't remember which, have (already) pushed past 5000 eV (55 million degrees), confinement times of ~ 800 seconds, and densities of 10^19 to 10^20 particles per M^3.

There has been criticisms that the Polywell is completely inadequate with confinement times of under a second, especially with claimed Tokamak times of ~ 800 seconds, but when density and temperature both are included the picture is completely different.

As such, if the stellarator is anticipated to achieve higher densities, shorter confinement times may actually suggest greater performance.

Dan Tibbets

Dan,
Density is more machine dependant in my opinion.
Whereas temp/time gives a quick sense of the where tje machine is at. Especi6when considering pre fusion work.
At he bottom line, triple product is mo' betta.

Iadajo,
Perhaps with a stable magnetic confinement (convex fields towards the plasma always), the temp and confinement time may serve as a good measure of position on the goal graph, but without considering density in unstable fields like the Tokamak*, the picture is much more uncertain. Controlling density driven edge instabilities seem to be the major concern in Tokamaks at present. If a stellarator can control most of the instabilities, even at increased densities, it has an exponential advantage, and direct comparison of the other two parameters alone (assuming density is a constant between the systems) are even more confounding. It effects not only the scientific goals but the economical goals as well. Greater density implies greater energy density and probably smaller and cheaper machines.

*The Lockheed machine seems to be a hybird, stable B fields face the vast(?) majority of the plasma, but that circulating outside the primary magnets face unstable B fields. The saving grace is, I understand, that here the plasma density is much less so the relative effects are tolorable.

Dan Tibbets

Dan,

Edge Instabilities are connected to confinement time, so longer confinement time means already a better control in edge instabilities (and W7-X should be able to handle this issue easily according design parameters).
As for densities, I partially agree with your point, but let's wait till they warm up the machine and start to make some real test shots.
I have been reading a lot of papers on W7-X, and there are good hints pointing to the fact that (in theory) Density importance has been overestimated in a Stellarator type machine. As I said, these are just "theories" so far, but will be easy and quick to check them once the machine is fully operational.

What he said.

Giorgio wrote:Dan,

Edge Instabilities are connected to confinement time, so longer confinement time means already a better control in edge instabilities (and W7-X should be able to handle this issue easily according design parameters).
As for densities, I partially agree with your point, but let's wait till they warm up the machine and start to make some real test shots.
I have been reading a lot of papers on W7-X, and there are good hints pointing to the fact that (in theory) Density importance has been overestimated in a Stellarator type machine. As I said, these are just "theories" so far, but will be easy and quick to check them once the machine is fully operational.

Indeed, I realized this as I was pontificating, but I managed to cut off my comments before they carried on to far. Edge instabilities is a major, if not dominate concern for achieving desired confinement times , especially in the unstable B field configurations. My point I am trying to make is that edge instabilities scale exponentially (I think) with density, so running a machine at low density will improve confinement times, perhaps dramatically. But, that does not imply that the confinement times are useful for fusion. You must have a certain confinement time at a specified density to know where you are in your scaling predictions (assuming the temperature is a constant). Some machines are very good at confining low density and effectively non collisional plasmas. Penning traps can confine a relative few particles for long times. This is useful for storing muons, anti particles, etc. But it is useless for any consideration of useful fusion power production unless you include how the system scales with density (and temperature).

Any one or two componets of the triple product can be approached individually or in pairs, and in a given machine with the excluded components considered as constants, improving that component is always a benefit in isolation. But not if it compromises another component(s) too much. In comparing different machines with different weightings for the three components, caution must be used when comparing. A Penning trap may have a confinement time of millions of seconds, but at a density of perhaps 10^6 particles per M^3 the amount of fusion possible is extremely limited. At best, even if you could reach breakeven, you would only have a billion dollar hand warmer.

The question is not which is better, a Tokamak with 100 sec. confinement times, a Stellarator with 1 second confinement times, or a Polywell with 0.05 sec. confinement times, or a DPF with 0.00000005 sec. confinement times. It is which confinement time is closest to the goal for that specific machine, ie- the triple product goal. Comparisons are misleading with only part of the picture reveled.

Dan Tibbets

A more valid comparison of the Chinese Tokamak may be its comparative confinement time to JET or the Japanese Tokamak. It could still be highly miss leading though because of the two other parameters.

I don't know what the 1 sec. confinement time for this new Stellarator could be compared to. Obviously computer modeling helps, but I understand that this is such a profound improvement in the Stellarator hardware, that comparing it to older efforts is challenging . Is one second confinement times, even in this early test much improved compared to older designs? And, of course the density and temperature ( or combined into pressure) parameters remain of paramount comparative importance. This Stellarator borders on being a different machine.I speculate that older Stellarator hardware performed only marginally better (if that) compared to Tokamaks, when talking about confinement time verses density. So the confinement time numbers were comparable. This new machine, with much or tremendously improved edge instability characteristics may be anticipated to operate at unobtainable densities compared to Tokamaks, so the confinement times are relatively much shorter.

Of course, Tokamak efforts are not stationary. There are indications that edge instabilities may be controllable to such an extent that densities may be pushed up accordingly. Even in Tokamaks, considering confinement times alone may be misleading.

Dan Tibbets

Dan,
Regarding your comments above, what temperature are the stored particles in a Penning trap?

We all agree that the triple product is the best comparison model, however, it was also conceived with steady state burning plasma machines in mind.
If you are talking non-burning plasma, non-steady state (pulsed) machines it becomes less relevant.

That said, a quick dirty check is the concept of being able to maintain highly energetics for a while (temp & time). Low temp is easy, high temp less so.
Now, if you want to do a more detailed look, then count the number of highly energetics in the contained volume.