Whats the holdup with Fusion Power?

[gdp]

...I am extremely skeptical of this claim by Nebel. Bremsstrahlung scales as the square of the ion charge, so bremsstrahlung off Boron is 25 times worse than bremsstrahlung off D or T, and six times worse than bremsstrahlung off He3.

Well in a perfectly neutral system. Bremmsstrahlung is proportional to electron density, electron temperature, and the ratio of electrons to ion Z. These virtual cathode systems are by definition not perfectly neutral, as the electron/ion ratio > 1 sets up the electrostatic well.

Sorry, no. IEC systems, while they do have a very slight charge imbalance, nevertheless do still satisfy the "quasineutrality" condition to an excellent degree of approximation, as shown by Rider in his thesis.

Since the fusion power scales as the product of the proton and boron ion densities, trying to beat bremsstrahlung by running a "lean mix" (lowering the boron ion concentration relative to the proton concentration) necessarily also decreases the output power, so it is a self-defeating strategy.

Only to a point, as Nebel suggested with the 'optimum' qualifier, as the power gain function is not linear in all its parameters.

I am still very skeptical, and I'd want to see the data. IIRC, Rider explicitly shows that in a quasineutral plasma, the bremsstrahlung loss rate and fusion power depend on the densities of the ion species in the exact same way, so that the ratio of bremsstrahlung losses to fusion gain is a constant, independent of any monkey-business with the ion mixture. Anything that decreases bremsstrahlung losses should therefore decrease the fusion power by the exact same fraction.

Has Nebel published any of these claims in a refereed journal, or is it the only source for Nebel's claims the blog exchange between Nebel and Carlson?

Red Herring. The 2nd Law limit on IEC comes from the necessary disequilibrium between the electron and ion distributions --- not from the secondary disequilibrium between ion species. Two-stream instability is a collective effect that increases the thermalization rate of the plasma --- but even if two-stream and other instabilities were somehow completely eliminated, the unavoidable coulomb collisions between the electrons and ions will still cause their energy distributions to relax toward equilibrium with each other, generating entropy during the process. To maintain the electron/ion disequilibrium will cost power. Rider shows that maintaining this disequilibrium will cost more power than will be gained from operating at an electron/ion disequilibrium.

As I understand it, though Rider/Nevins correctly point out the 2nd law issues in play, there are two areas where they fall short: 1) the electron confinement times for a virtual cathode device are shorter than the thermalization/collision time with ions so that the electron temperature never has the opportunity to rise enough to cause unsustainable Bremmstrahlung,

Rider deals with this. In effect, one is "refrigerating" the electrons by removing them from the system before they can equilibriate. Since the electrons are "cold" compared to the ions, the ions therefore continuously lose energy to the cold electrons through coulomb collisions, producing entropy, and requiring that additional power be recycled to maintain the ion distribution. Rider finds that the power expended to maintain the disequilibrium will exceed the additional gain from operating at disequilibrium.

There is no escape from the 2nd Law.

2) their mathematical treatment of collisionality is inadequate. That is, the FP model performed by Chacon et al 2000 improves power gain (Q) by 5 to 10x over that predicted by Nevins. Take this last part up with Chacon et al.

I have downloaded the paper, but have not yet had time to read it. However, I note already that according to their abstract, they are performing an "optimistic" (their term!) calculation that explicitly neglects electron-ion collisional interactions --- and neglecting electron-ion interactions is simply not physically realistic in these disequilibrium systems.

Even worse, they appear to be treating the electron distribution as a fixed, prescribed "background" that generates a potential-well of two assumed forms: square-well and parabolic --- neither of which are particularly physical.

It is like attempting to estimate the performance of an automobile by neglecting road and air friction, and concluding that top speeds of 900 mph should be possible. Well, of course you can get unphysically good results, if you throw away the most important physical limiting factors!

A truly "self-consistent" calculation of fusion gain in an IEC device will need to explicitly treat the continuous transfer of energy from the ion to the electron population, rather than ignoring collisions and treating the electron population as merely a fixed prescribed background that is unaffected by the ion population as Chacon and Miley appear to be doing in their paper.

 

[mheslep]

Chacon et al do indeed account for losses due to electrons radiating and escaping the system. The quote above refers to simplifications made for the FP calculations only. The electron losses are calculated in the familiar closed form way. I'll get back to this in detail later time permitting...

 

[mheslep]

...Has Nebel published any of these claims in a refereed journal, or is it the only source for Nebel's claims the blog exchange between Nebel and Carlson?...

Google scholar for "R. Nebel iec" will quickly show his publications in the area, but I am not sure what you are looking for. The Chacon et al paper is the most relevant to that blog discussion.

 

[Utwig]

Hi,

I think the time delay has a lot to do with the pressure, heat issues, but mostly because the only really large scale attempt, in Europe anyway, is happening in France. Yes, I am English.

Utwig

 

[TallDave]

Rider finds that the power expended to maintain the disequilibrium will exceed the additional gain from operating at disequilibrium.

Sure, but isn't that statement dependent on his other assumptions about gain, which the Chacon paper argues are not accurate?

 

[TallDave]

However, I note already that according to their abstract, they are performing an "optimistic" (their term!) calculation that explicitly neglects electron-ion collisional interactions --- and neglecting electron-ion interactions is simply not physically realistic in these disequilibrium systems.

FWIW, that was raised at T-P as well. Nebel's answer was:

At the risk of putting my foot in my mouth, the usual answer is that ion-electron collisions are much smaller than ion-ion collisions. It's much easier for particles of the same mass to transfer momentum to one another. This is the same effect as shooting pool with a cue ball that weighs the same as the other balls vs. shooting pool with a heavy cue ball. It's really hard to stop that heavy cue ball (it's just the combination of conservation of momentum and energy).

The best discussion I've seen of this is in chapter 4 of Glasstone and Lovberg (Controlled Thermonuclear Reactions, Robert E. Kriger Publishing Co. 1975.) but I suspect that it is out of print. Generally, calculating collisions of ions with electrons is more complicated than electron-electron collision, ion-ion collisions, or electron-ion collisions. The general rule of thumb is that electron distributions and ion distributions will equilibrate at a much faster rate than they will transfer energy to each other.

Anyways, to answer the OP: the reason fusion power is taking so long is that no one has a design that is economically viable. Even with some optimistic assumptions, ITER/DEMO's plant power density is way too low to compete with light-water fisison reactors -- and fuel for those won't run out for at least 1,000 years. IEC/FRC are a bit more promising in this regard, but they haven't seen much funding yet and have their own problems to resolve.