reddit: We are nuclear fusion researchers, ask us anything

[rcain]

so,

Tokamak == Stellarator <=> Grandmother == Grandfather

is what you are saying JC?

by the way, just reminded myself that this thread started off as a discussion of the Alcator C-Mod project. i've heard no further news therefore i'm supposing its still being wound down/snuffed out over the next year or so.

how do you rate the chances of the compact torus/spheromak approaches JC?

- though i get the distinct impression you think nothing will ever work in the fusion world.

I do not know what sex have TOKAMAK and Stellarator but several decades ago those were the competting approaches and TOKAMAK is winner while Stellarator - looser.
But all TOKAMAKs to day have divertor invented by Spitzer namely for Stellarator.
Alcator C-mode as well as Italian-Russian Ignitor as well as some larger than Alcator other MIT's projects are the so called "high field TOKAMAKs" and I like them. As they are more compact then ITER or DEMO, much cheapper as do not use superconductors and allow to reach ignition easier as hold in times more dense plasma. 5 times denser plasma makes 25 higher fusion power density. IGNITOR for example has about 10m^3 plasma while ITER - 840m^3
I doubt in Spheromac viability as capability of running at higher beta is good but technically very hard to place there all needed for commercial reactor equipment.
Also all so called "Advanced TOKAMAKs" use bootstrap mode in which current by the end of induction cicle is driven by beam. It is very hard to reach required temperature at which plasma has the reactivity enough for ignition in less than tens of seconds. As for ignition we need more power pumping and more intense beams drive higher current consequantly increasing poloidal field and we need in times higher toroidal field for keeping plasma stable. And 5.5T for ITER, about 7 for high field machines and may be a little higher is a technical limit. Though in USA was a project called "Ignitex" with 20T toroidal field http://www.ornl.gov/info/reports/1987/3445602690611.pdf
Also the way how neutral beams are created is very impractical for commercial reactors as in all TOKAMAKs vacuum chamber straightly is connected with gas filled neutralization chamber with vacuum absorbers at the walls cooled to cryogenic temperature. And after each shot then those absorbers need long time for desorbtion. Can we build commercial reactor with such design? I think that no.
There in USA is a strong team of Heavy Ions Fusion. But no real hardware as I know, but only theoretical researches yet except of comparatively small experiment of propagation of heavy ion beam through plasma column. As for this approach beams focusing on target is a challenge. This approach will be even more costly than TOKAMAK program. As estimation of practical reactor's dimension gives about 200-300 m.
So, I think that new idea is needed.

interesting summary JC. thanks.

i had forgotten ITER is quite so large - 840m^3!

personally, i am still quite a fan of Stellarator approach and Spheromaks/Compact Toks - both seem to apply intelligence to the stability/containment problem rather than brute force. i think there is some resurgence in interest in them - albeit for research rather than power generation per se.

ITER is just too expensive and slow to ever recoup its development costs or produce commercially competitive electricity. but it is a fine international collaboration and we learn much from it.

Unlike you, I believe we already have some good alternative approaches in IEC, FRC, Focus, etc. Not new perhaps, but certainly relatively neglected areas of research attempting to catch up with Toks right now.

I'm afraid I too disagree with most of your objections to Polywell feasibility, for the same reasons Tom, Ladajo. Dan, Kite and other's have tried to convince you. vis:

Polywell is a stockhastic IEC machine, not a beam machine, nor a mirror machine. The potential well has been observed and is maintained. Formation of 'Wiffle-ball' is critical for efficient operation and has been observed. Neutrons are observed. It basically works, as advertised.

It has remaining issues, as has been pointed out, but they are none of the ones you suggest - indeed if any of the issues you suggest were true, we would not have the experimental results currently obtaining. QED.

But currently we are not in possession of additional/deciding data. As is so often said - we wait to see.

i think you are just being negative. instead of 'it will not work because' ask perhaps 'what must be done to overcome/avoid/solve'? otherwise discussion is like a watching a dog chase his own tail.

[rcain]

by the way, just reminded myself that this thread started off as a discussion of the Alcator C-Mod project. i've heard no further news therefore i'm supposing its still being wound down/snuffed out over the next year or so.

The House Energy and Water Subcommittee restored domestic funding for fusion, increased funding for ITER above that requested by the DOE, and reduced funding for research and development related to renewable energy and energy efficiency, among other things. See here for the chairman's statement and here for the full text of the Energy and Water Development Approprations Bill. See also the news stories here and here.

Thanks for the update IvyMatt. I make from that I was right in my supposition that ITER funding got the go ahead. regrettable consequences for MIT and others. their protest page is still up: http://www.fusionfuture.org/

[KitemanSA]

[/quote]

You STILL haven't answered a simple, relevant question. What do you mean by "convex" and "concave" when it comes to magnetic containment fields?

As gradient of B by minor radius is positive for Stellarator and negative for TOKAMAKs.

This statement, which was the first definitive statement made by Joe, seems odd. Does anyone else agree that the gradient of a Tok B is negative with MINOR radius? I was under the impression that it was negative with MAJOR radius. It is stronger at the inside radius of the torus, not at the minor center of the torus, no?

[Ivy Matt]

Thanks for the update IvyMatt. I make from that I was right in my supposition that ITER funding got the go ahead. regrettable consequences for MIT and others. their protest page is still up: http://www.fusionfuture.org/

Er, not quite. As I said, funding for domestic fusion research was restored. It's the renewables budget that got slashed (by the House subcommittee). Alcator C-Mod is back in business...as long as the House passes the bill, the Senate passes it, the president signs it, etc.

[Joseph Chikva]

Does anyone else agree that the gradient of a Tok B is negative with MINOR radius? I was under the impression that it was negative with MAJOR radius.

Do you want now to arrange a poll? For your reference #1 poloidal field in TOKAMAK is created by an axial current. For your reference #2 such a field is in inverse proportion to radius. So, gradient dBp/dr<0 The radius of cross-section a torus is called as "minor radius". Enjoy.

[Joseph Chikva]

It has remaining issues, as has been pointed out, but they are none of the ones you suggest - indeed if any of the issues you suggest were true, we would not have the experimental results currently obtaining. QED.

And what my suggestions is wrong?

Plywell is not beam machine.

Reserch of Dr. Nebel confirms that electron-electron 2-stream instability has been investigated. With corresponding conclusions.

Polywell will run at high field (differnt numbers were mentioned: 2, 7 and even 10T) and high density (10^22 by Dan Tibbets).

Who has investigated electron-ion 2-stream at higher density. As condition (large enough angular momentums) will not kept initially for ions. Nobody.

alphas will not heat (thermaize) plasma

Have we found a magic wand? As in other ignition approaches the same order of mag field should confine alphas thus allowing them to keep self-sustained (ignited) mode.
I am tired to repeat 100 times the same.
As I see that rational arguments are powerless against blind belief.

[rcain]

Thanks for the update IvyMatt. I make from that I was right in my supposition that ITER funding got the go ahead. regrettable consequences for MIT and others. their protest page is still up: http://www.fusionfuture.org/

Er, not quite. As I said, funding for domestic fusion research was restored. It's the renewables budget that got slashed (by the House subcommittee). Alcator C-Mod is back in business...as long as the House passes the bill, the Senate passes it, the president signs it, etc.

oh, blimey yes, i misread. thanks for correction. that is good news

[rcain]

It has remaining issues, as has been pointed out, but they are none of the ones you suggest - indeed if any of the issues you suggest were true, we would not have the experimental results currently obtaining. QED.

And what my suggestions is wrong?

- that it cant work. (2-stream, thermalisation, take your pick).

(my apologies if i am crossing threads here - i have lost track)

Plywell is not beam machine.

Reserch of Dr. Nebel confirms that electron-electron 2-stream instability has been investigated. With corresponding conclusions.

- that is can be circumvented/avoided to the extent that basic geometry remains viable. ie. not a major concern, but a known issue - and secondary to basic device operation.

Polywell will run at high field (differnt numbers were mentioned: 2, 7 and even 10T) and high density (10^22 by Dan Tibbets).

Who has investigated electron-ion 2-stream at higher density. As condition (large enough angular momentums) will not kept initially for ions. Nobody.

i have not researched the question thoroughly, so i do not know. but Doc Bussard and others have considered it and their analysis suggest the effects should be minor/manageable. it may well effect scaling efficiency, etc, but should not prevent/limit basic operation.

the B field ranges have been discussed.

alphas will not heat (thermaize) plasma

Have we found a magic wand? As in other ignition approaches the same order of mag field should confine alphas thus allowing them to keep self-sustained (ignited) mode.

yes. first - we dont let them stay around long enough to thermalise (critically damage the distribution).
second there is no 'ignition' in conventional sense within Polywell - it is a velocity machine (pump), not a thermal machine.

thermalisation is a recognised issue and has well been accounted for.

I am tired to repeat 100 times the same.
As I see that rational arguments are powerless against blind belief.

i know. i am sorry Joseph. we do not believe we are believing blindly. i am sure you do not either.

most of us very aware that new problems could just appear in 'pushed' environments. and in retrospect someone is usually always able to say 'i told you so'.

currently we have only weak 'clues' as to what problems/issues Polywell team are 'actually' facing right now. we can only really hope that someone else eventually repeats the Polywell V8x series experiments and makes results available in the public domain, before we know for sure.

all best

[Joseph Chikva]

- that it cant work. (2-stream, thermalisation, take your pick).

These two do not contradict each other.
As initially when we have not thermalization of ions (ions have not angular momentums) ion-electron 2-stream. If that not destroying the virtual cathode at least that will not allow working at high beta.
Then thermalization inceasing plasma presure twice. As all here are calculating beta considering only electrons' partial pressure assuming ions partial pressure equal to zero.
And finaly, all here as I undestand considers particle losses exceeded expected casp losses as beta=1 overcoming. That's not so.
All the best you too.

[ladajo]

Plywell is not beam machine.

Reserch of Dr. Nebel confirms that electron-electron 2-stream instability has been investigated. With corresponding conclusions.

Show us.
Show us both that Nebel confirms 2-stream was investigated in Polywell.

and

Show us his "corrosponding conclusions".

Or is it just you...?

[Joseph Chikva]

Plywell is not beam machine.

Reserch of Dr. Nebel confirms that electron-electron 2-stream instability has been investigated. With corresponding conclusions.

Show us.
Show us both that Nebel confirms 2-stream was investigated in Polywell.

and

Show us his "corrosponding conclusions".

Or is it just you...?

Somewhere in this thread you can see the link on article published by him regarding to electron-electron 2-stream in Polywell.
"corrosponding conclusions" is such electron-electron is not issue for Polywell thanks to large angular momentums of background electrons (read thermal electrons).
So, Dr. Nebel proved that you are wrong and Polywell is a beam-plasma machine.
But where is investigation of electron-ions if you are going to increase number density on orders of magnitude and initially ions will not have large angular momentums?

[hanelyp]

Does anyone else agree that the gradient of a Tok B is negative with MINOR radius? I was under the impression that it was negative with MAJOR radius.

Do you want now to arrange a poll? For your reference #1 poloidal field in TOKAMAK is created by an axial current. For your reference #2 such a field is in inverse proportion to radius. So, gradient dBp/dr<0 The radius of cross-section a torus is called as "minor radius". Enjoy.

The toroidal component of the tokomak magnetic field varies as 1/major radius. The poloidal component, assuming a uniform current density through the torus, will increase in proportion with minor radius.

Looks to me like the toroidal components has unfavorable gradients for stability. Plasma pushing out will find a weaker magnetic field trying to hold it in. The poloidal component looks stable, except I see only the poor toroidal field keeping the plasma from squirming, carrying the current and poloidal field with it.

I recall the tokomak described as derived from the toroidal pinch discharge, adding a toroidal stabilization field, then increasing that field until it prevented the pinch effect.

Contrast with the modified cusp field of the polywell, with the magnetic field stronger in all direction from the center (so long as you don't press so far that you get a blowout).

[Joseph Chikva]

The toroidal component of the tokomak magnetic field varies as 1/major radius.

Toroidal component is created by bent around solenoid or set of short solenoids. What ypu can say about mag-field of solenoid? varies as 1/major radius? Wrong.

The poloidal component, assuming a uniform current density through the torus, will increase in proportion with minor radius.

Wrong. As this is magnetic field of long current filament that varies as 1/minor radius.

And are you sure on "a uniform current density through the torus"?

[rcain]

- that it cant work. (2-stream, thermalisation, take your pick).

These two do not contradict each other.

i did not mean they did - i meant 'pick any combination', others even of your objections to concept.

As initially when we have not thermalization of ions (ions have not angular momentums) ion-electron 2-stream. If that not destroying the virtual cathode at least that will not allow working at high beta.

indeed as you say, 'not' destroying virtual cathode - perturbing it yes, at very high beta (high B) perhaps significantly. the operating point of the device will change - perhaps we have to 'throttle it back', or perhaps make a larger device - but the basic 'spherical' (double well) geometry should still establish itself, albeit scattered at a wider different mean.

i do not have the proper time or knowledge to 'model' the relative magnitude of such effects - i am sure it could be done.

but the latest round of US Navy Polywell research has gone ahead on the assumption that a viable operating regime exists, before serious onset - ie. not expected to be a 'critical issue'.

now experiment could prove that assumption wrong, especially if they have to push B - but my feeling is they we would have heard more about the issue on the grape vine and in the existing documentation.

i don't think this discussion is going to resolve until we get some real numbers - either out of a sim or a lab.

Then thermalization inceasing plasma presure twice. As all here are calculating beta considering only electrons' partial pressure assuming ions partial pressure equal to zero.

- all here in this thread maybe - but i am certain 2-species model exists (1.5 d bounce averaged Fokker-Planck - it is linked somewhere, i cannot be bothered to retrieve it). this/similar + theoretical estimates + experimental results to date => worth pursuing experimentally. was the conclusion. which is what they are doing.

And finaly, all here as I undestand considers particle losses exceeded expected casp losses as beta=1 overcoming. That's not so.
All the best you too.

not sure i quite understand you there. you mean upscattering? ejection? recirculation? beta is on a knife edge it is true, but swimming pool analogy is not quite right - this swimming pool has parabolic sides - so we have some restorative slop.

[ladajo]

Reserch of Dr. Nebel confirms that electron-electron 2-stream instability has been investigated. With corresponding conclusions.

Show us.
Show us both that Nebel confirms 2-stream was investigated in Polywell.

and

Show us his "corrosponding conclusions".

Or is it just you...?

Somewhere in this thread you can see the link on article published by him regarding to electron-electron 2-stream in Polywell.
"corrosponding conclusions" is such electron-electron is not issue for Polywell thanks to large angular momentums of background electrons (read thermal electrons).
So, Dr. Nebel proved that you are wrong and Polywell is a beam-plasma machine.
But where is investigation of electron-ions if you are going to increase number density on orders of magnitude and initially ions will not have large angular momentums?

No, that is not Nebel's proof, but your conclusion.
Show me.
I can not be bothered to go find it myself. You lost that respect after I repeatedly provided you resources you completely failed to look at or attempt to analyse. For now, you are nothing but a yapping mouth emitting empty words. The burden is yours.