reddit: We are nuclear fusion researchers, ask us anything

[D Tibbets]

....
Convex field had all mirror kind of machines. All them are forgotten now.

If the Polywell were trying to confine the plasma with magnetic fields, it would be forgotten now too. But that is the GENIUS of the Polywell. It is confining the plasma with an electrodynamic well, not a magnetic field. ED wells are quite stable against minor perturbations.

Studying only Polywell history may be better if you get studied also what has been done in fusion research during past 60 years.

I have looked the general history a bit, enough to understand the failings of most of the big money machines to date. Polywell doesn't have those failings. It may have OTHERS, but not those....

Just to be picky, KitemanSA is accurate when he mentions the difference in the Polywell. I doubt it will chang Jos's vitrified viewpoints, but an expansion of what was said may be helpfull. First off mirror machines have not been abondoned, there are some adherants that hope some manipulation may still work. In the Pollywell there are two considerations. First cusp confinement is perhaps ~8-10 times better than biconic mirror machines in terms of the volume of plasma contained vs the cusp loss rates. This is an improvement but not a game changing one. The Wiffleball inflation is a game changer. High beta inflation of the Wiffleball increases the 'cusp' confinement an additional ~ 100 X (to values in the thousands) . This leakage would apply to the ions and electrons in a neutral plasma. But the plasma is not neutral. The excess electrons produce an electrostatic potential well that contains the ions to even better levels of perhaps several orders of magnitude or more. This requires MFP for the fuel ions that approach or probably exceed the diameter of the machine combined with annealing which combined retard the upscattering of ions so that the potential well can continue to retain enough of the ions so that the ~ 100x- 1000 X less efficient Wiffleball containment losses only applies to the relatively small portion of ions that have managed to overcome the annealing process.

The crossfield transport of these upscattered electrons may be an issue, but I suspect the cusp losses still dominate for the upscattered ions as the effective density of these ions are substantially less than the population of of electrostatically contained ions deeper within the magnet grid. These ions are not reaching the magnetic domain, so they are not contributing to the random walk of the upscattered ions doing the ExB drift through the magnetic field. The point is mostly moot though as either the drift or cusp escape of these upscattered ions is desireable. The ExB drift is one reason why magnatized plasma machines need to be large as ion confinement time is limited by this process (assuming macro instabilities do not occur). The Polywell overcomes this by only confining the electrons in this manner. Things changes some when the Wiffleball forms, but initially this applies. This allows for electron confinement to be dominated by the cusp losses, even in small machines where the thicknes of the magnatized layers small. With ions in a neutral plasma with no potential well the ExB drift losses may even dominate over cusp losses in these small machines. The non neutral plasma is the key. This mostly removes the ions from ExB drift considerations. They are electrostatically retained within the magnetic shell. This allows for the magnetic constraints to apply mostly only to the electrons and with their ~ 60X or greater difference in momentum the ExB drift limits allows for smaller machines with significantly increased densities- especially with Wiffleball confinement greatly improving the cusp confinement. This allows for larger contained electron densities at acceptable losses (with recirculation) and this leads to electrostatic ion containment at almost equal densities. This ~ 1000 fold increased density leads to n^2 fusion rates a million times higher, with constant B and volume. This occurs with loss rates of ExB transport of electrons and cusp loss of electrons similar to the starting lower density conditions of the non wiffleball configuration. This ~ 1000 fold improvement in density with resultant ~ 1 million fold improvement in fusion makes up for much of the deficiency of Elmore Tuck type fusors which is the baseline from which the Polywell is derived. Recirculation of the elecrons then pushes the energy balance into the positive region, if the system works as advertized. Add to that further gains from increasing the B field strength ( B* n^2). Density scales as the square of the B field. Fusion scales as the density squared, thus the B^4 scaling.

There are all sorts of permutations that effect the final configuration and interacting physics, thus the difficulty is modeling the system without extensive experimental data. The same has been found to apply to the tokamak. But the complications are hoped to be much less than in the tokamak (such as the macro instabilities, asymmetry conditions, MHD stability issues, etc. WB 8 will/ has answered much of these questions, but a larger machine would have done better, once again illustrating Bussard and Nebels reasoning for going to to a near full size machine. Also, it illustrates the comparative dollar costs based on ~ r^3 scaling between the Polywell and tokamak.

Dan Tibbets

[KitemanSA]

By the way, what do YOU mean by concave and convex fields? Just asking to be sure we are talking the same language.

Concave is concave and convex is convex.

This is a BS statement.

What are the characteristics of a "concave field" and a "convex field". If you can't answer this I will have final proof that you are indeed an idiot.

[Joseph Chikva]

By the way, what do YOU mean by concave and convex fields? Just asking to be sure we are talking the same language.

Concave is concave and convex is convex.

This is a BS statement.

What are the characteristics of a "concave field" and a "convex field". If you can't answer this I will have final proof that you are indeed an idiot.

Kiteman, assaulting me you will gain nothing. Field on Polywell as well as in any mirror machine are convex. There is the historical fact that mirror machines are forgotten as could not provide good parameters. In spite of the fact that they all had convex fields and all of them provided a “minimum B principle”. If this facts afflicts you, that’s your problem and not mine.

By the way, Kiteman, apples are round or square?
And your head?

[KitemanSA]

Concave is concave and convex is convex.

This is a BS statement.

What are the characteristics of a "concave field" and a "convex field". If you can't answer this I will have final proof that you are indeed an idiot.

Kiteman, assaulting (insulting?) me you will gain nothing. Field on Polywell as well as in any mirror machine are convex.

True, but what does that MEAN to you?

There is the historical fact that mirror machines are forgotten as could not provide good parameters. I did not dispute that. I merely pointed out that the Polywell is not a "mirror machine" so your statement is pointless.
In spite of the fact that they all had convex fields and all of them provided a “minimum B principle”. If this facts afflicts you, that’s your problem and not mine.

I did not dispute that. I merely pointed out that the Polywell is not a "mirror machine" regarding the plasma so your statement is pointless. PolyWELL is a ED potential WELL machine. It is enhanced in certain ways by the magnetic field but fundamentally it is a WELL machine.

By the way, Kiteman, apples are round or square?
And your head?

To paraphrase ladajo:
pwheeeet!! Absurdly Senseless Argument! 10 yd penalty.

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

[Joseph Chikva]

This is a BS statement.

What are the characteristics of a "concave field" and a "convex field". If you can't answer this I will have final proof that you are indeed an idiot.

Kiteman, assaulting (insulting?) me you will gain nothing. Field on Polywell as well as in any mirror machine are convex.

True, but what does that MEAN to you?

There is the historical fact that mirror machines are forgotten as could not provide good parameters. I did not dispute that. I merely pointed out that the Polywell is not a "mirror machine" so your statement is pointless.
In spite of the fact that they all had convex fields and all of them provided a “minimum B principle”. If this facts afflicts you, that’s your problem and not mine.

I did not dispute that. I merely pointed out that the Polywell is not a "mirror machine" regarding the plasma so your statement is pointless. PolyWELL is a EDpotential WELL machine. It is enhanced in certain ways by the magnetic field but fundamentally it is a WELL machine.

By the way, Kiteman, apples are round or square?
And your head?

To paraphrase ladajo:
pwheeeet!! Absurdly Senseless Argument! 10 yd penalty.

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

If you think that I do not know the difference between concave and convex it is a big question who from us is an idiot.
Asking "relevant" and kiddies question sometimes you also answer on questions being put to you.
For example where did you see the statement that Stellarators have the same concave field as TOKAMAKs? As gradient of B by minor radius is positive for Stellarator and negative for TOKAMAKs. As I understand despite my idiotism positive-negative differ each other similarly how black differs white, how concave differs convex.
But you could not answer and I am sure that only because that you started to splash a saliva.

[D Tibbets]

Some confusion that I perceive. The Polywell is a mirror machine, but this needs to be qualified. The Polywell is an opposing magnet biconic mirror machine.
This is one type of mirror machine and benefits from convex B fields. There are also solenoid type mirror machines that have some concave B fields. There is a tradoff between the macro instabilities associated with concave fields and the large equatorial cusp losses associated with the convex fields. Efforts to modify both types to overcome these concerns have generally failed with the possible exception of the Polywell and a few other approaches such as the efforts by a group of MIT graduates .
What is remarkable for the Polywell is multifactorial. The much improved cusp confinement. This applies to ions and electrons both. The incorporation of excess electron injection, which creates a non neutral plasma and allows for a potential well which contains the ions much better than the cusp confinement alone. This serves to make the electron losses the dominate loss issue. It also serves to make ExB transport for the large gyroradii ions much less of an issue in these small machines. There are side benifits such as the possibility of annealing due to ion velocity differences as they transit the potentialwell. This improved cusp confinement also allows for much easier Wiffleball inflation. I assume a typical biconic cusp mirror machine could also form a Wiffleball, but the electron currents would have to be much higher, and even if the Wiffleball could be formed the resultant losses would be greater in proportion to the cusp confinement losses at baseline. Thus losses with a theoretical biconic cusp machine may stay ~ 10 times higher than a Polywell. The recirculation issue and acheivement was a late developement that improves loss performance by another factor of ~10X or more.

In short, the Polywell succeeds in containing electrons much better, and it also decouples the ions from magnetic confinement issues (to a major extent.), and converts the ion confinement to an electrostatic issue. This results in other considerations and complications, but I have yet to see one that has not been addressed by Bussard, etel. I don't know if these issues are resolved, but they have been addressed, and resolution has been claimed. There are many engineering issues and perhaps some scaling issues (primarily on the cost side) but I have not seen any physics issues that are show stoppers, that have not been reasonably challenged. .
These include thermalization issues, cross section issues, confinement issues, macro instability issues, two stream instability issues, bremstruhlung issues, and other instability issues.
Reading through the posts in this forum and in another forum between R. Nebel and A. Carlson (both experianced plasma physicists) there has not been a single challange that has not been answered. The validity of the answers can be further challenged, with experimental data being the final arbitrator , but the challenge and defense has covered the all of the bases of which I am aware..

Dan Tibbets

[Joseph Chikva]

There is a tradoff between the macro instabilities associated with concave fields and the large equatorial cusp losses associated with the convex fields. Efforts to modify both types to overcome these concerns have generally failed with the possible exception of the Polywell and a few other approaches such as the efforts by a group of MIT graduates .

Advantage of convex fields against concave is not proved experimentally. Macro - instability are observed in any devices. And concepts using convex field had not showed better imunity aginst them. That is only the historical fact.
MIT people didn't look for the new forms of fields but develop thought up in 1956 by Lev Artsimovich and his group the concept of combination of a poloidal field created by the induced toroidal current and toroidal field created by the external coil that simply is the solenoid bent around into a ring. They worked on so called “high field TOKAMAKs” using resistive magnets. For example ALCATOR with as I know 7T on-axis field. Also they have made proposal of building another – bigger and stronger TOKAMAKs. But configuration of mag field in all devices are the same – absolutely identical to Artsimovich’s concept.

[D Tibbets]

There is a tradoff between the macro instabilities associated with concave fields and the large equatorial cusp losses associated with the convex fields. Efforts to modify both types to overcome these concerns have generally failed with the possible exception of the Polywell and a few other approaches such as the efforts by a group of MIT graduates .

Advantage of convex fields against concave is not proved experimentally. Macro - instability are observed in any devices. And concepts using convex field had not showed better imunity aginst them. That is only the historical fact.
MIT people didn't look for the new forms of fields but develop thought up in 1956 by Lev Artsimovich and his group the concept of combination of a poloidal field created by the induced toroidal current and toroidal field created by the external coil that simply is the solenoid bent around into a ring. They worked on so called “high field TOKAMAKs” using resistive magnets. For example ALCATOR with as I know 7T on-axis field. Also they have made proposal of building another – bigger and stronger TOKAMAKs. But configuration of mag field in all devices are the same – absolutely identical to Artsimovich’s concept.

You are referencing MIT work that is completely different from the work I mentioned. This has been discused some here in another thread. You can search for it if you wish. I am tired of feeding you references that you apparently ignor or dismiss as irrelevent. The work involved a team of MIT PhD graduates that utilized some Polywell concepts, though in a much different machine within the last few years .
Why don't you provide some references that denies significant MHD stability advantages of of convex magnetic fields.

And all tokamaks have the same physical configuration of magnets- that is the defining structure of tokamaks. If the configuration was different, it would be a spheromak, etc. That does not preclude variations in the B field strength over short and long time frames, playing with temperature, density, or active interventions to try to suppress or control instabilities, etc. You seem to conceive the Polywell as a differently named tokamak. That is nonsensical.

Dan Tibbets

[Joseph Chikva]

You are referencing MIT work that is completely different from the work I mentioned. This has been discused some here in another thread. You can search for it if you wish. I am tired of feeding you references that you apparently ignor or dismiss as irrelevent. The work involved a team of MIT PhD graduates that utilized some Polywell concepts, though in a much different machine within the last few years .
Why don't you provide some references that denies significant MHD stability advantages of of convex magnetic fields.

And all tokamaks have the same physical configuration of magnets- that is the defining structure of tokamaks. If the configuration was different, it would be a spheromak, etc. That does not preclude variations in the B field strength over short and long time frames, playing with temperature, density, or active interventions to try to suppress or control instabilities, etc. You seem to conceive the Polywell as a differently named tokamak. That is nonsensical.

Dan Tibbets

Thanks, Dan. I was a little aware what they (MIT people) do.
Spheromak is the same TOKAMAK with the same combination of poloidal and toroidal field but with lowest posibble aspect ratio (ratio between major and minor radii). As when major and minor raddii equal each other torus converts into sphere.
As TOKAMAK theory stands that lower aspect ratio allow running at higher beta.
But now read below disatvatages:

DisadvantagesThe ST has three distinct disadvantages compared to "conventional" advanced tokamaks with higher aspect ratios.

The first issue is that the overall pressure of the plasma in an ST is lower than conventional designs, in spite of higher beta. This is due to the limits of the magnetic field on the inside of the plasma, This limit is theoretically the same in the ST and conventional designs, but as the ST has a much higher aspect ratio, the effective field changes more dramatically over the plasma volume.[28]

The second issue is both an advantage and disadvantage. The ST is so small, at least in the center, that there is little or no room for superconducting magnets. This is not a deal-breaker for the design, as the fields from conventional copper wound magnets is enough for the ST design. However, this means that power dissipation in the central column will be considerable. Engineering studies suggest that the maximum field possible will be about 7.5 T, much lower than is possible with a conventional layout. This places a further limit on the allowable plasma pressures.[28] However, the lack of superconducting magnets greatly lowers the price of the system, potentially offsetting this issue economically.

The lack of shielding also means the magnet is directly exposed to the interior of the reactor. It is subject to the full heating flux of the plasma, and the neutrons generated by the fusion reactions. In practice, this means that the column would have to be replaced fairly often, likely on the order of a year, greatly affecting the availability of the reactor.[29] In production settings, the availability is directly related to the cost of electrical production. Experiments are underway to see if the conductor can be replaced by a z-pinch plasma[30] or liquid metal conductor[31] in its place.

Finally, the highly asymmetrical plasma cross sections and tightly wound magnetic fields require very high toroidal currents to maintain. Normally this would require large amounts of secondary heating systems, like neutral beam injection. These are energetically expensive, so the ST design relies on high bootstrap currents for economical operation.[28] Luckily, high elongation and triangularity are the features that give rise to these currents, so it is possible that the ST will actually be more economical in this regard.[32] This is an area of active research.

[vernes]

Couple of posts later, is KitemanSA sufficiently explained what is meant with concave and convex in this discussion?

And somewhat related, don't we have a ton of images that could serve as illustration?

[KitemanSA]

Couple of posts later, is KitemanSA sufficiently explained what is meant with concave and convex in this discussion?

And somewhat related, don't we have a ton of images that could serve as illustration?

That is the problem. As I stated earlier, I am only slightly familiar with "stellarators". And when Joe finally made a specific statement about what he meant by "convex" I was rather flabbergasted that someone who could make such a statement would apply it to "stellerators". Well, it seems that "stellerators" have undergone a SIGNIFICANT change in geometry since they started. I am still trying to fathom the field in the "WENDELSTEIN 7-X" which is called a stellarator, but... Nor have I grasped where the statement "As gradient of B by minor radius is positive for Stellarator" applies. Seems that for these new forms of stellarator, (the ones that look like wierd tokamaks rather than figure 8 race tracks) the B may have both a positive AND a negative gradient with minor radius depending on the angle. Still looking.

[Joseph Chikva]

the ones that look like wierd tokamaks

Thanks.
Several posts ago

Sorry, I know a little about the stellarator. As described in wikipedia.......

Mr. Kiteman, force lines of fields that you like to draw are only geometric abstractions for your better imagination. But you also can use another math abstractions as well expressed with the help of gradient.
And all last editions of Stellarators (Waldenshtein, Large Helical Device, Stellarator in Ukraine, etc.) were simple torus as rule having not any racetrack and all the more without twisted racetracks (8-shape). As required force lines twist can be reached namely with combination of gradiented poloidal and comparatively uniform toroidal field.
You can not draw such a system on the 2-D plane but if you are talking about advantage of convex field against concave, so, you admit so called "minimum B principle". And I have already explained you and others twice here what it does mean. And unlike TOKAMAKs Stellarators satisfy that principle but despite to that TOKAMAKs provide better confinement.
Understand?

[rcain]

the ones that look like wierd tokamaks

Thanks.
Several posts ago

Sorry, I know a little about the stellarator. As described in wikipedia.......

Mr. Kiteman, force lines of fields that you like to draw are only geometric abstractions for your better imagination. But you also can use another math abstractions as well expressed with the help of gradient.
And all last editions of Stellarators (Waldenshtein, Large Helical Device, Stellarator in Ukraine, etc.) were simple torus as rule having not any racetrack and all the more without twisted racetracks (8-shape). As required force lines twist can be reached namely with combination of gradiented poloidal and comparatively uniform toroidal field.
You can not draw such a system on the 2-D plane but if you are talking about advantage of convex field against concave, so, you admit so called "minimum B principle". And I have already explained you and others twice here what it does mean. And unlike TOKAMAKs Stellarators satisfy that principle but despite to that TOKAMAKs provide better confinement.
Understand?

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.

[Joseph Chikva]

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.

[Ivy Matt]

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.