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Re: Tokamak Energy news

Posted: Thu Feb 15, 2018 2:06 am
by Skipjack
Looks like Tokamak Energy may be the first ones to demonstrate break even, if their current timeline for 2020 holds up. JET might still beat them to it, if they demonstrate net gain in the next series of T+D tests. There is also JT-60 that might do it, though with D+D only, if I understand correctly (which would be quite a feat too).

Re: Tokamak Energy news

Posted: Wed Mar 14, 2018 10:38 am
by RERT
Tokamak Energy is investigating Spherical Tokamaks.

Reading a little yesterday, I couldn't come across a reason why high aspect-ratio tokamaks stopped at spherical.

Put a little more formally, one can conceive of an 'ellipsoidal' tokamak where the length of the central column is substantially greater than the diameter of the plasma loop in the torus. One merit of this idea might be that in principle the ratio of first-wall area to plasma volume can be tuned by changing the geometry.

Does anyone here know what might be the negatives are to that geometry?

Re: Tokamak Energy news

Posted: Fri Mar 16, 2018 4:42 pm
by crowberry
The shape of the plasma is important for good stability and confinement in tokamaks. In Lausanne, Switzerland there is the TCV tokamak (Tokamak à Configuration Variable) which has extra magnets for tuning the shape of the plasma. This is used for studying the properties of different plasma shapes.
Having extra magnets for a tunable plasma shape of course adds to the cost and complexity, so it probably only makes sense in a research device.
I have not read the TCV papers, so I don't know the difference in practice of the different plasma shapes shown on the webpage I quote.

https://spc.epfl.ch/research_TCV_Tokamak

Re: Tokamak Energy news

Posted: Sat Mar 17, 2018 7:25 pm
by RERT
Thanks for that. Apparently this feature is known as plasma 'elongation'. It seems that the question of Tokamak plasma shape has been an active field of research. Who knew...

Going back to Wiki on Spherical Tokamaks, I see this in the section on plasma stability :

"Using this in the critical beta formula above: βmax=0.072(1+κ²)ε/2"

and further on in the section on power scaling, it lists average pressure as related to βmax*(1+κ²).

Now κ is the elongation, and if consistent with definitions elsewhere is the ratio of the depth of the plasma parallel to the central column to its protrusion beyond the major radius perpendicular to the central column. So higher elongation is a more 'ellipsoidal' plasma.

The above formulae seem to imply very positive scaling as κ increases, on the face of it plasma pressure goes like (1+κ²)². I'm sure there must be a good reason why κ can't be increased further. I'll keep looking...

Re: Tokamak Energy news

Posted: Mon Mar 26, 2018 5:46 pm
by RERT
I’ve been at the Tokamak Energy conference at the Royal Society today, and I took the chance to ask a couple of people from TE ‘Why did you stop at spherical?’.

Two answers. 1) the plasmas naturally forms a ‘D’ shape as the current flows through the toroid. Constructing the coils in a ‘D’ shape minimizes the stress on the toroidal field coils.
2) if the ‘D’ was flat enough, the plasma would probably split into multiple ‘threads’ circulating the torus.

None of which actually confirms that spherical is the right geometry, of you have any slack on the mechanical strength of the coils, and the point at which the plasma splits is some way below spherical.

Dr. Alan Costley presented scaling laws which confirmed that the plasma performance improved with greater elongation.

Seems like it is a (slightly) open question.

Re: Tokamak Energy news

Posted: Mon Mar 26, 2018 6:00 pm
by RERT
Further to the conference first day: mostly consisted of lots of bigwigs from ITER arguing that (they weren’t stupid and) a Tokamak power plant had to be the size of ITER - about 8 m major diameter. The premise was that they needed to generate 1-2 GW electric, breed enough tritium to be self sufficient, and keep wall loading and divert or loading within reasonable bounds.

Costley countered that recent research showed that scaling was different to that used by ITER, with confinement time essentially independent of Beta. Also, with a low aspect ratio, enough tritium can be bred on the outer wall without requiring 1.3 m is breeding blanket around the central column. Also, shape crucially affects scaling, and with greater elongation and lower aspect ratio performance would be improved.

ITER guys said they didn’t believe the scaling, and an experiment was needed. TE said “That’s fine we’re building it’. ITER smiled through gritted teeth, and everyone went home.

Re: Tokamak Energy news

Posted: Mon Mar 26, 2018 6:01 pm
by RERT
Oh, and of course, TE don’t accept the premise of a multi GW plant, they propose multiple smaller units sharing turbine/balance of plant.

Re: Tokamak Energy news

Posted: Mon Mar 26, 2018 8:29 pm
by Skipjack
Thanks for the recap! Very good insights here! Wished I could have been there! Not sure, if they mention it, but the TE guys have MIT's Dennis Whyte and his team to back them up. As much as I like my old friend Gunter, I do hope that TE (and Whyte's team) can proof him wrong with their upcoming experiments.

Re: Tokamak Energy news

Posted: Thu Mar 29, 2018 11:15 am
by RERT
For completeness a few recollections of the second day of the Royal Society event.

Day began with John Menard and Dennis Whyte giving pretty upbeat presentations on low-aspect ratio Tokamaks and the impact of HTS. John's talk showed a ton of data on modelling and simulation, making a tough argument for the ITER folk. The most important take-away from Dennis's talk was that the change brought by new advances in HTS was the fact that critical current for the magnets is no longer an issue. Whereas for LTS it is that which binds the maximum field strength, for HTS it is actually the structural strength of steel which makes for the practical field strength limit. I think he said about 22T, not sure exactly whereabouts in the reactor. Given the importance of scaling with B^4, seems like there is bound to be work on supports more exotic than steel.

There was an interesting presentation from Tokamak energy on neutron shielding, especially for the narrow core. They are proposing a 45cm (additional) radius shield, comprising Tungsten, Tungsten Carbide and Tungsten Boride, interspersed with water cooling channels. The Tungsten reflects Neutrons, to help with Tritium breeding on the outside of the vacuum vessel. The Tungsten carbide - or at least the carbon part - slows neutrons, to an energy where the cross-section with Boron is very high, so the inner layer of WB is very effective. They said they had worked with Sandvik on the WC part, and they were confident in an industrial supply chain for those components. They said they thought the reactor - not sure which one, but must use HTS - would last for a year on full power with that shielding. Each additional 15cm of shielding reduces damage by an order of magnitude. It was an open question whether damage would be more severe at low temperatures due to a lack of annealing.

There were other talks on the complete lack of preparedness in most of the supply chain to produce and qualify materials and components, and a presentation on economics which pointed out that fusion power from Tokamaks has no particular niche market, and that ITER will be too late for such power to make a difference to global warming.

Final thing which sticks in my mind is a throw away remark by Guenter that if ITER suffered a complete loss of cooling, the chamber would melt because of residual radioactivity. Like Fukushima.... There was no discussion on that, it might be wrong and I sort of hope it was, but that's what I heard.

Sadly I couldn't find any slides on the internet, you may have to buy a 'Proceedings' to get the real info. My errors above are probably many and pernicious, but only ever accidental.

Re: Tokamak Energy news

Posted: Thu Mar 29, 2018 1:39 pm
by crowberry
Thanks RERT for the Royal Society event reports! What did Daniel Clery have to say on the alternative fusion projects?

Re: Tokamak Energy news

Posted: Thu Mar 29, 2018 7:32 pm
by Skipjack
RERT wrote:
Final thing which sticks in my mind is a throw away remark by Guenter that if ITER suffered a complete loss of cooling, the chamber would melt because of residual radioactivity. Like Fukushima.... There was no discussion on that, it might be wrong and I sort of hope it was, but that's what I heard.

Sadly I couldn't find any slides on the internet, you may have to buy a 'Proceedings' to get the real info. My errors above are probably many and pernicious, but only ever accidental.

Thanks bud! This is all really interesting!
The revelation about ITER, if correct, is rather shocking!
Wished there was video of this somewhere

Re: Tokamak Energy news

Posted: Fri Mar 30, 2018 9:25 pm
by RERT
Daniel Clery's talk was a very quick canter through most of the major competitors. NIF, TAE and GF got mentions, I don't believe Helion or Zap Energy did, but LPP did, maybe a slide or two on each. The overall impression was that this was a matter of form, to mention other approaches before completely ignoring them, with a room full of very eminent Tokamak engineers there to discuss advancing tokamak approaches.

Re: Tokamak Energy news

Posted: Sat Mar 31, 2018 4:05 pm
by crowberry
Thanks RERT. Of course the tokamak people are betting on their own horse and feel sceptical about the other concepts. This is of course justified as long as tokamaks are the best performing devices around. It will be interesting to see how the field develops and what kind of concept in the end will be the first to achieve break even.

Re: Tokamak Energy news

Posted: Tue Apr 03, 2018 8:22 pm
by RERT
On a closing note to the above, TE reports in a brief email exchange that elongation with Kappa greater than 3 produces a very unstable plasma, and that increasing the height of the toroid at fixed major radius will increase costs.

Re: Tokamak Energy news

Posted: Tue May 22, 2018 1:34 pm
by crowberry
Tokamak Energy has published a 1 min 38 s long time lapse video of preparing ST40 for hot plasma experiments. The video covers the period from the 12th of October 2017 to the 31st of January 2018. https://www.youtube.com/watch?v=Cjzok6h2-xA