Commonwealth Fusion's High Temp Superconducting Magnet Success

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Nathan Marshall
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Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Nathan Marshall »

I've been following the progress of Commonwealth Fusion Systems, a startup based out of MIT's Plasma Science and Fusion Center. Recently, they tested their high temperature superconducting magnet and achieved a promising >20 T field strength:

https://cfs.energy/news-and-media/cfs-c ... hts-magnet

They seem to be confident they can achieve Q > 1 using an array of these magnets in their tokamak reactor design. Time will tell. I will continue to follow their progress with skeptical enthusiasm.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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The deadbeat drumbeat goes on and on as hope springs eternal. There will always be "this new thing"! Thanks for keeping us informed on this new, "new thing".
Now. who believes we can have any chance of altering the entire planets econ system/weather within the next 300 years for the better? Remember, there is a star and only a star 93 million miles away keeping us alive. We have been and always will be at the mercy of what is called weather. Weather 100% is solar related. No star, no weather.

No one dare suggest that we kill 2 Billion people at one thrust to make the world a bit more human friendly, ecologically! Still, we might just do that by a nuclear misadventure or by direct, planned action. That, of course, would not involve nature's weather but man's nature. Weather and planetary seismic belches rarely takes out more than 50,000 at once during one of its worst strokes. Far too spasmodic an event to contribute in the paring down of global population. Just too many hogs at the trough despoiling the planet.

Watch Planet of the Humans to get a bit o' hard sayins'

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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Nicolas Krause »

I'm really curious about the modelling behind the relationship they talk about. As I understand it ITER was designed in ye olden days when stronger magnets meant bigger magnets, leading to ITER's huge size and all the associated problems. The new superconducting materials for CFS make their reactor smaller but is it the same underlying model used to make those predictions? I get nervous when people are making extrapolations about the behaviour of a larger device based on previous experimental results for much smaller devices, where they were unable to achieve the desired behaviour.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Richard Hull »

Good for you Nicolas. The suppositions are many as it seems like this or that is the answer, especially if it scales down the giant fusion leviathan ITER. It has and always will be a waiting game on fusion. Redux after redux to no effect is fusion's history. The new huge field in the small is a new twist on an old theme. I have seen the new energy folks chase the mobile perpetuum with fail after fail... the excuse...."If I only had access to much stronger magnets".

While it is generally agreed by the anointed that a plasma can be confined by a magnetic field in theory, it saddens them no end that such a field has yet to appear. This is indeed big news for them. Of course, we know they do indeed get excited easily as pronouncements flow regularly about "we have it now, we think".

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Richard Hester »

I am more interested in the implications for successful use of high temperature superconductors for generating intense magnetic fields for other endeavors besides fusion. Think of what this means for particle accelerators and MRI machines.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Rich Feldman »

By my superficial reading, they don't claim to have a 20 T electromagnet.
They tested a section of a scaled-down model, which if scaled up with plenty of material and money, could sustain 20 T.

The milestone test, conducted at MIT’s Plasma Science and Fusion Center, proved that the magnet built at scale can reach a sustained magnetic field of more than 20 tesla, enough to enable CFS’s compact tokamak device, called SPARC, to achieve net energy from fusion, a historic first.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Getting a practical electromagnet design using high-temp superconductors would be a boon to several industries. I'm curious as to how they can work with the materials (ceramics!) to make a windable electromagnet.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Rich Feldman »

HTS materials proved useful in one engineering niche where the B fields are very small:

High-Q passive RF filters for wireless telecom stations, using patterned planar designs.
According to one paper from the 2000's, the thin or thick films became practically useful when sheet resistance,
counting skin effect at 1 GHz, was less than 1 milliohm per square.
That's far better than copper at room temperature, or even copper at LN2 temperature.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Nathan Marshall »

Good catch on that scaling detail, Rich. My quick read through their post gave me the impression that they’ve actually built the thing. Well, we will see if it scales up as they expect.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Andrew Seltzman »

Weighing in from MIT:
The TFMC magnet sustained over 20T, the bore is smaller than the final TF coils that will be used on SPARC. The final magnet will use the same tech and field, it will just be bigger.
https://www.youtube.com/watch?v=rAv6p3grFVM
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Rich Feldman »

Thanks, Andrew.
Can you save us the trouble of hunting for a doc that's not a glitzy promo video,
to learn things like dimensions and turns-count, and about using HTS superconductors at 20K?
So techies can better grasp the details which are novel.
The 3D rendering is kind of ambiguous about whether the B arrow is inside the bore of future toroid,
or at edge of winding where field is much stronger and the superconductive material cares.
coil_20t.png
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Here are some guesses from basic principles & Internet searches about HTS, later compared with numbers from CF sites.

I figured the current to get B = 20 T at the surface of a round conductor bundle.
H = B / u0 = 15.9 million A/m. That should be the same as 200,000 oersteds.

If the bundle diameter is 20 cm (7.9") then it must conduct exactly 10 million ampere-turns.
Video by CF says 40 kA, so there would be 250 turns. Packed area 1.26 cm^2/turn.
Average current density 31.8 kA/cm^2.

Internet searches for HTS Magnet pop up values of that order. The critical current depends strongly on material and temperature and magnetic field strength. What fluid is used for freezing bath at 20 K?
High pressure helium gas! https://www.psfc.mit.edu/sparc/hts-magnet

As coil diameter goes down, required current density goes up.
For D = 12 cm (4.7") the circumference is smaller, so we need only 6 million ampere-turns.
150 turns at 40 kA. But now the area per turn is only 0.75 cm^2, and average J is up to 53 kA/cm^2.
Can that be handled by real coilable materials at the stated T and B?

Found more at
https://www.cambridge.org/core/journals ... 1764EF9FF0
The real SPARC will have a bore with B0 = 12.2 T and R0 = 1.85 m. So the toroidal magnet current must be 112,850,000 ampere turns. 18 units with 6.27 MA each.

Am reminded of advances in electric motor compactness during 20th century, driven as much by increased magnet wire temperature as by improvements in electrical steel.
In today's case, improved magnet grade HTS. CF estimates that their demo coil has 10% of the world inventory.
Last edited by Rich Feldman on Sat Sep 25, 2021 7:25 pm, edited 5 times in total.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Unfortunately I don't know as I work in the PSFC, not CFS. I believe a few papers will be forthcoming.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Rich Feldman »

Thanks again. Duck Duck Go helped my hunt, and Excel saved wear on the slide rule.
Here's a chart from the CF paper cited near bottom of my post full of numbers.
urn cambridge.org id binary 20200925120924322-0810 S0022377820001257 S0022377820001257_fig1.png
Skeptics will look at the Q curves and say "yeah, right".
Be interesting to look at charts of inertial confinement projects, from when NIF was at this stage of planning.

[edit] And the papers Andrew mentions will tell us all about the engineering numbers. And conductor fabrication, insulation, physical support, etc. Quick, what's the magnetic force on wire carrying 1 million amperes in a B field with transverse component of 1 tesla?
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Agree with RIchard Hester about the promise of HTS in other big magnet applications. What will Florida National Magnet Lab do? They are a leader in high field solenoids: at least 33 T with water-cooled multi-megawatt Bitter coils, and I think over 40 when resistive coil is surrounded by a traditional superconducting coil.

Apparently the HTS and surrounding regular conductor are produced in tape form, then wound in a spiral like recording tape to make one D-shaped layer. The demo magnet is a stack of 16 layers, each of which is said to be the biggest HTS magnet in the world today.

The SPARC intro paper gave me a chance to draw and learn. Here, overlaid on their drawing, is green circle representing torus with the given R0 and a dimensions. Smaller circles were overlaid to "measure" the winding inner and outer radii. Above that I drew a polar view with 18 torioidal-field coils.
sparc_overlaid.png
As mentioned in prev post, we know the current from basic principles. For 12.2 T field at R0, the total current is 112.85 million amps. 6.27 million amps per coil. Field will be 17.6 T at inside edge of torus and 9.3 T at outside edge.
What if 17 coils were powered down and one were operated by itself, as with the slightly smaller demo coil? Using formula for a circular current loop, we'd get 2.96 T at center of loop (my blue dashed line), 3.16 T at R0, and about 20 T when 7 cm away from the current filament.
https://tiggerntatie.github.io/emagnet/ ... ulator.htm
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Finn Hammer »

All,


Please bear with me on a small detour from Rich's excellent calculations of magnets.

One of the things that has disturbed me in the past is the claims of elevated temperatures to the tune of millions of degrees, inside the fusion devices competing for public interest and the associated funding.
On the path to understanding these claims, I came to the conclusion, that these temperatures are a measure of the speed of the particles involved.
But also that these speeds could only be acheived in a low pressure environment, so that the temperatures quoted lost their meaning to the layman audience relating it to an atmospheric context.
In short, if the particles had this speed out here in atmosphere, the result would be these increadible high temperatures, but it is happening in a near vacuum, so....
Within such a context, the claims of elevated temperatures seemed to be manipulative in nature, if not in fact bordering on the fraudulent.

However, from this source:

https://www.machinedesign.com/news/arti ... ear-fusion
I read this:

""During its 23 years of operation, the Alcator C-Mod reactor has repeatedly set the record for plasma pressure, increasing its fusion rate with each improvement. This time, it beat its 1.77-atmospheres record set in 2005 by 15%, reaching 2.05 atmospheres and temperatures twice as hot as the sun. ""

If it is correct, that the plasma is pinched and thereby compressed to 2.05 atmospheres, then the temperatures quoted, inside the plasma, is not just a speed number inside a void, but actually a measure of a "real" temperature, as it would have developed out here in the real world.

This definitely has changed my respect and awe of the status of the present research.

Cheers, Finn Hammer
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Richard Hester »

I did my thesis work right next to what used to be the highest field magnet in the world - it was a pair of nested solenoids, one a conventional Bitter magnet, and the other a superconducting solenoid. If I remember correctly, state of the art at the time ('74-'75) was 14T. I could also throw a rock (not very hard) and hit the current incarnation of MIT's Alcator tokamak, which at the time boasted conventional copper electromagnets cooled with LN2.

I was working with a group that was generating pulses of terahertz radiation using a CO2 laser and methyl fluoride gas. We used the magnet in one of our experiments, and I had the job of machining a tube to fit down inside the magnet bore.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Finn... Back in 2014 I did a FAQ in the fusor theory forum that dealt very completely with fusion temperatures that are little white lies told by scientists to reporters to impress the down at heel dullards in the public. Makes for good press and funding to talk in millions and billions of degrees. I wrote the FAQ when I failed to realize a lot of folks arriving here had no idea about temperature beyond "Ouch! burny-burn, That's hot Ouch!!...or...."Oh man, is it cold! It's freezing!" here is that FAQ.....

viewtopic.php?f=42&t=10424

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Fusion is the energy of the future....and it always will be
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Those high-field solenoids find use in testing practical superconductive wires and tapes
for critical current at various values of T, B, and B orientation.

Found this chart at the Florida lab's site, under:
Engineering Critical Current Density vs. Applied Field for Superconductors Available in Long Lengths
Je_vs_B-041118_1024x743_PAL.png
Note that in some curves the data stops at 20 T.
And that all are measured at 4.2 K.
Critical current densities are much lower at 20 K, and lower still (mostly nonexistent) at 77 K even at B=0.

As CFS has demonstrated, it looks like HTS technology greatly extends the B range of superconducting magnets,
esp when cooled far below critical temperature.
What are the economies of refrigeration to 20 K instead of 4.2 K? 4.2 K instead of 1.9 K?

Anyway, we can now talk about 100 million amps DC with a straight face. Bet the number is even bigger for ITER.

[edit] Back to resistive Bitter solenoids, I remember a power density number from some Florida design about 10 years ago.
13 watts per mm^3, that's 13 kW per cm^3. Would make a great tankless water heater. Advances have been in cooling and mechanical strength.
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

Post by Justin Fozzard »

A huge elephant in the room for devices such as CFS's are the material limitations of the plasma-facing first wall.
With current technology the first wall loading limits are 2MW per sq.m for neutrons and 20MW per sq.m for heat, meaning that to be economically viable, a device will require a large first wall area, and thus large volume in order to produce a reasonable amount of power.
For example, a toroidal configuration with a 1.5m major radius and 0.5m minor radius has a surface area of around 30 sq.m, so the neutron loading limit would be only 60MW; not enough power production to be of commercial interest, especially compared to the relative simplicity of gas turbine or diesel powered generator sets or even a couple of wind turbines.

Another rarely mentioned issue is the lithium/beryllium blanket that will have to surround the first wall to slow down and capture the neutrons to avoid damaging the superconducting magnets.

The forces between large superconducting magnets at 20T will be rather interesting as well....
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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Any strengthening or increased output solution at one end of a power system that is positive or efficacious will typically force other engineering challenges to ripple throughout the system. Excellent solutions at one end that have plagued development to any point will usually require seemingly insuperable problems across the board that often, even if solved, make the system too expensive or unable to warrant such costs on a planned mission goal

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Re: Commonwealth Fusion's High Temp Superconducting Magnet Success

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Justin mentioned:
>>The forces between large superconducting magnets at 20T will be rather interesting as well....

Yup. Each D shaped coil really wants to expand and form a circle,
and really wants to get cozy with neighboring D's.
Fortunately the attraction of left and right neighbors almost cancel each other, as long as all coils carry the same current.
Not counting tidal forces, which compete with the magnetic compressive (pinch) force within each coil bundle.

Suppose we energize just two neighboring SPARC coils with nominal 6.27 MA each.
The straight vertical sections near the tokamak axis are 0.314159 meters apart (1/18 of circumference of 1.8 m circle).
Each generates 4.0 T radial field at the neighboring vertical coil section.
The attraction per lineal meter is 25 MN (5.6 million lbf), 83% of Space Shuttle nominal total thrust.
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