nice new stellarator at PPPL with permanent magnets, 3d printed parts

Reflections on fusion history, current events, and predictions for the 'fusion powered future.
Post Reply
Alexander Ziemecki
Posts: 74
Joined: Mon Aug 21, 2023 6:25 pm
Real name: Alexander Ziemecki

nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Alexander Ziemecki »

anyone know anything about this? nice little reactor, almost our size. I wonder if we could build one.

attached at the bottom of the post is the paper they published on it.
https://www.pppl.gov/news/2024/return-r ... new-plasma


MUSE "Tony and Mike"
MUSE "Tony and Mike"
3d printed magnet holders left, 3d printed shell right
3d printed magnet holders left, 3d printed shell right
MUSE, "the first stellarator built at PPPL in 50 years and the first ever to use permanent magnets"
MUSE, "the first stellarator built at PPPL in 50 years and the first ever to use permanent magnets"
Attachments
Qian_2022_Nucl._Fusion_62_084001.pdf
(1.48 MiB) Downloaded 42 times
User avatar
Liam David
Posts: 540
Joined: Sat Jan 25, 2014 5:30 pm
Real name: Liam David
Location: PPPL

Re: nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Liam David »

I work in the building over from where MUSE is. I visited a few times during construction and it was neat to see things come together. Haven't had the chance to visit with everything fired up.

There is something on the order of 200lbs of magnetic force trying to push the 3D-printed holders apart. There's a whole assembly jig (not pictured). The team held "magnet parties" where grad students were invited to help install the 9736 permanent magnets into the holders.

The toroidal field coils are also something like 50 years old and we use them on a number of experiments here at PPPL, including the PFRC-2. Old but robust. The magnet power supplies cost more than your car.

Notably, the temperature (1-10eV electrons, room temperature ions) and density (1e15-1e17/m^3) are much, much too low for any fusion as it was never designed for that.

I can't release any pre-publication results, but current papers are at:
https://iopscience.iop.org/article/10.1 ... 326/ac6c99
https://www.cambridge.org/core/journals ... 22D9725BFF

Attached are some slides from an internal presentation.
Attachments
stats.png
muse.png
magnets.png
construction.png
assembly.png
e beam.png
User avatar
Richard Hull
Moderator
Posts: 15071
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Richard Hull »

Nice bit of kit and wonderful for what it was made for. What's more, it works at the level it was assembled for. Wow! All those magnets and the magnet parties, I am sure, hosted free pizza, chipos and sodas along with a scientific work ethic and discussion all melded into a memorable experience for later life.

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
Patrick Lindecker
Posts: 105
Joined: Mon Apr 09, 2018 9:47 am
Real name: Patrick Lindecker
Location: Maisons-Alfort France

Re: nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Patrick Lindecker »

Hello Liam,

If I understand well, this interesting device tends to "produce" a "neoclassical transport" which would be a big progress. If this is confirmed, the losses will be much lower than on a classical Stellarator (lets's say the W7X which ripples would be a big source of losses in real fusion conditions). However, even with a lower magnitude of losses, the toroidal field is rather low: 0.15 T. Do you know, if there is some hope that in the future the toroidal field be bigger (let's say 1 T) with permanent magnets. This will probably open the possibility to fusion (and so propulsion) for the future space crafts.

Patrick Lindecker
User avatar
Liam David
Posts: 540
Joined: Sat Jan 25, 2014 5:30 pm
Real name: Liam David
Location: PPPL

Re: nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Liam David »

Patrick, I'm sure you already know some of what I'm writing below, but I figured I'd clarify for a more general audience.

In stellarators the usual 2D (toroidal) symmetry that tokamaks enjoy is broken which makes closed and nested magnetic surfaces much more difficult to realize. These closed (field lines bite their own tails) and nested (field strength is a monotonic in "radius") surfaces are extremely important for confinement. Non-closed surfaces can just run into the wall and plasma follows. Closed surfaces with rational poloidal/toroidal magnetic symmetries different than the design symmetry cause the formation of magnetic islands, which break nesting.

All these effects (and a few others) lead to confinement properties that are worse than those for tokamaks, which themselves operate in the "neoclassical transport" regime. Up until now, the best stellarators have had much worse confinement than the best tokamaks.

The basic idea of quasisymmetry is to regain the tokamak symmetry. In a tokamak, the symmetry is the strength of the magnetic field in the toroidal direction. In a quasisymmetric stellarator, the symmetry is the strength of the magnetic field in the Boozer-coordinate toroidal direction. Boozer coordinates are quite a bit more complicated than e.g. spherical polar, but you can think of the Boozer-toroidal coordinate as wrapping through the center of the plasma analogously to the that in the standard toroidal coordinate system.

Breaking the symmetry in magnetic field strength is bad because particles can bounce off of regions of high magnetic field. Thus, instead of orbiting the device as so-called passing particles, these particles are locally trapped. These trapped particles experience a net drift outward precisely because they are trapped: the whole point of the twisting field of a stellarator or toroidal current in a tokamak is to swirl these outward-drifting particles back in. If quasisymmetry works, the transport of heat and particles in stellarators should approach that of tokamaks. Since stellarators, unlike tokamaks, have no net toroidal current, they do not suffer from things like disruptions and current-driven instabilities which are some of the reasons physicists find stellarators appealing.

There is some ongoing theory/computational work investigating the use of permanent magnets in larger stellatators, but there are some notable issues. For example, permanent magnets have upper limits in their magnetization and the fields of dipoles fall off as 1/r^3. Compare that to the 1/r field dependence of a current-carrying wire. Also, if I recall correctly, neutrons damage permanent magnets significantly faster than typical structural materials.

Regarding MUSE, I'm not aware of any serious plans to upscale things in the near future. Many more papers and results are still to come. For example, the team has mapped the magnetic fields using electron beams and there is great correspondence between design and experiment.
Patrick Lindecker
Posts: 105
Joined: Mon Apr 09, 2018 9:47 am
Real name: Patrick Lindecker
Location: Maisons-Alfort France

Re: nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Patrick Lindecker »

Hello Liam,

Thanks for the nice explanations. A very complex problem indeed.

Patrick Lindecker
Patrick Lindecker
Posts: 105
Joined: Mon Apr 09, 2018 9:47 am
Real name: Patrick Lindecker
Location: Maisons-Alfort France

Re: nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Patrick Lindecker »

Hello to all,

I was intrigued by the fact that neodymium magnets with a remanent magnetic field of at least 1 T (up to 1.4 T) can produce a so little field (0.15 T) inside the ring (torus here).

In this article https://www.scielo.org.mx/pdf/rmfe/v59n1/v59n1a2.pdf page 13, a toroidal field is produced by a permanent magnet in form of ring. The remanent magnetic field is equal to 1.086 T. It can be seen on the figure 6 that at the center of the ring B=0.2 T and the mean B inside the ring is equal to about 0.18 T, so only about 16% of the remanent field, which is not much. However, such weak field could be possibly used in amateur particles guns to slightly focus a particles beam.
Note that an interesting phenomena in two positions along the axis, at the exterior of the magnet: the field reverts (see figure 6 and comment on page 13 second column).

Patrick Lindecker
User avatar
Richard Hull
Moderator
Posts: 15071
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: nice new stellarator at PPPL with permanent magnets, 3d printed parts

Post by Richard Hull »

Around any magnet, permanent or electromagnet with no real defined, highly permeable magnetic path as with air or vacuum where much useful field strength is to be had. A polar field is intrinsically nothing at any really useful range. Backing a permanent magnet with a highly permeable plate, (iron), will increase the pole strength on the opposite side. Try it with a good gauss meter. Even then, there is no free ride for polar magnetism in air or vacuum. Too many "rels" standing in the way for a high polar field strength.

Using Rels in a magnetic circuit and you have a true Ohms-law related to electricity. We chose to use permeability which relates to Mhos in electricity. Why? We always tend to use iron or some super permeable material to almost join the two poles of the magnet to gain an intense field in between the poles. There little use for a naked polar field in air at range. It is like a one million volt battery made up of 9 volt batteries in a straight line. What is the electric field strength measured 6-inches from the positive terminal. arrange them in a circle where the positive and negative terminals are 5 meters apart. Now measure the electric field strength. (roughly 2000 v/cm or 200kv/meter)

Again, unfortunately, 1 mega volt is easily created. One mega gauss, 100 tesla, is not easy, and impossible in a permanent magnet.

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
Post Reply

Return to “Fusion --- Past, Present, and Future”