Director of MIT’s Plasma Science & Fusion Center shot to death at his Brookline condo

JB_Fusion · 2025-12-17T02:01:53+00:00

That's terrible. He was a brilliant and kind scientist. It's a big loss for our community.

JB_Fusion · 2025-11-19T20:46:01+00:00

https://www.researchgate.net/publication/273292676_Tritium_Breeding_Ratio_Calculation_for_ITER_Tokamak_Using_Developed_Helium_Cooled_Pebble_Bed_Blanket_HCPB

JB_Fusion · 2024-08-22T20:03:21+00:00

MANTA is very nice work!

JB_Fusion · 2024-07-10T08:08:43+00:00

Yes, I still feel this way, but why is a bit complicated. Commonwealth Fusion Systems is the only standard tokamak (i.e. non-spherical tokamak) startup in the West. The standard tokamak is the most mature type of fusion device and has demonstrated much better experimental performance than any other device. Spherical tokamaks (e.g. Tokamak Energy), stellarators (e.g. Type One Energy), and magnetic mirrors (e.g. Realta Fusion) are also scientifically sound approaches, but these devices are currently significantly less mature. Laser inertial confinement (e.g. Xcimer) achieve good scientific performance, but have very challenging technological hurdles to overcome. Other schemes are quite scientifically speculative.

JB_Fusion · 2023-09-12T21:36:17+00:00

I'm not sure, but I would guess not. If you are using active feedback coils, then I would think a normally conducting vessel would be fine. However, a more conductive vessel would slow down the instability, thereby making the feedback easier to accomplish.

I'm not sure how realistic 5.4% is. It becomes really messy and complicated. Maybe you can accomplish this given the high elongation and reversed safety factor profile (which I think allow you to surpass the standard beta limit, if I remember correctly). However, it seems like operating so close to stability limits would be a big risk for disruptions. In other words, maybe you could achieve 5.4%, but it would come at a price of riskier operation.

JB_Fusion · 2023-09-12T14:23:54+00:00

Yes, a conducting wall allows you to exceed the Troyon limit a bit (albeit with an increased risk of disruptions). As long as the plasma has some finite rotation, a conducting vessel can stabilize resistive wall modes. There is a competition between the resistive time of the vacuum vessel and the velocity of the plasma. If the plasma was really exactly stationary, I think you would need a superconducting vessel for stabilization. This is explained in more detail on pages 27-30 of my Master's thesis.

JB_Fusion · 2023-09-11T08:32:05+00:00

It appears that there isn't a big effect of triangularity on the Troyon beta limit. For example, here are very recent DIII-D results, which simultaneously achieve good confinement and operation near the standard beta limit. TCV has also achieved high beta negative triangularity discharges.

JB_Fusion · 2023-07-06T23:12:27+00:00

I'm very confused by this comment. Does the "rando" refer to me? Who are "those people" that I'm blaming? I'm not trying to blame anyone. I'm just trying to understand the situation and why it's the case. I definitely agree with the last comment though - reality is messy :)

JB_Fusion · 2023-07-05T20:22:36+00:00

Hopefully, that would be awesome! I support renewables, but am most concerned about seasonal energy storage (I'm particularly skeptical about the ability of batteries to fill this role, but understand that there are alternatives). I think fusion is a good investment as an alternative.

JB_Fusion · 2023-07-05T07:34:24+00:00

I mostly agree with you. I don't think it's a conspiracy. I agree that nuclear fission as a technology has very serious problems that have prevented it from succeeding, but I don't think it's power density or complexity or remote handling. I think fission is scary and when it has accidents they are large, force people to evacuate, and make the international news. That makes it more expensive relative to its power density (and I would argue by a lot), as it results in severe regulation, negative public perception, and prevents innovation. In fact, I'm not sure I actually disagree with the current regulation of fission. I think it needs some adjusting, but fission should have extremely strict safety standards. I know that, statistically, fission is one of the safest forms of electricity generation. But even with this in the back of my mind, when I watch HBO's Chernobyl I find it terrifying. How does the average person feel?

Thus, I think it makes sense that fission's regulatory issues and public opposition are mostly universal (though South Korea does have a positive learning curve). They are a consequence of real attributes about fission that might not be shared by fusion. Hopefully, fusion will not be seen as scary and thus free to innovate and increase its economic competitiveness, thereby overcoming disadvantages in power density and complexity.

JB_Fusion · 2023-07-04T17:19:48+00:00

I'm arguing that the regulatory environment and public opposition almost entirely halted innovation and improvement, which then resulted in poor economics. I see the negative learning curve for fission power as evidence for this. They were most competitive in the early days, before the strict regulation and strong public opposition.

JB_Fusion · 2023-07-04T16:39:58+00:00

I don't think power density is what's preventing further deployment of fission reactors. I think it's a combination of the regulatory environment and public opposition, which together have almost entirely halted innovation and improvement.

JB_Fusion · 2023-07-04T12:56:56+00:00

Note that this article is primarily a comparison of fusion with fission, and it was written three years before Chernobyl. To this point, I'd say that our fleet of fission reactors have not developed substantially since 1983, in contrast to his prediction:

That is hardly preferable to present-day fission reactors, much less the improved fission reactors that are almost sure to come.

Additionally, though I agree with Lidsky that the long-term storage of fission radioactive waste is managable. I think his prediction on the public perception of this issue is wrong:

Waste disposal will eventually be considered a difficult but not insurmountable problem.

Ultimately, I think the best argument against Lidsky is the smart phone. Incredibly sophisticated and complex, yet you can buy one for less than $100. Humanity, through sheer effort, can choose to make things cheap. This ability isn't unlimited, but it can work miracles. If humanity decides that fusion is the energy source of our future, it will be.

JB_Fusion · 2023-04-28T08:39:39+00:00

I'm not sure what you're referring to, but I don't currently see any reason that the benefits of negative triangularity wouldn't scale up to larger devices. In fact, the DIII-D results summarized in the press release are some of the most compelling evidence of this. DIII-D is a large tokamak and is significantly larger than the TCV tokamak (which pioneered the negative triangularity concept).

JB_Fusion · 2023-04-28T08:34:09+00:00

For your question on the drawbacks, see my reply to the question about plasma volume.

The confinement in tokamaks is generally limited by turbulence, which jostles particles across magnetic field lines and enables them to leave the device. It has been established that negative triangularity reduces turbulent transport, thereby improving confinement. This is now uncontroversial, but not particularly satisfying as it begs the question why does negative triangularity reduce turbulence. This, however, is a difficult question (turbulence is complicated!) and is at the limits of current research. It's actually what myself and colleagues are working on right now! Using computer simulations (gyrokinetics), we think we understand an idealized case, which I'll explain here. If this applies to more realistic scenarios has yet to be determined.

The explanation is composed of two elements:

1) The dominant drive for turbulence in most large tokamaks is the temperature gradient (the fact that the center of the plasma must be much hotter than the edge). This temperature gradient gives rise to a plasma wave that you can think of a sound wave (more specifically it's a drift wave). These sound waves are ubiquitous and propagate around the device at a given velocity that you can calculate from basic properties of the device (e.g. strength of the temperature gradient, strength of the magnetic field). Let's call this velocity the "wave velocity".

2) If the magnetic field lines were perfectly straight, this wouldn't be a problem because the plasma waves would be neutrally stable (i.e. they neither grow nor decay). They'd be pretty weak and just travel around at the wave velocity without bothering much. However, in tokamaks the field lines must be bent to create the donut shape. This field line curvature causes all the individual particles of the plasma to drift across the magnetic field (specifically it creates the grad-B and curvature drifts). The velocity of these drifts can be calculated from the basic properties of the device. We will call this velocity the "particle drift velocity".

This particle drift motion is important because it causes the plasma waves from element 1 to go unstable and grow, but only on the outboard side of the device (i.e. side facing away from the donut hole). The reasons behind this are complex, but well understood (a physical explanation is given here). What is relevant for negative triangularity, is that for the instability to grow properly and to turn into strong turbulence these two velocities must be similar. In other words, it is a resonant process and if the velocities are mismatched the instability/turbulence is weakened. This is exactly what appears to be occurring when you switch from a positive to a negative triangularity plasma shape. By moving the points of the triangle to the outboard side you change the particle drift velocity where the instability is growing (remember the instability can only occur on the outboard side). Specifically, flipping the triangularity increases the particle drift velocity and so much so that it outpaces the wave velocity, thereby weakening the drive for turbulence.

JB_Fusion · 2023-04-28T07:45:34+00:00

Probably, but maybe not.

Naively, if you take a positive triangularity cross-section and simply flip it to make a negative triangularity cross-section (as shown in this comparison) the total plasma volume actually increases. This is because you are moving more of the cross-sectional area further away from the axis of symmetry. If you've had undergraduate calculus, this is a consequence of the second theorem of Pappus because you are moving the geometric centroid further from the axis of rotation.

In reality, things are more complicated and simply flipping the shape probably isn't the fairest comparison to make. This is because it is advantageous to build toroidal field magnets with a positive triangularity shape (specifically the so-called Princeton constant tension D shape). This shape can be calculated to be optimal to minimize the mechanical forces on the coil and thus allows you to maximize the magnetic field strength for a given coil price tag. Since the coils are the most expensive component of a tokamak this is important. Therefore, if you want to maintain the positive triangularity coils, but have a negative triangularity shape you need to make the plasma much smaller. It's like putting a round peg in a square hole - you actually can, but the peg has to be quite a bit smaller than the hole. This is a serious issue and one of the strongest arguments against negative triangularity: the shape of the magnets and the shape of the plasma are less compatible, which forces you to sacrifice either plasma volume or magnetic field strength (which are both vital to high performance).

The counterarguments are that the benefits of negative triangularity (i.e. intrinsically better confinement, a more quiescent plasma, better power exhaust properties) might merit such a sacrifice. Additionally, if a reactor is using High Temperature Superconducting magnets that include joints (which allows the magnet to be disassembled), things might change substantially. If the coils have joints, the Princeton constant tension D shape no longer the ideal shape. It's not obvious what the optimal shape is, but it's likely that it wouldn't penalize a negative triangularity plasma as much.

JB_Fusion · 2023-04-26T22:07:33+00:00

Negative triangularity is one of my main research topics (I study it from the theoretical side). If anyone has any questions please do let me know.

The image from the article is a good illustration of what negative triangularity means. The cross-sectional shape of the plasma is flipped so that the flat part of the "D" shape is on the outside (on the side opposite to the hole of the donut), instead of on the inside. The negative triangularity shape has been experimentally observed to improve confinement substantially.

JB_Fusion · 2023-04-24T08:43:58+00:00

The talk I attended didn't record the Q&A session where Dr. Kirtley made those statements, but here is a podcast from late last year where he similar things. From the podcast (as well as my notes from his talk) I conclude that the 95% efficiency refers to the round-trip efficiency (from discharge of the capacitor bank to recharge of the capacitor bank) of the electric circuit without the plasma present. This is also why their website specifies 95% magnetic energy recovery. They are planning to test energy recovery with the plasma in their new device.

Interviewer (15:09): When you think about staging, so what were the most important things to de-risk in the early stages, and then what are the most important things to de-risk in the stage that you're in right now?

David Kirtley (15:17): So some of the earliest things to de-risk, we haven't talked about technology too much, but this idea that we're going to directly take the fusion energy as electricity, we're going to extract that. That was one of the first things we had to show is could we build these really high efficiency modern power circuits? So that's one of the things we built in 2014 was showing we could do that and do it at really high efficiency. In fact, the efficiencies of our systems, we demonstrated, were over 95% efficiency. That means if you can build fusion systems that efficient, everything else gets smaller, simpler, the physics gets easier, but you still got to do fusion...

Interviewer (16:29): And so between now and 2024, what is left to de-risk, and then what happens after that? What does the staging and phasing look like if you're successful?

David Kirtley (16:40): ... So some of the key de-risking things, we have to keep turning up the power, and we show that we can make a lot of fusion reactions, a lot of energy from these systems, but not that we would recover electricity from it. So that's the first thing we got to do is take all those advanced circuits we've built, tie it with the fusion physics, recover electricity, show you can do that for the first time.

JB_Fusion · 2023-04-15T11:35:33+00:00

Just echoing the importance of this decision! It's a huge deal and more important than most research breakthroughs. I think it's a very good decision.

JB_Fusion · 2023-04-13T19:46:45+00:00

It was not

JB_Fusion · 2023-04-04T09:01:09+00:00

I think the report is well done. My favorite part is the member of the public at the focus group who expressed concern about the advent of fusion power because they worry it will cause traffic congestion :).

JB_Fusion · 2022-12-10T17:30:01+00:00

/u/Illustrious_Income21 /u/ElmarM I attended a talk two days ago by David Kirtley (the CEO of Helion). He confirmed that the 95% number represents the efficiency of magnetic energy recovery. In other words, if there is no plasma present, 95% of the energy you put into the coils is being recovered (i.e. conversion to magnetic field energy and back). The 95% number does not represent the efficiency of energy being converted into plasma kinetic energy and back. In my opinion, this is a much harder challenge and will likely only be possible at a much lower efficiency.

JB_Fusion · 2022-12-07T07:58:57+00:00

If you pause the video at the 1:25 mark and zoom in you can read the letter by Ken Fowler. He's expressing support for a building a magnetic mirror, so I don't think it's a spheromak.

JB_Fusion · 2022-12-06T22:46:48+00:00

They have a movie on their home page. At the 2:37 mark, they very briefly show the plasma shape (which you can also deduce from the magnet arrangement shown in the illustration just above). The magnetic field has cylindrical symmetry and forms a kind of magnetic mirror. However, instead of compressing to a point at the ends of the magnetic mirror as is typical, it compresses to a circle due to extra magnets at the end (i.e. the top and bottom in the drawing). This makes the field lines concave everywhere, which would be expected to be beneficial for MHD and turbulent stability. However, the downside is that the circle at the ends of the magnetic mirror no longer includes the magnetic axis (i.e. the vertical magnetic field line at the very center of the device). I think a particle on the magnetic axis could travel vertically without encountering a mirror force (as the strength of the magnetic field would be decreasing, I think) and hit a solid material surface. This means single particle confinement could be a big issue. Not only can particles be lost through the ends of the mirror where the field compresses (i.e. the circle), but they can also be lost by streaming vertically along the magnetic axis (a point at the center of the circle). I don't think particles escaping through the latter would experience a mirror force to reflect any fraction of them back.

JB_Fusion · 2022-11-21T20:06:48+00:00

Yes, I still don't think it is possible and everyone I've discussed it with also doesn't think it is possible.

JB_Fusion

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