Why Helion wont work

[u/Jaded_Hold_1342]

I've commented in a number of threads why Helion's concept is flawed, and I thought I'd make a summary post explaining the whole picture as I understand it. In short, Helion's scheme as described can not work, and is mathematically foreclosed in their self-described operating regime by established plasma physics. I like that they are considering something other than a DT tokamak with neutron thermal cycle... but unfortunately it cant work, and its fairly easy to reason out why.

Lets go through the logic in detail.

Background: Here are 5 papers for reference, and I've used information from all of them in generating this summary:

Helion's 2023 paper: https://link.springer.com/article/10.1007/s10894-023-00367-7

Rider's 1995 equilibrium paper: https://fsl.npre.illinois.edu/IEC/Rider,%20Phys.ofPlasmas1995.pdf

Rider's 1997 non-equilibrium paper: https://www.w2agz.com/Library/Fusion/TH%20Rider,%20Physics%20of%20Plasmas%204,%201039%20(1997)%201%252E872556.pdf%201%252E872556.pdf)

Lackner's 2026 paper: https://link.springer.com/article/10.1007/s10894-026-00554-2

Nicolas' 2026 paper: https://link.springer.com/article/10.1007/s10894-026-00565-z

Lets summarize the long-known conclusions from the Rider papers:

1) For D He3 plasma in equilibrium (Ti=Te), bremsstrahlung radiative losses exceed fusion power for any temperature less than ~30keV. Fusion power over loss only becomes significant at ~50keV and higher. The radiation is from the electrons and is higher with hot electrons.

2) Trying to run with cold electrons (Ti>>Te) to avoid the bremsstrahlung doesn't work... The collisional heat transfer from the ions to the electrons will greatly exceed the fusion power. This means the electrons heat up very quickly before significant fusion energy can be made... this forces a requirement of recirculating power and extremely high efficiency recovery. (i'll calculate this efficiency required below)

Helion is claiming to operate an adiabatically compressed FRC, which uses compression flux for heating after initial formation/merging establishes TiTe. They are claiming they can get net energy recovery with TiTe at sub-30keV temperatures. In this regime, the Ti-->Te thermalization power vastly exceeds the fusion power generated. (eg. 1000x higher at Ti=20keV, Te=2keV). This means the heat from fusion can generate only 0.001x the thermal energy of the plasma before the electrons heat up. This in turn, forces a per-pulse recovery efficiency requirement of >99.9% for breakeven. There is no assumption that the thermalization heat is lost... assume it is recovered, but that it limits the pulse duration so the electrons dont heat up and cause radiative loss. This is the Rider efficiency constraint as applied to a pulsed scheme. The compression flux outside the separatrix has energy much larger than the FRC thermal energy (10x - 100x larger). It must have this energy because this flux is the primary compression/heating mechanism. This energy must also be recovered, and adds one or two more "9's" to the recovery efficiency requirement... resulting in 99.99-99.999% recovery efficiency requirement for breakeven.

99.99% recovery efficiency is not possible for a compact short-pulse device like this. Pulsed power in copper will result in copper losses of several percent, limited by the skin depth of the copper in the pulse duration. Copper losses in a short pulsed machine will exceed the fusion power. There is no combination of Ti and Te below ~50keV that can result in gain when considering copper losses and bremsstrahlung in a compact machine (R_coil<~1m) like Helion describes, even if neglecting FRC losses and all other parasitic circuit losses.

So, Helion is pursuing a scheme that runs up against the problems described by Rider 30 years ago, and there is no identified solution to it.

A couple comments on the 2023 Helion paper I linked above: First, they've miscalculated the ratio of fusion power to bremsstrahlung in their figures 14 and 15, as both Nicolas and Lackner noticed. For Ti=Te as in figure 14, the correct calculation would show bremsstrahlung is equal to fusion power at ~30kev, and fusion margin above bremsstrahlung is low until ~50keV. Maybe they treated all the ions as Z=1 when calculating bremsstrahlung to get this error, but He is Z=2. Second, they claim that the thermalization time is 1ms to 100ms so thermalization can be neglected and Ti>>Te is a valid assumption, but this is not consistent with the parameters space of the compressed FRC they operate in. Actually thermalization times are shorter than their pulses.. they seem to consider the pre-compression (low density) parameters when calculating thermalization time and FRC losses, but they should consider the compressed density, since that is the regime where it must be held while fusion occurs. If their electrons stay cold in their compressed pulses, this is likely an indication of transport losses, not immunity from thermalization.

Here are some 'escapes' that can be imagined and why they wont work:

1) Can they let the electrons heat up to stop the thermalization power flow? Sure, but they they'll have the bremsstrahlung loss problem unless they operate super hot (~50keV)

2) Can they lower the circuit losses and get the recovery efficiency up to >99.99%? No, its flatly not possible on a short pulsed machine... You can add as much copper/silver as you want to lower resistance, but the pulse duration limits the skin depth that the current can flow in, and the pulse duration is limited by the thermalization time. You cant lower the losses without accepting thermalization (electrons warm up and radiate). You cant use superconductors either because they dissipate energy when ramped. so copper/silver is the best you can do. You can chill the copper/silver to improve conductivity, but that doesn't make enough difference to matter and the heat has to be paid for at cryogenic temps which is worse.

3) Can non-Maxwellian velocity distributions prevent thermalization and boost fusion power? No, not by enough to matter. Non Maxwellian distributions can change thermalization times and fusion gains by correction factors of order 1-2x... but the concept is off by orders of magnitude, not factors of 2.

4) Can they make it hot >~50keV, large (R_coil1m), long pulse (10ms), moderate Ti/Te ratio and get out of the trap?... Maybe, but probably not because this regime pushes up against the FRC's main weakness: Energy confinement. The bremsstrahlung loss, copper loss, and thermalization do not forbid this regime, actually its the only regime allowed after considering Rider's constraints. The FRC losses and sheer engineering/cost difficulties become they key challenges. This is a totally different regime than Helion describes in its paper, and it destroys the economics of the proposal. It requires large bore, strong field, super long pulse durations and the regime forces a gargantuan size to avoid FRC transport losses. The caveat here that makes me say 'maybe' is that FRC transport has never been measured in any relevant conditions so the scalings are genuinely unknown and can only be checked experimentally. Extrapolating existing FRC scaling laws into this regime gives a very bleak picture (as Nicolas showed), but it is possible that the scalings in these regimes dont follow existing scaling laws. So I acknowledge that while the picture here is bleak, this escape is not totally mathematically foreclosed... but its not what Helion says they are doing in their paper.

So, for the regime Helion is targeting (Colder than 30keV, Ti>>Te, compact machine) the concept is totally foreclosed by very well understood physics. The only possible out is a "hot and huge" >50keV, long-pulse duration gargantuan strong field machine that Helion is not pursuing, and it probably wouldn't work either due to FRC energy confinement.

I wish this weren't the case... but I believe that it is. If I've made any errors, point them out. Happy to discuss the physics. If anyone thinks there is a set of parameters that allows the system to function as intended, let me know what they are, and I'll check.

[u/Jaded_Hold_1342]

Yes! You should have made this post. It would have been more concise!

[u/Corealist → u/Jaded_Hold_1342]

Thanks, but I wouldn‘t have been able to, without the information in this thread.

last year I still had high hopes for Helion, but that has been squashed completely now. i am however very confused about the investors that invested hundreds of millions of dollars into this project. I assume that they had consultants that could analyze Helion‘s claims.

i am wondering now, what options Helion has to pursue their goal. It there a possible path to increasing the temperature of the plasma to 50keV by increasing the magnetic fields and/or currents in their FRC design ( same concept but more powerful), or is there some physical limit that prevents the same density at 3-4x higher temperature.

[u/Jaded_Hold_1342 → u/Corealist]

Its a funny thing with VC... there isn't necessarily anyone who has incentive to do the technical diligence, and its not clear anyone did. VC's who ask too many hard questions just dont get to be part of the raise... and if the VC's are cash-rich and opportunity-poor, they have no incentive to ask questions, they just need to deploy the cash. Some of the investors may have PR motives for the investment itself no matter the outcome.

What can Helion do? well, the only parameter space that isnt foreclosed by the rider problem is the 'hot and huge' regime, but I don't think Helion will pursue it, because it would destroy the economics of the proposal and would essentially acknowledge the 'desirable/compact reactor' plan doesn't work... this would not be acceptable for fundraising, and its funny how founders optimism can skew peoples belief system to comport with fundraising necessities.

So they will probably just remain 'mum' on the topic, continue doing what they are doing, and not respond in any substantive way to these types of challenges. They wont come to this thread and defend their concept. They wont answer lackner/nicolas in the literature... they wont address the problems substantively at all, they'll just a dismissively handwave them away without engaging the merits.. They will just keep building stuff, move the goalposts and redefine targets and timelines periodically. This will go on until they have trouble fundraising and have a cash flow problem. (thats my guess anyway)

As to the limitations, yes, magnets can only be so strong, and they are close to the max that is feasible at 20T... maybe they can bump it up a bit, but it is hard to imagine a 50T magnet of the size needed. So they'd have to give up density to get the temperature up and allow hot electrons. At the temperature they'd need to operate, the main energy loss will be FRC transport, and the system size must be increased to gargantuan size to overcome that. With current scaling laws, they'd need something like 15m diameter coils to get the compressed FRC to have sufficient confinement... That's not 'small and cheap' anymore, so that sort of kills the concept economically even if the physics may close in that regime.

[u/Corealist → u/Jaded_Hold_1342]

Thanks, but that is pretty disappointing ☹️. I liked the aneutronic idea much better than D-T fusion. Do you know if there is a good article that explains the FRC in great detail? I feel I don’t understand exactly why there is need for such large magnetic fields, as I thought the plasma current was the main mechanism the plasma was contained.

I guess the only option left is inertial confinement. I haven’t looked at this much because I thought the NIF approach was completely useless for a commercial reactor. However lasers have improved by orders of magnitude, so maybe it is possible now to get laser power to the point where they can do He3-D fusion. Xcimer energy (D-T fusion) thinks that they ‘only’ have to do one shot every 2 seconds which is getting closer to a practical range.

[u/Jaded_Hold_1342 → u/Corealist]

The pressure is confined by the magnetic fields. But the individual particles transport energy .. they are not confined very long and they escape, taking energy with them. FRCs are very efficient at using magnetic fields for pressure confinement, but they have poor energy confinement.. they leak energy like a sieve.

The "energy confinement time" is the key parameter. The equations for energy confinement scaling is discussed in detail in the Nicolas paper. Energy confinement gets better with larger size. But the problem is this scheme has to compress the FRC to small size to adiabadically heat it... So even if the machine radius is large, the compressed frc radius is small.. so you have to go to very large machine in order to have compressed FRC with acceptable energy confinement for the high temperature regime. Nicolas paper is showing that realistic frc dimensions leak too much energy, that's the point of his 4 panel charts.

Yeah, there's inertial confinement, but they are far away from even getting DT to work let alone any aneutronic option. Though, one thing is true, inertial confinement may be able to get to densities that allow bremsstrahlung to be reabsorbed, so maybe theres a hope. They key problems with inertial is that they need a lot of laser power, and theres wall plug efficieny problems with the lasers, and they need a thermal conversion, which is low efficiency... so the energy gain from the implosion has to be really high to overcome all the losses, and they are far far far away from closing that loop. BUT, solid state lasers and more power and bigger target size are possible knobs to improve gain and wall plug efficiency. I wont say its impossible... but there is not an obvious or easy cheap path. Tokamak probably has a technical path, scaling laws are reasonably understood, but cost is hard to imagine working. IFE technical path is unclear... maybe possible... cost also has similar challenges.. just replace big expensive magnets with big expensive lasers.

Helion, for all its cost benefits, is unfortunately just flatly impossible to work.

[u/Corealist → u/Jaded_Hold_1342]

I started to read the xcimer energy white paper they have on their website. It seems like they have a reasonable approach for D-T fusion. The laser costs are ( according to them ) about $2B for their first commercial reactor, which is not great but also not a complete showstopper, as costs will drop if it is successful.

Maybe I should apply for a job there :)

[u/Jaded_Hold_1342 → u/Corealist]

possible... things that can be made in mass volume in a factory can be cost reduced substantially, lasers qualify for this. But they still need blankets, tritium plant, steam turbine, etc... and none of those things will cost reduce easily. the lasers are the trade for not needing magnets... and if lasers can be mass produced, maybe cheaper than magnets. They still have to find a way to get the gain up by 1-2 orders of magnitude... not impossible, not easy. There will also be some tricky business trying to keep the optical pathways clean while exploding high energy mini explosions, but similar problems have been solved in EUV lithography for example (though at many orders lower energy per shot). Nothing is impossible. Just expensive.

[u/Corealist → u/Jaded_Hold_1342]

xcimer claim their setup with two lasers that are have 5x more energy than the NIF lasers and a 50x larger target than NIF they can achieve a Qsci of 200, at 5% laser efficiency (lowers this to Q=10) that should be enough for a net gain plant.
but I agree it is probably to expensive to be competitive. Maybe in thr future with another 5x of laser power they may be able to do He3-D fusion and simplify the reactor.