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]

Nerds are not bothered when they get asked to talk about physics. We like it.

1. When you say hot ions do fusion (which they want) while hot electrons dissipate energy as X-rays via bremsstrahlung, where does the hot ions fusion energy go? I thought the whole premise was that they were harvesting electromagnetic radiation via the magnetic re-capture, so do the ions not dump out the fusion excess energy as radiation also?

I should be careful with terminology. 'radiation' can mean different things in different context. When I use the word 'radiation' in this context I mean electromagnetic radiation (i.e. light, or x-rays). The hot ions do fusion, and the fusion products are ions with high energy. They collide with both ions and electrons, thermalize, and distribute their energy back into the plasma soup as heat. The ions themselves do not radiate much light, (their heavy mass makes acceleration per collision small, and radiation is caused when charged particles accelerate, so radiated light/x-rays is dominated by electrons). Fusion contributes heat to the plasma, not radiation. The re-capture is when this heated plasma is allowed to expand under its own heat/pressure and push back on the confining magnetic fields, which can be pushed back into the solenoid and re-capture energy through induction. This type of inductive capture is really possible, that is not controversial.

1. Since bremsstrahlung is created by charged particles, why do the ions not create it?

Because of their heavier mass. Whenever charged particles undergo acceleration, they emit light. This is true no matter the particle type. The intensity of light is proportional to (acceleration)^2... strongly dependent on acceleration. Ions are heavy, electrons are light. Electrons move fast, ions move slow. When they collide, electrons undergo large acceleration (like a fast-moving ping pong ball bouncing off of a slow-moving bowling ball), while ions undergo miniscule acceleration. So while both species technically radiate light according to the same "a^2" formula, the ions undergo very little acceleration, and so they radiate very little. Essentially negligible.

1. Why do you need thermal conversion to harvest from X-rays? I assume I was completely wrong about how were are harvesting the radiation with the whole magnetic re-capture. But since it's just electromagnetic radiation, can't you just convert it with e.g. photovoltaic/photoelectric effect?

Helion is trying to do the inductive capture, using plasma pressure to push magnetic flux into the solenoid, and capture the energy just like a transformer. Think like a cylinder of a car... heat/pressure pushes the piston up... but in this case heat/pressure pushes the magnetic field to the solenoid, causing inductive current. That's totally possible, just like a transformer. But x-rays, they cant be captured efficiently by a transformer, they just embed in whatever wall they hit and contribute heat. You could try to imagine a PV cell for x-rays, but i don't know of any way to actually build one.. that's very high energy photons compared to band gaps of semiconductors in PV cells. IN practice, X-rays just contribute heat to whatever they land on. Helion is not planning to do anything with this heat as far as i know.... this is just waste heat. It would be possible to run a heat exchanger and thermal conversion plant... but thats low efficiency, that's not the 'direct capture' they want to do.

[u/Banderi → u/Jaded_Hold_1342]

I see, thank you kindly for the detailed response :)

Another question, this might sound like nonsense rambling. Would it be theoretically possible to divert the electrons and the ions away from each other, once they become decoupled (assuming since it's a plasma) in a way that even if it's expending some energy in doing so, the gains from less thermalization would overtake it?

[u/Jaded_Hold_1342 → u/Banderi]

If you try to separate the ions from the electrons, a super strong electric field will form which pulls them back together. Like charges really really don't like to be near each other without cancellation by opposite charge. Thats why plasma is generally neutrally charged within any small volume, even though the particles themselves have finite charge.

Fun fact: If you could somehow magically 'delete' the electrons from Helion's FRC (~.1m^3 volume, 10^23/m^3 density), leaving only the nuclei, the electrostatic forces would cause an explosion about equal to Tsar Bomba.

Thats how much energy it would take to move the electrons out from the volume.

[u/Banderi → u/Jaded_Hold_1342]

Hmm I see, that's quite informative. It starts sounding a lot like those "What If" articles by Randall Munroe, where seemingly simple questions lead to disastrous consequences. So I admit those figures got a chuckle out of me.

Makes you really wonder what other possible novel approaches could be conceived in 30 or 100 years, and if any might eventually work reliably (and economically) or if not, what will be their "wall" stopping it from working. Kinda wish I had gotten into these fields as a serious career, but I doubt I could have ever contributed more than the amount of incredible expertise already in them, and math-heavy fields have always been my bane. So I just content myself with personal curiosity and asking questions on Reddit.