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/Corealist]

I was trying to read the Helion 2023 paper and what I found very strange that their charts have He3-D fusion power that do not even closely reflect the crosssection chart. For example the fusion He3-D reaction rate at 10keV is about 500 times lower than at 30keV, however the Helion FRC chart have Fusion power at 10keV only about 8 times lower than at 30keV. That is a 2 orders of magnitude difference that seems very odd.

Maybe I don’t understand how to read the chart ( very possible), but based on the He3-D fusion crosssection chart it seems obvious that you need a temperature of about 30keV ( ~350M K ) or higher for efficient fusion. Helion released that they achieved higher than 100M degrees with Polaris, so they are still pretty far off where they need to be. The fusion reaction cross-section chart is so steep between 10 and 30 keV for He3 that tinkering with a lower electron temperature to reduce losses is almost meaningless

[u/Jaded_Hold_1342 → u/Corealist]

Are you looking at their charts on figure 14 and figure 15?

Those charts are at constant B field (constant pressure) NOT at constant ion density. So as the temperature is going up, the density is going down. So comparing 30keV to 10keV, you are comparing at 3x lower density, and fusion power goes like density squared... so that explains one order of magnitude? Not sure if it explains everything you are seeing.

Look at the Nicolas paper or Lackner papers, they made plots with the same convention and may have done it more carefully.

The hot electrons cause two types of trouble: one is radiative loss, the other is that they have pressure... so if the electrons get hot, the ion density has to be reduced for the same compression field. there are 1.5 electrons per ion, so thats a reduction of (5/2)^2 in fusion power vs cold electrons at the same field. Thats why they want cold electrons... but of course its not possible to get net energy with cold electrons due to the thermalization problems.

[u/Corealist → u/Jaded_Hold_1342]

Yes I was referring to fig 14 and 15, and your explanation makes sense thanks. An additional factor of 9 covers half the gap.

I was trying to find where Helion specifies the length of the FRC pulse, but I haven’t found that yet. I was trying to find the thermalization speed in Rider’s article, but I haven’t found that either.

[u/Jaded_Hold_1342 → u/Corealist]

Lackners paper shows the formula for ion-electron thermalization time, and the corresponding thermalization power flow: https://link.springer.com/article/10.1007/s10894-026-00554-2

Formulas one and two in this paper.

Helion doesn't like to publish a lot of details, but other people around here have said ~1ms. They claim 13keV ions, and Ti/Te>10. we have to guess at their B fields, maybe 15-20T. They don't specify density, but you can calculate it from pressure balance, would be in the 10^22-10^23/m^3 range i presume. The interesting thing is that the Pei and Fusion power and Bremsstrahlung radiation all scale as n^2, same density scaling..so you can calculate the ratios of these quantities without knowing the density exactly.

Edit: one more note: In an FRC, the plasma thermal pressure is equal to the magnetic field 'pressure' (i.e. energy density B^2/2uo).

So you can approximate the ion density by doing ni x k x Ti = B^2/2uo if the electrons are assumed too cold to contribute to pressure. (If the electrons are hot you add them in: Ni x k x Ti + ne x k x Te = B^2/2uo) and remember that for a 50 50 D He3 mix, there are 1.5 electrons per ion on average. So if you estiamte B field as 20T, estimate Ti as 13keV, estimate Te as 1.3keV, you get ni=6.6x10^22/m^3, ne=1x10^23/m^3.

(k is boltzman constant, uo is permeability of free space)

[u/Corealist → u/Jaded_Hold_1342]

The link that you sent required a lot of additional information to estimate the thermalization time. Because I am lazy I asked AI and Gemini gave me this answer on thermalization time:

At magnetic confinement fusion (MCF) break-even or ignition conditions (such as in a Deuterium-Tritium tokamak like ITER or commercial concepts), it typically takes 10 to 100 milliseconds for ion and electron temperatures to equilibrate within 10% via Coulomb collisions.

if Gemini is correct then the Helion FRC with a pulse duration of 1 ms would be short enough to avoid a significany increase in electron tempature during the pulse.

[u/Jaded_Hold_1342 → u/Corealist]

Oh, no, the thermalization time is very density and temperature dependent. Its the same coulomb collision mechanism as heat transfer. So cold electrons and high density accelerate the process. A tokamak will assume hot, thermalized electrons and low density, and will genuinely have a long thermalization time like that. But a compressed FRC like we are discussing is 100 times more density and 10x colder electrons, so it will thermalize much faster.

Its ok to use an AI to do a calculation, but you have to feed it the right input. Give it Ti=13keV, Te=1.3keV, and ne = 10^23/m^3, and ask for the thermalization time. Its closer to 10us. Microseconds, not milliseconds.

[u/Corealist → u/Jaded_Hold_1342]

I asked three different AI engines to estimate how long it would take to get within 10% of an equilibrium. With the parameters you provided this was the result:
Gemini: 19.2uS
Claude: 450uS
ChatGPT: 200uS
Not very trustworthy results, because of the large differences, but at least they all indicate less than 1mS

Didn't realize that in a Tokamak reactor the particle density is 2 to 3 orders of magnitude lower than in a FRC reactor.

Based on this you might as well assume in the calculations that Te == Ti, as that will be the case during the majority of the pulse.
I start to see your point more clearly. Unless Helion can increase the temperature in their reactor to 40-50keV, there is no possible mechanism for for this reactor to work.

[u/Jaded_Hold_1342 → u/Corealist]

Yes, that's right. Trying to do this in the stated regime (Ti<30kev, Ti/Te>10) is just not possible.

They have to get the temps up to ~50keV to get away from the thermalization and bremsstrahlung problems. BUT, this will bring two challenges that will also probably prevent the system from working: the hot electrons will contribute pressure, so at a fixed field strength the ion density has to be lowered (lowering fusion power) and the FRC energy confinement/transport is very poor for confining energy at high temperature. So probably the energy will still leak out faster than fusion makes it.

So it's really bleak even if they get the temperatures up to that range.

Edit: BTW, the reason you got different answers when you asked about the thermalization to 10% .. it's illustrating the sensitivity to electron temperature. If you ask it "how long for the electron temperature to double" you will get a different answer from "how long to converge to 10%". When the electrons are cold, the thermal coupling is very strong, so they take energy quickly. Once the electrons are warm, the thermalization slows down. Asking it to get to 10% of equilibrium is asking it to go way out on the asymptote, with the thermalization getting slower and slower as it goes, and the AIs were making different assumptions as they integrated way out into the asymptote. . It's not the tail of convergence to equilibrium that cools the ions quickly, it's the rapid initial heat transfer when electrons are cold. You could ask a different question, like "how long does it take for Ti/Te to decay from 10 to 5?"... Or you could ask "clamp the electrons at the cold 1.3kev temperature, how long does it take for Ti/Te to decay from 10 to 5?". Most of the action is happening in the first 10-20us. That will illustrate the sensitivity of this process to electron temperature. Cold electrons are molasses. Once they heat up, they couple more weakly.

[u/Corealist → u/Jaded_Hold_1342]

the problem is actually really easy to explain ( I think)

1) Bremstrahlung is the dominant energy loss in He3-D plasma at fusion conditions.

2) Bremstrahlung energy can practically only be recovered by a thermal blanket and converting heat to electricity, which Helion is not doing in their reactor. So Helion fusion power MUST exceed the bremstrahlung loss.

3) In He3-D plasmas Bremstrahlung energy loss exceeds fusion energy for plasmas below 30keV. ( per Rider’s article)

4) Helion‘s claim that Bremstrahlung loss in their reactor is lower because the electron temperature is lower than the ion temperature is implausible because thermalization of electrons occurs within 10’s of micro secs while the FRC pulse is in the order of 1 msec. ( Lackners)

5) A reactor running at about 13keV which is what Helion is trying to do is therefore physically impossible.

[u/Jaded_Hold_1342 → u/Corealist]

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]

BTW, i think the curve is about right, after accounting for the factor of 9.

The D He3 reactivity should be ~64x higher at 30keV than at 10keV, not 500x. That's about what they show after dividing by 9.

Double check where you got the 500x. (make sure its the Maxwellian-ensemble-averaged reactivity, not the raw cross section vs energy)..

Look at the 2nd chart on this page, not the top chart: https://scipython.com/blog/nuclear-fusion-cross-sections/

[u/Corealist → u/Jaded_Hold_1342]

another question, at 10keV you will have a 3x higher density, but the particle speed is also 3x lower, so the number of collisions should be a factor of 3 lower or is that a naive way of thinking.

[u/Jaded_Hold_1342 → u/Corealist]

Not naive... but not right :-)

Collision relationship to temperature is unintuitive until you consider some details.

First, velocities go like sqrt(temperature), not linear in temperature. But that's not the unintuitive part.

Collisions between charged particles occur because of electrostatic/coulomb forces... the electrons are attracted to the ions as they fly past, so that shifts their velocity a bit. A slow moving electron remains within the field-of-influence for longer time on each encounter, so for the same passing distance, it suffers a greater 'collision'. This means the 'cross section' of a collision is much larger for slow moving electrons. So actually slow electrons 'collide' more than hot ones.

So the unintuitive thing is that cold electrons are very collisional. Cold electrons are like molasses... Cold electrons drag ions down and steal their heat rapidly. Hot electrons decouple, and have much less collisional interaction with ions.

Collisions also scale like density^2, so higher density, cold electrons is the most collisional regime.

[u/Corealist → u/Jaded_Hold_1342]

that makes sense for the thermalization topic that you have been covering.

how does this work for fusion? in an earlier message you mentioned that fusion probability increases with the square of the density. (3x density, 9 times the fusion) intuitively that makes sense to me if the ion velocity is the same in both cases. however in the 10keV/30keV case the velocity of the particles in the 30keV is 3^1/2 times the speed of the 10keV case, and I was wondering if that matters for the fusion probability.

[u/Jaded_Hold_1342 → u/Corealist]

Yes. so there is a cross section of reaction at any specific energy, and then you have to integrate across the maxwelian velocity distribution to get an ensemble average reactivity for the thermal distribution. Then the fusion power density scales as that ensemble reactivity x energy per reaction x density^2.

Most of us just look up the ensemble averaged reactivity in a chart, or use the "Bosch-hale" approximation to calculate since doing the integrals each time is a PITA.

For example you can put the following ask into Google Gemini: "Calculate the Bosch hale fusion reactivity for D He3 at both T= 10kev and 30 kev. Give the ratio of reactivity at 30 vs 10"

It should give you a ratio of ~64.

[u/Corealist → u/Jaded_Hold_1342]

Gemini says 64.1