Part of the reason D-T fusion is the primary candidate is that it occurs at temperatures and pressures orders of magnitude lower than He-3 based fusions. This makes Helions approach more difficult from a materials, heating, and magnetic field perspective.

I do research tangential research and one thing I've heard (take with a grain of salt) is that they don't publish nearly as many results as other fusion companies. This may not mean anything, or it could mean they don't have favorable results thus far.

Have a look at this graphic https://en.wikipedia.org/wiki/Lawson_criterion#/media/File:Fusion_tripleprod.svg

Reaching fusion conditions for D-He3 and especially for D-D is much much harder then for D-T.

There's two main bottlenecks I see:

1. Energy extraction. The way their system works, it is basically impossible to be efficient extracting energy from the heat produced like how ITER (or, planned future commercial reactors using the principles in ITER) is planning. This is because the extremely strong, temporary magnetic fields they are producing use up enormous amounts of energy, which recycling from heat would be horribly inefficient - ITER uses much weaker semi-permanent fields (in comparison to Helion, ITER's fields are no joke either), which are much less energy-intensive to maintain. Therefore, Helion proposes to generate energy directly from the magnetic fields involved - the fusion process itself produces much of the energy from the motion of charged particles, which the magnetic confinement will capture as magnetic flux - thereby also recycling the energy from the magnetic fields they produce. So far, they haven't demonstrated capturing the energy back out, which will be very difficult (but, at my cursory look, physically reasonable, just difficult to do) and very difficult to do efficiently enough for their proposal to work.

A slightly different but equivalent explanation for the efficiency issue is that Helion uses much higher temperatures for their fusion - this means more energy in, which needs to be recaptured to be efficient, and following thermodynamics, the higher temperature at which the reactor can capture the energy, the higher efficiency of recycling. Magnetic fields capture the energy immediately after fusion at peak temperatures. Heat capture would be much lower temperature in comparison, which makes everything horribly inefficient.

2. Scaling up production of energy. Helion makes relatively low-energy "bursts" of fusion - to make the energy generation appreciable, they have said they need the bursts to cycle at something like 1000+ times per second (I forget the exact number they gave). Right now, they've demonstrated fusion at 1 time per minutes, and the magnetic confinement and lauch (without fusion) at closer to, but still far from, that frequency. This means everything involved - the production of magnetic fields using their superconducting electromagnetics powered by massive capacitor banks, injection of fuel, launch process, collection of energy probably back into capacitor banks, needs to long-term reliably happen at these frequencies. As an example, in practice, one thing that probably needs to happen is the capacitors need to be done away with altogether (outside of startup and net energy capture) and the electrical energy generated needs to mostly directly power the electromagnets - cycling that much energy at those frequencies would overheat capacitors, needing multiple banks to run instead.

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