Claim Check: Realta Fusion claims to be the first private fusion company in history to demonstrate direct energy conversion of plasma kinetic energy into electricity

Verdict: Did they directly convert plasma kinetic energy into electricity? Probably. Were they the first private fusion company to do so? Probably not. Was it at a power level relevant to fusion reactors? Not at all. Rather than being a significant technical achievement, what Realta has done here is highlight the arbitrariness of some milestones.

What happened

On June 30, Realta announced that it had demonstrated direct energy conversion on the Wisconsin HTS Axisymmetric Mirror (WHAM) during experiments on June 19. Charged particles escaping through the mirror ends passed through two grids and reached a positively biased ion collector. Slowing and collecting the ions produced multiple amps at around 100 volts.

Realta was clear that this was not net fusion electricity. Nearly all the recovered energy had first been supplied by WHAM’s plasma-heating systems, not by fusion reactions.

Why it matters

Mirrors are well suited to direct conversion because they naturally channel escaping charged particles toward their ends, which are much freer than toroidal systems to expand as needed for direct energy conversion.

At low fusion gain, a converter could recycle some externally supplied heating power. At high gain, it could recover part of the alpha-particle energy. But in D-T fusion, approximately 80% of fusion energy appears in neutrons and only 20% in charged alpha particles. Most of the plant must still use a blanket and thermal cycle.

What is actually new

Very little.

Direct energy conversion has been demonstrated in magnetic mirrors. Lawrence Livermore began work on plasma direct converters in the 1970s, with tests reaching 100 kilovolts and 6 kilowatts. In 1983, a converter on the Tandem Mirror Experiment recovered 67 watts from approximately 140 watts of incident ion power, for a measured efficiency of 48%. A separate experiment with a 22-stage converter had previously reported approximately 86% electrostatic recovery.

Realta’s claim is narrower: that no private fusion company had previously converted plasma kinetic energy into electricity. Putting on my pedantic physicist hat, that is false as written.

The Migmacell II counterexample

While there may be earlier counterexamples, this was the best evidence I could find against the claim that Realta was “the first private fusion company in history to successfully demonstrate direct energy conversion (DEC) of plasma kinetic energy into electricity.”

In 1977, the Migma Institute and Fusion Energy Corporation published “Fusion products detection system in Migmacell II”. They used silicon detectors to measure protons and tritons from D-D reactions involving 0.55 MeV deuterons. The charged fusion products deposited kinetic energy in the silicon, creating electron-hole pairs and electrical charge pulses. Fusion Energy Corporation had therefore converted charged-particle kinetic energy into electricity 49 years before Realta. The output was microscopic and diagnostic. Nonetheless, it meets the criteria of Realta’s claim.

What the numbers imply

The following examines how direct energy conversion affects the ultimate plant power output.

Let $\eta_{\mathrm{DC}}$ be the direct-conversion efficiency and $\eta_{\mathrm{th}}$ the thermal-cycle efficiency. If the power not recovered directly becomes useful heat, the effective efficiency of the converter stream is:

$\eta_{\mathrm{stream}}=\eta_{\mathrm{DC}}+\left(1-\eta_{\mathrm{DC}}\right)\eta_{\mathrm{th}}$

The first term is electricity recovered directly. The second is electricity produced from the remaining heat.

If $f_{\mathrm{DC}}$ is the fraction of total fusion power delivered to the converter, gross plant efficiency is:

$\eta_{\mathrm{gross}}=\left(1-f_{\mathrm{DC}}\right)\eta_{\mathrm{th}}+f_{\mathrm{DC}}\eta_{\mathrm{stream}}$

Substituting the expression for $\eta_{\mathrm{stream}}$ gives:

$\eta_{\mathrm{gross}}=\eta_{\mathrm{th}}+f_{\mathrm{DC}}\eta_{\mathrm{DC}}\left(1-\eta_{\mathrm{th}}\right)$

The improvement over an all-thermal plant is therefore:

$\Delta\eta_{\mathrm{gross}}=f_{\mathrm{DC}}\eta_{\mathrm{DC}}\left(1-\eta_{\mathrm{th}}\right)$

This shows why a high converter efficiency does not produce an equally large increase in total plant efficiency. The benefit is multiplied by the fraction of reactor power that actually reaches the converter.

Using Realta’s optimistic assumptions:

$f_{\mathrm{DC}}=0.20$

$\eta_{\mathrm{th}}=0.45$

$\eta_{\mathrm{DC}}=0.90$

The effective efficiency of the converter stream is:

$\eta_{\mathrm{stream}}=0.90+\left(1-0.90\right)(0.45)=0.945$

Whole-plant gross efficiency becomes:

$\eta_{\mathrm{gross}}=0.80(0.45)+0.20(0.945)=0.549$

Thus, 90% direct conversion of the entire alpha-particle energy stream raises gross efficiency from 45% to 54.9%, not to 90%.

What would have to be true

For direct energy conversion to be useful in a magnetic mirror:

A large fraction of alpha and injected heating power must leave axially as collectable charged particles.
The converter must remain efficient across broad particle energies and pitch angles.
Grids, collectors, and insulators must survive high voltage, particle flux, radiation, and harsh plasma conditions.
The converter must not compete with the plasma potentials and expander geometry needed for confinement and stability.
The value of the recovered electricity must justify the additional system’s capital cost and maintenance burden.

What is still missing

Realta has not released any substantial information on the plasma conditions under which this energy conversion was achieved. The fusion contribution was presumably negligible. This is only their first step along the technology roadmap.

To put it into perspectiv: External heating power is about 1 MW on WHAM. The results presented here are about 500 W. Scaling to WHAM-relevant conversion of plasma kinetic energy into electricity would therefore require an increase of about a factor of 1,000, followed by another factor of about 100 to reach reactor-level conditions. These are very large leaps in plasma conditions and power density, and achieving them will be no small technological feat.

This achievement is on par with the high power plasma diagnostics. For example, the Langmuir probes I used on the Alcator C-Mod tokamak operated at up to a few amps and a few hundred volts, the same regime as Realta’s first generation energy conversion system.

The claim beneath the claim

Good milestones matter.

As I stated in last week’s Fusion Fortnightly, I see this as an attempt by Realta to scoop Helion’s milestone of demonstrating the first direct conversion of plasma energy into electricity. I find this announcement a good-natured way of taking the wind out of Helion’s sails over a milestone that makes a good headline but might not be a substantial technical achievement.

This is an opportunity to raise a broader point about fusion-industry milestones: Many are set, achieved, and communicated in ways that make them sound bigger than they really are. This is a problem. Fusion energy is a complicated field, and those outside it have a hard time separating meaningful progress from modest milestones.

What Realta has done here is demonstrate that an arbitrary milestone such as “the first X to do Y” can sometimes be achieved relatively easily in a way that represents only a small step toward what a fusion reactor actually requires. Here, I have taken that a step further and shown that the supposed “first” had already occurred decades earlier in an even more trivial fashion.

Bottom line

Realta performed a real and worthwhile experiment, but wrote the historical claim too broadly.