Deep Dive - Helion’s Direct Drive Energy Recovery System
Helion is taking bold steps toward fusion energy breakthroughs.
Most electrical generation systems consume fuel to produce heat, which boils pressurized water. The superheated water usually goes through a heat exchanger, where the water in a secondary loop is turned to high-pressure steam, which spins a turbine, driving a generator. The negative of the “boiling water” approach is that there is inevitably some energy loss, and the plants are large, complex, and expensive due to the ongoing fuel consumption.
Solar energy avoids most of this loss because photovoltaic (PV) solar panels prevent this issue by directly creating electricity; they only have any losses in converting light energy to electricity. What if there was a way to get the 24/7 dispatchable power of traditional fossil/nuclear power plants with the direct energy production of solar energy?
Direct Energy via Field Reversed Configuration
This is the concept of Helion’s Direct Drive Energy Recovery System. They are trying to use a Field Reversed Configuration (FRC) approach to drive a direct power creation model that offers several efficiency improvements over traditional steam-based systems used in conventional power generation to eliminate the need for large cooling towers and steam turbines to improve overall system efficiency. Let’s start with Helion’s approach to the fuels it uses to create fusion energy. Most approaches to fusion energy today utilize the “D-T fusion” process, which fuses deuterium (a proton plus one neutron) and tritium (a proton plus two neutrons), producing a helium-4 atom (two protons and two neutrons) plus a free high-energy neutron, as shown to the right. It is a reasonably straightforward reaction, requiring a (relatively) low ignition temperature of 13.6 thousand electron volts (keV).
The approach that Helion is using to produce fusion is known as a “Deuterium-Helium3” (D-He3) reaction. Helium-3 differs from the (more conventional) helium-4 because it has two protons but only one neutron. Unlike deuterium (which makes up about 0.0156% of all hydrogen on earth), helium-3 only makes up about 0.000137% of the helium on earth; however, helium-3 is created by the radioactive decay of tritium over a half-life of roughly 12 years (along with an electron).
Unfortunately, tritium is not particularly plentiful, but we know how to produce tritium in “breeder” reactors. Finally, the D-He3 reaction requires significantly more energy than the D-T reaction (58 keV vs 13.6 keV) and considerably higher temperatures. So why is D-He3 so “promising,” given the relative rarity of its fuel? The answer is simple (and you heard it in last week’s article): D-He3 is aneutronic (it does not produce any high-energy neutrons).
A Different Set of Particles Produces a Different Cocktail
The other reason that D-He3 is promising is how energy is “carried away” from the reaction. In the case of the D-T reaction, the primary energy produced by the reaction is contained in the free neutron at 14.1 MeV, followed by the ionized helium particle (also known as an “alpha particle”) at 3.5 MeV. The problem with most of the energy being in the high-energy free neutron (which tends to produce structural and biological damage) is that the only way to recover the neutron’s kinetic energy is by converting it to thermal energy. This is done by colliding the neutrons with a material with a high-neutron cross-section, such as ceramic lithium pellets inside a water-cooled cavity. The ceramic lithium pellets get heated up from the neutron collisions, which heat the high-pressure water in the cavity.
As we explained at the start of this article, the hot water has to go through a heat exchanger and a turbine/generator to produce electricity, as shown in the illustration. This conversion of kinetic energy to electricity is a lossy process, as is the neutron damage occurring from the high-energy neutrons. This is where the other benefit of the D-He3 reaction comes in. The D-He3 reaction also creates nearly twice the energy of the D-T reaction. More importantly, four of the five particles created are charged particles: protons at 14.7 MeV, alpha particles (He4 nucleus) at 3.6 MeV, helions (He3 nucleus) at 0.8 MeV, and tritons (tritium nucleus) at 1.0 MeV. The neutron produced only has an energy of 2.45 MeV, significantly less than the energy produced by the D-T reaction. Because most of this energy is contained in charged particles, it is easy to confine them within the Helion device’s magnetic fields after the response.
Putting Theory Into Practice – The Helion “Hourglass”
However, the coolest aspects of Helion’s approach lie in how Helion plans to reach the temperatures and pressures required to fuse deuterium and helium-3 together. They use a device that looks somewhat like an hourglass on its side, surrounded by many high-power magnets. The steps in the reaction are as follows:
1. Fuel is injected into both ends of the fusion machine: Once the deuterium and helium-3 are injected into the ends of the machine, it is heated to plasma conditions and confined in two field-reversed configurations (FRCs), essentially forming donut or torus shapes, which are self-confined at each end of the hourglass. Under the right conditions, very little plasma ions leave the cyclotron orbits in the toruses.
2. The toruses are accelerated towards each other and collide: Utilizing extremely powerful magnets analogous to electron guns, the two toruses accelerate towards each other at speeds approaching one million miles per hour. At the same time, the toruses are squished longitudinally into cylinders until they collide in the narrowest part at the center of the reactor (the “interaction chamber”).
3. The combined toruses are further compressed until they begin fusion: Upon reaching the center of the machine, the combined torus (now more of a ball) is highly compressed by the central magnets of the machine, heating the plasma to greater than 100 million degrees, where fusion between the deuterium and helium-3 nuclei occurs.
4. The plasma rebounds, inducing a reverse current in the compression magnets, where new things occur. The now-fusing plasma (primarily of charged particles) “pushes back” to induce a current in the central magnets that compressed it in Step 3, producing more current.
This resultant current is (hopefully) more significant than the current used to compress the plasma, directly yielding net energy initially. The resulting “ash” from the fusion reaction (primarily helium-4) is then diverted out of the machine, and the cycle repeats itself, essentially producing pulsed power, which can be “evened out” using energy storage devices such as capacitor banks.
Dr. David Kirtley Explains Helion’s Approach
As with most engineering endeavors, the real “magic” is optimizing all parameters to produce a system that meets its goals. For Helion’s machine, there are some very complex hydrodynamics occurring during the compression and expansion of the plasma; the parameters of plasma length, diameter, compression, volume, and time have huge impacts on the efficiency of the fusion burn, as well as the efficiency of the energy recovery process. To do this material “justice,” I will not try to explain the details of this (to be honest, I was never particularly good nor particularly enjoyed thermodynamics, which is why I became an electrical engineer).
Instead, you can watch a video presentation by Dr. David Kirtley (CEO of Helion) that he presented at the Fall 2024 meeting of the Division of Plasma Physics of the American Physical Society.
Swinging for the Fusion Fences
Helion’s approach to Fusion energy is not without skeptics, and much of the world is still skeptical about fusion in general, which is something all of us in the business must contend with every day. Helion is, without a doubt, swinging for the fences with new fusion technology, new power generation methods, and some very aggressive timelines. They have drawn their share of skeptics.
Their linear fusion reactor design combines aspects of magnetic and inertial confinement fusion, aiming to create a more efficient and cost-effective solution. Helion's method eliminates the need for traditional steam turbines and cooling towers by directly capturing electricity from the fusion reaction, potentially reducing overall system complexity and cost. The company's direct-drive energy recovery systems could lower capital costs and be more easily deployed than larger, conventional fusion concepts. If successful, this technology promises to produce electricity at a low price, making fusion power economically competitive with existing energy sources while offering the benefits of clean, safe, and abundant electricity production.
Those who have audacious dreams attract vocal critics. If Helion succeeds in beginning the first commercial, they (and the fusion energy industry) will join a list of people (Musk, Ford, Wright, etc.) who can say, “You're only saying that because it has never been done before. Watch me.”