Author: Stephanie Thomas

The internet was abuzz last week with the news that the National Ignition Facility had achieved that elusive goal: a fusion experiment that achieved net (scientific) energy gain. This facility, which uses 192 lasers to compress a peppercorn-sized pellet of deuterium and tritium, released 3 MJ of energy from 2 MJ of input heat.

We have to use the caveat that this is “scientific” gain because it does not account for the total amount of energy needed to make the laser pulse. As a matter of fact, the lasers require 400 MJ to make those 2 MJ that reach the plasma. If we account for this energy, we can call it the “wall plug” gain or “engineering” gain since it includes all the components needed. This gain for laser-induced fusion is still less than 1%, because the lasers are very inefficient.

Nonetheless, this is great news for all fusion researchers. Since we often get asked: Has anyone achieved net (scientific) gain yet? Now we can say: Yes! It is physically possible to release net energy from a fusing plasma, to get more energy output than direct energy input. This advance has been achieved through various new technology: machine learning to select the best fuel pellets, wringing more energy from the lasers, more exact control over the laser focusing. Modern technology, especially computing for predicting plasma behavior, explains why progress in fusion energy development is now accelerating.

Tokamaks have also come close to net gain, and in fact the JT-60 tokamak achieved conditions that could have produced net gain, if it had used tritium [1].

The reason JT-60 did not use tritium in those shots is very relevant to our fusion approach, the PFRC. Tritium is radioactive, rare, expensive to handle, and releases damaging neutrons during fusion. Tritium is also part of the easiest fusion reaction to achieve in terms of plasma temperature, the deuterium-tritium reaction. It makes sense for fusion experiments to use such a reaction, but this reaction presents many difficulties to a future working power reactor.

The PFRC is being designed to burn deuterium with helium-3, rather than with tritium, precisely to make the engineering of a reactor easier. The deuterium-helium-3 reaction releases no neutrons directly. Some deuterium will fuse with other deuterium to produce neutrons and tritium, but the PFRC is small enough easily expel tritium ash. This results in orders of magnitude less neutrons per square meter reaching the walls. Once we have scientific gain, like the NIF has now demonstrated for laser fusion, we have an easier path to engineering gain — that is, net electricity.

So while the laser fusion milestone doesn’t directly impact our work on the PFRC, it is important to the field. We will continue to follow the progress of all our peers as we work to achieve higher plasma temperatures in our own experiments!

[1] T. Fujita, et al. “High performance experiments in JT-60U reversed shear discharges,” Nuclear Fusion 39 1627 (1999). DOI: 10.1088/0029-5515/39/11Y/302

We have commenced operations at 1.8 MHz in PFRC-2, after installing new capacitors over the summer to allow us to lower the frequency from the previous value of 4.3 MHz. A lower frequency should allow the RF system to directly heat the plasma ions, not just the lighter electrons.

With each new operating frequency, we need to explore how the plasma responds: to fill pressure, RMF power, magnetic field, mirror ratio, and more. We have now achieved “big bright flashes” with Argon plasmas in PFRC-2! The seed plasma, on the left, is a dimly glowing column. The RMF heated plasma, on the right, produces a bright flash.

This bright light is atomic or molecular line emission, depending on the fill gas. This occurs in the PFRC when the plasma gets dense and energetic due to the RMF current drive. With Argon, we have achieved bright discharges at about 50 kW, or 1/4 of the total RF power available. Argon gas produces a higher density plasma in the PFRC because it has a lower ionization energy.

We are now working to find the parameters which will produce these bright, energetic discharges in our target operating gas of hydrogen. The hydrogen gas must dissociate as well as get ionized. We can experiment with other gases too, like helium and neon, to learn more about the system.

We recently learned of this great article written on FRCs and the PFRC in particular: “Small-scale fusion tackles energy, space applications”. It was posted on the website for the Proceedings of the National Academy of Sciences (PNAS) in 2020.

https://www.pnas.org/doi/10.1073/pnas.1921779117

The article is well written and provides information on the PFRC innovation, fusion fuel choice, and development plan. It does a great job explaining the heating methods of the main FRC approaches in industry today: the RF-heated PFRC, the beam-heated TAE approach, and the merged-and-compressed Helion Energy approach. Dr. Sam Cohen, Stephen Dean, Michl Binderbauer (TAE), and Michael Paluszek are quoted.

Cohen, for his part, has been pursuing his Princeton Field Reversed Configuration (PFRC) design since 2002, with a strong emphasis on simplicity and compactness… The idea, says Cohen, is to drive oscillating currents through these coils in a way that sets up a rotating magnetic field inside the tube: a loop of flux that whirls through the plasma like a flipped coin and drags the plasma particles around and around the waist of the cylinder. In the process, he says, “the fields create, stabilize, and heat the FRC”—all in a single deft maneuver.

Small-scale fusion tackles energy, space applications, M. Mitchell Waldrop, January 28, 2020, 117 (4) 1824-1828

Read and enjoy!

We will be at the 2022 ARPA-E Summit in Denver, CO next week, May 23-25! PFS will have booths for both of our projects, WIDE BAND GAP SEMICONDUCTOR AMPLIFIERS FOR PLASMA HEATING AND CONTROL and Next-Generation PFRC. This post has links to the documents that we will have at our booth both physically and on the summit mobile app!

Wide Band Gap Amplifiers (GAMOW)

ARPA-E Innovation Summit in Denver, CO

Next-Generation PFRC (OPEN 2018)

The Space subcommittee of the Fusion Industry Association, of which we are a member, has prepared a new white paper recommending government funding for a fusion propulsion development program, styled similarly to ARPA-E and DARPA.

https://www.fusionindustryassociation.org/post/fia-proposes-funding-for-fusion-for-space-propulsion

The goal is to provide funding not just for “paper studies,” but enough funding to build real hardware and start to test fusion propulsion concepts. We want the US to remain competitive in the upcoming Deep Space Race – building a human presence on the Moon, and then Mars, and beyond.

The PFRC is directly applicable, configured as Direct Fusion Drive – a variable thrust, variable specific impulse rocket in the 1 to 10 MW range. With sufficient funding, we could build a PFRC-3 to test a fully superconducting configuration’s ability to achieve fusion-relevant plasma temperatures, and a separate propulsion testbed to develop the thrust augmentation system. This is the actual mechanism to transfer the energy from the fusion products to a rocket propellant – a fusion reactor is not a rocket until you have accelerated a propellant! For more on the Direct Fusion Drive, see our related videos: