Bright RMF pulses at 1.8 MHz

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.

RMF pulse at 1.8 MHz with Argon

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.

Great article from National Academy of Sciences on PFRC

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.

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!

New FRC Journal Paper is an Editor’s Pick at Physics of Plasmas

PFRC inventor Dr. Sam Cohen and his student Taosif Ahsan have published a new journal paper, “An analytical approach to evaluating magnetic-field closure and topological changes in FRC devices,” in Physics of Plasmas (Phys. Plasmas 29, 072507 (2022)). The paper is an Editor’s Pick and has important implications for confining plasma in Field-Reversed Configurations (FRCs).

We describe mathematical methods based on optimizing a modified non-linear flux function (MFF) to evaluate whether odd-parity perturbations affect the local closure of magnetic field lines in field-reversed configurations. Using the MFF methodology, quantitative formulas are derived that provide the shift of the field minimum and the threshold for field-line opening, a discontinuous change in field topology.

Paper Abstract

This paper follows up on a 2000 paper by Cohen and Milroy, which made qualitative assertions about changes in magnetic field topology, e.g., movement of the center of separatrix, separator line, and other geometric parameters. Ahsan and Cohen developed the modified flux function (MFF) mathematical tool to quantitatively understand the effects of perturbations on a Solov’ev FRC field structure.  The analytical results from this function have reproduced the previous numerical observation that small odd-parity perturbation preserves FRC field structure. In particular, the contours around the equilibrium stay closed.

Closure of magnetic field lines limits plasma losses that would occur due to charged particles leaving the FRC by traveling along open field lines. The paper points out that in a reactor-scale FRC where ions have a large gyroradius relative to the field structure, but electrons have a small radius and follow the field lines, particle and energy losses on the open field lines outside the FRC will be significant. Hence, ensuring closure of field lines is a crucial step toward improved plasma confinement in FRCs.

3D contours of a perturbed FRC using the modified flux function (MFF)

ARPA-E 2022 Summit

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)

Next-Generation PFRC (OPEN 2018)

PFRC video including animation of how it works
7-minute tech demo video of PFRC-2 experiment from 2021 Virtual Summit

Posters Presented at 2021 APS Division of Plasma Physics

Our team presented a number of posters at the 63rd Annual Meeting of the APS Division of Plasma Physics, representing work supported by our ARPA-E OPEN contract and other supporting programs.

Magnetic Fusion Energy Session

Inferring electron temperature in warm hydrogen plasmas from Balmer series spectral line ratios using a collisional radiative model, Sangeeta Vinoth,

Undergraduate research

Inferring electron temperature using the collision radiative model, plasma radius = 5 cm

Modeling Spatially Resolved Neutral Atom Densities in the PFRC-2 Using DEGAS 2, Catherine Biava:

Electrostatic Energy Analyzer and Gas Stripping Cell to Measure Ion Temperature in the PFRC-2, Matthew Notis:

Consideration of Vacuum Vessel Properties Required for PFRC-type Fusion Reactors, Miles Kim,

The pulse-pile-up tail artifact in pulse-height spectra, Taosif Ahsan,

Collaborator Research

Overview of TriForce: Projects, Progress, and Plans, Adam Sefkow,

Integration of a portable spectroscopy system on the PFRC-2 device, Drew Elliott,

Kinetic simulations of the PFRC-2 using the VPIC code, Mehmet Demir,

FIA Proposes Funding Fusion for Space Propulsion

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.

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:


As a follow-up to the TriAgency workshop on Compact Fusion which took place on April 28, PFS was invited to join several Fusion Industry Association members on an AIAA ASCENDx summit on June 15, “Accelerating Pathways to Space”:

Our panel on “New Opportunities in Fusion for Space Power and Propulsion” was moderated by Julie Reiss of Aerospace Corp and included us, Helicity Space, NearStar Fusion, and Tokamak Energy. You can register to rewatch our panel discussion anytime!

A key takeaway from the TriAgency workshop was that investment in compact fusion is strategic for both space and defense applications. NASA’s Ron Litchford was quoted as saying:

Compact fusion stands as a well deserving candidate for an aggressive whole-of-government R&D initiative.

Ron Litchford, Principal Technologist of NASA’s Game Changing Development Program, April 2021

We appreciated the opportunity to participate in the panel and will continue to advocate for more investment in compact fusion!

Thomson scattering

Thomson scattering is the industry standard diagnostic for measuring electron temperature, typically requiring, large, expensive, custom lasers. We are fortunate that ARPA-E has funded a team from Oakridge National Laboratory to build a portable Thomson system from commercial parts, and they are now installing and calibrating the diagnostic on the PFRC-2!

These measurements will give us critical insight on the plasma temperatures we are achieving with RMFo as a function of input power!

Portable Diagnostic: from ORNL to PPPL

Our team is mentioned in this press release from ORNL about the “traveling” high-temperature plasma diagnostic they are building:

The ORNL diagnostic team
The ORNL diagnostic team

ARPA-E is supporting the development of several such portable diagnostics in tandem with their other fusion efforts, including our OPEN project. The ORNL team hopes that their “suitcase Thomson scattering” diagnostic will be on its way to us this summer!

When installed, it will measure the electron temperature and density profiles in the PFRC-2 experiment as a function of radius, as often as once per millisecond. The profiles measured will allow us to probe the internal structure of the plasma, but beyond that will also allow us to better interpret the results of our other diagnostics!

Princeton Propeller Talk

Princeton Propeller is a series of talks supported by the Princeton Area Alumni Association (PA3) to showcase technical innovation. Our team was recently invited to give a talk on the new frontier of commercial fusion development! Dr. Swanson and Ms. Thomas took the podium on February 11, 2020, at the Quadrangle Club on Princeton’s campus.

Download our slides at this link:
“Frontiers in Commercializing Fusion Development” Slides

We are now joined by over a dozen private companies in the UK and the US pursuing commercial fusion development. Recent programs in the Department of Energy and their ARPA-E advanced projects division have supported teams in magneto-inertial confinement (ALPHA), compact tokamaks, pinches, and our PFRC (OPEN), and public-private partnerships between labs and the private companies (INFUSE). It’s a great time to be in fusion research!