Polywell Fusion

Combining Magnetic and Inertial Electrostatic Confinement Fusion Approaches

Polywell Fusion.

 Device Interior showing a vacuum chamber, 5 out of 6 cusp coils, and a modular support structures from the WB-8 Polywell test device.

The Polywell design combines key elements of fusor and magnetic cusp fusion devices for a highly efficient fusion reactor.

What is a Polywell Fusion Device?

Polywell was invented by the late Dr. Bussard in 1985. It is a unique approach to fusion that combines fusor or IEC (inertial electrostatic confinement, by Farnsworth, Hirsch, Elmore, Tuck, and Watson) with high beta magnetic cusp (by Grad and his team at New York University).

Polywell Fusion =  Polyhedral Magnetic Fields (electron confinement) + Potential Well (ion acceleration and enhanced ion confinement)

The Polywell fusion device operates with a set of electromagnets (e.g., Polyhedral coils in a cube or dodecahedron configuration) generating a cusp magnetic field that traps energetic plasma particles at a high fuel pressure. When electron beams are injected, they create a potential well (i.e., a negative voltage in the central region) that accelerates fuel ions to 100 million degrees or higher for fusion reactions. The potential well also enhances the magnetic confinement of ions in a symbiotic relation with the magnetic cusp.

Principles of high beta cusp confinement

  • Left Image: Vacuum magnetic field lines from a 6-coil cubical Polywell cusp device
  • Middle Image: Magnetic field lines at low plasma pressure (i.e., beta << 1). The plasma can leak through the seams of cusp magnetic fields and confinement is poor.
  • Right Image: Magnetic field lines at a high plasma pressure (i.e., a high beta state or beta = 1, where beta is the ratio between the plasma pressure and the magnetic field pressure with the theoretical maximum value of beta is 1 for confined plasma inside the magnetic field). As the internal magnetic fields are isolated from the outside region, electrons and ions cannot escape from the central region of a Polywell device. This is the principle of high-beta cusp confinement where the plasma confinement is achieved by the modification of the magnetic field structure due to the current induced by the plasma pressure.
  • This change in cusp magnetic field structure was originally theorized by Harold Grad and his team in the mathematics department at New York University in the 1950s, leading to active fusion energy programs based on magnetic cusps.

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In 2013, EMC2 became the third team that successfully mastered the high beta cusp start-up process and the first to demonstrate the confinement property of a high beta cusp device in experiments. EMC2 achieved this breakthrough by employing an innovative plasma start-up method with 700 MW of pulsed power. The results are published in a high-impact peer-reviewed journal (https://link.aps.org/doi/10.1103/PhysRevX.5.021024).

In 2019, EMC2 conducted first-ever first principles simulations resolving electron gyroradius scale to elucidate the scientific foundation of high beat cusp confinement utilizing ~ 200 million virtual particles. The results are published in a peer-review journal (https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2019.00074/full).

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Principles of fuel ion heating by potential well

To generate fusion reactions, fuel ions need to be accelerated to high energy to overcome repulsive electrical forces. In a fusor or IEC device, this can be achieved by physical grids or a virtual cathode (i.e. excess electrons in the device center as shown in the figure, also known as a potential well). In Polywell, electron beams are utilized to create a potential well inside the magnetic cusp that can heat fuel ions. This feat was achieved in 1995 and the results are published in a peer review journal (https://doi.org/10.1063/1.871103).

                                   

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Bussard's legacy: Polywell fusion = A symbiotic relationship between high beta cusp and potential well

Bussard’s remarkable innovation is recognizing the complementary nature of high beta cusp and potential well. To generate net fusion energy, energetic particles inside a fusion reactor must be confined at exceptionally high efficiencies. For example, the ITER fusion reactor is designed to operate with a plasma confinement time in excess of a few seconds, during which energetic electrons and ions will move around the donut-shaped ITER device for millions of times (an equivalent confinement efficiency of better than 99.9999%). The same is true for Polywell fusion devices. This remarkable efficiency can be achieved in Polywell ONLY WHEN

  1. a high beta condition is satisfied with the successful plasma start-up process,
  2. a potential well is formed by electron beam injection when the electron confinement efficiency is high (i.e. after the cusp plasma reaches a high beta state), and
  3. fuel ions are accelerated to 100millions of degree or higher by the potential well, while the potential well enhances the ion confinement of high beta magnetic cusp.

In recent years, EMC2 was finally able to test Bussard's vision of Polywell fusion in first-principles simulations by satisfying all three conditions simultaneously. Encouraged by the results, EMC2 looks forward to building and demonstrating a commercial prototype Polywell fusion device in the near future.

" But fusion works. All you have to do is go outside at night and look up. There are billions of fusion reactors. Every star is a fusion reactor, every single one of them, and not one of them is 'Toroidal'. "
Dr Robert Bussard - Google Tech Talk
Plasma Stability

Magnetic Cusp Confinement.

This approach to plasma confinement provides a uniquely stable enironment for the operation of continuous fusion devices.
Driver Efficiency

Electron Beam Heating.

E-beam heating (related to Farnsworth's electronic television) to drive a Polywell device relies on cost effective and mature technology
Compact Confinement

High Beta
Operation.

Fusion power output scales with the ^2 of plasma density (Beta). This allows a Polywell fusion reactor to produce 100 MW - 1 GW electricity output in a compact and economical device.
EMC2 has built and operated 20 Polywell test devices since 1985

The Activity in our Lab

1st vacuum vent operation of WB-8.

 Nick Krall & the late Mike Skillicorn (Chief Eng).

WB-11.

EMC2 will build the next Polywell device designed from first-principles simulations.

Magnetic cusp coils.

 3D nature of a Polywell device is an essential feature for high performance.

WB-8 & WB-X.

 Experimental data aided HPC code development & the design of next-generation Polywell devices.

Control Room Ops.

 Fully automated operation, including diagnostics, interlocks and remote safety features.

Modular Design.

  Polywell device utilizes a modular design approach for practical fusion devices.

Generation of Neutrons for use-cases in the Fusion Industry supply chain

High Flux
Neutron Source

- Irradiation damage on material degrades its properties.
- Helium and Hydrogeon bubbles cause swelling and mechanical property degradation.
- Materials are radio-activated after reactor shutdown.
Fusion Industry Supply Chain Component

Neutron Source.

 Required for a variety of uses in the emerging Fusion Industry supply chain.

In 2023, US DOE invited interested parties to provide technology solutions to meet the needs of the Fusion Energy Sciences (FES) program for a Fusion Prototypic Neutron Source (FPNS) and on potential ways to accelerate the construction and delivery of such a facility, including partnerships with the private sector.

A proposed FPNS, a Billion Dollar class facility, is considered "Absolutely Central" to the US fusion program as it will address the fundamental question of whether materials retain adequate properties and integrity when exposed to high levels of energetic neutron bombardment over an extended period inside a fusion power plant.

EMC2 has partnered with SHINE Technologies (a current leader in commercial neutron generators - https://www.shinefusion.com/blog/shine-phoenix-combine-for-world-record) to answer the call for action with a modified design of Polywell device that can deliver the requested neutron production performance for FPNS. Our joint proposal can be found here, https://www.regulations.gov/comment/DOE-HQ-2023-0038-0025.

"SHINE has evaluated various plasma target options and identified EMC2’s Polywell design, shown in Figure 1(b), as the best choice for the following reasons: The Polywell is capable of operating at high density within a compact size due to its utilization of a magnetic cusp configuration,a well-established system known for its ability to provide plasma stability. "

The Poywell neutron generator operates differently from its cousin - a Polywell fusion power reactor. Instead of focusing on high fusion efficiency, its design is optimized for a compact size (about the size of a basketball for its reactor core) to maximize the neutron flux onto a test sample volume.  A compact size beam-driven fusion device can produce sufficient neutron flux at a modest temperature and confinement time (~0.5 keV or 50 million degree temperature, a few milliseconds of confinement time for target plasma, and ~70 microseconds of beam ion confinement) corresponding to a fusion efficiency of 0.1 (500 kW of fusion power vs. 5MW of input power). In comparison, a fusion power plant would require a high plasma temperature and long confinement time (over 10 keV and ~1s or longer) corresponding to a fusion efficiency of 10 (500 MW of fusion power vs. 50 MW of input power).

EMC2 look forward to deliver a much needed technology solution to deliver a neutron generator for FPNS and is currently working with the US DOE for their evaluation of proposed FPNS technology solutions with the anticipated due data of March 2025 for the report.

Leveraging High-Performance Computing (HPC) Capabilities

Accelerate Polywell R&D by leveraging HPC capabilities

First principles simulations using Particle-in-Cell (PIC) code:

High Performance Computing simulation of fusion devices is the key to rapid development.

EMC2 is one of the oldest private fusion companies in the world. Since 1985, it has built and tested 20 Polywell devices to advance Polywell fusion technology. Despite significant knowledge and progress from these experiments, the pace of Polywell R&D proved too slow and a new direction was needed.

Since 2017, EMC2 has recognized the potential of high-performance computing (HPC) to accelerate the development of Polywell technology. In collaboration with the late Professor Giovanni Lapenta at KU Leuven, EMC2 developed a customized version of the ECsim Particle-in-Cell (PIC) code. This code enables first-principles simulations of Polywell fusion devices using data from past test devices. Developed by Professor Lapenta, the ECsim code is the first semi-implicit PIC code to enforce energy conservation with a precision of over 1x10^-15(https://doi.org/10.1016/j.jcp.2017.01.002).

ECsim PiC can utilize more than 48,000 processors simultaneously, a requirement for handling billions of virtual particles and simulating fusion devices with high fidelity. These capabilities are crucial for modeling a Polywell fusion reactor, with the needed plasma confinement efficiency exceeding 99.9999% for achieving fusion energy generation.

EMC2 has developed designs for a next-generation test device incorporating all three critical elements of the Polywell fusion approach: high beta cusp start-up, potential well formation leading to fuel ion heating, and symbiotic plasma confinement. Additionally, EMC2 has created a modified Polywell device designed for a compact high-flux neutron generator for the Fusion Prototpyic Neutron Source (FPNS) program to support the fusion industry supply chain.

EMC2 believes that the combination of advanced plasma simulation codes and a focused experimental R&D program offers the best path forward for achieving fusion energy in a timely and cost-effective manner.We are optimistic that the end of our quest for Polywell fusion energy that began in 1985 is within striking distance.
ECsim simulation results demonstrating the formation of the high beta cusp or 'wiffleball' confinement in a polywell fusion device. Initially, ions can escape along the magnetic field lines. As plasma pressure builds inside the device, the exit avenues via the magnetic field lines become sealed. This provides exceptional plasma confinement at high 'Beta'.