OpenStar – Levitated Dipole

From 2004-2011 the Levitating Dipole Experiment (LDX) operated at MIT, using a ~500kg NbSn levitated superconducting magnet. The superconductor is cooled to 4.5K by a pool of liquid helium, which is boiled into a surrounding inconel pressure vessel. Liquid nitrogen provides intermediate cooling and radiation shielding. This enabled a float-time of 8 hours, followed by a 30 minute cooldown period.

OpenStar are currently in the process of comissioning a similar device, using a magnet just shy of 500kg, within a 5.2m vacuum chamber (compared to 5m for LDX). OpenStar’s device will be using high temperature superconductors, kept at a temperature of 40-60K. For reference REBCO superconductors have a transition temperature around 90K, although the maximum permissible current increasing significantly with reducing temperature.

Below are some slides showing the design of LDX taken from Garnier, 1999.

A D-He3 Fusion Reactor Based on a Dipole Magnetic Field

The following was taken from Hasegawa et al., 1990, as an example of a conceptual power producing dipole reactor. The design calls for a 20MA magnet suspended within a 24m radius vacuum vessel, generating 70MW of fusion power from a De-He3 reaction.

*Beta refers to the ratio of plasma pressure to magnetic pressure. In effect it is the ‘leverage’ that the magnetic field can exert on the plasma, and is a useful indicator of plasma density.

*ω_pe/ω_ce is the ratio of the electron plasma frequency (oscillation frequency of electrons) to the electron gyrofrequency (frequency of circular motion of ions around a magnetic field line). The latter depends on field strength, and the former, to my understanding, on electron density within the plasma.

Below are the descriptions from Openstar’s website, reproduced here for the sake of easier reading.

Company Timeline

30 Years Away – Until in 2004 a team at MIT sparked a plasma around the Levitated Dipole Experiment. LDX started the countdown to real, useful, power-positive fusion. Even if no one could see it at the time.

6 Years Away – OpenStar was founded to build a levitated dipole reactor using the new technology that has developed in the 20 years since LDX. By the end of 2023 OpenStar will spark a plasma around our first Marsden Class device constructed using High Temperature Superconductor, the critical step to show LDRs can be scaled to produce fusion energy.

3 Years Away – Once HTS has been used to build dipoles, OpenStar will build the key experiments to show that fusion is in reach. Our second class of device, Rutherford, will produce the fusion reactions to probe the remaining physical unknowns. Meanwhile, rapidly deployed Marsden class devices increase broad understanding of dipole physics.

0 Years Away – Our first power-positive Hasegawa Class devices come online and begin providing useful power to the grid, eliminating the need for fossil fuels and improving reliability of power for industry. Hasegawa will be just the beginning, as we scale and iterate to drive the world into a new age of energy abundance.

Better Physics

Turbulent Pinch – The superiority of dipole plasmas lies in a phenomenon called the turbulent pinch, whereby convective cells drive peaked pressures, rather than dispersing them. This leads to scalable physics, with fewer things to go wrong or control.

Natural Stability – The stability of dipole plasma is based on the most fundamental principle of physics: entropy. As the energy in the plasma increases, so does the entropy. Unlike other fusion concepts where entropy destabilises the plasma, the turbulent pinch in dipole plasmas keeps them stable as they are heated to fusion conditions.

Beautiful Simplicity – Turbulent pinch is beautifully simple. As a result, the plasmas are easy to control and be confined by the superconducting magnets. This reduces the need for complexity, even as they are heated and scaled to fusion temperatures.

Strong, Effective Magnets – Turbulent pinch gives rise to plasma pressures far in excess of the magnetic pressures being used to confine them. This surprising property makes achieving fusion pressures easier, and possible with smaller magnets. These reactors need less superconducting wire, resulting in cheaper magnets. This not only reduces the cost of each reactor, but dramatically improves how many reactors can be deployed per year in the race to decarbonise the grid and combat climate change.

Rapidly Mixing Fuel – Fusion reactors are not magic, and like anything that burns fuel, their performance boils down to three questions:

• Can you get exhaust out before it chokes the reaction?
• Can you get the right fuel mix?
• Does the fuel get hot and stay hot?

The dipole is the only magnetoconfinement approach that holds onto heat for longer than it holds onto ash. This critical and often overlooked property means dipoles can be designed to burn advanced fuels. This is a critical step in solving the neutronics problem in fusion.

Better Engineering

Modular Reactors – Unlike other concepts, the levitated dipole has loosely coupled architecture. The magnets, the chamber, and the heating system are easy to seperate, giving rise to a number of major engineering advantages.

Faster Iterations – Because the dipole magnet is not integrated with the vacuum vessel, we are able to rapidly develop and test different magnet designs without having to build a new reactor for each iteration. This allows for a rapid iteration cycle for magnet designs enabling an accelerated R&D process and reducing our time to market.

Parallel Development – The modular nature of our magnets, vacuum vessels, and heating and diagnostics systems allows them to be designed and tested in parallel, mixing and matching the technologies as we see fit. This parallel development of our key technologies allows us to further accelerate our R&D process.

Simple Assembly and Maintenance – If any one of the individual coils making up the magnet has a fault or quenches, we can quickly and easily replace it with a new coil, minimising the downtime of the magnet and reducing the costs of malfunctions.

Cheaper Reactors – Dipole reactors have a strong power scaling law between their power output and the reactor size. Due to the decoupling of our reactors and their magnets, the reactors can be made large and powerful whilst still using small amounts of expensive superconducting wire. The major costs then scale like civil infrastructure, leading to cheaper reactors.

Better Problems

Dipole plasmas are inherently stable with no disruptions, and the risk that nature will throw some unexpected result at us is low. Instead, we have a reactor with interesting engineering problems to solve. We have achieved breakthroughs on many of our engineering problems, but still have a green field of problems worth tackling.

Unlike many other concepts, the core components of a dipole reactor; the magnets, the chamber, and the heating system; are all decoupled from each other. This gives rise to a number of major engineering advantages over other fusion concepts.

Resources

https://www.openstar.tech/

NZ Company Register

Hasegawa, A., Chen, L., & Mauel, M. E. (1990). A D- ³ He fusion reactor based on a dipole magnetic field. Nuclear Fusion, 30(11), 2405–2413. https://doi.org/10.1088/0029-5515/30/11/018

D. Garnier and M. Mauel, L. Bromberg and J. Kesner, & J. M. Dawson. (1998). The Dipole Fusion Confinement Concept: A White Paper for the Fusion Community. https://dspace.mit.edu/bitstream/handle/1721.1/93713/98rr005_full.pdf?sequence=1

Garnier, D. (1998). Overview of the Levitated Dipole Experiment.

https://www-internal.psfc.mit.edu/ldx/pubs/presents/dpp99_garnier.pdf

https://en.wikipedia.org/wiki/Levitated_Dipole_Experiment

http://sites.apam.columbia.edu/CTX/index.html

Device and Data. (n.d.). Retrieved September 4, 2023, from http://sites.apam.columbia.edu/CTX/ctx_device.html

Levitated Dipole Experiment. (n.d.). Retrieved September 4, 2023, from https://www-internal.psfc.mit.edu/ldx/

Other References