Figure courtesy IPP Garching
In early 1951, Argentine President Juan Perón made a startling claim that a group of scientists under his patronage and headed by the Austrian scientist Ronald Richter had achieved controlled thermonuclear Fusion in their laboratory (1). This turned out to be a big hoax. Nevertheless, the goal of replicating the thermonuclear fusion process that happens in the Sun in the laboratory was being pursued in a veil of secrecy in the 1950s.
Many ideas bloomed in this period. Lyman Spitzer in Princeton, leading the secret “project Matterhorn” conceptualized the Stellarator — a torus twisted into a figure-eight shape — creating a twisted magnetic field meant to get rid of the violent magnetohydrodynamic instabilities which lead to plasma loss. Scientists at the Lawrence Livermore National Laboratory built a “magnetic mirror” device with an axial magnetic field peaking at both ends to form a magnetic bottle containing plasma. British scientists in Harwell invented the “pinch” device to compress the plasma by rapidly rising currents. Los Alamos Laboratory pursued the “Theta Pinch”, where a rapidly rising magnetic field compresses the plasma. “Astron”, evocative of stars, was a magnetic trap created by currents from intense particle beams.
“Pinches, mirrors, torii, traps so diverse, pellets of ice to be lit by lasers; fusion in bubbles and alchemist’s jars chasing the dreams that remind you of stars”
Soviet scientists Andrei Sakharov and Igor Tamm realized the twisting magnetic field by combining the poloidal magnetic field produced by a current through the plasma in a doughnut-shaped machine with an external toroidal magnetic field. This was the tokamak (a prosaic name in Russian meaning a chamber with a magnetic field).
The ‘Atoms for Peace’ conference in Geneva in 1955, was a lament of failures in producing hot plasma for Fusion. Secrecy was found to be futile in a game where nobody was winning. Following this, fusion research was declassified during the Second Geneva Conference on the Peaceful Uses of Atomic Energy held in 1958 (2).
A decade later, at the 1968 Novosibirsk fusion conference, Lev Artsimovich’s talk on reaching high densities and plasma temperatures exceeding 1 Million degrees in the tokamaks T-3 and TM-3, higher than those realized in other machines, was heard in scepticism. A Culham team confirmed these claims using laser scattering diagnostics and reinvigorated hopes in the fusion world. Convinced that the tokamak was the way to fusion nirvana, the fusion-faring nations started a frenzy of building tokamaks. The continuing success of tokamaks culminated in a plan for ITER, a large enough machine to demonstrate Fusion’s feasibility. The International Tokamak Experimental Reactor is under construction in France under a seven-nation collaboration.
Cost, complexity, and delays have been the bane of inertial-confinement Fusion as well, the leading alternative to magnetic confinement. In this approach, high-power laser beams implode frozen fuel pellets. Unfortunately, even after years of effort and colossal funding, scientists at the National Ignition Facility at Lawrence Livermore National Laboratory in Livermore, California, have not delivered on their promises (3).
Many plasma physicists believe that ITER is too complex and unwieldy to become a reactor and want to pursue alternative options. These had lost out due to the tokamak frenzy. The fusion mavericks argue for simpler solutions: adopt sound engineering for a simple, cheap reactor that will be attractive to utility companies. Over the past fifteen years, many companies have sprung up to pursue this path and seem to attract venture finance.
The mavericks include Government Laboratories. Stellarator, using external magnetic fields for both the toroidal and the poloidal fields, is a significant alternative to tokamaks, though demanding extreme precision in its engineering realization. It is a steady-state device without the need for internal currents. Recent experiments in the Wendelstein 7X experiment at the Institute fur Plasma Research at Garching, Germany, have achieved 100 Million degree temperatures and record confinement times. UKAEA has launched the STEP project (Spherical Tokamak for Energy Production) that aims to create a compact prototype that would deliver real power to the national grid by 2040 (3).
Another approach employs a plasma avatar called a compact toroid (CT), particularly a field-reversed configuration (FRC). First observed in the 1950s, this entity involves a plasma configuration similar to a smoke ring. The internal current forms a self-magnetic field which confines it, inhibiting plasma diffusion. Despite lasting only for a few microseconds, they were intriguing because they seemed to provide an elegant means to confine the plasma. As a result, some start-up companies have adopted the FRC approach.
General Fusion aims at Magnetised Target Fusion, a compromise between the magnetic confinement and inertial confinement fusion. The former requires cryogenic high field magnets, while the latter depends on the shock waves generated by energy-guzzling lasers. The centrifugal expansion of a spinning vortex of liquid lead and Lithium creates a cavity at the centre, where a magnetized plasma is created. This will then be compressed inwards by an array of 400 pneumatic pistons. If this happens within a few microseconds, the plasma can reach fusion conditions. The Lithium wall breeds Tritium by neutron bombardment. A small prototype with pistons activated by explosives has verified the idea, and GF has raised about $50 million from VCs and the Canadian government. They plan to scale up the system to meet the compression levels needed for fusion (4).
Polywell is a 3-dimensional magnetic cusp promoted by San Diego based EMC2. Six electromagnets, arranged on the sides of a cube inside a vacuum chamber, create a magnetic field, zero at the centre and increasing outwards. Electrons are injected into the centre, forming a virtual cathode with a steep potential gradient between the centre and the outside (5). This gradient accelerates ions injected into the cube to speeds necessary for Fusion when they converge at the centre.
Tokamak Energy, though employing the tokamak concept, approaches the technology in a novel manner. They are building a spherical tokamak, a very compact version of the conventional tokamak. Its diameter of 3.5 metres is one-fourth of ITER. The magnets are wound with ribbons of high-temperature superconducting (HTS) material. This produces magnetic fields more intense than those produced by conventional superconducting magnets used by ITER. In addition, the HTS magnet can be cooled by liquid nitrogen, unlike the liquid Helium cooling required for the ITER magnets.
FirstLight Fusion, spun off from the Oxford University is pursuing an inertial fusion variant (3). A free-falling fuel target is compressed by being hit by a projectile fired by an electromagnetic rail gun at enormous speed. The required speed to achieve Fusion is 50 kilometres per second — twice that achieved in current experiments. The unique target design creates converging shockwaves that create momentary pressure levels nearly a billion times the atmospheric air pressure. After a thermonuclear burn, the plasma will spread out and dissipate its energy.
An even more radical idea is to pursue neutron-less Fusion. D-T is preferred because it needs only 100 Million degrees for ignites. But 80% of the fusion energy resides in the fusion neutrons. They can damage the steel vessels and make them highly radioactive. Energy generation through converting neutron energy into heat and running a conventional steam cycle has low (30–40%) efficiency.
Prof Norman Rostoker from the University of California, Irvine, co-founded Tri Alpha in 1998 to pursue proton-Boron 11 fusion (4). Igniting this fuel would require temperatures of about a billion kelvin, ten times that needed for ITER. However, the reaction products would have no neutrons: just three α-particles. Their charge could be extracted as electricity with 90% efficiency.
Tri Alpha Energy morphed into TAE Technologies. FRCs are fired from both ends of a linear device towards the mid-region with a magnetic field at nearly 1 million kilometres per second. When they merge to form a cigar-shaped FRC about 3 m long and 40 cm across, their kinetic energy is converted into heat. The plasma is held there by magnetic fields while tangential beams of boron beam inject angular momentum into the FRC to keep it spinning, stabilizing and healing. Proton beams provide additional heating from four particle accelerators bombarding the plasma. TAE collaborates with Google to develop AI-based control systems to keep the plasma in place.
Helion Energy in Redmond, Washington is developing a linear reactor that would fire a steady stream of plasmoids from both sides into a chamber, where magnetic fields crush them to fusion conditions. The fusion products are swept away just as the next plasmoid-pair arrives. This is the plasma version of the diesel engine, where, on each stroke, the injected fuel is compressed to ignition by the piston, and the explosion pushes back the piston. When the target plasma undergoes fusion burn, it will expand, pushing against the confining magnetic field and compressing it. The changing magnetic flux induces a current in solenoids wound around the reactor. This approach does away with the neutron-based steam cycle to generate power. The reactor, called Polaris, is expected to start working by 2024.
Another contender in the radical arena is the dense plasma focus (DPF) invented in the 1960s. DPF has two coaxial electrodes inside a vacuum vessel filled with a fusile gas. A high voltage pulse across the electrodes ionises the gas-forming tiny plasma filaments. Electromagnetic forces due to the discharge current drive the plasma towards the end of the electrodes where the filaments merge into a tiny plasmoid. The collapse of the magnetic field generates beams of electrons and ions, which heat the plasmoid to fusion temperatures. The plasmoid only lasts for 10 billionths of a second but achieves a plasma density close to that of a solid. A company, Lawrenceville Plasma Physics in Middlesex, New Jersey, recently reported an ion temperature of nearly three billion degrees and is aiming for hydrogen-boron Fusion.
Reaching for the stars is a metaphor for achieving the impossible. The Fusion Mavericks in their quest to bring starfire on earth in their magnificent machines are fulfilling an atavistic human urge to kindle the very fires which gave birth to the elements which made us.
1. Max Schulz, The Fusion Illusion, https://www.thenewatlantis.com/publications/the-fusion-illusion 2. ITER: The Giant Fusion Reactor, Michael Claessens, 2020, Springer ISBN 978–3–030–27580–8 3. The Chase for Fusion Energy, https://www.nature.com/immersive/d41586-021-03401-w/index.html 4. Plasma physics: The fusion upstarts: M. Mitchell Waldrop, Nature volume 511, pages 398–400 (2014) 5. Alternatives to tokamaks: a faster-better-cheaper route to fusion energy?
Daniel Clery https://royalsocietypublishing.org/doi/10.1098/rsta.2017.0431 04 February 2019