With abundant technical arguments, this website has concluded that the mainstream approach to commercial fusion reactors will likely fail. Alternative fusion techniques have existed for several decades but until recently the primary focus of R&D funding has been on this mainstream approach of magnetic confinement fusion with tokamaks and D-T fuel. Now, there is great revived interest in alternatives, likely because many entrepreneurs have long ago arrived at similar conclusions. 


Fusion companies based on private finance

More than 30 start-up fusion companies, with a miscellany of ideas, have appeared in the last decade and Fusion Companies Survey by the Fusion Industry Association (FIA) provides details.

Private fusion companies (taken from FIA survey)

The Fusion Industry Association survey describes 33 companies which have received a total funding of $4.9 billion, of which 98% is from private sources. The HQ of these companies are mostly in the USA (21/33) with 3 in the UK and 3 in the EU.

In this survey, 7 companies ("big-seven") have funding >$200 million by 2002 and, in sum, received funding of $4.5 billion, 93% of the total.

Commonwealth Fusion SystemsCompact TokamakD-T
TAE TechnologiesField Reversed Config.p-11B
HelionMagneto-inertialD-3He
General FusionMagneto-inertialD-T
Tokamak EnergySpherical TokamakD-T
Zap Energy Sheared flow Z-PinchD-T
ENN Spherical Tokamakp-11B

Of these seven significant sized companies, four plan to use the same fusion reaction of D-T as the mainstream approach and three plan on using alternative fuels, two p-B which avoids the tritium supply problem and one using D-3He. Among the seven, three are using compact Tokamaks with High Temperature Superconductors for the magnets, while four have very different techniques. 

A common feature of the private fusion companies, as reported in the FIA survey, is an incredibly optimistic timescale The survey introduction says: "... the vast majority of companies predicting fusion will first power the grid at some point in the 2030s ...". All the big-seven companies quote such a timescale independent of technology or present status. These claims do not seem to be based on any reality. David Jassby in his articles: Fusion Frenzy and Voodoo Fusion Energy expresses sceptical views on these timescales based on their history. For instance, in 2011 TAE were saying they would produce a working commercial reactor between 2015 and 2020 while now the prediction is 2030s. Similarly in 2012, General Fusion was claiming there would be a working reactor by 2020, and now the claim is 2030. Helion was in 2014 advertising profitable fusion energy in 2019 and now their statement is the end of 2020s.


Alternative Fuels

Given the tritium situation described on the page, Impossibility of Tritium Fuel Supply, it is not surprizing that 11/33 of the new fusion companies are exploring alternative fuels to Deuterium-Tritium. 

The book: The Future Of Fusion Energy by J. Parisi and J. Ball,  presents an important discussion on the difficulty of the alternative fusion reactions p-11B, D-D and D-3He. Their discussion is based on the increased energy loss from bremsstrahlung (breaking radiation) in these reactions. The temperature required for these alternative reactions is much higher than for the D-T reaction and this leads to higher losses from bremsstrahlung. A further, even bigger problem for p-11B, is that the boron atom has five electrons which add to the plasma for only one fusion nucleus giving even more bremsstrahlung. The table below shows the calculations of the ratio of fusion power to losses from bremsstrahlung (pfusion/pbrem) indicating that the alternatives have much bigger power losses than D-T and that p-11B even has a net loss of power.

Conditions of fusion for different reactions. (Ion and electron temperatures, cross-sections and fusion power compared to losses)


Magnetic confinement fusion

Among the new fusion companies 15/33 are based on magnetic confinement fusion (MCF), with three of the big-seven companies proposing MCF approaches based on the use of High Temperature Superconductors (HTS). These companies have projects with compact conventional tokamaks, spherical tokamaks and stellarators. The innovations in these new developments address the problems of mainstream fusion discussed in the pages: Maintenance, Breakdowns and Availability and Pharaonic Size and Byzantine Complexity, but not others because the projects use D-T fuel. 

Compact Tokamaks

Commonwealth Fusion Systems (CFS), which is the largest private fusion company with $2+ billion funding and 300+ employees although it was only formed in 2018, is building a compact Tokamak using HTS. Their approach benefits from the experience of mainstream fusion, adding innovations which could well solve many of the difficulties. 

The design is described in the paper: ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets. The Affordable, Robust, Compact (ARC) reactor will be a tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, an on-axis magnetic field of 9.2 T and will produce about 200 MW of electricity. The machine will have rare earth barium copper oxide (REBCO) superconducting toroidal field coils, which have joints to enable disassembly. It will use D-T fusion and so retains the fuel supply problem as well as the radiation damage problems.  ARC will be about half the physical size of ITER but with comparable fusion power. A further innovation of ARC compared to ITER, is the breeder blanket which will be outside the vacuum vessel. The fully liquid blanket is a molten salt called FLiBe where the liquid is a continuously recycled, offering significant improvement on the tritium breeder ratio compared to other designs.

The CFS: Affordable, Robust, Compact (ARC) reactor 

The joints in the magnet allow a vastly easier maintenance and component replacement regime compared to the ITER/DEMO plans. This innovation, combined with the smaller size could enable a much faster and less costly development cycle.

Exploded view of ARC, illustrating the innovative assembly and disassembly scheme, which enables easy component replacement and maintenance

CFS plans for a first smaller reactor, SPARC, operational in 2025 and the full ARC in the early 2030s. The use of D-T fuel means that ARC will have initial tritium supply problems, as all other fusion reactors operating after 2030, although this is denied in the CFS publicity.

Spherical Tokamaks

The Spherical Tokamak (ST) concept has been studied for many decades but now has revived interest with two of the big-seven private fusion companies: Tokamak Energy in the UK and ENN in China, as well as the publicly funded STEP project in the UK. The figure below illustrates the different plasma aspect ratio of an ST compared to a conventional tokamak (CT).

Tokamak Energy (TE) was launched in 2009 and now has $0.25 billion funding and 190 employees. TE has built a desktop ST prototype, ST25, which operated in 2012. Another prototype ST40, of 1m scale, operated in early 2022 and now is being upgraded. Despite minimal available details on designs and results Tokamak Energy advertises a pilot power plant in the early 2030s.

The interest of spherical tokamaks is that the modified plasma aspect ratio gives improved plasma confinement parameters compared to that of a conventional tokamak. Unfortunately, the changed aspect ratio presents engineering difficulties for the ST because of the smaller central bore diameter which leads to several practical problems. In a conventional tokamak, the central bore contains: neutron shielding, breeder blanket, the toroidal magnet coils and the central solenoid. A spherical tokamak design must either eliminate these elements or make them much thinner, hence the ST programs contain R&D programs to do this in various ways.

Stellarators

The stellarator magnetic confinement concept was invented in 1951 before the tokamak concept. The stellarator has very complex shaped field coils which allows the stellarator to avoid the electric current of the tokamak. The lack of toroidal symmetry of the stellarator has various disadvantages over the tokamak but the stellarator has many advantages with better plasma stability. In practice, as shown in the figure below, stellarators have achieved longer pulse durations than tokamaks.

The triple product vs. duration of plasma discharge for various projects. Circles: tokamaks, Crosses Stellarators

As well as the operational W7-X stellarator in Germany, 4 of the private fusion companies use the stellarator concept, the biggest being Renaissance Fusion in France.


Inertial Confinement Fusion

Inertial confinement fusion (ICF) has been pursued in parallel with MCF since the 1950s and since their invention in the 1960s, lasers have been used for this. Most of the funding for inertial fusion comes from defence budgets because of the close relationship to weapons technologies. The main ICF activities are now at the National Ignition Facility (NIF) at the Lawrence Livermore Laboratory in the USA. In France the Laser Mégajoule facility is part of the defence division of the CEA. Eight of the smaller private fusion companies now have inertial confinement activities.

Schematic of Laser Mégajoule facility in France

There has been much recent interest in inertial confinement fusion because of a success at NIF in 2021, Burning plasma achieved in inertial fusion. The figure below shows the "indirect-drive" technique pursued for this experiment. 

Schematic of the indirect-drive inertial confinement approach to fusion. Laser beams enter hohlraum through entrance holes. At centre of hohlraum, capsule is bathed in X-rays, which ablates outer surface of capsule. The pressure generated drives the capsule inward upon itself, compressing and heating fusion fuel. 

This paper, and several others, celebrate a single inertial fusion shot that produced 1.3 MJ of energy on 8 Aug 2021 which was eight times the previous record. Although a reason for optimism, a commercial power plant with this technique is a very long way off. ICF can operate about one fusion shot per day while a power plant would need about 10 shots per second to be economic.


Magneto-inertial fusion

The magneto-inertial fusion technique uses both magnetic confinement and inertial implosion to achieve fusion. It is employed by two of the big-seven fusion companies: General Fusion and Helion.

General Fusion (GF) is one of the older private fusion companies, formed in 2002, with funding of $300+ million and 200+ employees. Their method is very complicated, with a Magnetized Target technique which is between MCF and ICF in method as well as in plasma density and in confinement time, as shown in the table below from the book of Parisi & Ball.

The review paper, Alternatives to tokamaks: a faster-better-cheaper route to fusion energy?,  by Daniel Clery explains the GF method. The machine has a first part consisting of a plasma injector that forms Field Reversed Configurations (FRC) and fires them out at speed into the centre of the second part, a spherical reaction chamber. The chamber contains some liquid lithium which is spun up to coat the inner wall. This serves several purposes: it absorbs the high-energy neutrons produced in fusion reactions, preventing them from damaging the reactor structure; it absorbs the energy of the neutrons and carries it out to be harvested and converted into electricity; and the neutrons convert some of the lithium into tritium, which can be collected and used as fuel for the reaction. The third part of the system provides the compression which implodes the FRC once it reaches the centre of the chamber. It is comprised of up to 400 pneumatic pistons positioned all round the reaction chamber like spines on a sea urchin. These slam down in unison to produce a converging shock wave in the chamber centred on the FRC. As with ICF this technique has a long progression in performance. At present the GF prototypes will be able to perform around one shot per day but a working power plant would need to perform one per second. Nevertheless the company advertise commercial fusion in about 10 years.

Field-reversed configuration. Ring of plasma (blue) which rotates (red arrow) creates magnetic field (green) which helps to confine it.

General Fusion's reactor showing the multiple mechanical pistons to compress the central chamber.