Compact spherical tokamak would be 100 times smaller than ITER and has a chance to start operating decades earlier

Next Big Future May 31, 2016

Startup company Tokamak Energy has published three papers showing size is not an important factor in fusion reactors and proving that a compact spherical tokamak reactor can produce high power. This turns the pursuit of fusion into a series of engineering challenges. The Tokamak Energy plan will overcome these challenges, such as the development of magnets made from high temperature superconductors, delivering a fusion power gain within five years, first electricity within ten years and a 100 MWe power plant within 15 years.

The best-performing tokamak in the world is JET, producing 16 MW of fusion power with 24 MW input in 1997 – i.e. 65% as much energy out as was put in. It holds the world record for total fusion power produced and for getting closest to breakeven. To reach this point, fusion research followed a Moore’s law-like path. The temperature, density and energy confinement time, which indicates fusion performance, was increasing at a faster and faster rate up until the JET experiments.

But since then it seems that progress has stalled. There have still been experiments built and much learned, but progress towards energy breakeven has slowed. We still haven’t actually reached energy breakeven almost 20 years after we nearly got there.

Traditional designs have moved to larger dimensions, culminating in the ITER experiment currently under construction in the south of France. This will be over 30m tall and weigh about 23,000 tonnes. The demonstration reactor that follows, dubbed DEMO, will likely be slightly bigger again. When ITER was being designed in the 1990s, it was believed that the only feasible way to increase fusion power was to increase machine size. But the size and complexity of ITER has led to very slow progress in the fusion program, with first fusion set for the mid 2020s. Tired of waiting so long and recognising the inherent difficulties of such a big project, some have been questioning the possibility of a smaller way to fusion.

Fusion reactor development could proceed much more rapidly by scaling down the size of reactors being developed, potentially helping the first compact fusion pilot plants to be ready to produce electricity for the first time within the next decade.

Theoretical calculations show that a Spherical Tokamak using high fields produced by HTS magnets could be significantly smaller than other fusion machines currently proposed. For example, a compact ST power plant would have a volume up to 100 times smaller than ITER – the successor to JET currently being built in France at a cost of €15bn – so would be approximately room-sized rather than aircraft-hangar-sized.

DEMO (DEMOnstration Power Station) is a proposed nuclear fusion power station that is intended to build upon the ITER experimental nuclear fusion reactor. The objectives of DEMO are usually understood to lie somewhere between those of ITER and a “first of a kind” commercial station. While there is no clear international consensus on exact parameters or scope, the following parameters are often used as a baseline for design studies: DEMO should produce at least 2 gigawatts of fusion power on a continuous basis, and it should produce 25 times as much power as required for breakeven. DEMO’s design of 2 to 4 gigawatts of thermal output will be on the scale of a modern electric power station.

To achieve its goals, DEMO must have linear dimensions about 15% larger than ITER, and a plasma density about 30% greater than ITER. As a prototype commercial fusion reactor, DEMO could make fusion energy available some 15 years after ITER. ITER schedule is slipping. DEMO will not start tests before 2035. It is estimated that subsequent commercial fusion reactors could be built for about a quarter of the cost of DEMO

PROTO is a beyond-DEMO experiment, part of European Commission long-term strategy for research of fusion energy. PROTO would act as a prototype power station, taking in any remaining technology refinements, and demonstrating electricity generation on a commercial basis. It is only expected after DEMO, beyond 2050, and may or may not be a second part of DEMO/PROTO experiment.

Tokamak Energy Progress to date

Nuclear Fusion journal – On the power and size of tokamak fusion pilot plants and reactors

It is generally accepted that the route to fusion power involves large devices of ITER scale or larger. However, we show, contrary to expectations, that for steady state tokamaks operating at fixed fractions of the density and beta limits, the fusion gain, Qfus, depends mainly on the absolute level of the fusion power and the energy confinement, and only weakly on the device size. Our investigations are carried out using a system code and also by analytical means. Further, we show that for the two qualitatively different global scalings that have been developed to fit the data contained in the ITER ELMy H-mode database, i.e. the normally used beta-dependent IPB98y2 scaling and the alternative beta-independent scalings, the power needed for high fusion performance differs substantially, typically by factors of three to four. Taken together, these two findings imply that lower power, smaller, and hence potentially lower cost, pilot plants and reactors than currently envisaged may be possible. The main parameters of a candidate low power (~180 MW), high Qfus (~5), relatively small (~1.35 m major radius) device are given

Nuclear Fusion Journal – Heat deposition into the superconducting central column of a spherical tokamak fusion plant

A key challenge in designing a fusion power plant is to manage the heat deposition into the central core containing superconducting toroidal field coils. Spherical tokamaks have limited space for shielding the central core from fast neutrons produced by fusion and the resulting gamma rays. This paper reports a series of three-dimensional computations using the Monte Carlo N-particle code to calculate the heat deposition into the superconducting core. For a given fusion power, this is considered as a function of plasma major radius R0, core radius rsc and shield thickness d. The deposited power shows an exponential dependence on all three variables to within around 2%. The additional effects of source profile, the outer shield and shield material are all considered. The results can be interpolated to 2% accuracy and have been successfully incorporated into a system code. A possible pilot plant with 174 MW of fusion is shown to lead to a heat deposition into the superconducting core of order 30 kW. An estimate of 1.7 MW is made for the cryogenic plant power necessary for heat removal, and of 88 s running time for an adiabatic experiment where the heat deposition is absorbed by a 10 K temperature rise.

IEEE Transactions on Plasma Science – Recent Advances on the Spherical Tokamak Route to Fusion Power

Stambaugh developed the Peng-Hicks concept of a fusion reactor based on a solid copper center-post spherical tokamak (ST). Using the promising results from the START experiment, they produced a vision for a path to fusion power. This path had two elements such as the ability to produce high fusion gain from an ST and of equal importance, the ability to demonstrate this in a small (and therefore relatively low cost) pilot plant device. In this paper, we review various attempts to pursue this vision, and try to elucidate the reason why success has not yet been achieved. However, we show that the advent of high temperature superconductors may overcome some of the problems, and we suggest a revised version of the small, low entry cost route to fusion power.