7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Spherical Tokamak path to Fusion – History and the Next Step
M Gryaznevich, Yu Takase
Tokamak Energy Ltd Abingdon UK
e-mail (speaker): mikhail.gryaznevich@tokamakenergy.co.uk.
The Spherical Tokamak (ST) path to Fusion offers key
elements needed to enable magnetic confinement fusion
to make the transition from pure government-funded
research programs to the commercial marketplace andattract commercial cost sharing at an affordable level andwith low financial risk: a low-cost, low-power, small-size
market entry vehicle and a strong economy of scale inlarger next step devices. This ST path has certain specifics
that makes it different from the conventional JET-ITERDEMO path: 1) The ST approach can progress from thepilot plant to the power plant just by increasing the linear
dimensions of the device without significant changes intechnology [1]; 2) The use of High Temperature
Superconductors (HTS) in an ST allows both high fieldand compact size. The high beta potential of the ST is sogreat that the physics of this device will not determine itssize; 3) The development path to a Power Plant based on
ST can be faster and cheaper than the conventional JETITER-DEMO path, so attractive to the private investors.
According to the report published by the Fusion Industry
Association, private investments into fusion development
have already exceeded $4.7 billion in total, spread over
several alternative to the mainstream approaches.
It was found that the physics and technical demands of theST approach diminish with device size. It was shown [2]that the power efficiency of a Fusion reactor, Q, dependsmainly on the Fusion power produced, but not on thedevice size. That is why at Tokamak Energy Ltd, a privatecompany in Oxfordshire, UK, we started our path toFusion with a very compact but high field prototype, ST40,
which has already demonstrated Fusion relevant plasma
temperatures exceeding 100 M degrees [3] confirming asignificant improvement of performance with the increasein the toroidal field in STs. The use of HTS in tokamak
magnets, the possibility of which was demonstrated byTE.Ltd in the full-HTS tokamak, and by achieving
magnetic fields of up to 24T in tokamak magnet relevant
magnet prototypes, allows reduction in the size of thetokamak central post allowing more space for neutron
protecting shielding and smaller size overall.
Most of recent DEMO and reactor designs assume pulsed
operations. Advantages of pulsed reactors, including
economics, have been discussed recently in detail ([4,5]for conventional tokamaks, [6] for STs). ST reactors haveeven more advantages due to possibility of high bootstrap
current, stronger increase in the confinement with thetoroidal field, good stability, and lower requirements on
the volt-second capability of the central solenoid for theplasma current sustainment. These advantages will bediscussed in detail.
In this talk, the history of Spherical Tokamaks and of theST path to Fusion, as well as advances in the ST physics
and technology R&D will be also overviewed.
References[1] R Stambaugh et al., Fusion Technol. 33 1 (1998)[2] A Costley et al., Nuclear Fusion 55 033001 (2015)[3] S A M McNamara et al., Nucl. Fusion 63 054002(2023)[4] D J Segal et al., Nucl. Fusion 61 045001 (2021)[5] S J Frank et al., Nucl. Fusion 62 126036 (2022)[6] M Gryaznevich et al., Plasma 5(2) 247 (2022)