On the size of tokamak fusion power plants
Professor Hartmut Zohm, Max-Planck-Institute of Plasma Physics, Germany
Studies for conventional tokamak power plants usually end up with a major radius R0 of the order 8-9 m, ie significantly beyond the largest existing tokamak, JET (R0 = 3m), and also in excess of the size of ITER (R0 = 6.2 m). This can be understood from a simple 0-D scaling  by the need to operate in a state of large power amplification Q=Pfus/PAUX > 30, where Pfus is the fusion power and PAUX the auxiliary heating power to compensate for residual plasma energy loss. However, these considerations assume a technical limit of the confining magnetic field B in line with the limits of the ITER design. Recent advances in High Temperature Superconducting Coil technology have led to proposals based on higher B, leading to more compact devices, i.e. smaller R0 , .
In this contribution, Professor Zohm argues that the definition of tokamak ‘size’ should include the magnetic field to remove the ambiguity in the discussion of ‘size dependence’ of the performance of fusion power plants , . He also analyses the possibilities that higher B would offer, using an extension of the 0-D model used in . Different routes of taking advantage of higher field are discussed. It is shown that, one also has to consider consistently the assumptions about plasma performance, such as confinement quality, operational limits or exhaust schemes. Finally, Professor Zohm discusses some of the significant implications for future R&D needed to make higher magnetic field in reactor-grade devices a reality.
 H. Zohm, Fusion Sci. Technology 58 (2010) 613.
 B. Sorbom et al., Fus. Eng. Design 100 (2015) 378.
 A. Costley et al., Nucl. Fusion 56 (2016) 066003.
 W. Biel et al., Nucl. Fusion 57 (2017) 038001.
Smaller and quicker with STs and HTS
Dr Melanie Windridge, Tokamak Energy UK Ltd, UK
Research in the 1970s and 80s by Sykes, Peng, Jassby and others showed the theoretical advantage of the spherical tokamak (ST) shape. Experiments on START and MAST at Culham throughout the 1990s and 2000s, alongside other international STs like NSTX at the Princeton Plasma Physics Laboratory, confirmed their increased efficiency (namely operation at higher beta) and tested the plasma physics in new regimes. However, whilst interesting devices for study, the perceived technological difficulties due to the compact shape initially prevented STs being seriously considered as viable power plants.
Then, in the 2010s, high temperature superconductor (HTS) materials became available as a reliable engineering material, fabricated into long tapes suitable for winding into magnets. Realising the advantages of this material and its possibilities for fusion, Tokamak Energy proposed a new spherical tokamak path to fusion power and began working on demonstrating the viability of HTS for fusion magnets. The company is now operating a compact tokamak with copper magnets, R0~0.4m, R/a~1.8, and target Ip=2MA, Bt0=3T, whilst in parallel developing a 5T HTS demonstrator tokamak magnet.
Here Dr Windridge will discuss why HTS can be a game-changer for tokamak fusion. She will outline Tokamak Energy’s solution for a faster way to fusion and discuss plans and progress, including benefits of smaller devices on the development path and advantages of modularity in power plants. She will indicate some of the key research areas in compact tokamaks and introduce the physics considerations behind the ST approach, to be further developed in the talk by Alan Costley.
Towards an ST fusion pilot plant
Dr Alan Costley, Tokamak Energy UK Ltd, UK
System code and analytical studies have shown that in addition to the conventional large size, high aspect ratio approach to realising fusion power, there could be an approach based on the low aspect ratio spherical tokamak at much smaller size . Small devices would enable accelerated tokamak development because they offer relatively rapid and less expensive development cycles. However, small devices require novel technology and advanced engineering. High temperature superconductors are potentially the enabling technology because they can provide and withstand the necessary high fields used in the toroidal magnets. Other areas are important too, for example plasma start-up and ramp down with a limited, or without, a solenoid, divertor loads, and stresses in the magnet structure. Tokamak Energy is pursuing a development programme that aims to realise this alternative route to fusion power. In this presentation, the physics basis of the approach will be outlined and the key technology and engineering aspects will be highlighted and potential solutions identified. A technology roadmap to deal with the physics and engineering challenges on this path to fusion is under development and will be briefly introduced.
 A. E. Costley, J. Hugill and P Buxton, 2015, ‘On the power and size of tokamak fusion pilot plants and reactors’, Nuclear Fusion 55, 033001.