With steady progress on ITER project and the design of DEMO, the international community is now entering an era in which fusion power on the grid could become a reality within the next 20 – 30 years. In this environment the UK has started the ambitious Spherical Tokamak for Energy Production (STEP) programme, aiming to develop a compact prototype reactor based on the spherical tokamak (ST) concept by 2040 [1]. This endeavour is split into three tranches: (1) concept design & supplier base generation; (2) detailed design; (3) build. Whilst tranche 1 (2020-2024) is already entirely funded publicly to provide a concept design for a STEP prototype reactor and define a way forward to a STEP commercial reactor, the later tranches will have substantial and increasing industrial involvement. The STEP programme has taken an innovative approach by embedding a strong central team within one of the world’s leading fusion labs and strongly engaging with universities and industry. UKAEA has the unique experience of hosting and operating JET (the only DT capable tokamak in the world), operating one of the largest spherical tokamaks MAST-U [2] (which prototypes a novel alternative divertor solution), a world leading remote handling lab and world-class material research facilities. The key enabler for a viable concept design is the plasma scenario. The ST concept makes it possible to maximise fusion power and bootstrap fraction in a compact device at relatively low toroidal field by allowing operation at high normalised pressure  and high elongation  but it also poses unique challenges [1]. The compactness restricts significantly the available inductive flux for the plasma pulse and therefore the required plasma current of needs to be predominantly generated, maintained, and ramped-down non-inductively. The non-inductive current must be driven mainly off axis as MHD stability and operation at high  require broad current profiles with low internal inductance and elevated safety factor . At the high density beneficial for fusion, conventional microwave current drive techniques are inefficient and techniques such as electron Bernstein waves (EBW) may need to be employed. A ~2 MW EBW system is being implemented for MAST-U, with strong university involvement, to validate the heating and current drive predictions using this technique for STEP and grow capability in the UK. MAST-U will also be key for testing novel divertor concepts needed to handle power exhaust on compact divertor targets with relatively small wetted area – especially at the inner target. SOLPS-ITER calculations show that acceptable heat loads in the flat-top may be obtainable with Ar seeding and divertor pressures of the order of 10 Pa. Operating in a highly self-organised high  non-inductive scenario with a high radiation and high bootstrap fraction requires sophisticated and novel control concepts. A particular challenge is the prediction of the confinement in the conditions relevant for STEP. Scaling laws and present reduced transport models may give an indication but for a high  ST plasma such as STEP they are well outside their domain of experimental validation. Indeed, parameter dependencies differing from those in the IPB98(y,2) scaling have already been observed in present day large STs and reduced models such as TGLF need to be upgraded to describe transport from the electromagnetic turbulence that is expected to be present. Linear gyrokinetic (GK) modelling [3] has shown that the turbulence in STEP reference flat-top plasmas is dominated by micro-tearing modes (MTM) and kinetic ballooning modes (KBM).  Diamagnetic flow shear is stabilising for KBMs and other modes at higher  and the turbulence is likely to be dominated by MTMs at low , which have very extended eigenfunctions in ballooning space. These slowly growing low n modes make the non-linear GK modelling very challenging. The actuators to control the ensuing turbulent transport are being investigated, to seek routes to optimised confinement.  Reduced models capturing the magnetic flutter driven transport are being tested and extended. Substantial work to generate the physics base for a preferred plasma scenario has been performed within the last 3 years centred around the JINTRAC/JETTO modelling suite adapted to non-inductive operation and ST geometry as assumption integrator. A fast low fidelity version of JETTO employing genetic algorithms for parameter optimisation has been used to quickly explore a multitude of flat top operating points in different prototype concept families bases on initial system code assessment. Higher fidelity modelling has sketched the path of the largely non-inductive ramp-up as well as validating the heating and current drive assumptions. The  operating point with  allows access to both ECCD and EBW.  Vertical stabilisation and resistive wall mode control schemes have been developed to allow operation at  and  slightly above the no-wall limit. The exhaust challenge for  is met with alternative configurations at both the inner and outer divertor legs allowing fully detached operation with Ar seeding.  A brief overview of the STEP programme will be given with a detailed discussion of the current state of the physics basis. The key plasma scenario challenges and the work that allows the quantification and reduction of the associated risks will be presented. Whilst many challenges remain this work gives confidence that an electricity producing prototype compact fusion power plant based on the plasma performance of an ST is likely feasible.

References:

• H Wilson, et al “STEP – on the pathway to fusion commercialisation” Chapter 8 of W J Nuttall “Commercialising Fusion Energy” (IoP Publishing, 2020)
• H Meyer “The mega amp spherical tokamak”, Chapter 12 of G. H. Neilson “Magnetic Fusion Energy: From Experiments to Power Plants”(Elsevier Inc., 2016).
• Patel Confinement physics for a steady state net electric burning spherical tokamak. PhD thesis. University of York, 2021 https://etheses.whiterose.ac.uk/28991/ – submitted to Nucl. Fusion (see https://arxiv.org/abs/2108.11169)