Plasma control for the STEP prototype power plant

In 2019 the UK launched the STEP programme to design and build a prototype electricity producing nuclear fusion power plant, aiming to start operation around 2040. The plant should lay the foundation for the development of commercial nuclear fusion power plants. The design is based on the spherical tokamak principle, which opens a route to high pressure, steady state, operation. While facilitating steady state operation, the spherical design introduces some specific plasma control challenges: i) All plasma current during the burn phase should to be generated through non-inductively means, dominated by bootstrap current. This leads to operation at high normalised plasma pressure β_"N" with high plasma elongation, which in turn imposes effective active stabilisation of the vertical plasma position. ii) The tight aspect ratio means very limited space for a central solenoid, imposing that even the current ramp up must be non-inductively generated. iii) The compact design leads to extreme heat loads on plasma facing components. A double null design has been chosen to spread this load, putting strict demands on the control of the unstable vertical plasma position. iv) The heat pulses associated with unmitigated ELMs are unlikely to be unacceptable imposing ELM free operation or active ELM control. v) To reduce and spread heat loads, core and divertor radiation and momentum loss has to be controlled, aiming to operate with simultaneously detached upper and lower divertors. vi) High pressure operation is likely to require active resistive wall mode stabilisation. vii) The conductivity distribution in structures near the plasma must be carefully selected to reduce the growth rates for the vertical instability and the resistive wall mode without damping the penetration of the of magnetic fields from active control coils too much. This article describes the initial work carried out to develop a STEP plasma control system.