  STEP is a spherical tokamak prototype power plant that is being designed to
demonstrate net electric power. The design phase involves the exploitation of
plasma models to optimise fusion performance subject to satisfying various
physics and engineering constraints. A modelling workflow, including integrated
core plasma modelling, MHD stability analysis, SOL and pedestal modelling, coil
set and free boundary equilibrium solvers, and whole plant design, has been
developed to specify the design parameters and to develop viable scenarios. The
integrated core plasma model JETTO is used to develop individual flat-top
operating points that satisfy imposed criteria for fusion power performance
within operational constraints. Key plasma parameters such as normalised beta,
Greenwald density fraction, auxiliary power and radiated power have been
scanned to scope the operational space and to derive a collection of candidate
non-inductive flat-top points. The assumed auxiliary heating and current drive
is either from electron cyclotron systems only or a combination of electron
cyclotron and electron Bernstein waves. At present stages of transport
modelling, there is a large uncertainty in overall confinement for relevant
parameter regimes. For each of the two auxiliary heating and current drive
systems scenarios, two candidate flat-top points have been developed based on
different confinement assumptions, totalling to four operating points. A lower
confinement assumption generally suggests operating points in high-density,
high auxiliary power regimes, whereas higher confinement would allow access to
a broader parameter regime in density and power while maintaining target fusion
power performance.
