Confinement physics for a steady state net electric burning spherical tokamak

Spherical tokamaks have many desirable properties that make them a suitable candidate for
a compact fusion reactor. Such a device could accelerate the timeline of fusion and reduce
capital costs, allowing fusion to have a more significant impact on the world. The feasibility of
a compact spherical tokamak able to generate net electricity needs to be examined as well as
the modelling tools currently available. Extrapolating to reactor relevant conditions requires
a great deal of trust in these models.
This work begins by identifying steady state plasma equilibria and applying empirical limits
to characterise the available parameter space for a given machine design and scale. This
is done with a consistent calculation of the neoclassical currents, allowing for the auxiliary
current drive requirements to be determined. A baseline scenario was identified with a major
radius of 2.5m and fusion power of 1.1GW. An important result found is that a minimum
current drive efficiency is required given the empirical limits used. Neutral beam injection was
found to have a sufficient current drive efficiency, with 94MW of power needed to drive all
the required current. The validity of reduced physics neutral beam models was also examined
and it was found that reasonable predictions were made provided the beams were aligned with
the magnetic field.
The performance of a tokamak is generally limited by the turbulent transport so the linear
gyrokinetic stability of a baseline ST reactor plasma scenario was investigated. The baseline
equilibrium showed some desirable properties as the electron scale turbulence was found to
be stable. In the ion scale, kinetic ballooning modes and micro-tearing modes were found
to co-exist on multiple flux surfaces. Through exploring the drives of these modes it was
possible to optimise the equilibrium to minimise their growth rates. Moreover, the credibility
of quasi-linear transport models was explored with a new tool developed that is better able
to capture the instabilities in this regime, though further development is still needed.