The physics of spherical confinement systems

Spherical torus magnetic confinement systems, covering spheromaks and spher ical tokamaks (STs), are reviewed. As well as being potentially very import ant for fusion, spherical tori research is enhancing our understanding of m agnetic confinement systems with wider applications than fusion research. T he studies contribute to the conventional tokamak, for example, ITER via a range of scalings, as well as to our understanding of 'quiescent' plasmas a nd those subject to 'turbulent magnetohydrodynamic (MHD) relaxation'. The t heoretical and experimental properties are described, showing how these var y with configuration and contrasting them with the conventional aspect rati o tokamak. Topics covered include equilibrium, refuelling, helicity injecti on, influence of trapped particle fraction, plasma heating, confinement, st ability (including pressure limits and energetic particle instabilities) an d disruption resilience.