Injection and sustainment of plasma in a preexisting toroidal field using a coaxial helicity source

The spheromak device SPHEX has been modified by adding a current-carrying rod along the geometric axis, providing a preexisting toroidal field. We show that plasma can be successfully injected into such a field from a helicity source; the field assists plasma ejection from the gun and improves the coupling between gun and plasma, so that ${\mathit{T}}_{\mathit{e}}$, ${\mathit{T}}_{\mathit{i}}$, and the toroidal current all increase with rod current. The mechanism of plasma sustainment appears to be the same as that of the spheromak. These results represent a step towards the achievement of steady-state tokamak operation.