Non-solenoidal tokamak startup using localized current sources near the outboard midplane

The development of a cost-effective non-solenoidal startup technique would advance the plasma physics research capabilities and fusion energy application of the spherical tokamak (ST) concept. To that end, non-solenoidal tokamak startup is examined on the ultra-low aspect ratio (A > 1.13) Pegasus Toroidal Experiment using localized current injection from three gas-injected washer-stack plasma guns mounted near the outboard midplane. The plasma guns are biased relative to an external anode and current is injected along helical magnetic field lines. A region of near-zero average poloidal magnetic field is predicted to form for sufficiently large injected toroidal current and is shown to correlate with the relaxation of the magnetic topology to a configuration that resembles the closed and nested flux surfaces of typical tokamak plasmas.;The plasma current (Ip) is driven to values as large as 0.1 MA via the helicity injected by the biased plasma guns and poloidal field magnetic induction (Vind ∼ 1.5 V). The outboard-limited plasmas are observed to expand into the vacuum region as Ip increases and the plasmas are sufficiently turbulent so that the maximum Ip for a plasma of a given geometry is dictated by the Taylor relaxation criteria. This is supported by observations that suggest the maximum Ip matches the predicted dependence of the relaxation limit with IinjITF/w where ITF is the toroidal field rod current and w is the radial width of the average poloidal magnetic flux that intersects both the plasma guns and the external anode.;Intermittent n = 1 MHD activity is observed during periods of strong edge current drive. Equilibrium reconstructions indicate that each burst of activity leads to the rapid expansion of the plasma volume and a drop in the plasma inductance. The plasmas become significantly more quiescent when the strong edge current drive is removed. Evidence of an increase in the plasma temperature implies that the magnetic topology relaxes to a stable tokamak plasma with closed magnetic flux surfaces, which provides a viable target discharge for traditional current drive schemes. This is demonstrated by coupling a gun-driven discharge to induction from a central solenoid.