Advances in Transport Studies in the Spherical Tokamak (ST) PI3 at General Fusion Inc

The ST PI3 is a medium-size device in operation at General Fusion Inc., aiming to study transport, equilibrium, stability, plasma-wall interaction, while developing core & edge diagnostics for the unique environment created by the lithium (Li) coating of the vacuum vessel, thus guiding the future devices where Li liners will be used for plasma compression.

The PI3 plasmas are formed exclusively via coaxial-helicity injection (CHI) and last ~30ms. They can be close-fitted or slightly detached from the aluminum spherical vacuum vessel of 1.00m inner radius and 50mm thickness, acting as a flux-conserver. This wall is regularly coated with Li using 4 heated evaporators.

Typical values of Pi3 plasmas in early phases of discharge (~3ms) are: R_o=0.67m, a=0.37m, A=1.8, k=1.7, δ~0.4, B_T(R_o)=0.23T, I_p =250-500kA, n_e(bar)=2-3x10¹⁹m^-3, T_e(0)=300eV and T_i(bar)=400eV.

Equilibrium reconstruction is performed by Flagship code using constraints from poloidal pickup coils inside of the vacuum vessel wall and polarimeter signals.

Experimental results of the thermal energy confinement time tauE and the thermal diffusivity Chi will be presented. The former is calculated from kinetics diagnostic and magnetic power balance methods. Preliminary results from kinetics method in medium plasma densities (N_G=0.5) show tauE=13ms, thus much higher than ~8ms expected from the ITER97-L-mode scaling. Analysis of tauE in new regimes at higher B_T(R_o), I_p, and T_e(0), comparison with H-modes scenario (scaling and L-H power transition), and the impact of pressure constrained equilibria, as well as few simulations with standard transport codes will also be presented.