Magnetic Field Generation and Energy Confinement with T$_{e} >$ 500 eV in the SSPX Spheromak*

The understanding of confinement and energy transport in spheromaks is key the understanding the physics of spheromak formation and self-organization as well as addressing the feasibility of the concept as a reactor scenario. In the Sustained Spheromak Physics eXperiment (SSPX), increased understanding of the physics in building and sustaining self-organized magnetic equilibria has resulted in record electron temperatures T$_{e} \quad >$ 500 eV and plasma currents of $\sim $ 1 MA on the magnetic axis. We find that the highest edge magnetic field magnitudes (and correspondingly high T$_{e})$ is achieved when $\lambda =\frac{\mu _0 I_{gun} }{\Psi _{gun} }$ is near (but slightly below) the Kruskal-Shafranov instability limit $\lambda _{KS} \cong \frac{2\pi }{L}\cong 12.6\,m^{-1}$ where L is the length of the flux-conserver (0.5 m). Building on previously reported results, power-balance analysis has shown levels of electron thermal transport $\chi _e <$ 1 m$^{2}$/s, indicating good confinement and closed flux surfaces. With the addition of a modular capacitor bank we are able to highly tailor the gun current to take advantage of the sensitive dependence of spheromak performance on the plasma $\lambda $. When in this optimum operating range we also find that the efficiency of field build-up (defined as the ratio of edge poloidal magnetic field to gun current) is increased 20{\%} over prior results, to $\sim $1.0 T/MA. Additionally this brings the efficiency of spheromak formation into numerical agreement with results from the NIMROD 3-D MHD code. Plasma energy evolution has been studied by taking time-resolved measurements of T$_{e}$(r) and n$_{e}$(r) indicating a distinct and robust feature of spheromak formation; a hollow-to-peaked temperature transition with an inverse relationship to the electron density. This feature, as well as sub-microsecond transport, is being studied with the upgrade of the Thomson scattering diagnostic to double-pulse operation. We also present recent results of the impact of charge-exchange losses on overall power balance and estimates of the plasma ion temperature as measured with a neutral particle analyzer.