Global energy confinement studies on the Pegasus Toroidal Experiment

Recent studies have shown that low-recycling walls significantly enhance L-mode energy confinement [1]. Discharges on Pegasus suggest a low-recycling regime is obtained using titanium gettering and cryogenic pumping. When the external gas supply is terminated during an established discharge, tangential H$_{\alpha }$ and visible light signals drop to 5{\%} of their initial levels within 5 ms. Wall recycling is measured using the density decay rate, and its effect on particle and energy confinement on Pegasus is explored. Initial global energy confinement times of $\tau _{E}$ = 2 -- 4 ms were calculated for I$_{P} \quad \sim $ 0.15 MA L-mode discharges. Scans of plasma current and line-averaged density are used to benchmark $\tau _{E}$ measurements against empirical L-mode scaling laws. These initial $\tau _{E}$ measurements indicate the H-mode power threshold as given by the ITPA04 scaling [2] can be exceeded in diverted Ohmic discharges on Pegasus. \newline [1] Majeski, R. \textit{et al.} Phy. Rev. Lett. \textbf{97} 2006 075002 \newline [2] Takizuka, T. \textit{et al.} Plasma Phys. Control. Fusion \textbf{46} 2004 A227