Design of the Thomson Scattering Diagnostic on the \textsc{Pegasus} Toroidal Experiment

A critical question concerning use of point-source helicity injection for non-inductive startup is whether, as $I_p $ increases, energy confinement is dominated by cross-field transport or by parallel losses due to field line stochasticity. Furthermore, resistively-driven helicity dissipation during plasma formation must be characterized. Both of these topics are important for predictive scaling to larger tokamaks. In addition, $T_e $ and $n_e $ profiles are needed for accurate magnetic equilibrium reconstructions at high $\beta _T $ and $I_N $. To resolve these issues, a Thomson scattering diagnostic is being developed for the \textsc{Pegasus} Toroidal experiment. The design is guided by systems on MST\footnote{ J.A. Reusch, et al. RSI \textbf{79}, 10E733 (2008)} and HSX.\footnote{ K. Zhai, et al. RSI \textbf{75}, 10 (2004)} Scattered light from an incident Nd-YAG laser ($\lambda =1064$ nm) will be detected by a polychromator system. Implementation on \textsc{Pegasus} will measure $n_e $ and $T_e $ at $\ge $ 10 radial locations for plasmas with $n_e \ge $ 10$^{19}$ m$^{-3}$ and $T_e \quad \sim $ 10 eV -- 1 keV, with radial resolutions of $\sim $1.75 cm and 5 cm for fine and coarse configurations, respectively.