Effects of Impurity Transport in Tokamak Startup Using LHI on the Pegasus-III Experiment

Local Helicity Injection (LHI) is a solenoid-free tokamak startup technique and one of the major research thrusts of the new Pegasus-III facility. Efficient startup with LHI requires low resistive dissipation and radiated power to increase the plasma current and temperature to create a plasma suitable for RF and neutral beam current sustainment. Fast transport at the startup phase can be responsible for electron cooling due to impurity collisional excitation even at low impurity concentrations if low-charge state species can reach the plasma core. The enhanced radiation losses from these low-Z impurities can be used to explain observed hollow T_e profiles during LHI discharges at low toroidal field B_T, which transition to peaked when B_T increases. Impurity transport in LHI will be studied in Pegasus-III using multiple diagnostics and tools and access to increased B_T < 0.6 T. A newly developed AXUV diode array is used to measure the radiated power during LHI discharges. VUV and visible spectral surveys are planned to identify the impurity species. Information from those diagnostics as well as magnetic equilibria are used as inputs for the impurity transport modeling with STRAHL to quantify limits on impurity sourcing during LHI startup.