Characterization of Te and neProfiles of Discharges Driven Purely by Helicity Injection in the Pegasus Toroidal Experiment

Understanding the electron confinement and transport in plasmas driven purely by local helicity injection (LHI) is critical to the demonstration of high-performance discharges. Given the proper operating conditions, purely LHI-driven discharges can feature peaked T_e profiles with T_{e , 0} ∼ 150 eV. Ohmic discharges in Pegasus at the same field level, B_T ∼ 0 . 15 T exhibit similar T_e profiles albeit with higher n_e. At lower levels of B_T, LHI discharges feature hollow T_e profiles that increase in <T_e> as the effective loop voltage, V_LHI, is increased. The increase in <T_e> scales with V_LHI rather than the injector electrode voltage, V_inj, contrary to predictions from open field line theory. The hollowing of the T_e profile is hypothesized to be a combination of low ηj² heating power due to the hollow current profile and low-Z impurity radiation losses. Approximations of Z_eff in LHI discharges from voltage balance assuming purely Spitzer and neoclassical resistivity are ∼ 3 and ∼ 1 , respectively. Thomson scattering and magnetic probe measurements indicate a pressure-free region between the kinetic and magnetic boundaries, possibly indicative of separate Ohmic and stochastic confinement regions. Overall scaling of I_p with V_LHI appears to be consistent with linear Ohmic confinement scaling assuming auxiliary ion and electron heating from magnetic reconnection.

Work supported by US DOE Grants DE-SC0019008 and DE-SC0020402.