Initial characterization of electron temperature and density profiles in PEGASUS spherical tokamak discharges driven solely by local helicity injection

Abstract

Local helicity injection (LHI) is a non-solenoidal startup technique that utilizes electron current injectors at the plasma edge to initiate tokamak discharges. Viable non-solenoidal startup techniques require high central T e to combat resistive losses and enhance coupling to auxiliary methods of current drive/heating. Thomson scattering measurements of LHI discharges in Pegasus showed peaked T e profiles at I p ∼ 0.15 MA and B t ∼ 0.15 T with T e, 0 ∼ 100- 150 eV. These results are similar to T e profiles observed with Ohmic induction. At lower levels of B t, LHI T e profiles were hollow with T e, 0 ∼ 40 eV and T e, max ≤ 120 eV depending upon the helicity input. Regardless of the B t level and helicity input, the electron pressure profiles were flat/peaked with hollow J R profiles. Equilibrium reconstructions and measurements of core absolute extreme ultraviolet radiation suggest the hollow T e profiles are a result of very low resistive heating power in the core due to the edge-localized nature of LHI and low-Z line radiation losses. Estimates of Z eff from the plasma conductivity indicate averaged values of ∼1 or ∼3 assuming neoclassical or Spitzer conductivity, respectively. When auxiliary heating power from magnetic reconnection is considered, this observed LHI performance is comparable to expectations from a linear Ohmic confinement scaling estimate and a collisional stochastic confinement scaling estimate of the core plasma region.