Current Drive Scaling of Local Helicity Injection in the PEGASUS Toroidal Experiment

Local Helicity Injection (LHI) is a non-solenoidal startup technique that utilizes electron current injectors at the plasma edge to initiate a tokamak-like discharge. In order to determine the scalability of LHI to MA-class facilities, it is necessary to identify the key parameters that dictate LHI performance . Injection on the high-field-side (HFS) allows for the creation of discharges driven purely by helicity injection. Ohmic and stochastic confinement scalings predict a favorable non-linear relationship between I_p and drive voltage V_LHI. Recent experiments have indicated a linear current drive scaling suggesting a constant impedance. This scaling has been observed over different levels of B_T and MHD activity. Thomson measurements at low B_T indicate hollow T_e profiles that increase in <T_e> and decrease in <η> as the input power is increased. Despite this decrease in <ηglt;, the current drive scaling remains linear. At higher levels of B_T, peaked T_e profiles (T_{e , 0} 100 eV) and higher I_p are observed for the same amount of V_LHI . These results have been compared to the first Thomson documentation of Ohmic discharges in PEGASUS which feature T_e <= 250 eV. Calculation of neoclassical resistivity and plasma impedance from equilibrium reconstructions and Thomson data suggest this scaling result may be attributed to an increase in Z_eff .

Work supported by US DOE Grants DE-FG02-96ER54375 and DE-SC0019008.