Taylor Limit Studies for Local Helicity Injection Plasma Startup on Pegasus-III*

Creating and sustaining I_p in tokamaks requires a supply of magnetic helicity, such as from an Ohmic solenoid. DC helicity injection provides an alternative, solenoid-free approach to tokamak startup which is a critical technology for the spherical tokamak. Local helicity injection (LHI) utilizes small, modular current sources to drive open field line current I_inj in a narrow edge layer, which is redistributed via helicity-conserving turbulent processes to generate tokamak-like plasmas with high current multiplication (I_p >> I_inj). This turbulence relaxes the system toward a minimum energy state described by Taylor relaxation theory. This imposes a global I_p limit, referred to as the Taylor limit I_TL, that scales as (I_injB_T/w_inj)^1/2, where B_T is the toroidal field at the injector and w_inj is the radial width of the injected open field line region. Pegasus-III is a new experiment studying the scaling of I_TL with increased B_T, up to 0.6 T on axis, and an enhanced LHI system. Initial experiments have tested scaling relations for LHI in the expanded Pegasus-III operating space. These tests have verified the expected I_TL ∝ B_T^1/2 scaling is observed up to 0.3 T and attained I_TL ≤ 0.225 MA. Experiments have also demonstrated the ability to alter I_TL through injector design by varying w_inj. Discharges with varied injector diameters d_inj find I_TL ∝ d_inj^-1/2, consistent with w_inj ∼ d_inj . During operation with I_p < I_TL, input helicity is balanced by resistive dissipation and radiative losses. However, dissipation mechanisms for excess helicity input at I_TL is an outstanding fundamental question for LHI that has been explored in these studies. In over-driven plasmas at I_p = I_TL, T_e(R) is comparable or greater than those with lower input power, despite increased magnetic fluctuation activity. It is most notably observed on the low field side of the machine in a low frequency (∼10s kHz) n = 1 mode associated with line-tied kink motion of the injected current streams.