Predictive Modelling and Helicity Dissipation Scaling Studies for Local Helicity Injection Non-Solenoidal ST Startup*

A 0D power balance model is being tested on the Pegasus ST to develop predictive capability, interpret experiments, and inform future system design for Local Helicity Injection (LHI). The model calculates I_p(t) by balancing LHI effective drive (V_LHI), helicity dissipation, and inductive effects while enforcing the Taylor relaxation current limit. Experimentally constrained drive inputs (plasma geometry, ℓ_i, β_p, injector parameters) allow for prediction of upper bounds on I_p. Namely, predictive modeling suggests nonlinear increases in achievable I_p are possible by higher B_T and/or I_inj to increase the early-phase Taylor limit. This motivates a new injector design and facility enhancements to further test LHI scalability. However, proper treatment of the helicity dissipation term is still a model uncertainty. Thus far, helicity dissipation has been attributed to neoclassical resistivity. This has been challenged by experiments showing I_p scales linearly with V_LHI while Thomson scattering indicates a variety of T_e profiles (from hollow to peaked, 40<T_e,0 <150 eV) depending on B_T, n_e, and injector parameters. Systematic scaling studies of T_e with discharge parameters are underway to resolve this model uncertainty.