Local Helicity Injection Studies at Increasing Toroidal Field on Pegasus-III

Local helicity injection (LHI) is a startup technique that uses small current sources to inject current streams I_inj along field lines which relax into a tokamak-like plasma with I_p >> I_inj. However, Taylor relaxation provides an absolute I_p limit for LHI plasmas: I_p ≤ I_TL ∝ (B_T I_inj / w_inj)^1/2 where w_inj is the radial width of the injected current. As such, it is possible to increase I_TL, and thus I_p, by increasing B_T or by decreasing w_inj. Previous LHI experiments with B_T = 0.3 T have already achieved I_p ≥ 0.2 MA, following I_TL ∝ B_T^1/2 scaling using 4×4 cm² circular injectors. To better study LHI at reactor-relevant fields, the Pegasus-III Experiment has been upgraded to have B_T = 0.6 T, a 4× increase in toroidal field capability, which projects I_TL ≥ 0.3 MA for the circular injector array. A port-mounted noncircular injector (LHI-nc) is also expected to increase I_TL by injecting current as a thin-sheet beam, minimizing w_inj while maintaining injector surface area A_inj. This design will allow LHI-nc to provide the same helicity drive V_LHI ∝ A_injV_inj as the circular injector array at similar injector voltage V_inj, while the change in w_inj (∼1 cm compared to w_inj ∼ 2.2 cm for the circular array) projects I_TL ≥ 0.35 MA.