Implementation of a Noncircular Local Helicity Injector on Pegasus-III

Local helicity injection (LHI) is a startup method that uses small active arc current sources to inject current streams I_inj along field lines which relax into a tokamak-like plasma with I_p>>I_inj. Taylor relaxation limits I_p with I_p ≤ I_TL∝(B_TI_inj/w_inj)^1/2 where w_inj is the width of the injected current. Helicity drive is also necessary to overcome helicity dissipation and limits I_p through LHI loop voltage V_LHI∝V_injA_inj, where V_inj is injector voltage and A_inj is injector area. I_p is thus limited by the minimum of I_TL and helicity drive. A noncircular injector (LHI-nc) with a large, narrow aperture has been deployed and tested to I_inj≤4kA, V_inj≤700V, and I_p≤0.1MA . The noncircular geometry is designed to increase both I_TL and V_LHI by reducing w_inj and increasing A_inj. LHI-nc was planned to operate at I_inj≤16kA and I_p≤I_TL~0.3MA, but these limits were not realized due to gas flow limitations and dissipation from impurities, respectively. n_arc measurements are uniform within ~25% and higher resolution measurements are planned for the next injector iteration to confirm full utilization of increased A_inj, which is expected to give comparable V_LHI to a 4x4cm² injector array. Results from this campaign will inform advanced injector design.