Engineering Design of a Coaxial Helicity Injection System for Non-Solenoidal Startup Studies on Pegasus-III*

DC helicity injection is a promising way to address a critical challenge facing the spherical tokamak: non-solenoidal startup. Coaxial helicity injection (CHI) is one such technique studied on several devices. An engineering design is in process for a CHI system for Pegasus-III that will address outstanding issues for this startup scheme, including: eliminating the need for an axisymmetric vacuum vessel break; the scaling of I_p with injector and/or flux footprint shape and separation; minimizing impurity sourcing from the electrodes; the degree of axisymmetry required to achieve high I_p; and understanding the roles of reconnection and dynamo drive mechanisms. This design features two coaxial, segmented, floating electrodes in the upper divertor region. Their size and position enables I_p scaling studies with projected maximum I_p > 0.3 MA. Segmented electrodes enable rapid changes to shape and plasma facing material for impurity control. Current is sourced by six power feeds, enabling diagnosis and control of the drive symmetry. These features enable experiments to test the reconnection and dynamo current drive scalings that set the achievable peak I_p with CHI.