A Coaxial Helicity Injection System for Nonsolenoidal Startup Studies on the PEGASUS-III Experiment

Initiating current without using magnetic induction from a central solenoid is a critical scientific and technical challenge facing the spherical tokamak (ST). One such technique that has shown promise on several devices is coaxial helicity injection (CHI). In CHI, a dc voltage applied to large area coaxial electrodes injects current and helicity into the vacuum vessel for plasma startup and plasma current ( $I_{p}$ ) sustainment. Major outstanding issues for CHI include eliminating the need for a vacuum vessel break; the scaling of $I_{p}$ with injector and/or flux footprint shape and separation; mitigating plasma material interaction (PMI) and minimizing impurity injection; and the degree of axisymmetry required to achieve high $I_{p}$ . Thus, a first of its kind, CHI system is being installed on PEGASUS-III. It utilizes two coaxial, segmented, floating electrodes located entirely within the vacuum vessel in the upper divertor region. The design enables $I_{p}$ scaling studies as the electrode shape, coupled with a new 480 kA/288 kA/244 kA-turn divertor coil triplet, and allows for variation of the flux footprint shape and location simply through manipulation of coil currents. Together, they are projected to allow over 50 mWb connected flux and $I_{p} >300$ kA. The segmented electrodes facilitate simple changes to their shape, position, and plasma-facing material. This flexibility may be critical for mitigating PMI or impurity sourcing from the electrodes. Independent current feeds to each segment enable tests of the impact of axisymmetric drive on $I_{p}$ .