Integrated Study of Solenoid Free Tokamak Startup on the PEGASUS and URANIA Experiments

Introduction – Initiating current without using magnetic induction from a central solenoid is critical for the development of the spherical tokamak (ST) as a reactor-relevant configuration, and may benefit the advanced tokamak as well. The PEGASUS program has focused on developing the physics and technology basis for non-solenoidal tokamak startup using local helicity injection (LHI). LHI utilizes compact, edge-localized current sources (Ainj > 8 cm2, Iinj £ 8 kA, Vinj £ 1.5 kV) for plasma startup and sustainment, and can initiate > 200 kA of plasma current (Ip) in a low-field (BT(0) £ 0.15 T), near-unity aspect ratio (A) ST [Fig. 1(a)]. LHI initiated plasmas have been successfully handed off to Ohmic (OH) H-mode sustainment, resulting in the highest stored energy in PEGASUS to date [Fig. 1(b)] [1]. Recent work has focused on assessing: Ip scaling; current drive mechanisms; characteristics of LHI plasmas; and the physics and engineering tradeoffs inherent in the choice of injector location. Two helicity injection systems with similar capabilities have been used for the experiments, one on the low-field-side (LFS) near the outboard mid-plane and a second on the high-field-side (HFS) in the lower divertor region [Fig. 1(a)]. These experiments inform a next generation LHI system design and machine upgrade to further advance LHI startup. Physics Basis for LHI – If conditions for magnetic relaxation and radial force balance are met, a force-free current directed along the field can relax through helicity-conserving magnetic turbulence to form a tokamak-like plasma [2]. First-principles simulations of LHI with the NIMROD code have been performed [3]. These simulations reproduce this initial relaxation and current drive process, providing insight into the early phase LHI dynamics.