Current distribution modification measurements in the PBX-M tokamak

Measurements of the flux surface shapes in a tokamak with a non-circular cross-section have been made using a tangentially viewing soft X ray pinhole camera. These measurements allowed the derivation of the value of the axial safety factor, q(0), and the shape of the plasma current profile, during equilibrium modification experiments. Two different equilibrium modification techniques were used: neutral beam current drive and plasma current ramp. In the neutral beam current drive experiment, tangential counter-injection was found to modify the equilibrium significantly, broadening the current profile and raising the value of q(0) above unity. In tangential co-injection discharges, q(0) values as low as 0.4 were observed. Reproduction of the measured q(0,t) using a radial current diffusion model strongly suggests there was a non-classical component to the cross-field current diffusion. A plasma current ramp of 2.5 MA/s was also shown to broaden the current profile; however, the value of q(0) was not altered.