Radially resolved measurements of ''q'' on the ATC tokamak

This paper describes a new technique for directly measuring the safety factor, or q, profile in a tokamak. A tightly collimated neutral beam capable of injecting approximately 100 mA of 30 keV deuterium atoms was mounted on the ATC tokamak. By proper tangential aiming of the beam a strong concentration of circulating fast ions was created at a chosen plasma minor radius. By examining the orbits of these ions with a multi-sight-line charge exchange detector, it was possible to measure the shifts of the orbits off of magnetic surfaces, and thereby to make local measurements of q. This experiment showed that when q on axis reached unity, strong internal turbulence began, and further lowering of q at the outer edge of the plasma did not lower q(0). A second result was that the measured q(0) was approximately 1.8 times larger than the q(0) deduced from the T/sub e/(r) profile, assuming z/sub eff/ to be independent of minor radius. This suggests a significant peaking of z/sub eff/ on axis in ATC