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Abstract
Measurements of the shapes of the flux surfaces 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, $q({\it 0\/})$, and the shape of the plasma current profile. This method of obtaining $q({\it 0\/})$ has been used to study the effect of three different equilibrium modification methods on a tokamak plasma. The theoretical basis for the use of shape measurements to constrain the equilibrium, and the importance of measuring the plasma current profile, is also reviewed.
The camera consists of a foil filter in front of a pinhole which images the plasma onto a phosphor screen. The images are detected with a nonstandard video camera and are related to the plasma equilibrium using a forward modeling technique. A series of model equilibria, incorporating external magnetic measurements, and differing only in the value of $q({\it 0\/})$, are created with a numerical equilibrium analysis code. The relationships between the x-ray emissivity and the poloidal flux is derived by Abel-inverting the image data on the midplane, and comparing them to the calculated midplane flux. Using the derived emissivity vs. flux relations and the derived equilibria, a series of model images are created. The data are fitted to the models in a least-squares fashion to yield a best-fit value of $q({\it 0\/})$.
Experiments were performed using three different equilibrium modification techniques: neutral beam current drive, plasma current ramp, and deuterium pellet injection. 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 $q({\it 0\/})$ above unity. An analysis of the current diffusion suggested that there is a component which is significantly anomalous. A plasma current ramp of 2.5 MA/s was also shown to broaden the current profile; however, the value of $q({\it 0\/})$ was not altered. Pellet injection was shown to change the MHD character of the plasma, but no significant changes in the plasma current profile were found.
