Gyroradius-Scale Ion Gradients in NSTX*

The spherical torus geometry is well suited to compare experimental profiles on a fundamental transport scale. The low-field region in the low-aspect ratio NSTX along with a high spatial resolution charge exchange spectroscopy system make gyroradius-scale ion measurements possible. Ion temperature and density gradients with gyroradius-scale widths have been measured in some NSTX plasmas. Measured ion temperature changes of $\Delta T_i = 250-500$ eV are observed over $\Delta r \sim 1$ cm with total Larmor radius $\sim 1$ cm. Minimum gradient widths comparable to the larger banana orbit width would normally be expected from ``random walk'' arguments of neoclassical theory. Since radial electric field ($E_r$) gradients also vary on a gyro-radius scale, distortion of the gyro orbit is expected to cause orbit shrinking or orbit expansion [1], depending on the sign of $E_r$. Orbit shrinking removes the apparent discrepancy in the gradient width. \newline \newline [1] K. C. Shaing, A. Y. Aydemir, R. D. Hazeltine, Phys. Plasmas 5, 3680 (1998).