Power Balance in Neutral Beam Heated Discharges on the National Spherical Torus Experiment (NSTX)

The understanding of power balance is an important aspect of magnetic confinement. It is particularly important to understand loss mechanisms and transport. The inherently low toroidal field of low aspect ratio machines means that much of the analysis of standard aspect ratio devices must be modified. Typically, analyses of confinement and transport in large aspect ratio devices use the approximation that the gyroradius is very small compared to the system size. In a low aspect ratio machine, this is frequently violated because the low magnetic field significantly increases the gyroradius. For example, the minimum possible transport in a conventional aspect ratio tokamak is closely approximated by neoclassical estimates. However, in a low aspect ratio machine, such as NSTX, the gyroradius can approach or exceed the banana width, thus invalidating the approximation that the gyroradius is