Current profile modification during lower hybrid current drive in the Princeton Beta Experiment-Modification

The physics of current profile modification with lower hybrid waves has been investigated in the Princeton Beta Experiment-Modification tokamak by comparing experimental internal magnetic field profiles with ray tracing code predictions. When the nil spectrum of the launched waves was varied, local changes in the current profile were observed according to equilibria reconstructed from motional Stark effect polarimetry measurements. Changes in the central safety factor (q) were also determined to be a function of the applied radiofrequency (RF) power. Calculations of the noninductive current with the Tokamak Simulation Code/Lower Hybrid Simulation Code were unable to duplicate the measured current profiles. Instead, a current broadening mechanism like a finite fast electron diffusion coefficient had to be hypothesized to fit the data. This study was possible only with recent improvements in diagnostics and numerical analysis tools, and it constitutes an important contribution to our understanding of non-inductive current drive for future fusion reactors.