Timing system for neutral beam injection on the DIII-D tokamak

Eight neutral beam (NB) long pulse sources mounted on four beamlines are in use on the D3-D tokamak, a magnetic confinement fusion experiment at General Atomics. The main purpose of NB injection is to heat the bulk plasma with high energy injected neutral particles which undergo ionizing collisions with plasma ions and electrons. NB injection is also used as a diagnostic tool for charge exchange recombination spectrometry to determine ion temperatures and rotation velocities and for neutral particle exchange spectrometry to determine spatial distributions of beam ions. This paper describes recent modifications to the D3-D central timing system hardware and software to allow more flexible and complex timing waveforms than previously available to control NB injection. In the past, for each plasma discharge, the start time and the pulse duration for each NB source could be set. The new NB timing system allows the experimental session leader to specify more than one pulse for each NB source to make efficient use of multiple physics experiments in a single discharge. Also, pulse trains with programmable duty cycles can be independently specified for each NB source to make NB injection a more useful diagnostic tool. Experimental results using features of the new NB timing system are presented. 5 refs., 3 figs