Upgraded alignment control for the DIII-D Thomson scattering laser system

Abstract

The Thomson scattering system measures electron density and temperature with the aid of eight pulsed Nd:YAG lasers on the DIII-D tokamak. DIII-D is the United States National Fusion Facility for magnetic confinement fusion experiments. The Thomson system probes the tokamak in three separate regions. The measurements from the different regions produce density and temperature profiles during plasma discharges. The laser light must be aimed through the DIII-D vessel with 1 mm of spatial accuracy to produce correct density profiles. Recent upgrades to the alignment system reduce the effort required to perform the initial alignment before plasma operations, and improve the monitoring and control of the alignment. The upgraded hardware allows for monitoring and control of the alignment from the Thomson control room during operations and from multiple key locations along the 35 m long optical path. In future upgrades the adjustment of the mirrors will be automated by utilizing feedback from a computerized beam analysis system that relays beam position information to the control computer. The newly established methods for setting up the YAG beams using the CCD cameras instead of burn paper offer a faster and more reliable method to prepare the system for plasma-operations. The YAG lasers are aimed with repeatable accuracy and any drifts can be detected and corrected during the small warm up period before each shot.