Development of a laser sweeping system for precise positioning of Thomson measurement in KSTAR

Abstract

Thomson diagnostics are essential for diagnosing electron density (ne) and electron temperature (Te) profiles in most plasma machines. However, precise alignment of the laser path with the collection optics focus is crucial to obtain the Thomson signal. Misalignment can lead to inaccuracies in measuring the signal and may introduce errors in plasma parameters. Therefore, precise control of the laser beam position inside the vacuum vessel is necessary. Many researchers involved in Thomson diagnostics carefully adjust the laser beam path and position to ensure accurate measurements.

The KSTAR Thomson system traditionally aligned the laser beam with the collection optics focus manually using LED backlighting. However, this alignment process is challenging and time-consuming due to radiation in the vacuum vessel. To address this issue, a laser sweeping system was recently installed to enhance alignment accuracy and minimize working time in the vacuum vessel. This system includes a step motor installed on the last laser guiding mirror, enabling remote adjustment of the laser beam position from the diagnostics room.

The Rayleigh signal was measured while sweeping the laser in both the vertical and horizontal directions under a 40 mbar nitrogen gas condition. The obtained signal was analyzed using a three-pulse fitting method. The vertical and horizontal sweeping intervals were set to 20 steps. As a result, for vertical sweeping, both the core and edge regions showed peak signals around the +60 step region. For horizontal sweeping, the core region exhibited a peak at +60 steps, whereas for the edge region, the signal increased as the position moved below -60 steps. This indicates that the horizontal focal position of the edge lens is aligned more inward than initially expected. This study demonstrates that precise alignment can be achieved by horizontally and vertically sweeping the mirror from the laser injection side. And the purpose of laser sweeping in this study is to verify and correct slight misalignments that may occur after the baking process.