In-vessel design of a two-color heterodyne laser interferometer system for SPARC

This article covers the in-vessel design of the SPARC interferometry diagnostic system, highlighting unique aspects of the systems design and port plug integration in preparation for "day-1" plasma operations as a critical diagnostic for density feedback control. An early decision for the diagnostic was to deploy two lasers in the infrared wavelength spectrum, allowing the system to have a higher optical throughput. The optimization of the in-vessel geometry for the diagnostic follows a similar approach, focusing on de-risking possible damage to the plasma facing optical components by moving them further from the plasma with an orientation that provides a greater possibility for protective features to be added. The inclusion of in-vessel optical assemblies requires detailed design efforts of custom all-metal parts, designed to remain functional when subjected to harsh operational conditions, in many cases for the entire SPARC lifetime. The details presented here were included in the design to ensure that the assemblies can not only withstand a major electromagnetic disruption or thermal event but also maintain good stability through normal operations. The design also addresses more nuanced effects, such as the transient heat loading of the plasma facing mirrors. Through the utilization of modeling and design tools, these effects were brought into the design and simulation workflow, further reducing uncertainty as the system moves toward system commissioning.