SPARC Laser Aided Plasma Diagnostics for Q > 1 Campaign

To help ensure SPARC achieves its operational and scientific milestones, two laser-based diagnostics are included in its early campaign diagnostic set: the Two-Color Heterodyned interferometer (INTF) and Core Thomson Scattering (CTST). The INTF system is essential for tokamak operations providing active density feedback control from day-1 via real-time streaming of line averaged core plasma density at 10 kHz to the plasma control system (PCS). Reliance on INTF for density feedback control informs the design to be as robust as possible, prompting the focus on a single plasma leg with active alignment tracking, ≥ 10 W laser power, NIR & CO₂ wavelengths to minimize optical losses, and redundant systems to maximize signal reliability & minimize downtime. SPARC CTST is built with the same principles in mind, utilizing the early stages of the first campaign to commission & validate the diagnostic performance. Initially, CTST will operate at a 100 Hz laser repetition rate with a few spatial positions over 0.0 < r/a < 0.6, before expanding to higher spatial resolution and possibly a future Edge TS. Neutron shielding requirements on SPARC preclude direct line of sight to the plasma for laser beamlines and collection optics, requiring multiple mirrors inside and outside the vacuum chamber. For INTF and CTST this means a careful consideration of optical substrates, coatings, in-vessel location, and protective structures to maintain effective diagnostic operation over the course of multiple SPARC campaigns.