Design of a diamond-based in-vessel soft x-ray camera system for the SPARC tokamak

Soft x-ray tomography has been used in tokamaks for decades to interrogate a wide variety of phenomena; however, the standard silicon diodes coupled with beryllium filters will not be able to survive the high heat and neutron fluxes present in burning plasma devices like SPARC. Presented here is the design for a set of soft x-ray tomography cameras that use chemical vapor deposition (CVD) single-crystal diamonds and graphene filters for photodetection. Four of these cameras will be placed in the port plugs of SPARC with a large enough view of the poloidal cross section to enable tomographic inversion and sufficient time resolution to resolve the dynamics of low-m and -n magnetohydrodynamic modes. CVD diamonds have been used successfully for the detection of both vacuum ultraviolet and soft x-ray photons at the Frascati Tokamak Upgrade and have demonstrated the ability to maintain greater than 50% of their initial responsivity at a neutron fluence of 1016 cm-2. The neutron dose in SPARC will exceed this limit, providing a demonstration of how CVD diamonds fail in place while adding value to the early operation of a new tokamak. Numerical simulations indicate that graphene spectral filters will filter nearly as effectively as traditional beryllium ones and are able to resist significantly higher heat fluxes due to graphene's unique thermal properties. Simulated plasma emissivity profiles have been used to optimize both the camera placement and detector properties.