Design of a Diamond-Based In-Vessel Soft X-Ray Detector for the SPARC Tokamak

Silicon diodes used for soft x-ray (SXR) detection in tokamaks are sensitive to neutron damage, making them unsuitable for use in tokamaks like SPARC. Furthermore, they are typically placed in beamlines several meters away from the plasma, limiting their field of view to a small cross-section. Presented here is the design for an array of chemical vapor deposition (CVD) single-crystal diamonds which will be placed in the upper and lower port plugs of the SPARC tokamak with a large enough view of the poloidal cross-section to enable tomographic inversion. CVD diamonds have been used successfully for the detection of both vacuum ultra-violet and SXR photons at the Frascati Tokamak Upgrade [1], and have been shown to withstand a 14.8 MeV neutron fluence of at least 2.0×10¹⁴ n/cm² [2], much higher than that of silicon diodes. In addition to their neutron tolerance, these diamonds are evaluated for their rise time and responsivity for the purpose of developing an array calibration method. The array design presented here is optimized to provide a wide field of view of the poloidal cross-section while maintaining its positional calibration when exposed to high temperatures during bake-in and g-forces during disruptions. Simulated plasma conditions are used to estimate the x-ray signal that this array will receive and to fine-tune the detector placement within the array.

[1] F. Bombarda, M. Angelone, G. Apruzzese, C. Centioli, S. Cesaroni, L. Gabellieri, A. Grosso, M. Marinelli, E. Milani, S. Palomba, V. Piergotti, G. Pucella, G. Rocchi, A. Romano, A. Sibio, B. Tilia, C. Verona, and G. Verona-Rinati, Nucl. Fusion 61, 116004 (2021).

[2] M. Pillon, M. Angelone, G. Aielli, S. Almaviva, M. Marinelli, E. Milani, G. Prestopino, A. Tucciarone, C. Verona, and G. Verona-Rinati, J. Appl. Phys. 104, 054513 (2008).