Prototyping activities supporting the design of a spectrometric neutron camera for SPARC

The SPARC tokamak, now under construction in Devens, MA by Commonwealth Fusion Systems (CFS), is predicted to access the burning plasma regime Q_p > 5 producing up to 140 MW of DT fusion power or ∼ 5 × 10¹⁹ neutrons per second. A poloidal neutron camera capable of resolving neutron emission in time, space, and energy is being designed for the device. Neutron cameras have been fielded on a variety other magnetic confinement fusion devices (JET, TFTR, LHD, MAST-U) but the SPARC neutron camera will be the first to operate with energy-resolved detector units, enabling more accurate emissivity reconstructions and measurement of the ion temperature profile. This spectral capability is provided by two modern detector technologies: single-crystal chemical vapor deposition diamonds and deuterated-xylene liquid organic scintillators. As part of the development of the SPARC neutron camera, a variety of laboratory prototyping activities have been undertaken. In this work we discuss how these activities, combined with synthetic diagnostic modeling, are being used to characterize the instrument's performance and robustly quantify its uncertainty. These activities include quantification of spectral capabilities using DT and DD beam-target neutron generators, scintillator characterization with a fast LED pulser, evaluation of detector gamma ray sensitivity and rejection capabilities, neutronics modeling with OpenMC, and irradiation of an integrated camera channel prototype including a collimator and both detector types. A workflow for quantifying the uncertainty in both emissivity and ion temperature profile reconstructions using the ToFu code [1] is outlined.

[1] D. Vezinet et al. 2016 Nucl. Fusion 56 086001