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Description
The ITER Radial Neutron Camera (RNC) is a key diagnostic system designed to provide time resolved measurements of the neutron source profile through the application of reconstruction techniques to the line-integrated neutron fluxes. It is composed of two subsystems, the In-Port RNC and Ex-Port RNC located, respectively, inside and outside the Plug of Equatorial Port #01. The Ex-Port RNC features 16 Lines of Sight (LoS) along which the neutron flux is measured by a set of 3 detectors, two of which are scintillators (EJ-276G plasƟc and 4He gas) coupled to photomulƟplier tubes (PMTs). PMTs suffer gain variations (due to change of the incoming neutron flux, temperature drifts and PMT aging) that need to be monitored and corrected via software reconstruction. This is done using a light source of well-known characteristics (amplitude, shape, duration), typically LED signals. In the RNC the light source is foreseen to be coupled to the PMTs through optical fibers. This work presents the design of a novel system of PMT gain monitoring using a 520 nm LASER diode as replacement of the traditional LED-based system that is not suitable in the ITER environment due to the insufficient power, given the expected radiation-induced reduction of the optical fiber transmission
throughout the ITER lifetime. An overall description of the LASER system is provided including layout, positioning, optical components, fibers, detectors and PMTs. The LASER can generate customizable pulses to be routed through radiation-hard optical fibers to the detectors. Experimental tests run with PMTs coupled to the scintillators enabled us to determine the optimal set-up of the LASER pulses in terms of LASER current, pulse shape and width. The LASER is found to produce stable and tunable signals suitable for PMT monitoring, achieving LASER pulse height resolutions as low as 2.4%.
The use of a single LASER diode for monitoring the whole set of 32 RNC PMTs simplifies the system architecture and improves maintainability in the ITER’s remote and radiation-intensive environment.
