• enea
• fusion reactors
• imas
• iter
• neutron diagnostics
• radial neutron camera
• rnc
• synthetic diagnostic
• tokamak

During this thesis work a software model of an ITER neutron diagnostic, the Radial Neutron Camera (RNC), was developed and tested within a dedicated ITER framework (IMAS, Integrated Modelling & Analysis Suite). The software model represents a "synthetic diagnostic" and simulates the RNC measurement process based on the geometrical layout of the system, the characteristics of the detectors and the ITER plasma neutron source. The simulations were conducted on the ITER cluster server system through “No Machine” virtual machine.
The RNC is a system designed to monitor in real-time the ITER neutron emission profile for plasma control purposes. The diagnostics consists of a set of 22 collimated Lines of Sight (LOS) covering a whole poloidal cross section of the plasma, distributed in two subsystems: the In-port RNC (located inside an ITER equatorial port and looking at the plasma edge) and the Ex-port RNC (located in the port interspace in front of the same equatorial port and looking at the plasma core). Each In-port LOS is equipped with two detectors (an 238U fission chamber and a single crystal diamond matrix detector) while Ex-port LOS are equipped with 3 detectors (a plastic scintillator, a 4He scintillator and a single crystal diamond matrix detector), for a total of 60 detectors. The detectors provide line-integrated count-rate measurements from which the 1D local emissivity profile (neutron emitted by the plasma per unit time and volume, n s^-1 m^-3) can be derived by applying an inversion algorithm.
The creation of a synthetic diagnostic integrated into an ITER framework is an important step in the development of the RNC, since it enables the study of the diagnostic performance based on a shared database of plasma scenarios and allows the comparison of the RNC measurements with those of other plasma diagnostics. Moreover, it makes the results of RNC measurements available for physics/engineering analyses, in particular by the ITER team responsible for developing and implementing the technologies and systems needed to keep the plasma stable, controlled and confined within the reactor.
The work of thesis involved three successive phases. As a first step the data structure of the RNC was set up according to IMAS rules; such structure contains data such as detector positions, collimator positions and areas, active areas of the detectors and detector efficiencies with associated energy thresholds.
As second step the actual physical model of the RNC was implemented in C++ and subsequently integrated with IMAS. The physical model is the IMAS block that calculates the reconstructed 1D emissivity profile for a given plasma scenario available in IMAS by:
• Retrieving the reference neutron emissivity and the magnetic equilibrium data for the scenario.
• Using the reference emissivity, the equilibrium and the RNC data structure to simulate the line-integrated measurements provided by the detectors. The synthetic measurements include error terms associated to: Poisson statistics of detector counts, detectors’ background, uncertainty on detector efficiencies, uncertainty on collimator lengths/diameters.
• Applying to the line-integrated RNC measurements an inversion algorithm based on Tikhonov regularisation with a first derivative objective functional.
As a third and final step, the code was validated by applying it to a set of plasma scenarios spanning the expected range of the ITER DT neutron emissivity. The 1D emissivity profiles were reconstructed taking into account the time resolution and the spatial resolution requirements of RNC real-time measurements (10 ms time bin and a/10 with a equal to the ITER minor radius, respectively) and compared with the reference emissivity profiles of the scenarios.