MITICA neutron diagnostic: Conceptual design

This report presents the conceptual design of the CNESM neutron diagnostic for MITICA. It is submitted as Deliverable P1 of Task 4.10, F4E contract F4E-RFX-PMS_A-WP-2017, Work Programme2017. It is an updated version of RFX_MITICA_TN_099_r5, deliverable of F4E-RFX-PMS_A-WP-2016.The main changes made in this version can be found in chapter 2.5 and 8, where a new nGEM detector design and the updated Monte Carlo model of MITICA beam dump and CNESM diagnostic system arepresented. The CNESM system for SPIDER has been the object of a detailed design study leading to the overall system description reported in RFX_SPIDER_TN_169_r4. The present conceptual study presents the evolution of the CNESM concept required when moving from SPIDER to MITICA. On MITICA the CNESM diagnostic will be particularly useful to resolve the horizontal profile of the beam intensity. The CNESM detection system aims at providing the map of the neutron emission on the beam dump surface by placing a detector in close contact with the emitting surface. It is placed right behind the MITICA beam dump panels and it uses nGEM as neutron detectors. These are Gas Electron Multiplier detectors equipped with a cathode that also serves as neutron-proton converter foil. On MITICA the spatial resolution requirement is driven by the possible changes in the horizontal profile of the beam power due to e.g. deviations from optimal divergence or alignment. The spatial resolution of CNESM on MITICA depends on this distance and other effects investigated with the help of simulations. Simulations of the different steps from the deuteron interaction with the beam dump to the neutron detection in the nGEM were carried out. Neutron transport across the beam dump was performed with the MCNP611 code. In 2014 the first simulations at a neutron emission angle equal to 0° were performed in order to investigate the neutron energy distribution impinging on the pixelated detector surface. In 2015, new simulations were performed in order to evaluate the scattering contribution in the beam dump at higher emission angles. The scattering contribution is very high but can be limited by considering only high energy neutrons and by using a cathode that also serves as a n-p converter and as a p energy selector. A cathode composed of an Al and a CH2foil was implemented and its effect was deeply simulated in 2016 and 2017. Such a cathode is designed for detection of neutrons emitted with an angle around 20° with respect to the deuterium beam axis. This gives a spatial resolution of the CNESM diagnostic system of about 5 cm in the horizontal direction, which is compatible with the MITICA requirements.