Neutron diagnostics for the study of fast ion confinement and scenario developments in the high-field compact spherical tokamak ST40

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In fusion plasmas, fast ions have energies much higher than the thermal plasma background. Fast ions are generated by external auxiliary heating such as Neutral Beam Injection (NBI) and Ion Cyclotron Resonance Heating(ICRH) or by the fusion reactions themselves. In the former cases, fast ions are hydrogen isotopes with energies in the range from tens of keVs up to a few MeVs. Fusion reactions produce, in addition to hydrogen isotopes, alpha particles with energies in the MeV range. Fast ions play an important role in heating the plasma, maintaining the high temperatures necessary to sustain the fusion reactions and crucial in achieving a burning plasma. NBI heating is also important for current drive, that is for long pulse operation of tokamaks beyond the inductive regime and therefore for the realization of a fusion reactor. Confining fast ions in the plasma for time long enough so that they can transfer their energy to the background plasma is therefore crucial for achieving the goal of a power plant based on thermonuclear fusion reactions. However, fast ion confinement is degraded by plasma instabilities some of which are triggered by the fast ion themselves. In this case, energy exchange between the fast ions & the instabilities results in the redistribution and loss of fast ions, ultimately reducing the performances of fusion reactors. Furthermore, the loss of fast ions in the plasma can result in the damage of the reactor first wall, an issue particularly for the very energetic alpha particles that will be produced in DT fusion reactions.The high-field compact spherical tokamak ST40 is a compact device designed to operate at high toroidal magnetic fields up to 3 T (much higher than present day STs) and 2 MA plasma current thus ensuring the confinement of the fast ions generated by the NBI system. These unique capabilities allow to achieve improvement in the performances thanks to the scaling of the fusion power with the 4th power of the magnetic field and to address key questions relative to the capability of reaching break-even conditions (i.e. when the amount of produced fusion power equals the input power) in high fields that are not achievable in other present day STs as demonstrated in recent results. The confinement of the fast ions is expected to be reduced by plasma instabilities such as the Alfvén modes while first orbit losses are estimated to lead to the loss of a significant fraction of particles from DT reactions and both causes the reduction in the generated fusion power. In order to develop scenarios with reduced fast ions redistribution & losses it is essential to measure how well they are confined in ST40. At present,ST40 is equipped an activation system to monitor to the total neutron yield and a liquid scintillator for the collimated observation of the fast ion population. Diamond detectors for 2.45 and 14.1 MeV neutrons (from DD and DT reactions respectively) will be installed.This project will be carried out in two phases. The first phase will be focused on the characterization of all three neutron diagnostics (activation system, liquid scintillators and diamonds) to provide an integrated characterization of different operating regimes in ST40 with NBI heating. In this phase, the correct modelling of the detectors' response functions will be carried out using radiation transport codes such as NRESP, MCNP, FLUKA and GEANT4. The response functions will then be evaluated in calibration facilities able to provide the relevant neutron fields such as the high yield, 14 MeV NESSA (NEutron Source inUppSAla) facility and NPL. Crucial in this phase is also the development of on-line data acquisition and processing based on FPGAs. The second phase will be dedicated to the modelling of the neutron emissivity using plasma transport codes such as TRANSP/NUBEAM, ASCOT/AFSI and DRESS which can then be compared to the experimental observations. This would allow to verify & constrain the predictions