Off-axis neutron camera for MAST Upgrade

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 and the instabilities result 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 ITER and DEMO.
MAST Upgrade is equipped with an on-axis and an off-axis neutral beam injector. The on-axis NBI, injects fast ions on MAST Upgrade equatorial plane in tangential direction resulting in fast ion deposition profile that is peak in the plasma core. The large fast ion pressure gradient resulting from the on-axis NBI system is known to drive plasma instabilities that reduce the fast ion confinement and thus the plasma performances. The off-axis NBI is also injected tangentially but approximately 60 cm above the equatorial plane, thus broadening the fast ion deposition profile and reducing the fast ion pressure gradient suppressing the drive of these plasma instabilities. Proof-of-concept of this was demonstrated on MAST and confirmed by preliminary results on MAST Upgrade.
Among the wide range of diagnostics dedicated to the study of fast ions, neutron diagnostics provide information on the confined fast ions thanks to the fact that on MAST Upgrade, neutron emission is dominated by beam-thermal nuclear reactions. Neutron emission on MAST Upgrade is measured by a recently installed 6-channels collimated neutron flux monitor looking in tangential direction on the equatorial axis.
The project will focus on the design, development, construction, installation and commissioning of a multi-channel, collimated neutron flux monitor dedicated to the measurement of the neutron emissivity off-axis, where the fast ions are being deposited by the off-axis NBI.
Analysis of initial observations are expected to be part of this project.