First results on runaway electron studies using the FTU neutron camera

Highlights: • Upgrade of the neutron camera of the Frascati Tokamak Upgrade with a digital acquisition system. • Set-up of the diagnostic in order to use it as a hard X-rays (HXR) profile monitor for runaway electrons (RE) studies: calibration, neutron–gamma separation and background evaluation. • Preliminary results on HXR profile measurements during different phases of RE discharges (plasma current ramp-up, flattop and ramp-down; disruptions). - Abstract: A digital upgrade of the analogue electronics of the Frascati Tokamak Upgrade (FTU) neutron camera was carried out in order to enable studies on runaway electrons by measuring the hard X-rays produced in the bremsstrahlung interactions between runaway electrons and plasma ions. This "gamma camera" system is based on six radial lines of sight equipped with liquid organic scintillators capable of neutron/gamma discrimination. The digital acquisition system is composed by three digitizers (14-bit, 400 Msamples/s) enabling the separation between neutron and hard X-ray events also in conditions of very high count rate (MHz range). The first measurement results indicate the capability of the diagnostic to provide HXR profiles for energies >0.1 MeV and information on the runaway population during the I_p ramp-up, flattop and ramp-down phases. Disruption events can also be analyzed with sub-millisecond time resolution, although in such phases the data are affected (mainly in the inner three detectors) by the presence of a strong HXR background not originating in the plasma.