Beamline duct monitoring of the TJ-II neutral beam injectors

•The beam reionization problem has been addressed.•Two beam diagnostics in the beam duct have been commissioned: infrared thermography and Fast Ion Gauge pressure measurements.•An upper limit for beam reionization for a well-conditioned duct has been obtained by infrared thermography.•The gas re-emission from the graphite plates has been analyzed from pressure signals. In the stringent geometry of TJ-II NBI ducts, beam reionization can reduce beam transmission by a significant fraction. The re-emitted gas from direct beam impact areas acts as “seed” gas for reionization losses. In order to monitor and control reionization losses two diagnostics have been commissioned: infrared thermography of the beam path and Fast Ion gauge measurements of the pressure buildup at the duct exit during beam pulses in the absence of magnetic field. An upper limit of 20% for reionization losses in a well-conditioned duct has been obtained from infrared thermography. The gas dynamics has been studied, obtaining the re-emitted gas flow (6–12mbarls−1) and a limit for the characteristic time constant of the emission process (