A new dedicated signal processing system for gamma-ray spectrometers in high power deuterium-tritium plasma scenarios in tokamaks.

The most performant deuterium-tritium (DT) plasma discharges realized by the Joint European Torus (JET) tokamak in the recent DT campaign have produced neutron yields on the order of 1018 n/s. At such high neutron yields, gamma-ray spectroscopy measurements with scintillators are challenging as events from the neutron-induced background often dominate over the signal, leading to a significant fraction of pileup events and instability of the photodetector gain along with the consequent degradation of the reconstructed spectrum. Here, we describe the solutions adopted for the tangential lanthanum bromide spectrometer installed at JET. A data acquisition system with free streaming mode digitization capabilities for the entire duration of the discharge has been used to solve dead-time related issues and a data reconstruction code with pileup recovery and photodetector gain drift restoration has been implemented for off-line analysis of the data. This work focuses on the acquired data storage and parsing, with a detailed explanation of the pileup recovery and gain drift restoration algorithms.

A new tangential gamma-ray spectrometer for fast ion measurements in deuterium and deuterium-tritium plasmas of the Joint European Torus.

A new tangential gamma-ray spectrometer has been developed for fast ion measurements in deuterium and deuterium-tritium plasmas of the Joint European Torus (JET). The instrument is based on a LaBr3 crystal with a photo-multiplier tube and replaces a pre-existing bismuth germanate detector, providing enhanced energy resolution and a counting rate capability in the MHz range. The line of sight is equipped with a LiH attenuator, which reduces the background due to 14 MeV neutron interactions with the crystal by more than two orders of magnitude and enables the observation of gamma-ray emission from confined α particles in JET deuterium-tritium plasmas. Thanks to its tangential line of sight, the detector can distinguish co- and counter-passing ions. The performance of the instrument is demonstrated through the results of recent JET fast ion experiments in deuterium plasmas.

Measurements of neutron fluxes from tokamak plasmas using a compact neutron spectrometer.

A compact neutron spectrometer based on the BC-501A liquid organic scintillator was applied to neutron measurements at the TUMAN-3M tokamak. The spectrometer was calibrated using measurements from the ion beam of the cyclotron accelerator. Neutron spectra were measured during discharges using a neutral deuterium beam injection into the TUMAN-3M D-plasma. An energy distribution of the neutrons from the plasma that hit the spectrometer was obtained from the measured BC-501A instrumental spectra by the DeGaSum code using detector response functions obtained in the course of the calibration. This allowed for the estimation of the 2.45 MeV neutron yield and the evaluation of both the time evolution of the DD fusion rate and the characteristic time of the injected deuterium slowing down in discharges with neutral beam injection heating.

High resolution gamma-ray spectrometer with MHz capabilities for runaway electron studies at ASDEX Upgrade.

A new gamma-ray spectrometer with MHz capabilities has been developed to measure the bremsstrahlung emission spectrum in the gamma-ray energy band generated by MeV range runaway electrons in disruption experiments at ASDEX Upgrade. Properties of the runaway electrons are inferred from the measured bremsstrahlung spectrum by a deconvolution technique, particularly with regard to their maximum energy. Changes induced to the runaway electron velocity space are unambiguously observed both in massive gas injection and resonant magnetic perturbation experiments with the detector.