The upgraded JET gamma-ray cameras based on high resolution/high count rate compact spectrometers

A gamma ray spectrometer with Compton suppression on the HL-2A tokamak

A new broad-energy, high-resolution gamma ray spectrometer (GRS) with Compton suppression function has been developed recently in the HL-2A tokamak to obtain the gamma ray information in the energy range of 0.1-10 MeV. This is the first time to develop an anti-Compton GRS for a magnetic confinement fusion device. The anticoincidence detector consists of a large-volume high purity germanium (HPGe) crystal (Φ63 × 63 mm²) as the primary detector and eight trapezoidal bismuth germinate (BGO) scintillators (trapezoid crystal with 30 mm thickness) as the secondary detector. The anti-coincidence data processing is implemented by a digital-based data acquisition system with fast digitization and software signal processing technology. Using radioisotope gamma ray sources and Monte Carlo N-Particle code, the energy and efficiency of the spectrometer have been calibrated and quantitatively tested. The Compton continuum suppression factor reaches 4.2, and the energy resolution (Full Width at Half Maximum) of the 1.332 MeV full energy peak for ⁶⁰Co is 2.1 keV. Measurements of gamma ray spectra with Compton suppression using the spectrometer have been successfully performed during HL-2A discharges with different conditions. The performance of the spectrometer and the first experimental results are presented in this paper.

A high-resolution neutron spectroscopic camera for the SPARC tokamak based on the Jet European Torus deuterium-tritium experience

Dedicated nuclear diagnostics have been designed, developed, and built within EUROFUSION enhancement programs in the last ten years for installation at the Joint European Torus and capable of operation in high power Deuterium-Tritium (DT) plasmas. The recent DT Experiment campaign, called DTE2, has been successfully carried out in the second half of 2021 and provides a unique opportunity to evaluate the performance of the new nuclear diagnostics and for an understanding of their behavior in the record high 14 MeV neutron yields (up to 4.7 × 10¹⁸ n/s) and total number of neutrons (up to 2 × 10¹⁹ n) achieved on a tokamak. In this work, we will focus on the 14 MeV high resolution neutron spectrometers based on artificial diamonds which, for the first time, have extensively been used to measure 14 MeV DT neutron spectra with unprecedented energy resolution (Full Width at Half Maximum of ≈1% at 14 MeV). The work will describe their long-term stability and operation over the DTE2 campaign as well as their performance as neutron spectrometers in terms of achieved energy resolution and high rate capability. This important experience will be used to outline the concept of a spectroscopic neutron camera for the SPARC tokamak. The proposed neutron camera will be the first one to feature the dual capability to measure (i) the 2.5 and 14 MeV neutron emissivity profile via the conventional neutron detectors based on liquid or plastics scintillators and (ii) the 14 MeV neutron spectral emission via the use of high-resolution diamond-based spectrometers. The new opportunities opened by the spectroscopic neutron camera to measure plasma parameters will be discussed.

Role of neutron attenuators for gamma-ray measurements in deuterium-tritium magnetic confinement plasmas

The Joint European Torus (JET) is the only tokamak in the world able to operate in Deuterium-Tritium (DT) plasmas. A successful DT experimental campaign, the DTE2, has recently been carried out, providing unique opportunities for studying both physics and technological aspects. In particular, it allowed us to investigate and benchmark the solutions adopted to attenuate the significant 14 MeV neutron flux, needed to enable high-resolution gamma-ray spectroscopy measurements on a tokamak. While in inertial confinement experiments, gamma-rays and neutrons are discriminated through time-of-flight techniques; in magnetic confinement experiments, the neutron attenuators are a key element to allow gamma-ray measurements in order to reestablish the 1 × 10⁵ to 1 background to signal ratio. In this paper, the role of the reference neutron attenuators at JET, based on LiH, has been analyzed and described.

First spatially resolved measurements of the D-3He α-particle source with the upgraded JET gamma-ray camera

The Joint European Torus (JET) gamma-ray camera has been recently upgraded with the installation of new gamma-ray detectors, based on LaBr₃(Ce) scintillation crystals, which add spectroscopic capability to the existing system allowing measurements with good energy resolution (5% at 0.622 MeV), a dynamic range from hundreds of keV up to about 30 MeV, and high counting rate capabilities of MCps. First gamma-ray measurements during the C38 campaign of the JET have been successfully carried out, in particular, in D-³He plasmas from three-ion ion cyclotron resonance heating experiments, where the detection of 16.4 MeV γ-rays from D + ³He → γ + ⁵Li reactions with the gamma-ray camera upgrade allowed determining the spatial profile of alpha particles born in D + ³He fusion reactions.

Novel compact hard x-ray spectrometer with MCps counting rate capabilities for runaway electron measurements on DIII-D

A novel compact spectrometer optimized for the measurement of hard x rays generated by runaway electrons is presented. The detector is designed to be installed in the fan-shaped collimator of the gamma-ray imager diagnostic at the DIII-D tokamak. The spectrometer is based on a 1 × 1 cm² cerium doped yttrium aluminum perovskite scintillator crystal coupled with a silicon photomultiplier. The detector dynamic energy range is in excess of 10 MeV, with an energy resolution of ∼10% at 661.7 keV. The fast detector signal (≈70 ns full width at half maximum) allows for operation at counting rates in excess of 1 MCps. The gain stability of the system can be monitored in real time using a light-emitting diode embedded in the instrument. The detector is expected to be deployed in the forthcoming DIII-D runaway electron experimental campaign.

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.

Influences of fusion neutrons on Compton suppressed γ-spectrum analyses at the HL-2A tokamak

The newly-built Compton suppression system at the HL-2A tokamak works in a harsh fusion neutron field especially when the second neutral beam injection system is put into application. The present paper performed Geant4 simulations to study the influences of fusion neutrons on Compton suppressed γ-spectrum analyses. The simulation data show that the influence of fusion neutrons on suppressed γ-spectrum shape is limited, while the influence on detection efficiencies is considerable. Consequently, the changes of detection efficiencies caused by the coexisting neutrons must be taken into account in the subsequent γ-spectrum analyses. Hence, the empirical formulas of γ-ray detection efficiencies of the Compton suppression system, which had considered the influences of fusion neutrons, were put forward and then applied to unfold the experimental γ-spectra. This work lays a superior foundation for the further analyses of the Compton suppressed γ-spectra at the HL-2A tokamak.

Development of gamma ray spectrometer with high energy and time resolutions on EAST tokamak

A new gamma ray spectrometer with high energy and time resolutions has been developed and installed on the EAST tokamak to study fast ion and runaway electron behaviors. The spectrometer is based on a LaBr₃(Ce) scintillator detector and a fully digital data acquisition system that is based on a digitizer with digital pulse processing algorithms. The energy resolution of the spectrometer is about 3.9% at 662 keV, and the spectrometer can operate stably at a counting rate as high as 1 MHz, monitored by using a light emitting diode monitoring system. The measured gamma ray spectrum is simulated based on Geant4 and unfolded with the high-resolution boosted Gold deconvolution algorithm, aiming at reconstructing the energy distribution functions of fast ions and runaway electrons.

Design of gamma-ray spectrometers optimized for fast particle studies at ITER

A set of gamma ray spectrometers has been designed for ITER within the Radial Gamma Ray Spectrometer (RGRS) project. The aim of this project is designing a system, integrated with the ITER radial neutron camera, which is able to measure the gamma-rays emitted from the plasma with a good energy resolution (about 1.5% at 4.44 MeV) and at high counting rates (in excess of 1 MHz). The RGRS will be able to operate both in the D phase and in the full-power DT phase and will measure gamma rays from (i) reactions between fast ions, such as α particles, and light impurities and (ii) bremsstrahlung emission generated by runaway electron interactions with both plasma bulk and tokamak walls. The RGRS detectors are arranged in nine lines of sights (able to cover a radial region with r < a/3), each featuring a large LaBr₃ scintillator crystal. Due to the high neutron flux and magnetic field, several solutions have been adopted to guarantee a good signal to background ratio and MHz counting rate capabilities. The RGRS is capable to combine space and energy distribution measurements of α particles and runaway electrons, which will help the study of the fast particle physics in a burning plasma.

JET diagnostic enhancements testing and commissioning in preparation for DT scientific campaigns

In order to optimize the scientific exploitation of JET (Joint European Torus) during the upcoming deuterium-tritium experiments, a set of diagnostic systems is being enhanced. These upgrades focus mainly on the experimental and operational conditions expected during tritium campaigns. It should be stressed that measurements relevant for burning plasmas are specifically targeted. Previously non-available capabilities, such as a current measurement system fully covering all poloidal field circuits, are described in detail. Instrument descriptions, performance prediction, testing, and initial commissioning results of these systems are presented.

Development of a new compact gamma-ray spectrometer optimised for runaway electron measurements

A new compact gamma-ray spectrometer was developed in order to optimise the measurement of bremsstrahlung radiation emitted from runaway electrons in the MeV range. The detector is based on a cerium doped lutetium-yttrium oxyorthosilicate (LYSO:Ce) scintillator coupled to a silicon photomultiplier and is insensitive to magnetic fields. A dedicated electronic board was developed to optimise the signal readout as well as for online control of the device. The detector combines a dynamic range up to 10 MeV with moderate energy non-linearity, counting rate capabilities in excess of 1 MHz, and an energy resolution that extrapolates to a few % in the MeV range, thus meeting the requirements for its application to runaway electron studies by bremsstrahlung measurements in the gamma-ray energy range.

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.