M. Tardocchi

The TOFOR neutron spectrometer and its first use at JET

A time-of-flight neutron spectrometer (TOFOR) has been developed to measure the 2.45 MeV d+d→3He+n neutron emission from D plasmas. The TOFOR design features the capability to operate at high rates in the 100 kHz range, data collection with fast time digitizing and storing, and monitoring of the signals from the scintillation detectors used. This article describes the principles of the instrument and its installation at JET and presents preliminary data to illustrate the TOFOR performance as a neutron emission spectroscopy diagnostic.

Facility for fast neutron irradiation tests of electronics at the ISIS spallation neutron source

The VESUVIO beam line at the ISIS spallation neutron source was set up for neutron irradiation tests in the neutron energy range above 10MeV. The neutron flux and energy spectrum were shown, in benchmark activation measurements, to provide a neutron spectrum similar to the ambient one at sea level, but with an enhancement in intensity of a factor of 107. Such conditions are suitable for accelerated testing of electronic components, as was demonstrated here by measurements of soft error rates in recent technology field programable gate arrays.

Diagnosis of physical parameters of fast particles in high power fusion plasmas with high resolution neutron and gamma-ray spectroscopy

High resolution neutron emission spectroscopy (NES) and gamma-ray spectroscopy (GRS) measurements of fast ions in high power fusion plasmas are reviewed. NES is a well established diagnostics of the velocity distribution of fast fuel ions and was recently used to investigate the interaction of energetic ions with MHD instabilities. High energy resolution GRS on fusion plasmas is a more recent application and was shown to provide information on the distribution function of fast minority ions accelerated by ICRH, such as 4 He and 3 He. Starting from measurements on today’s high power D plasmas, fast ion measurements with NES and GRS in a DT burning plasma of next step tokamaks, such as ITER, are discussed. The enhanced neutron and gamma-ray fluxes expected on ITER will allow for time-resolved measurements of the fast fuel and minority ion dynamics in the ms time scale. The intensity of the alpha knock-on component in the 14 MeV neutron spectrum and of the 4.44 and 3.21 MeV gamma-ray peaks from the 9 Be(α,n γ) 12 C reaction is studied as a diagnostics for the α particle slowing down distribution in a DT plasma. The results show that the two techniques are sensitive to different regions of the α particle phase space and thus provide complementary information. (Some figures may appear in colour only in the online journal)

Measurements of fast ions and their interactions with MHD activity using neutron emission spectroscopy

Ion cyclotron radio frequency (ICRF) heating can produce fast ion populations with energies reaching up to several megaelectronvolts. Here, we present unique measurements of fast ion distributions from an experiment with 3rd harmonic ICRF heating on deuterium beams using neutron emission spectroscopy (NES). From the experiment, very high DD neutron rates were observed, using only modest external heating powers. This was attributed to acceleration of deuterium beam ions to energies up to about 2-3 MeV, where the DD reactivity is on a par with that of the DT reaction. The high neutron rates allowed for observations of changes in the fast deuterium energy distribution on a time scale of 50 ms. Clear correlations were seen between fast deuterium ions in different energy ranges and magnetohydrodynamic activities, such as monster sawteeth and toroidal Alfven eigen modes (TAE). Specifically, NES data showed that the number of deuterons in the region between 1 and 1.5 MeV were decaying significantly during strong TAE activity, while ions with lower energies around 500 keV were not affected. This was attributed to resonances with the TAE modes.

High Resolution Gamma Ray Spectroscopy at MHz Counting Rates With LaBr

High resolution γ-ray spectroscopy measurements at MHz counting rates were carried out at nuclear accelerators, combining a LaBr 3(Ce) detector with dedicated hardware and software solutions based on digitization and off-line analysis. Spectra were measured at counting rates up to 4 MHz, with little or no degradation of the energy resolution, adopting a pile up rejection algorithm. The reported results represent a step forward towards the final goal of high resolution γ-ray spectroscopy measurements on a burning plasma device.

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Recent studies indicate the resonance detector (RD) technique as an interesting approach for neutron spectroscopy in the electron volt energy region. This work summarizes the results of a series of experiments where RD consisting of YAlO3 (YAP) scintillators were used to detect scattered neutrons with energy in the range 1–200 eV. The response of YAP scintillators to radiative capture γ emission from a 238U analyzer foil was characterized in a series of experiments performed on the VESUVIO spectrometer at the ISIS pulsed neutron source. In these experiments a biparametric data acquisition allowed the simultaneous measurements of both neutron time-of-flight and γ pulse height (energy) spectra. The analysis of the γ pulse height and neutron time of flight spectra permitted to identify and distinguish the signal and background components. These measurements showed that a significant improvement in the signal-to-background ratio can be achieved by setting a lower level discrimination on the pulse height at ab...

Scintillators for Fusion Plasma Applications

High-resolution ?-ray measurements were carried out on the Joint European Torus (JET) in an experiment aimed at accelerating 4He ions in the MeV range by coupling third harmonic radio frequency heating to an injected 4He beam. For the first time, Doppler broadening of ?-ray peaks from the 12C(d, p?)13C and 9Be(?, n?)12C reactions was observed and interpreted with dedicated Monte Carlo codes based on the detailed nuclear physics of the processes. Information on the confined 4He and deuteron energy distribution was inferred, and confined 4He ions with energies as high as 6?MeV were assessed. A signature of MHD activity in ?-ray traces was also detected. The reported results have a bearing on diagnostics for fast ions in the MeV range in next step fusion devices.

YAP scintillators for resonant detection of epithermal neutrons at pulsed neutron sources

A new high efficiency, high resolution, fast γ-ray spectrometer was recently installed at the JET tokamak. The spectrometer is based on a LaBr3(Ce) scintillator coupled to a photomultiplier tube. A digital data acquisition system is used to allow spectrometry with event rates in excess of 1 MHz expected in future JET DT plasmas. However, at the lower rates typical of present day experiments, digitization can degrade the energy resolution of the system, depending on the algorithms used for extracting pulse height information from the digitized pulses. In this paper, the digital and analog spectrometry methods were compared for different experimental conditions. An algorithm based on pulse shape fitting was developed, providing energy resolution equivalent to the traditional analog spectrometry method.

High-resolution gamma ray spectroscopy measurements of the fast ion energy distribution in JET 4He plasmas

The fast-ion distribution from third harmonic ion cyclotron resonance frequency (ICRF) heating on the Joint European Torus is studied using neutron emission spectroscopy with the time-of-flight spectrometer TOFOR. The energy dependence of the fast deuteron distribution function is inferred from the measured spectrum of neutrons born in DD fusion reactions, and the inferred distribution is compared with theoretical models for ICRF heating. Good agreements between modelling and measurements are seen with clear features in the fast-ion distribution function, that are due to the finite Larmor radius of the resonating ions, replicated. Strong synergetic effects between ICRF and neutral beam injection heating were also seen. The total energy content of the fast-ion population derived from TOFOR data was in good agreement with magnetic measurements for values below 350 kJ.

Energy resolution of gamma-ray spectroscopy of JET plasmas with a LaBr3 scintillator detector and digital data acquisition.

Observations made in a JET experiment aimed at accelerating deuterons to the MeV range by third harmonic radio-frequency (RF) heating coupled into a deuterium beam are reported. Measurements are ba ...