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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.

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.

JET plasmas show fluctuations from a large variety of MHD instabilities. A combination of spectral analysis, mode number analysis, and determination of mode radial position has the potential as a diagnostic for the plasma q profile. Mode frequencies and mode numbers are obtained from spectral and phase analysis from fast magnetic pickup signals. Cross correlation of magnetic and multiarray signals from ECE, SXR, and reflectometer systems provide the mode localization. The present status of the diagnostic systems used for fluctuation analysis is summarized, with particular emphasis on the fast magnetic pickup system. A new data collection system allows the magnetic signals to be recorded at 2 MHz for up to 32 s in each pulse. An example of q-profile validation from the observation of Alfven eigenmode cascades and a snake is given. At JET, Alfven cascades are used to determine q(min) in optimized shear discharges, while a variety of low frequency modes (such as the snake) allow the determination of the radial position of q integer values in monotonic and reversed shear q profiles. (C) 2004 American Institute of Physics.

In neutral beam injection and ion cyclotron resonance frequency (NBI+ICRF) heated high-temperature Joint European Torus (JET) plasmas, electron cyclotron emission (ECE) spectra measured by the Michelson interferometer are inconsistent with the electron bulk distribution being Maxwellian. This conclusion follows from new, accurate modeling of the ECE spectra over the full measured frequency range, covering the first four harmonics of the cyclotron frequency. A model distribution function obtained by fitting a measured ECE spectrum reveals a distortion of the Maxwellian that is sharply localized at ∼1.5×thermal momentum. A consequence deduced from this distribution is that the electron temperature measured by JET’s Thomson scattering diagnostic, which weighs the bulk electrons differently, should be lower than the ECE one in the spatial region where this distortion exists. This prediction appears to be confirmed by temperature profile comparisons showing that core Thomson scattering electron temperatures on JET are up to 20% lower than ECE values for plasmas with strong NBI+ICRF heating. The possibility of addressing experimentally the question of whether the plasma has a Maxwellian bulk is also discussed. This goal can be accomplished by observing the emission at different angles with respect to the magnetic field (oblique ECE). Simulations of oblique ECE spectra using the model distribution function obtained previously show an angular variation of the emission that is characteristic of the distorted bulk.

Electron cyclotron emission measurements are routinely performed in TJ-II stellarator by means of a multichannel heterodyne radiometer. The radiometer is absolutely calibrated and measures the temperature profile with high temporal resolution. The description of the radiometer, the calibration procedure, and some data taken during the occurrence of fast phenomena in the plasma (edge localized mode-like events and central temperature crashes) are presented in the article.

The capabilities of the Joint European Torus (JET) electron cyclotron emission (ECE) diagnostics have recently been extended with an upgrading of the heterodyne radiometer. The number of channels has been doubled to 96 channels, with a frequency separation corresponding to 2.6 m for the X-mode due to harmonic overlap) at almost all magnetic field values used at JET (1.7 T

An improved correlation reflectometer technique has been developed and implemented on TJ‐I to measure the radial correlation lengths of the density turbulence. The method uses homodyne detection and is based on the slow sweeping of one oscillator while the other remains at fixed frequency. The decay of the coherence shows maxima and minima as the distance between the two probing points increases. The maxima correspond to the relevant coherence values. The correlation length for the overall turbulence as well as the correlation length for the different frequency intervals can be directly determined in a single shot. By the operation in the Q‐band (33–50 GHz) extraordinary mode, the system reaches most of the plasma radius. Correlation lengths obtained at the edge are compared with those measured at deeper positions in the plasma. The proposed method can provide the values of the differential phase delay along the sweep, avoiding the effect of lost fringes even in the presence of turbulence. This opens the possibility of obtaining the information on the density profile if several channels are used.

Electron density profile measurements have been obtained on PBX‐M by amplitude modulation reflectometry. With this technique, the measurement of the time delay is obtained from the phase delay of the modulating envelope (200 MHz in PBX‐M). The system operates with the extraordinary mode, in the range 32–50 GHz. Under these conditions the density profile of PBX‐M can be probed from the scrape‐off layer up to typically r/a=0.7. With a final bandwidth of 40 kHz, the reflectometer is able to obtain the edge profile in 1 ms. The profiles obtained are relatively noise free and in good agreement with Thomson scattering measurements. Perturbations due to the strong‐edge turbulence are kept to a minimum, and no software filtering or other signal processing was necessary to extract the time delay information from the raw data. Profiles have been measured for ohmic and Neutron Beam Injection heated discharges.

One of the main problems which arises in density profile measurements by reflectometry is the need for continuous tracking of the phase delay; fast density fluctuations and strong modulation on the amplitude of the reflected signal can cause ‘‘fringe jumps’’ in the phase signal. Amplitude modulation reflectometry performs a time delay measurement by the determination of the phase delay of the modulating envelope of a millimeter wave reflected by the plasma. The phase delays involved are small, the measurement is not affected by the fluctuations and can be directly performed without complicated fringe counters; the method provides a promising possibility for real time determination of the plasma position and density profile. In the present paper the principles of the method are presented as well as the analysis on the effects of phase and amplitude fluctuations, dispersive effects, and accuracy requirements.

The European Fusion Development Agreement's mission for JET is the development of ITER scenarios exploiting the specific properties of the device. This task requires significant improvements in the measuring techniques. The most innovative diagnostic upgrades are in the fields of edge measurements, detection of fast magnetohydrodynamics modes and "burning plasma" diagnostics. The importance of plasma-wall interactions, and, in particular, the issue of tritium inventory promoted the development of the quartz microbalance, a detector with improved time resolution to measure material redeposition in the remote areas of the inner divertor. Measurement of Alfven cascades with unprecedented spectral resolution, reaching a toroidal n number of up to 16, was obtained using an O-mode microwave reflectometer as an interferometer. For the diagnosis of the fusion products, a new approach is being developed to measure the He ash based on double charge exchange between thermalized particles and neutrals from JET beams. There have been several upgrades of the neutron diagnostic systems, one of which, the new time of flight neutron spectrometer designed for high counting rates is described. (c) 2006 American Institute of Physics.

A two-dimensional code based on the Wentzel–Kramers–Brillouin approximation is used to simulate the ability of reflectometry techniques to reproduce the average density profile in an ignition device to test engineering concept (ITER) size plasma with density turbulence. Analysis is mainly performed for frequency modulation [(FM) swept frequency] systems, with some comparison with the amplitude modulation techniques. The effects of turbulence with nonperpendicular reflection and plasma rotation are analyzed. Perturbation effects seem to appear for the FM technique, although the real impact for reflectometry in ITER will strongly depend on the parameters of turbulence and rotation. The influence of the turbulence level, fluctuation wavelengths, and antenna beam size on the density profile determination has been studied in a static plasma with perpendicular launching. The vector average of the reflected e field improves the resultant profiles in turbulent plasmas, while an inappropriate choice of the antenna...

A method for determination of radial correlation lengths of the electrostatic turbulence has been developed and applied to the TJ‐I tokamak [T. Estrada, J. Sanchez, B. Branas, and A. P. Navarro, Rev. Sci. Instrum. 61, 3034 (1990)]. The method uses homodyne detection and is based on the slow sweeping of one oscillator while the other remains at fixed frequency. The decay of the coherence can be observed as the distance between the two probing points increases. By the operation in the Q band (33–50 GHz) extraordinary mode, the system reaches most of the plasma radius. Correlation lengths obtained at the edge are compared with those measured at the plasma center. A critical issue for this comparison is the detailed knowledge of the local density gradient. The proposed method can provide the density profile information, avoiding the effect of lost fringes, even in the presence of turbulence. This fact can be used to determine the local density gradient which is needed to evaluate the separation between both r...