In this paper we present the design of a photon pre-amplifier based on a photo-cathode coated Thick Gas Electron Multiplier (THGEM). Such device is crucial in application where a weak light signal produced in a radiation detector must be amplified so that it can be carried to a photo-detector by means of optical fibres. An example of a device where a light signal must be amplified is a gamma-ray Cherenkov detector for fusion power measurements in magnetic confinement devices. In such application the active part of the detector must be located very close the plasma, typically in a harsh radiation environment where standard photodetectors cannot operate. The photon pre-amplifier allows to increase the signal generated in the active part of the detector so that it can be easily detected by the photodetector located outside the harsh environment. We present the conceptual design of a THGEM based photon pre-amplifier supported by Garfield++ simulations. The device working principle is the following: primary photons impinge on the photo-cathode and extract electrons that are accelerated by the THGEM electric field. Upon collisions with the accelerated electrons, the gas molecules in the pre-amplifier are brought to excited states and de-excite emitting scintillation photons. Since each electron excites multiple gas molecules, the scintillation photons outnumber the primary photons, leading to the amplification. In addition, we present the first observation of measurements of Nitrogen gas scintillation in a THGEM device. We devised an experimental setup consisting of a vacuum chamber containing a THGEM and an alpha particle source. The vacuum chamber is filled with pure nitrogen and is coupled to a photomultiplier tube via a view-port to detect the scintillation photons generated in the THGEM. For sake of simplicity the electrons that induce the scintillation are generated by the ionization track of an alpha particle rather than by the THGEM photo-cathode coating. A good qualitative agreement between simulations and experiment has been found, however no quantitative conclusions can be made due to the lack of N2 excitation cross sections in the Garfield++ code.

As of today, the standard method employed in tokamaks for the absolute measurement of the neutron flux (thus of the nuclear fusion power) is based on activation foils, being the most robust and unbiased technique for the absolute determination of neutron fluence. However, this technique is not able to provide real-time data useful for the control of future fusion plants like DEMO. In this paper, we present some preliminary results about the R&D activity aimed at developing the Single-crystal Diamond Detectors (SDD) used for fast neutron measurements into an absolute neutron flux monitor. Tests have been conducted at the new NILE neutron source of the Rutherford-Appleton Laboratory, a facility with compact neutron generators with a maximum yield of 109 n/s and 1010 n/s for 2.5 MeV and 14 MeV neutrons, respectively. A series of neutron spectra and flux measurements have been taken with different SDD and associated DAQ. Comparisons with standard activation foils (and namely Fe, Zr, Al and Nb foils for 14 MeV neutrons and In for 2.5 MeV neutrons) and with other reference detectors are presented and discussed. Also discussed is the stability of the SDD over time when employed at high neutron rates in realistic neutron environment, and the effects of neutron irradiation on both the counting rate and detector resolution.

Silicon carbide detectors represent an alternative to diamond detectors for fast neutron detection in harsh environments, especially fusion plasmas. Previous studies on thin prototypes (either 10 mi m or 100 mi m thick) suggested that thicker active volumes might be better suited for spectroscopy measurements, due to the higher chance of retaining the neutron interaction products inside the active volume. Therefore, in this work two 250 mi m SiC prototypes are tested with alpha particles following the same process conducted in the past for thinner prototypes. A stable detection is demonstrated, along an energy resolution that, if projected to DT neutrons, could become the lowest achieved so far with a SiC detector (1.3%). Some difficulties in reaching a full depletion are highlighted, as long as perspectives of a partial polarization operation of the detectors.

Interferometry is the commonly exploited technique for electron density measurements in magnetically confined fusion plasma experiments. Reliable electron density measurements are fundamental both for machine protection and for plasma physics understanding. In the last years, attention was drawn on the dispersion interferometer concept, because of its robustness and simplicity. Nevertheless, the heterodyne version of this configuration, which has several advantages over the homodyne scheme, loses one of the main benefits of the dispersion interferometer technique, that is its inherent insensitivity to vibration errors. In this paper, two methods are proposed and theoretically investigated to reduce the vibration noise in the electron density measurements performed with heterodyne dispersion interferometers.

The future DEMO tokamak will be equipped with a suite of diagnostics which will operate as sensors to monitor and control the position and operation parameters of DT plasmas. Among the suite of sensors, an integrated neutron and gamma-ray diagnostic system is also studied to verify its capability and performance in detecting possible DEMO plasma position variations and contribute to the feedback system in maintaining DEMO DT plasma in stable conditions. This work describes the progress of the conceptual study of the gamma-ray diagnostic for DEMO reactor performed during the first Work-Package contract 2015-2020. The reaction of interest for this Gamma-Ray Spectrometer Instrument (GRSI) consists of D(T, ?)5He with the emission of 16.63 MeV ? rays. Due to DEMO tokamak design constraints, the gamma and neutron diagnostics are integrated, both featuring multi-line of sight (camera type), viewing DEMO plasma radially with vertical (12) and horizontal (13) viewing lines to diagnose the ? and neutron emission from the DT plasma poloidal section. The GRSI design is based on the investigation of the reaction cross sections, on the calculations performed with GENESIS and MCNP simulation codes and on the physics and geometry constrains of the integrated instrument. GRSI features long collimators which diameters are constrained by the neutron flux at the neutron detectors of the Radial Neutron Camera (RNC) system placed in front, which are key to control DEMO DT plasma position. For these reasons, only few GRSI parameters can be independently selected to optimize its performance. Among these, the choice of the collimator diameters at the back side of the neutron detector box up to the GRSI detector, the use of LiH neutron attenuators in front of the GRSI detectors, the GRSI detector material and shielding. The GRSI detector is based on commercial LaBr3(Ce) inorganic scintillating crystal coupled with a photomultiplier tube or a silicon photomultiplier. They are designed to operate at high count rate although GRSI geometry constraints severely impact on this feature. The GRSI can also provide an independent assessment of DEMO DT fusion power and T burning.

At present, the only method for assessing the fusion power throughput of a reactor relies on the absolute measurement of 14 MeV neutrons produced in the D-T nuclear reaction. For ITER and DEMO, however, at least another independent measurement of the fusion power is required. The 5He* nucleus produced in the D-T fusion reaction has two de-excitation channels. The most likely is its disintegration in an alpha particle and a neutron, D + T -> 5He* -> ? + n, by means of the nuclear force. There is however also an electromagnetic channel, with a branching ratio ~10-5, which leads to the emission of a 17 MeV gamma-ray, i.e. D + T -> 5He* -> 5He + ?. The detection of this gamma-ray emission could serve as an independent method to determine the fusion power. In order to enable 17 MeV gamma-ray measurements, there is need for a detector with some coarse energy discrimination and, most importantly, capable of working in a neutron-rich environment. Conventional inorganic scintillators, such as LaBr3(Ce), have comparable efficiencies to neutrons and gamma-rays and they cannot be used for 17 MeV gamma-ray measurements without significant neutron shielding. In order to overcome this limitation, we here propose the conceptual design of a gamma-ray counter with a variable energy threshold based on the Cherenkov effect and designed to operate in intense neutron fields. The detector geometry has been optimized using Geant4 so to achieve a gamma-ray to neutron efficiency ratio better than 105. The design is based on a gas Cherenkov detector and the photo-sensor is still to investigated.

O-mode reflectometry, a technique to diagnose fusion plasmas, is foreseen as a source of real-time (RT) plasma position and shape measurements for control purposes in the coming generation of machines such as DEMO. It is, thus, of paramount importance to predict the behavior and capabilities of these new reflectometry systems using synthetic diagnostics. Finite-difference time-domain (FDTD) time-dependent codes allow for a comprehensive description of reflectometry but are computationally demanding, especially when it comes to three-dimensional (3D) simulations, which requires access to High Performance Computing (HPC) facilities, making the use of two-dimensional (2D) codes much more common. It is important to understand the compromises made when using a 2D model in order to decide if it is applicable or if a 3D approach is required. This work attempts to answer this question by comparing simulations of a potential plasma position reflectometer (PPR) at the Low Field-Side (LFS) on the Italian Divertor Tokamak Test facility (IDTT) carried out using two full-wave FDTD codes, REFMULF (2D) and REFMUL3 (3D). In particular, the simulations consider one of IDTT's foreseen plasma scenarios, namely, a Single Null (SN) configuration, at the Start Of Flat-top (SOF) of the plasma current.

The fusion power produced in a DT thermonuclear reactor is currently determined by measuring the absolute 14 MeV neutron yield of the D(T, ?)n fusion reaction. Measurements of 17 MeV gamma rays born from the much less probable D(T, 5He)? reaction (branching ratio of ~10-5) have been proposed as an alternative independent method to validate the neutron counting method and also to fulfill the requests of the nuclear regulator for licensing ITER DT operations. However, the development of absolute 17 MeV gamma ray emission measurements entails a number of requirements, such as: (i) knowledge of the 17 MeV gamma ray to 14 MeV neutron emission branching ratio; (ii) the simulation of the gamma ray transport from the extended plasma source to the gamma ray detectors; (iii) a careful determination of the absolute efficiency of previously calibrated gamma ray spectrometers. In this work, we have studied the possibility to infer the global gamma ray emission rate from measurements made with a 3? × 6? LaBr3 spectrometer installed at the end of a collimated tangential line of sight at the JET tokamak and using the neutron emission from deuterium plasmas of the most recent experimental campaigns. Results show that 17 MeV gamma ray fluxes at the end of this tangential line of sight have a weak dependence (less than 5%) on the plasma profile and can therefore be used to infer the total emission from the plasma.

Unfolding techniques are employed to reconstruct the 1D energy distribution of runaway electrons from Bremsstrahlung hard X-ray spectrum emitted during plasma disruptions in tokamaks. Here we compare four inversion methods: truncated singular value decomposition, which is a linear algebra technique, maximum likelihood expectation maximization, which is an iterative method, and Tikhonov regularization applied to 2 and Poisson statistics, which are two minimization approaches. The reconstruction fidelity and the capability of estimating cumulative statistics, such as the mean and maximum energy, have been assessed on both synthetic and experimental spectra. The effect of measurements limitations, such as the low energy cut and few number of counts, on the final reconstruction has also been studied. We find that the iterative method performs best as it better describes the statistics of the experimental data and is more robust to noise in the recorded spectrum.

Laser interferometer/polarimeter systems are used in magnetically confined fusionexperiments for simultaneous measurements of the line-integrated electron density and of thecurrent-induced magnetic field.In this work, we present the design of the interferometer/polarimeter system for the Divertor Tokamak Test facility (DTT), a new tokamak device dedicated to investigate alternative powerexhaust solutions for the nuclear fusion DEMOnstration Power Station (DEMO).The optical design is based on the exploitation of a 7+7 chords scheme, which allows deter-mining density and poloidal field, contributes to evaluate the plasma magnetic equilibrium and canprovide the real time estimate of the q profile. Since the optical scheme is thought to be compatible with a possible Double Null divertor configuration, an equatorial port is recommended. In order to protect the in-vessel optics, each chord employs a back reflecting mirror installed in the high field side inner wall close to the divertor, where some plasma-free space is available, and one retroreflector installed in the space behind the low field side outer first wall.With respect to polarimetric measurements and low effects of density gradients, the optimal laser source solution would be 100/50?m. With this setup, in low/medium density conditions, the longer wavelength will provide a good magnetic field measurement, while the shorter wavelength will allow vibration compensation for density measurements. In high-density regimes, the short wavelength alone can provide both magnetic field information from Faraday rotation and density measurements from the Cotton-Mouton effect. The two wavelengths are close enough to each other also to provide a good sharing of optical components.

An important instrumental development work has been done in the last two decades in the field of neutron and gamma ray spectroscopic measurements of magnetic confinement plasmas. Starting from the present state of the art instrumentation installed at JET, this paper reviews the recent development that has been carried out within the EUROFUSION programme for the forthcoming high power JET D and DT campaign. This development was dedicated to the realization of new compact neutron and gamma-ray spectrometers which combine very high energy resolution (typically better than 5%) and MHz counting rate capabilities allowing for time resolution in the 10 ms time scale. One of the advantages offered by the compact dimensions of these spectrometers is to make possible their use in multiple sight-line camera configurations, such as for future burning plasma reactors (ITER and DEMO). New compact neutron spectrometers based on single crystal diamond detectors have been developed and installed at JET for measurements of the 14 MeV neutron spectrum. Measurements on a portable DT neutron generator have shown that neutron spectroscopy of the accelerated beam ions at unprecedented energy resolution (~1% at 14 MeV) is possible, which opens up new opportunities for diagnosing DT plasmas. For what concerns gamma ray measurements, the JET gamma ray camera has been recently upgraded with new compact spectrometers based on a LaBr3 scintillator coupled to Silicon Photomultiplier with the dual aim to improve the spectroscopic and rate capabilities of the detectors. The upgrade camera system will reconstruct the spatial gamma ray emissivity from the plasma in the MeV energy range at MHz counting rates and energy resolution in the 2-4% range. This will allow physics studies of gamma rays produced by the interaction of fast ions with impurities in the plasma and bremsstrahlung emission from runaway electrons.

In the Divertor Tokamak Test (DTT) facility two Thomson scattering (TS) systems are under design for the measurements of Te and ne in the core plasma region and in the divertor respectively. The divertor TS system under study is a conventional TS system based on a Nd:YAG laser source, a fiber optic based light collection system and a set of filter polychromators equipped with Si APD detectors. The laser beam and the collection optics share an aperture between adjacent cassettes of the lower divertor and the scattering signal is collected from a set of scattering volumes close to one of the divertor legs by a collection optics system located under the divertor dome and is carried to the polychromators by fiber optic bundles. The filter polychromators are designed to measure Te as low as 1 eV. Measurements with a spatial resolution of 10 mm are possible, with accuracy limited by the plasma ne and the background light. For the core TS system, two options are under consideration: a conventional system, similar to that designed for the ITER core TS, in which Te and ne are measured along a large fraction of a laser beam crossing the plasma near the equatorial plane and the detection system is again based on fiber optic coupled filter polychromators. The spatial resolution is 5 cm in the central region and 1 cm at the plasma edge. Alternatively a TS system based on the LIDAR concept, previously implemented in JET, is under consideration. Recent advancements in laser and detector technology allow achieving a spatial resolution similar to that of a conventional system, but with a simpler and reliable experimental set-up and possibly at a lower cost.

A synthetic diagnostic has been developed for the JET lost alpha scintillator probe, based on the ASCOT fast ion orbit following code and the AFSI fusion source code. The synthetic diagnostic models the velocity space distribution of lost fusion products in the scintillator probe. Validation with experimental measurements is presented, where the synthetic diagnostic is shown to predict the gyroradius and pitch angle of lost DD protons and tritons. Additionally, the synthetic diagnostic reproduces relative differences in total loss rates in multiple phases of the discharge, which can be used as a basis for total loss rate predictions.

The measurement of the energy spectra and densities of alpha-particles and other fast ions are part of the ITER measurement requirements, highlighting the importance of energy-resolved energetic-particle measurements for the mission of ITER. However, it has been found in recent years that the velocity-space interrogation regions of the foreseen energetic-particle diagnostics do not allow these measurements directly. We will demonstrate this for gamma-ray spectroscopy (GRS), collective Thomson scattering (CTS), neutron emission spectroscopy and fast-ion D-alpha spectroscopy by invoking energy and momentum conservation in each case, highlighting analogies and differences between the different diagnostic velocity-space sensitivities. Nevertheless, energy spectra and densities can be inferred by velocity-space tomography which we demonstrate using measurements at JET and ASDEX Upgrade. The measured energy spectra agree well with corresponding simulations. At ITER, alpha-particle energy spectra and densities can be inferred for energies larger than 1.7 MeV by velocity-space tomography based on GRS and CTS. Further, assuming isotropy of the alpha-particles in velocity space, their energy spectra and densities can be inferred by 1D inversion of spectral single-detector measurements down to about 300 keV by CTS. The alpha-particle density can also be found by fitting a model to the CTS measurements assuming the alpha-particle distribution to be an isotropic slowing-down distribution.

In spite of its very high reliability, LIDAR Thomson Scattering (LIDAR-TS) has so far only been installed on JET. The new Divertor Tokamak Test (DTT) facility at Frascati will be similar in size to JET and thus is clearly suited for a LIDAR-TS system. The proposal here follows the concept of a no vignetting region, which circumvents the need for relative density calibrations and provides much better signals than those on the JET system, particularly at the outer boundary. The scattered spectrum reaches the spectrometer through a train of relay lenses. The spectrometer uses fast MCP-PMTs for detection of the scattered signal. The fast detectors are generally limited to a spectral range below 900 nm. Hence, a laser at a single wavelength slightly below 900 nm would be ideal. Here we investigate using an Nd:YAG laser, emitting simultaneously both the fundamental and the second harmonic wavelength. The expected performance is analyzed in a simulation program, confirming the ability of this choice to cover the full temperature range. The spatial resolution of the system on JET was about 7 cm. For DTT it is possible to improve this value both by using more modern hardware and by deconvoluting the measured signals. Using a commercially available Nd:YAG two wavelength system with 100 ps pulses in combination with 180 ps MCP-PMTs a resolution of less than 4 cm is achieved. Deconvolution works well at the outer boundary of an H-mode plasma. With deconvolution of the spectral channel signals, the effective spatial resolution in this region can be reduced to ~2 cm.

The paper describes a new compact neutron spectrometer optimized for the detection of 2.5 MeV neutrons for fusion plasma applications. The first neutron spectroscopy measurements on a nuclear fusion plasma experiment (EAST) with this detector are also reported together with the data interpretation. The detector allowed separating the different neutron emission components from the plasma and to validate the effectiveness of the Neutral Beam Injection (NBI) heating. A possible improvement of the diagnostic has been also identified in order to increase the detector sensitivity to weak components of the neutron spectrum, such as those due to the Ion Cyclotron Resonace Heating (ICRH). The relatively simple response function of the (CLYC)-L-7 detector to 2.5 MeV neutrons together with its good capability in the n/gamma discrimination, makes this detector an interesting spectrometer for deuterium (D) plasma diagnostics. In particular, its compactness allows for integration in a multi-line of sight camera where space constraints are present.

During the last years the Collective Thomson Scattering (CTS) system installed on the Frascati Tokamak Upgrade (FTU), operating at 140 GHz, has been exploited to investigate different kinds of phenomena: high-frequency daughter waves by Parametric Decay Instabilities, which, under certain conditions, are recently presumed to be conceivable even under routine ECRH scenarios, and bulk ions thermal emissions stimulated by a 140 GHz gyrotron beam. A new receiving line, giving rise to a second further line of sight from the plasma during the scattering experiments and integrated in the CTS antenna, has been recently completed and installed. Such new line will be useful to recognise signals originating in the volume of beams cross from the ones coupled outside or also to monitor the gyrotron's spectrum in real-time during the shot. The present status of this new installation is shown in the paper.

The recent implementation of data processing tools and the code activity, aimed at the interpretation of spectra measured in the tokamak FTU with the renewed Collective Thomson Scattering (CTS) system, are presented in this paper. Such instrumental renovation allowed investigating anomalous emissions originating from parametric decay instability (PDI) of an electron cyclotron (EC) pump wave in presence of magnetohydrodynamics (MHD) rotating islands, as foreseen by recent theoretical models. Aim of these experiments is to study the possible effects of these anomalous phenomena on the EC power absorption and on the CTS used as standard thermal diagnostic. The codes here presented have been implemented for the calibration of the CTS spectra and for the proper visualization of signals, for direct comparison with the MHD spectrograms and with the plasma parameters. The software also allows analysing data acquired simultaneously with two radiometric systems as is possible in FTU since the beginning of 2016. The calibration of the spectra is necessary for comparing the actual power of the signals, that can span orders of magnitude, from the lowest compatible with thermal (CTS) radiation and providing information on the ions dynamic, to the strong anomalous emission theoretically expected from PDIs. The Thermal Collective Scattering (TCS) code, able either to predict or to interpret thermal ion spectra, has been optimized and integrated in the overall data analysis tools. The analysis of the data and the interpretation of the scenarios achieved in the experiments are presently underway using the tools here presented.

A new receiving antenna for collecting signals of the Collective Thomson Scattering (CTS) diagnostics in FTU Tokamak has been recently installed. The squared corrugated section and the precisely defined length make it possible to receive from different directions by remotely steering the receiving mirrors. This type of Remote-Steering (RS) antennas, being studied on FTU for the DEMO Electron Cyclotron Heating (ECH) system launch, is already installed on the W7-X stellarator and will be tested in the next campaign. The transmission of the signal from the antenna in the tokamak hall to the CTS diagnostics hall will be mainly realized by means of oversized circular corrugated waveguides carrying the hybrid HE11 (quasi-gaussian) waveguide mode, with inclusion of a special smooth-waveguide section and a short run of reduced-size square-corrugated waveguide through the tokamak bio-shield. The coupling between different waveguide types is made with ellipsoidal focusing mirrors, using quasi-optical matching formulas between the gaussian-shaped beams in input and output to the waveguides. In this work, after a complete study of feasibility of the overall line, a design for the receiving line will be proposed, in order to realize an executive layout to be used as a guideline for the commissioning phase.

The COMPASS tokamak at IPP Prague is a small-size device with an ITER-relevant plasma geometry and operating in both the Ohmic as well as neutral beam assisted H-modes since 2012. A basic set of diagnostics installed at the beginning of the COMPASS operation has been gradually broadened in type of diagnostics, extended in number of detectors and collected channels and improved by an increased data acquisition speed. In recent years, a significant progress in diagnostic development has been motivated by the improved COMPASS plasma performance and broadening of its scientific programme (L-H transition and pedestal scaling studies, magnetic perturbations, runaway electron control and mitigation, plasma-surface interaction and corresponding heat fluxes, Alfvenic and edge localized mode observations, disruptions, etc.). In this contribution, we describe major upgrades of a broad spectrum of the COMPASS diagnostics and discuss their potential for physical studies. In particular, scrape-off layer plasma diagnostics will be represented by a new concept for microsecond electron temperature and heat flux measurements - we introduce a new set of divertor Langmuir and ball-pen probe arrays, newly constructed probe heads for reciprocating manipulators as well as several types of standalone probes. Among optical tools, an upgraded high-resolution edge Thomson scattering diagnostic for pedestal studies and a set of new visible light and infrared (plasma-surface interaction investigations) cameras will be described. Particle and beam diagnostics will be covered by a neutral particle analyzer, diagnostics on a lithium beam, Cherenkov detectors (for a direct detection of runaway electrons) and neutron detectors. We also present new modifications of the microwave reflectometer for fast edge density profile measurements.