Within the European development of a future tokamak demonstration fusion power plant (DEMO) [1] the pre-conceptual studies on the plasma diagnostic and control (D&C) system are progressing to prepare the basis for reliable plasma operation at high overall performance [2]. A variety of plasma diagnostics will be employed on DEMO together with advanced control techniques in order to provide an accurate knowledge of the plasma state, which is needed to maintain plasma operation within the allowed physical and technical limits. The integration of diagnostic front-end components has to cope with strong adverse effects arising from neutron and gamma irradiation, heat loads, impinging particles and forces. In this environment, the quality of measurements can only be ensured for longer periods by using robust diagnostic components, mounting them in sufficiently protected (retracted) locations, and any maintenance can only be performed via remote handling. Major open issues are the durability of magnetic measurements in the presence of irradiation induced effects and the feasibility of detachment control under DEMO conditions. In parallel to diagnostic developments, the details of the main control issues are being formulated and investigated by quantitative plasma control simulations. To obtain the envisaged performance DEMO operates close to some physics limits where even small disturbances, if not properly controlled, can trigger major variations of the plasma parameters. Equilibrium control requires high control power and can drive the poloidal field coil system to its operational limits. Within this paper, we will provide an overview on the current status of the ongoing D&C developments for the European DEMO concept.

In the last years, a new inversion method has been adapted to JET diagnostics. It is based on the Maximum Likelihood (ML) approach and has been applied to most systems of interest: the neutron, gamma ray and bolometric tomographies. In addition to its accuracy and reliability, the main competitive advantage of the ML inversion method is the fact that it can provide reliable estimates of the uncertainties in the reconstructions. The potential of this approach to analyse all the main emissivity types encountered on JET during the flat top phase of the discharges has been verified with both synthetic data and experimental measurements. Recently the same approach has been applied to the investigation of the current quench. Of course, during this phase an additional difficulty is posed by the errors in the magnetic topology; indeed given the ill-pose character of the tomographies on JET, a good reconstruction of the equilibrium is an essential input to any tomographic inversion on JET. In this perspective, particular attention has been devoted to producing good quality, high time resolution equilibria. Moreover, the effects of the uncertainties of the magnetic topologies on the ML tomographic inversions have been assessed. Application of the ML inversion technique to the signals of the hard X rays and the bolometric diagnostics has given very good results for example for the investigation of the runaways beam of electrons. The developed routines constitute a good complement to the other diagnostics, such as the cameras, typically used to investigate the current quench phase of the discharge. The ML tomographic technique is therefore becoming an important additional tool to investigate disruptions on JET, particularly the shatter pellet injector experiments, a crucial programme in support to ITER.

Parameters of the post-disruption runaway electron (RE) beam in the low density background plasma achieved after secondary deuterium injection are investigated in DIII-D. The spatially resolved RE energy distribution function is measured for the first time during the RE plateau stage by inverting hard x-ray bremsstrahlung spectra. It has maximum energy up to 20 MeV and a non-monotonic feature at 5-6 MeV observed only in the core of the beam supporting the possibility of kinetic instabilities. Results of Fokker-Plank modelling qualitatively support the formation of the non-monotonic distribution function. The RE current profile is reconstructed for the first time using the spatially resolved RE energy distribution. It is found to be more peaked than the pre-disruption plasma current, with higher internal inductance, suggesting preferential formation of REs in the core plasma or potentially a radially inward motion of REs. The accessed relatively low current (180 kA) RE beam is found to be MHD stable, likely due to its elevated safety factor profile. From this base stable equilibrium, an internal beam instability is accessed by ramping up the current. The instability leads to a sawtooth-like relaxation of the RE current profile, but drives no RE loss. An internal kink mode proposed as a candidate instability is supported by results of MARS-F modelling. Electron cyclotron emission (ECE) spectrum measured during the low density RE plateau is found to be bifurcated, with a break point at & x224d; 100 GHz, suggesting resonant absorption of the ECE at low frequencies.

Gamma-ray spectroscopy (GRS) has become an established technique to determine properties of the distribution function of the energetic particles in the MeV range, which are fast ions from heating and fusion reactions or runaway electrons born in disruptions. In this paper we present a selection of recent results where GRS is key to investigate the physics of MeV range particles. These range from radio-frequency heating experiments, where theoretical models can be tested with an unprecedented degree of accuracy, to disruption mitigation studies, where GRS sheds light on the effect of the actuators on the runaway electron velocity space. We further discuss the unique observational capabilities offered by the technique in deuterium?tritium plasmas, particularly with regard to the inference of the energy- and pitch-resolved distribution function of the ? particles born from fusion reactions in the plasma core.

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.

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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.

Organic NE213 liquid scintillator neutron detectors are commonly used at accelerator facilities for neutron beam diagnostics. In recent years, they have also been installed at nuclear fusion facilities to measure the neutron energy spectra from Deuterium (D) and Deuterium-Tritium plasmas, e.g., at the ASDEX Upgrade (Garching, Germany) and at the Joint European Torus (JET, Culham, UK). The NE213 equivalent scintillating material (now BC501A) of the detector is sensitive to neutron and gamma radiation, so pulse discrimination techniques are applied in order to obtain the measured neutron pulse height spectrum (PHS). If the neutron detector is stable, controlled, and well-characterized (i.e., its response function to incoming neutrons of known energy is determined with high accuracy), it can be used as a neutron spectrometer. The measured PHS can then be analyzed using standard techniques such as unfolding to determine the incoming neutron energy spectrum. This article focuses on the unfolding of neutron data measured in D plasma experiments at JET by a compact broadband neutron spectrometer using the maximum entropy unfolding code MAXED. A general method for optimizing MAXED input parameters is described and applied to the measured PHS to diagnose the effects of the auxiliary heating of JET D plasma on the neutron energy spectra.

For the future JET deuterium-tritium (DT) campaigns numerous gamma-ray diagnostics, in particular the JET Gamma-ray Camera (GC) and the JET Gamma-ray Spectrometer (GS), were upgraded in the last few years. The upgrade of GC and GS included replacement of detector modules in order to allow operation at expected high count rate of ~0.5 Mcps on the scintillator front surface of 506 mm2 for each GC crystal and 7 squared inches for GS as well as to improve energy resolution to equal or better than 5% at gamma-ray energy above 1.1 MeV for both diagnostics. Within the JET4 Enhancements Project, performance of a number of photodetectors including multi-pixel photon counter (MPPC), PIN diode and photomultiplier tube (PMT) connected to scintillators was investigated. Upgraded detector modules are based on fast scintillators with a decay time of about 20 ns, LaBr3:Ce and CeBr3, coupled to MPPC and PMT for GC and GS, respectively. Results on energy resolution obtained with upgraded diagnostics, already installed at JET, are presented and compared to those collected in laboratory conditions.

The study of Runaway Electron (RE) physics and their response to mitigation strategies is crucial to safeguard ITER structural integrity. During their motion REs collide with background ions before hitting the inner vessel of the machine and thus they emit Bremsstrahlung photons in the gamma range of the spectrum. It is possible to detect such radiation using a LaBr3(Ce) spectrometer with counting rate capability in the MHz range and high energy resolution [1][5]. The measured spectra contain information about the RE energy distribution, which can be reconstructed using specific inversion (or econvolution) algorithms. The deconvolution operation is computationally faster than first principles simulations and its use in RE studies might be many fold: it can be used to improve synthetic diagnostic calculations or as a preliminary method for RE spectra analysis.

The Divertor Tokamak Test (DTT) facility, which is under design for construction in Frascati (Italy), will produce neutron yield up to 1.3*1017 n/s at full power (H-mode scenario). This calls for an accurate design and selection of the 2.5 MeV neutron diagnostic systems and detectors which can give the comprehensive exploitation of the high neutron fluxes. Measurements of 14 MeV neutrons (which are about 1% of the total neutron yield) coming from the triton burn-up will also be performed. DTT will reach its best performances after a preliminary phase, needed to assess and improve the machine parameters. Here we present the neutron and gamma-ray diagnostics systems which are under design for the initial start-up phase of DTT. The design work benefits from the experience gathered by the community on high power tokamak such as JET. These systems, also called day-1 diagnostics, are: i) Neutron flux monitors which measure the 2.5 and 14 MeV neutron yields, ii) Neutron/Gamma camera for the reconstruction of the neutron and gamma ray emission profile of the plasma, iii) Hard x-ray monitors for measurements of the bremsstrahlung radiation produced by runway electrons in the 1-40 MeV energy range.

An optimized hard X-ray (HRX) spectrometer was designed to collect information from Bremsstrahlung emission in the MeV range runaway electrons (RE) generated during disruptions. The detector is based on a cerium doped lanthanum bromide scintillator crystal (LaBr3:Ce) coupled with a photomultiplier tube. The diagnostic allows for measurements of high hard X-ray fluxes in excess of 1 MHz with a wide dynamic range up to 20 MeV. The diagnostic was tested at the tokamak ASDEX Upgrade. The results achieved are promising and suggest the possibility of inferring information on the runaway electron energy distribution in tokamaks using deconvolution techniques.

A high resolution neutron spectrometer (HRNS) system has been designed as a neutron diagnostic tool for ITER. The HRNS is dedicated to measurements of time resolved neutron energy spectra for both deuterium and deuterium-tritium (DT) plasmas. The main function of the HRNS is to determine the fuel ion ratio n t/n d in the plasma core with 20% uncertainty and a time resolution of 100 ms for a range of ITER operating scenarios from 0.5 MW to 500 MW in fusion power. Moreover, neutron spectroscopy measurements should also be possible in the initial deuterium phase of ITER experiments. A supplementary function of the HRNS is to provide information on the fuel ion temperature. Furthermore, the HRNS can be used as an additional line-of-sight (LOS) for the radial neutron camera. To meet these requirements, a set of four spectrometers positioned after each other along a single LOS has been designed. The detector techniques employed include a thin foil proton recoil spectrometer (TPR), a neutron diamond detector (NDD), a back-scattering time-of-flight system (bToF) and a forward time-of-flight system (fToF). The TPR system, positioned closest to the plasma, provides data at high fusion powers. For plasma conditions producing intermediate fusion power two neutron spectrometers are installed: NDD and bToF. The NDD is installed as the second instrument along the HRNS LOS after the TPR. The fToF spectrometer is dedicated for low tritium densities and pure deuterium operation. The paper summarizes the current state of the art of neutron spectroscopy useful in plasma diagnostics and the possibility of installing a dedicated HRNS for ITER in the designated diagnostic port. We conclude that the proposed HRNS system can fulfil the ITER measurement requirements over a broad range of plasma operational scenarios, including full power DT, start-up, ramp-down and pure D operations.

The PRIMA project aims at the construction of two ITER-NBI facilities in Padova (Italy). The first one is called SPIDER which is negative H/D 100 keV RF source, while the second one (MITICA) will be a full scale 1 MeV deuterium beam injector as the one that will be used in ITER. In order to resolve the horizontal beam intensity profile in MITICA and one of the eight beamlets groups in SPIDER, the Close-contact Neutron Emission Surface Mapping (CNESM) system is being developed. The goal of this device is to reconstruct the D - beam evaluating the map of the neutron emission due to interaction of the deuterium beam with the deuterons implanted in the beam dump surface. For this reason, the CNESM diagnostic, which is based on nGEM detectors for fast neutrons, will be placed right behind the SPIDER and MITICA beam dump, i.e. in an UHV (Ultra High Vacuum) environment. Since the nGEM detectors need to operate at atmospheric pressure a vacuum sealed detector box has been designed to be installed inside the vacuum vessel and able to sustain atmospheric pressure inside. This paper describes the status of the CNESM diagnostic and underlines the different phases followed during the realization and installation of the diagnostic on the SPIDER beam dump as well as its imaging performances.

The plasma diagnostic and control (D&C) system for a future tokamak demonstration fusion reactor (DEMO) will have to provide reliable operation near technical and physics limits, while its front-end components will be subject to strong adverse effects within the nuclear and high temperature plasma environment. The ongoing developments for the ITER D&C system represent an important starting point for progressing towards DEMO. Requirements for detailed exploration of physics are however pushing the ITER diagnostic design towards using sophisticated methods and aiming for large spatial coverage and high signal intensities, so that many front-end components have to be mounted in forward positions. In many cases this results in a rapid aging of diagnostic components, so that additional measures like protection shutters, plasma based mirror cleaning or modular approaches for frequent maintenance and exchange are being developed. Under the even stronger fluences of plasma particles, neutron/gamma and radiation loads on DEMO, durable and reliable signals for plasma control can only be obtained by selecting diagnostic methods with regard to their robustness, and retracting vulnerable front-end components into protected locations. Based on this approach, an initial DEMO D&C concept is presented, which covers all major control issues by signals to be derived from at least two different diagnostic methods (risk mitigation).

Two scintillator neutron yield detectors have been tested and characterized for installation on the SPIDER beam test facility in Padova, Italy. SPIDER is a part of the facility designed to create a full scale 1 MV neutral beam injector prototype for direct application to ITER. SPIDER will produce 2.45 MeV neutrons via D-D reactions of the Dbeam impinging on the deuterium already embedded in the beam dump. The scintillator detectors will be used to provide an independent measurement of the global neutron yield produced. These detectors are comprised of a new plastic scintillator EJ276, capable of discriminating between neutron and gamma radiation, coupled with a photomultiplier tube. Tests to determine the stability, functionality, and accuracy of the device, as well as energy scale calibration and diagnostic firmware validation were performed in a laboratory setting using two gamma sources (Co60 and Cs137) and an LED. The detectors are found to have acceptable stability and energy resolution, and the firmware has been validated. Some final gamma source measurements are required, along with neutron/gamma measurements for confirming the pulse shape discrimination functionality. These further tests, as well as installation on SPIDER are expected to be completed in early 2019.

The Gamma-Ray Camera Upgrade (GCU) project aims at installing a new set of 19 scintillators with multi-pixel photon counter (MPPC) embedded, capable to meet the high fluxes expected during deuterium-tritium plasmas while improving the diagnostic spectroscopic capabilities. GCU will benefit from the Advanced Telecommunications Computing Architecture (ATCA)-based Data Acquisition System (DAQ), successfully installed and commissioned during the JET-EP2 enhancement. However, to cope with the new GCU detector signals, the DAQ Field Programmable Gate Array (FPGA) codes need to be rebuilt. This work presents the FPGA code upgrade for Gamma Camera (GC) DAQ, capable to sustain the expected fast response of new detectors, while exploiting the full capabilities of the DAQ. Dedicated codes were designed, capable to acquire the new signals at full rate, and simultaneously processing them in real-time through suitable algorithms, fitted to the new signals shape. First results of real-time processing codes applied to data from detector prototypes are presented.

The Joint European Torus (JET), the largest magnetic confinement plasma physics experiment in operation, has a large amount of key diagnostics for physics exploration and machine operation, which include several Gamma-Ray Diagnostics. The Gamma-Ray Spectrometer (GRS), Gamma Camera (GC) and Gamma-Ray Spectrometer Upgrade (GSU) diagnostics have similar Control and Data Acquisition Systems (CDAQ) based on the Advanced Telecommunication Computing Architecture standard, featuring Field Programmable Gate Arrays for data processing and management. During past JET-EP2 enhancements, the GRS and GC diagnostics were successfully installed and commissioned. However, the installed CDAQ software that interfaces these diagnostics to JET Control and Data Acquisition System is different, requiring higher maintenance costs. Benefiting from the Gamma Camera Upgrade (GCU) and new GSU installation and commissioning, the upgrading of the software and controller hardware used in the GRS and GC was evaluated, aiming at software standardization between all three diagnostics for easier maintenance. This paper describes the software standardization process between the diagnostics towards the usage of the same CDAQ software as well as the same Operating System (OS) for the controllers, which allows the operator to minimize the maintenance time, avoiding the need for system specific expertise. The rationale behind the choice of MARTe framework as CDAQ software and Scientific Linux as OS will also be presented.

Compact DT neutron generators based on accelerators are often built on the principle of a mixed beam operation, meaning that deuterium (D) and tritium (T) are both present in the ion beam and in the target. Moreover, the beam consists of a mixture of ions and ionized molecules (D, T ions, and ionized D-D, T-T and D-T molecules) so the relevant source components come from T(d, n), D(t, n), D(d, n) and T(t, 2n) reactions at different ion energies. The method for assessing the relative amplitudes of different source components (DD, DT, TT) is presented. The assessment relies on the measurement of the neutron spectrum of different DT components (T(d, n) and D(t, n) at different energies) using a high resolution neutron spectrometer, e.g. a diamond detector, fusion reaction cross-sections, and simulations of neutron generation in the target. Through this process a complete description of the neutron source properties of the mixed beam neutron generator can be made and a neutron source description card, in a format suitable for Monte Carlo code MCNP, produced.