The neutral beam injectors of the ITER experiment will be based on negative ion sources for the generation of beams composed by 1 MeV H/D particles. The prototype of these sources is currently under testing in the SPIDER experiment, part of the Neutral Beam Test Facility of Consorzio RFX, Padua, Italy. Among the targets of the experimentation in SPIDER, it is of foremost importance to maximize the beam current density produced by the accelerator. The SPIDER operating conditions can be optimized thanks to a cavity ring-down spectroscopy diagnostic, which provides line-integrated measurements of negative ion density in proximity of the accelerator apertures. The specific implementation in SPIDER shows a drift in ring down time measurements, which develops in a time scale of few hours, thus possibly affecting the negative ion density estimates in plasma pulses of 1 h duration, as required by ITER. Possible causes and solutions are discussed. Regarding the source performance, this paper presents how negative ion density is influenced by the RF power used to sustain the plasma, and by the magnetic filter field present in SPIDER to limit the amount of co-extracted electrons. In this study, SPIDER was operated in hydrogen and deuterium, in Cs-free conditions.

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.

JT-60SA will complement ITER in resolving key issues to finally decide an acceptable DEMO design. Diagnostics play a key role in this mission. The electron temperature and density profiles are measured by a core and an edge Thomson scattering (TS) diagnostics with high spatial resolution, needed to identify the pedestal parameters and small profile structures. The two systems use a common tangential Nd:YAG laser beam path in the plasma equatorial plane. The collection optics for the edge system (low field side) is hosted in a lower oblique port and that for the coresystem in a horizontal port. The optics fit in the port plug tube and image the scattering volumes into an array of fiber bundles. They both are exposed to a high neutron dose of1016n/cm2over 13years of operation. The optics are supported by a mechanical structure decoupled from the cryostat.A set of filter polychromators with avalanche photodiode (APD) detectors spectrally analyze the scattered radiation. The development of the TS systems is carried out by a joint Japan-EU team.The conceptual design of the edge TS system is presented here. Simulations of the TS signals show acceptable accuracy down to1×1019m-3electron density, sufficient to measure the edge gradient and even a small region outside the separatrix.

Dispersion Interferometers (DI) present the fundamental advantage over conventionalones to be insensitive to mechanical vibrations without requiring a second wavelength interferometerto measure path length variations. On the other hand, their optical setup requires duplication ofnearly all optical components for any measuring chord. This makes the realization of a multi-channel interferometer that is needed to obtain density profiles via Abel inversion of line integralmeasurements more complicated. To overcome such drawback of the DI, in this work we proposeto join the dispersion technique to the beam scanning one, which has been already successfullyimplemented in the conventional mid-infrared two-colour interferometer. In particular, we presenta preliminary design of a DI scanning interferometer for the new Divertor Test Tokamak (DTT)facility, presently in construction. DTT is designed to study a large suite of alternative divertormagnetic configurations in order to ensure acceptable conditions at the walls while maintainingsufficient core performance. In this contest, measuring plasma parameters in the divertor regionis very important though it often presents various difficulties. To improve divertor measurementsthe proposed interferometer will measure the density along the divertor legs from the strike pointsup the X-point. The interferometer will use a CO2laser (?=10.6?m) and a double pass opticalscheme. Phase modulation method will be used to improve the resolution of the measurement andto extend the measuring range above the 1020m-2line integral limitation of the standard homodyneimplementation. Both improvements are important in this application, considering the wide densityrange expected in the DTT divertor region. Comparing to shorter wavelengths, more commonlyused in the DI interferometers, the CO2wavelength improves density resolution while providinggood immunity to the diffraction effect due to the expected high density gradient.

Magnetized plasmas in compact trap may become experimental environments for the investigation of nuclear ?-decays of astrophysical interest. In the framework of the project PANDORA (Plasmas for Astrophysics, Nuclear Decays Observation and Radiation for Archaeometry) the research activities are devoted to demonstrate the feasibility of an experiment aiming at correlating radionuclides lifetimes to the in-plasma ions charge state distribution (CSD). The paper describes the multidiagnostics setup now available at INFN-LNS, which allows unprecedented investigations of magnetoplasma properties in terms of density, temperature and CSD. The developed setup includes an interfero-polarimeter for total plasma density measurements, a multi-X-ray detectors system for X-ray spectroscopy (including time resolved spectroscopy), a X-ray pin-hole camera for high-resolution 2D space resolved spectroscopy and different spectrometers for the plasma-emitted visible light characterization. A description of recent results about plasma parameters characterization in quiescent and turbulent Electron Cyclotron Resonance-heated plasmas will be given. A complete characterization has been already performed, studying, in particular, the time evolution of X-ray spectra and the change of plasma morphology, including the balance between radiation originated in the plasma core and the one due to plasma losses. Finally, the experimental setup is going to be further upgraded in order to allow measurements of nuclear decays in magnetoplasmas.

The SPIDER H-/D- ion source is currently in operation at the Neutral Beam Test facility (NBTF) in Consorzio RFX (Padova, Italy) to prove the possibility of generating up to 40 A of negative ions, with a maximum extracted current density of 350 A/m2 (H)/285 A/m2 (D) and a fraction of co-extracted electrons not greater than 0.5 (H)/1 (D). These performances are required for the realization of the ITER Heating Neutral Beam (HNB), which should deliver 16.7 MW to the plasma by means of negative ions accelerated up to 1 MeV and neutralized before being injected into the ITER tokamak. In order to obtain such high extracted current densities and low co-extracted electron fractions it is necessary to lower the work function of the surface of the acceleration system grid facing the source; this will be accomplished by coating the surfaces with Cs, routinely evaporated by three ovens. The functionality of the ovens has been tested at the CAesium oven Test Stand (CATS), hosted at NBTF. The test stand is equipped with several diagnostics, among which a Laser Absorption Spectroscopy (LAS) diagnostic. Using a tunable laser diode, the LAS diagnostic gets the high resolution absorption spectrum of the Cs 852 nm D2 line along a line of sight to measure Cs density at ground state. The paper describes the test stand and the LAS diagnostic, together with the characterization of the three ovens to be installed in SPIDER. The paper will also study the systematic underestimation effect on measurements caused by Cs ground state depopulation, as function of laser intensity and Cs density, in the perspective of correcting the deviation of measurements.

The SPIDER H-/D- ion source is currently in operation in the Neutral Beam Test facility (NBTF) at Consorzio RFX (Padova, Italy) to prove the possibility of generating up to 40 A of negative ions, with a maximum extracted current density of 350 A/m2 (H)/285 A/m2 (D) and a fraction of co-extracted electrons not greater than 0.5 (H)/1 (D). These performances are required for the realization of the ITER Neutral Beam Injector (NBI), which should deliver 16.7 MW to the plasma by means of negative ions accelerated up to 1 MeV and neutralized before being injected into the ITER tokamak. In order to obtain such high extracted current densities and low co-extracted electron fractions it is necessary to lower the work function of the surface of the acceleration system grid facing the source; this will be accomplished by coating the surfaces with Cs, routinely evaporated by three ovens. The functionality of the ovens has been tested at the CAesium oven Test Stand (CATS), hosted at NBTF. The test stand is equipped with several diagnostics, among which a Laser Absorption Spectroscopy (LAS) diagnostic. Using a tunable laser diode, the LAS diagnostic gets the high resolution absorption spectrum of the Cs 852 nm D2 line along a line of sight to measure Cs density at ground state. The paper describes the test stand and the LAS diagnostic, together with the characterization of the ovens to be installed in SPIDER. The paper will also study the systematic density underestimation effect caused by Cs ground state depopulation, as a function of laser intensity and of Cs density, in the perspective of correcting the density evaluation.

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.

The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measure- ments of the electron temperature and density profiles which are critical to the understanding of the ITER plasma. The diagnostic must satisfy the ITER project requirements, which translate to requirements on performance as well as reliability, safety and engineering. The implications are particularly challenging for beam dump lifetime, the need for continuous active alignment of the diagnostic during operation, allowable neutron flux in the interspace and the protection of the first mirror from plasma deposition. The CPTS design has been evolving over a number of years. One recent improvement is that the collection optics have been modified to include freeform sur- faces. These freeform surfaces introduce extra complexity to the manufacturing but provide greater flexibility in the design. The greater flexibility introduced allows for example to lower neutron throughput or use fewer surfaces while improving optical performance. Performance assessment has shown that scattering from a 1064 nm laser will be sufficient to meet the measurement require- ments, at least for the system at the start of operations. Optical transmission at ? < 600 nm is expected to degrade over the ITER lifetime due to fibre darkening and deposition on the first mirror. For this reason, it is proposed that the diagnostic should additionally include measurements of TS 'depolarised light' and a 1319 nm laser system. These additional techniques have different spectral and polarisation dependencies compared to scattering from a 1064 nm laser and hence provide greater robustness into the inferred measurements of T e and n e in the core.

The ITER project requires additional heating provided by two neutral beam injectors using 40 A negative deuterium ions accelerated at 1 MV. As the beam requirements have never been experimentally met, a test facility is under construction at Consorzio RFX, which hosts two experiments: SPIDER, full-size 100 kV ion source prototype, and MITICA, 1 MeV full-size ITER injector prototype. Since diagnostics in ITER injectors will be mainly limited to thermocouples, due to neutron and gamma radiation and to limited access, it is crucial to thoroughly investigate and characterize in more accessible experiments the key parameters of source plasma and beam, using several complementary diagnostics assisted by modelling. In SPIDER and MITICA the ion source parameters will be measured by optical emission spectroscopy, electrostatic probes, cavity ring down spectroscopy for H- density and laser absorption spectroscopy for cesium density. Measurements over multiple lines-of-sight will provide the spatial distribution of the parameters over the source extension. The beam profile uniformity and its divergence are studied with beam emission spectroscopy, complemented by visible tomography and neutron imaging, which are novel techniques, while an instrumented calorimeter based on custom unidirectional carbon fiber composite tiles observed by infrared cameras will measure the beam footprint on short pulses with the highest spatial resolution. All heated components will be monitored with thermocouples: as these will likely be the only measurements available in ITER injectors, their capabilities will be investigated by comparison with other techniques. SPIDER and MITICA diagnostics are described in the present paper with a focus on their rationale, key solutions and most original and effective implementations.

A method to determine the gamma-ray emissivity profile from measurements along a few multiple collimated lines of sight in thermonuclear plasmas is presented. The algorithm is based on a generalisation of the known Abel inversion and takes into account the non circular shape of the plasma flux surfaces and the limited number of data points available. The method is applied to synthetic experimental measurements originating from parabolic and non parabolic JET gamma-ray emissivity profiles, where the aim is to compare the results of the inversion with the original, known input parameters. We find that profile parameters, such as the peak value, width and centre of the emissivity, are determined with an accuracy between 1 and 20% for parabolic and 2 to 25% for non parabolic profiles, respectively, which compare to an error at the 10% level for the input data. The results presented in this paper are primarily of relevance for the reconstruction of emissivity profiles from radiation measurements in tokamaks, but the method can also be applied to measurements along a sparse set of collimated lines of sight in general applications, provided that the surfaces at constant emissivity are known to have rotational simmetry.

An experimental campaign aiming to investigate electron cyclotron resonance (ECR) plasma X-ray emission has been recently carried out at the ECRISs-Electron Cyclotron Resonance Ion Sources laboratory of Atomki based on a collaboration between the Debrecen and Catania ECR teams. In a first series, the X-ray spectroscopy was performed through silicon drift detectors and high purity germanium detectors, characterizing the volumetric plasma emission. The on-purpose developed collimation system was suitable for direct plasma density evaluation, performed "on-line" during beam extraction and charge state distribution characterization. A campaign for correlating the plasma density and temperature with the output charge states and the beam intensity for different pumping wave frequencies, different magnetic field profiles, and single-gas/gas-mixing configurations was carried out. The results reveal a surprisingly very good agreement between warm-electron density fluctuations, output beam currents, and the calculated electromagnetic modal density of the plasma chamber. A charge-coupled device camera coupled to a small pin-hole allowing X-ray imaging was installed and numerous X-ray photos were taken in order to study the peculiarities of the ECRIS plasma structure. (C) 2016 AIP Publishing LLC.

Single-crystal Diamond Detectors (SDDs), due to their high radiation hardness, fast response time and small size, are good candidates as fast neutron detectors in those environments where the high neutron flux is an issue, such as spallation neutron sources and the next generation thermonuclear fusion plasmas, i.e. the ITER experiment. Neutron detection in SDDs is based on the collection of electron-hole pairs produced by charged particles generated by neutron interactions with 12C. Recent measurements have demonstrated the SDD capability of measuring the neutron flux with a good energy resolution and at high rates. In this work a novel detector based on a 12-pixels SDD matrix will be presented. Each pixel is equipped with an independent electronic chain: the fast shaping preamplifier coupled to a digitizer is able to combine the high rate capability and the good energy resolution. Two CAEN digitizers are compared and the possibility of performing good energy resolution measurements (<2%) and at high rates (>1MHz per channel) is described. Each pixel was characterized and calibrated using an 241Am source: the energy resolution was evaluated and gives a mean value of 1.73% at 5.5MeV. The good energy resolution achieved and its uniformity between pixels are the demonstration of the capability of this novel detector as a spectrometer. This system will be installed during the next Deuterium-Tritium campaign on a collimated vertical line of sight at JET for 14MeV neutron measurements.

At INFN-LNS innovative techniques of plasma heating are under investigation in order to produce growing intensities of beams of multi-charged ions for particle accelerators. Recently, signatures about electromagnetic-to-electrostatic waves (EMW-ESW) conversion, leading to the formation of an overdense plasmas characterized by high energy content, have been observed. A new X-ray pin-hole camera, allowing two-dimensional resolved spectroscopy and imaging, has been designed and used, showing the formation of hot electron layers when the plasma is heated by ESWs. Electrostatic probes have been used in parallel to measure the plasma bulk density and temperature, and a new microwave interferometer is under design.