The fusion reaction between deuterium and tritium, D(T,n)4 He is the main source of energy in future thermonuclear reactors. Alpha-particles (4 He-ions) born with an average energy of 3.5 MeV transferring energy to the thermal plasma during their slowing down, should provide the self-sustained D-T plasma burn. The adequate confinement of alpha-particles is essential to provide efficient heating of the bulk plasma and steady burning of a reactor plasma. That is why the fusion-born alpha-particle studies have been a priority task in the second D-T experiments (DTE2) on the Joint European Torus (JET) to understand the main mechanisms of their slowing down, redistribution and losses and to develop optimal plasma scenarios. JET with Be-wall and W-divertor, enhanced auxiliary heating systems and improved energetic-particle diagnostic capabilities, producing significant population of alpha-particles, provided the possibility for comprehensive studying of the alpha-particle behaviour. Selected results of the confined and lost alpha-particle measurements, evidence of alpha-particle self-heating and assessments of the fusion performance are presented in this paper giving an opportunity for further modelling and extrapolation to the International Thermonuclear Experimental Reactor and burning plasma reactors.

Capacitive plasma pickup is a well-known and difficult problem for plasma-facing edge diagnostics. This problem must be addressed to ensure an accurate and robust interpretation of the real signal measurements vs noise. The Faraday cup fast ion loss detector array of the Joint European Torus (JET) is particularly prone to this issue and can be used as a testbed to prototype solutions. The issue of separation and distinction between warranted fast ion signal and electromagnetic plasma noise has traditionally been solved with hardware modifications, but a more versatile post-processing approach is of great interest. This work presents post-processing techniques to characterize the signal noise. While hardware changes and advancements may be limited, the combination with post-processing procedures allows for more rapid and robust analysis of measurements. The characterization of plasma pickup noise is examined for alpha losses in a discharge from JET’s tritium campaign. In addition to highlighting the post-processing methodology, the spatial sensitivity of the detector array is also examined, which presents significant advantages for the physical interpretation of fast ion losses.

An integrated energetic particle transport model has been constructed in JET plasmas constrained by experimental fast ion loss measurements. The model incorporates a synthetic fast ion loss detector identical to JET’s thin-foil Faraday cup fast ion loss detector array. The loss model combines analyses from the TRANSP and ORBIT-kick codes with enhanced features for producing the synthetic diagnostic. Extensions to the ORBIT code framework allow a full-orbit representation within the vacuum region that can map particles directly to an installed detector geometry. Since synthetic fast ion loss detectors are plagued by weak loss statistics, a novel reverse integration biasing scheme has been implemented to boost computational efficiency. The model is validated against experimental loss measurements induced by long-lived kink modes and is found to be in good agreement. This confirms the development of a fully integrated transport/loss model which can be quantitatively verified against experiment allowing for future validation and predictive studies. The model is particularly useful for more complicated plasma scenarios that involve multiple fast ion species such as JET’s 2021 DT-campaign.

Upgrades to electronic hardware and detector design have been made to the JET thin-foil Faraday cup fast ion loss detector [Darrow et al., Rev. Sci. Instrum. 75, 3566 (2004)] in anticipation of the upcoming deuterium-tritium (DT) campaign. An improved foil stack design has been implemented, which greatly reduces the number of foil-to-foil shorts, and triaxial cabling has mitigated ambient noise pickup. Initial tests of 200 kHz digitizers, as opposed to the original 5 kHz digitizers, have provided enhanced analysis techniques and direct coherence measurements of fast ion losses with magnetohydrodynamic activity. We present recent loss measurements in JET deuterium plasmas correlated with kink modes, fishbone modes, edge-localized modes, and sawteeth. Sources of systematic noise are discussed with emphasis on capacitive plasma pickup. Overall, the system upgrades have established a diagnostic capable of recording alpha particle losses due to a wide variety of resonant fast ion transport mechanisms to be used in future DT-experiments and modeling efforts.

JET equipped with ITER-like wall (a beryllium wall and a tungsten divertor) can provide auxiliary heating with power up to 35MW, producing a significant population of α-particles in DT operation. The direct measurements of alphas are very difficult and α-particle studies require a significant development of dedicated diagnostics. JET now has an excellent set of confined and lost fast particle diagnostics for measuring the α-particle source and its evolution in space and time, α-particle energy distribution, and α-particle losses. This paper describes how the above mentioned JET diagnostic systems could be used for α-particle measurements, and what options exist for keeping the essential α-particle diagnostics functioning well in the presence of intense DT neutron flux. Also, α-particle diagnostics for ITER are discussed.

Thin film Faraday cup detectors can provide measurements of fast ion loss from magnetically confined fusion plasmas. These multilayer detectors can resolve the energy distribution of the lost ions in addition to giving the total loss rate. Prior detectors were assembled from discrete foils and insulating sheets. Outlined here is a design methodology for creating detectors using thin film deposition that are suited to particular scientific goals. The intention is to use detectors created by this method on the Joint European Torus (JET) and the National Spherical Torus Experiment-Upgrade (NSTX-U). The detectors will consist of alternating layers of aluminum and silicon dioxide, with layer thicknesses chosen to isolate energies of interest. Thin film deposition offers the advantage of relatively simple and more mechanically robust construction compared to other methods, as well as allowing precise control of film thickness. Furthermore, this depositional fabrication technique places the layers in intimate thermal contact, providing for three-dimensional conduction and dissipation of the ion-produced heating in the layers, rather than the essentially two-dimensional heat conduction in the discrete foil stack implementation.

Fast ions were analysed in experiments focusing on fundamental He-3 minority and mode conversion (MC) in the ion cyclotron resonance range of frequencies (ICRF) in H plasmas and on second harmonic heating of He-3 ions at 2.65 T mimicking D-T plasma heating in ITER at half its nominal toroidal magnetic field. Gamma-ray spectrometry, neutral particle analysers and fast-ion loss diagnostics provided information on the generation of fast-ion populations and on the distribution of ICRH power among the species in various heating scenarios and for a large range of He-3 concentrations. In the scenario with the fundamental He-3 minority and MC wave heating at B-T(0) = 3.41 T and f approximate to 32 MHz, fast He-3 ions accelerated by ICRH in the MeV energy range were detected in discharges with low He-3 concentration. In the experiments with a He-3 concentration scan it was found that at a He-3 concentration of approximate to 2.2% the He-3 ion losses disappeared while a population of energetic D ions gradually built up due to a redistribution of the ICRH power between species on reaching the first MC regime. Under those conditions the ICRF-heated D beam ions effectively absorbed the wave power at their Doppler shifted resonance, which was close to the plasma centre. In discharges with second harmonic heating of He-3 ions at B-T(0) = 2.65 T and f approximate to 52 MHz, the confined energetic He-3 ions were found in the MeV energy range. There is some evidence that the D ions were also accelerated by ICRF. This paper also demonstrates that the synergy of the various fast ion diagnostics allows making a broad picture of the physics of the redistribution of the absorbed ICRH power in complicated heating scenarios of JET.

We review the operational status and present a few recent results from the JET lost alpha particle diagnostic KA2. We summarize the results of our study of the correlation between KA2 signals and JET magnetic fields and the correlation between KA2 signals and the scintillator-based lost alpha probe KA3. In addition we quantify the relative insensitivity of the diagnostic to intense neutron and gamma ray background radiation fields and our observations of the electro-mechanical noise in the diagnostics signals. We present recent observations carried out during the 2011 JET campaign following the installation of the beryllium ITER-like wall. These observations include lost energetic deuterons during both ion-cyclotron and neutral beam heated plasmas; in particular we have measured relatively intense fluxes of energetic particles during sawtooth plasmas.

We have examined the observed currents in the front foils of the JET Faraday cup lost alpha particle diagnostic KA-2. In particular, we have sought to understand the currents during Ohmic plasmas for which the ion flux at the detectors was initially assumed to be negligible. We have considered two sources of this current: plasma ions (both deuterium and impurity) in the vicinity of the detector (including charge exchange neutrals) and photoemission from scattered UV radiation. Based upon modeling and empirical observation, the latter source appears most likely and, moreover, seems to be applicable to the currents in the front foil during ELMy H-mode plasmas. A very thin gold or nickel foil attached to the present detector aperture is proposed as a solution to this problem, and realistic calculations of expected fluxes of lost energetic neutral beam ions during TF ripple experiments are presented as justification of this proposed solution.

The Doppler broadening of individual γ-ray lines was measured with a high purity germanium detector in JET plasma experiments. High-resolution γ-ray spectrometry of nuclear reactions between energetic D, 3He and 4He ions accelerated by ion cyclotron resonance heating and main plasma impurities such as carbon and beryllium has been used. The nuclear reactions giving rise to γ-rays have been identified and an effective temperature of the heated ions has been obtained in JET discharges. This technique could be used for fast ion and fusion alpha-particle studies in ITER.

The loss of MeV alpha particles from JET plasmas has been measured with a set of thin foil Faraday cup detectors during third harmonic heating of helium neutral beam ions. Tail temperatures of ∼ 2 MeV have been observed, with radial scrape off lengths of a few centimeters. Operational experience from this system indicates that such detectors are potentially feasible for future large tokamaks, but careful attention to screening rf and MHD induced noise is essential.

This paper presents recent results on fast ion studies on JET. A set of diagnostics for both confined and lost fast ions was employed for investigating the response of fast ions to MHD modes and for studying their behaviour in plasmas with toroidal field ripple and in shear-reversed plasmas. A dependence of the losses on MHD mode amplitude was deduced from the experimental data. A study of various plasma scenarios has shown that a significant redistribution of the fast ions happens during changes in the profile of the safety factor from shear-reversed to monotonic. Significant changes in the losses of ICRH accelerated protons were found to be associated with L–H confinement transitions in plasmas. After an L–H transition, an abrupt decrease in the ICRH proton losses was observed. In plasmas with an internal transport barrier, the loss of ICRH accelerated ions was found to increase as the barrier forms. Further results concerning fast ion losses were obtained during JET experiments in which the magnitude of the TF ripple was varied. The ripple losses of fusion products appear similar to classical losses, and are in agreement with modelling.

At the JET Tokamak a major goal is the production and measurement of high levels of neutron emission from d-d and d-t fusion reactions. A variety of neutron diagnostics are used independently at JET to measure both fast and thermal ion behavior. The available neutron diagnostics include a neutron profile monitor consisting of a vertical and horizontal camera. The JET neutron cameras, a unique instrument among similar diagnostics available at large fusion research facilities, consist of two concrete shields of which each includes a fan-shaped array of collimators. These collimators define a total of 19 lines of sight, grouped in two cameras (horizontal and vertical). The plasma coverage allows neutron the tomographic reconstruction of neutron emissivity spatial profiles in two dimensions. The reconstructions are useful for the study the thermal and beam-induced sources of neutron emission and to analyze the evolution of fast ion populations. However, due to the existence of only two fairly coarse views of the plasma, the tomographic problem is a highly limited data set one. This restricts the set of tomographic methods which can be used for the reconstruction. A reconstruction method based on the maximum likelihood (ML) principle was developed for solving the reconstruction problem during the year 2008 [1]. In 2009 the main research topics were focuse on: Improvement, adaptation for JET tomographic geometry and implementation of several reconstruction methods maximum entropy (ME), a Tikhonov regularization (TR) approach, and a Monte Carlo back-projection algorithm (MCBP). Assessment of the quality of the tomographic methods for reconstruction of neutron and gamma emissivity. Development of a comparative study. Diagnostics support in JET experimental campaigns based on tomographic validated methods.

JET-EFDA Culham Science Centre OX14 3DB, Abingdon, UK ERM-KMS, B1000 Brussels, Belgium, Partner in the Trilateral Euregio Cluster Max-Planck-Institut fur Plasmaphysik, EURATOM-Assoziation, D-85748 Garching, Germany Association EURATOM/ENEA, Consorzio RFX, 4-35127 Padova, Italy EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK. Association EURATOM-IPP.CR, CZ-182 21 Prague 8, Czech Republic European Commission, Joint Research Centre, Institute for Reference Materials and Measurements (IRMM), Retieseweg 111, B-2440 Geel, Belgium Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany Laboratori Nazionali del Gran Sasso, S.S. 17/bis km 18+910, I-67010 Assergi (AQ), Italy SCK•CEN, Boeretang, B-2400 Mol, Belgium Colorado School of Mines, 1500 Illinois Street, Golden, Colorado, USA Association EURATOM-VR, Department of Physics and Astronomy, Uppsala University, SE-75120 Uppsala, Sweden The National Institute for Laser, Plasma and Radiation Physics, Association EURATOM-MEdC, Magurele-Bucharest, Romania Princeton Plasma Physics Laboratory, James Forrestal Campus, Princeton, NJ 08543, New Jersey, USA Associazione EURATOM-ENEA sulla Fusione, IFP Milano, Italy Helsinki University of Technology, Association EURATOM-Tekes, P.O.Box 4100, FIN-02015 TKK, Finland Association EURATOM-Osterreichische Akademie der Wissenschaften (OAW), Austria Forschungszentrum Julich GmbH, Institut fur Plasmaphysik, EURATOM-Assoziation, Trilateral Euregio Cluster, D52425 Julich, Germany See the Appendix of F. Romanelli et al., Proceedings of the 22nd Fusion Energy Conference, Geneva, Switzerland, 2008, IAEA, (2008)

A study of magnetically confined fast ions in tokamaks plays an important role in burning plasma research. To reach ignition and steady burning of a reactor plasma an adequate confinement of energetic ions produced by NBI heating, accelerated with ICRF and born in fusion reactions is essential to provide efficient heating of the bulk plasma. Thus, investigation of the fast ion behaviour is an immediate task for present-day large machines, such as JET, in order to understand the main mechanisms of slowing down, redistribution and losses, and to develop optimal plasma scenarios. Todays JET has an enhanced suite of fast ion diagnostics both of confined and lost ions that enable to significantly contribute to this important area of research. Fast ion populations of p, d, t, He-3 and He-4, made with ICRF, NBI, and fusion reactions have been investigated in experiments on JET with sophisticated diagnostics in conventional and shear-reversed plasmas, exploring a wide range of effects. This paper will introduce to the JET fast-ion diagnostic techniques and will give an overview of recent observations. A synergy of the unique diagnostic set was utilised in JET, and studies of the response of fast ions to MHD modes (e.g. tornado modes, sawtooth crashes), fast We-ions behaviour in shear-reversed plasmas are impressive examples of that. Some results on fast ion losses in JET experiments with various levels of the toroidal field ripple will be demonstrated.

The JET scientific programme is directed towards the development of ITER relevant scenarios. In support of this, significant effort has been made to develop diagnostics to better characterise the power deposition on the plasma facing components, to investigate in more detail the radiation losses particularly in the divertor region and to better detect Magneto Hydrodynamic Modes (MHD) instabilities and their effects on fast ion confinement. A new wide-angle infrared camera provides for the first time the opportunity to perform infrared thermography in the JET main chamber, even during fast events like ELMs and disruptions. A completely new bolometric system, with better spatial resolution particularly in the divertor, is now used to investigate the total radiation losses and their influence on the ELM behaviour. A new set of microwave waveguides has improved by 20dB the signal to noise ratio of the JET X-mode reflectometers, that are now routinely used to detect MHD instabilities and in particular to localise the location of Alfvén Eigenmodes. This improved diagnostic capability to monitor MHD instabilities is complemented by two new diagnostics to detect lost fast particles. Both the new scintillator probe and a poloidal array of Faraday cups have already shown clear correlations between MHD activity and ion losses at the edge.

We present evidence for energetic charged particles emanating from partiallydeuterided titanium foils (TiDx) subjected to non-equilibrium conditions. To scrutinize emerging evidence for low-temperature nuclear reactions, we investigated particle yields employing three independent types of highly-sensitive, segmented particle detectors over a six-year period. One experiment measuring neutron emission from TiDx foils showed a background-subtracted yield of 57 ± 13 counts per hour. (The neutron experiments are discussed in a separate paper in this proceedings.) A second experiment, using a photo-multiplier tube with plastic and glass scintillators and TiDx registered charged particle emissions at 2,171 ± 93 counts/hour, over 400 times the background rate. Moreover, these particles were identified as protons having 2.6 MeV after exiting the TiDx foil array. In a third experiment, coincident charged particles consistent with protons and tritons were observedwith high reproducibility in two energy-dispersive ion-implanteddetectors located on either side of 25-micron thick Ti foils loaded with deuterium.