[Show abstract][Hide abstract] ABSTRACT: The migration of wall material or seeding impurities plays an important role in the formation of mixed materials, the impurity contamination of the plasma and tritium retention. First, this work presents an improved model for the sputtering from mixed material surfaces in WallDYN. Second, we present dynamic SDTrimSP and WallDYN simulations of the nitrogen implantation in Be and the migration of nitrogen in tokamaks with a Be main wall. The simulations with the binary collision code SDTrimSP predict that N accumulates directly at the surface and that the Be erosion decreases with increasing N surface content. A first application of WallDYN to the nitrogen migration with an ITER-like wall indicates that the Be main wall may cause wall pumping of N by co-deposition with Be.
[Show abstract][Hide abstract] ABSTRACT: A selected set of samples from JET-ILW divertor tiles exposed in 2011–2012 has been analysed using thermal desorption spectrometry (TDS). The highest amount of deuterium was found on the regions with the thickest deposited layers, i.e. on the horizontal (apron) part and on the top part of Tile 1, which resides deep in the scrape-off layer. Outer divertor Tiles 6, 7 and 8 had nearly an order of magnitude less deuterium. The co-deposited layers on the JET tiles and the W coatings contain C, O and Ni impurities which may change the desorption properties. The D2 signals in the TDS spectra were convoluted and the positions of the peaks were compared with the Be and C amounts but no correlations between them were found. The remaining fractions of D in the analysed samples at ITER baking temperature 350°C are rather high implying that co-deposited films may be difficult to be de-tritiated.
[Show abstract][Hide abstract] ABSTRACT: Experiments in the JET tokamak equipped with the ITER-like wall (ILW) revealed that the inner and outer target plate at the location of the strike points represent after one year of operation intact tungsten (W) surfaces without any beryllium (Be) surface coverage. The dynamics of near-surface retention, implantation, desorption and recycling of deuterium (D) in the divertor of plasma discharges are determined by W target plates. As the W plasma-facing components (PFCs) are not actively cooled, the surface temperature (T
surface) is increasing with plasma exposure, varying the balance between these processes in addition to the impinging deuteron fluxes and energies. The dynamic behaviour on a slow time scale of seconds was quantified in a series of identical L-mode discharges (JET Pulse Number (JPN)) by intra-shot gas analysis providing the reduction of deuterium retention in W PFCs by 1/3 at a base temperature (T
base) range at the outer target plate between 65 °C and 150 °C equivalent to a T
surface span of 150 °C and 420 °C. The associated recycling and molecular D desorption during the discharge varies only at lowest temperatures moderately, whereas desorption between discharges rises significantly with increasing T
base. The retention measurements represent the sum of inner and outer divertor interaction at comparable T
surface. The dynamic behaviour on a fast time scale of ms was studied in a series of identical H-mode discharges (JPN ) and coherent edge-localized mode (ELM) averaging. High energetic ELMs of about 3 keV are impacting on the W PFCs with fluxes of which is about four times higher than inter-ELM ion fluxes with an impact energy of about E
im = 200 eV. This intra-ELM ion flux is associated with a high heat flux of about 60 MW m−2 to the outer target plate which causes T
surface rise by Δ T = 100 K per ELM covering finally the range between 160 °C and 1400 °C during the flat-top phase. ELM-induced desorption from saturated near-surface implantation regions as well as deep ELM-induced deuterium implantation areas under varying baseline temperature takes place. Subsequent refuelling by intra-ELM deuteron fluxes occurs and a complex interplay between deuterium fuelling and desorption can be observed in the temporal ELM footprint of the surface temperature (IR thermography), the impinging deuteron flux (Langmuir probes), and the Balmer radiation (emission spectroscopy) as representative for the deuterium recycling flux. In contrast to JET-C, a pronounced second peak, 8 ms delayed with respect to the initial ELM crash, in the D
radiation and the ion flux has been observed. The peak can be related to desorption of implanted energetic intra-ELM D+ diffusing to the W surface, and performing local recycling.
[Show abstract][Hide abstract] ABSTRACT: JET neutron profile monitor ensures 2D coverage of the gamma and neutron emissive region that enables tomographic reconstruction. Due to the availability of only two projection angles and to the coarse sampling, tomographic inversion is a limited data set problem. Several techniques have been developed for tomographic reconstruction of the 2-D gamma and neutron emissivity on JET, but the problem of evaluating the errors associated with the reconstructed emissivity profile is still open. The reconstruction technique based on the maximum likelihood principle, that proved already to be a powerful tool for JET tomography, has been used to develop a method for the numerical evaluation of the statistical properties of the uncertainties in gamma and neutron emissivity reconstructions. The image covariance calculation takes into account the additional techniques introduced in the reconstruction process for tackling with the limited data set (projection resampling, smoothness regularization depending on magnetic field). The method has been validated by numerically simulations and applied to JET data. Different sources of artifacts that may significantly influence the quality of reconstructions and the accuracy of variance calculation have been identified.
Full-text · Article · Jan 2016 · Review of Scientific Instruments
[Show abstract][Hide abstract] ABSTRACT: Sawtooth stabilisation by fast ions is investigated in deuterium (D) and D-helium 3 (He3) plasmas of JET heated by deuterium Neutral Beam Injection combined in synergy with Ion Cyclotron Resonance Heating (ICRH) applied on-axis at 3rd beam cyclotron harmonic. A very significant increase in the sawtooth period is observed, caused by the ICRH-acceleration of the beam ions born at 100 keV to the MeV energy range. Four representative sawteeth from four different discharges are compared with Porcelli's model. In two discharges, the sawtooth crash appears to be triggered by core-localized Toroidal Alfvén Eigenmodes inside the q = 1 surface (also called “tornado” modes) which expel the fast ions from within the q = 1 surface, over time scales comparable with the sawtooth period. Two other discharges did not exhibit fast ion-driven instabilities in the plasma core, and no degradation of fast ion confinement was found in both modelling and direct measurements of fast ion profile with the neutroncamera. The developed sawtooth scenario without fast ion-driven instabilities in the plasma core is of high interest for the burning plasmas. Possible causes of the sawtooth crashes on JET are discussed.
[Show abstract][Hide abstract] ABSTRACT: The Joint European Torus (JET, Culham, UK) is the largest tokamak in the world. JET has been upgraded over the years and recently it has also become a test facility of the components designed for ITER, the next step fusion machine under construction in Cadarache (France). At JET, the neutron emission profile of Deuterium (D) or Deuterium-Tritium (DT) plasmas is reconstructed using the neutron camera (KN3). In 2010 KN3 was equipped with a new digital data acquisition system (DAQ) based on Field Programmable Gated Array (FPGA). According to specifications, the DAQ is capable of high rate measurements up to 0.5 MCps. A new compact broadband spectrometer (KM12) based on BC501A organic liquid scintillating material was also installed in the same year and implements a similar DAQ as for KN3. This article illustrates the observations on the DAQ high count rate performance of both KN3 and KM12 in the latest JET D plasma experiments related to hybrid scenario and runaway electrons. For the latter, >1 MCps event rate was achieved with consequences on the behavior of the FPGA and on the reliability of the measurements.
[Show abstract][Hide abstract] ABSTRACT: In Hybrid plasma operation in JET with its ITER-like wall (JET-ILW) it is
found that n>1 tearing activity can significantly enhance the rate of on-axis
peaking of tungsten impurities, which in turn significantly degrades discharge
performance. Core n=1 instabilities can be beneficial in removing tungsten
impurities from the plasma core (e.g. sawteeth or fishbones), but can
conversely also degrade core confinement (particularly in combination with
simultaneous n=3 activity). The nature of MHD instabilities in JET Hybrid
discharges, with both its previous Carbon wall and subsequent JET-ILW, is
surveyed statistically and the character of the instabilities is examined.
Possible qualitative models for how the n>1 islands can enhance on-axis
tungsten transport accumulation processes are presented.
[Show abstract][Hide abstract] ABSTRACT: A Bayesian model of the emission spectrum of the JET lithium beam has been
developed to infer the intensity of the Li I (2p-2s) line radiation and
associated uncertainties. The detected spectrum for each channel of the lithium
beam emission spectroscopy (Li-BES) system is here modelled by a single Li line
modified by an instrumental function, Bremsstrahlung background, instrumental
offset, and interference filter curve. Both the instrumental function and the
interference filter curve are modelled with non-parametric Gaussian processes.
All free parameters of the model, the intensities of the Li line,
Bremsstrahlung background, and instrumental offset, are inferred using Bayesian
probability theory with a Gaussian likelihood for photon statistics and
electronic background noise. The prior distributions of the free parameters are
chosen as Gaussians. Given these assumptions, the intensity of the Li line and
corresponding uncertainties are analytically available using a Bayesian linear
inversion technique. The proposed approach makes it possible to extract the
intensity of Li line without doing a separate background subtraction through
modulation of the Li beam.
[Show abstract][Hide abstract] ABSTRACT: Disruptions are a major operational concern for next generation tokamaks, including ITER. They may generate excessive heat loads on plasma facing components, large electromagnetic forces in the machine structures and several MA of multi-MeV runaway electrons. A more complete understanding of the runaway generation processes and methods to suppress them is necessary to ensure safe and reliable operation of future tokamaks. Runaway electrons were studied at JET-ILW showing that their generation dependencies (accelerating electric field, avalanche critical field, toroidal field, MHD fluctuations) are in agreement with current theories. In addition, vertical stability plays a key role in long runaway beam formation. Energies up to 20 MeV are observed. Mitigation of an incoming runaway electron beam triggered by massive argon injection was found to be feasible provided that the injection takes place early enough in the disruption process. However, suppressing an already accelerated runaway electron beam in the MA range was found to be difficult even with injections of more than 2 kPa.m 3 high-Z gases such as krypton or xenon. This may be due to the presence of a cold background plasma weakly coupled to the runaway electron beam which prevents neutrals from penetrating in the electron beam core. Following unsuccessful mitigation attempts, runaway electron impacts on beryllium plasma-facing components were observed, showing localized melting with toroidal asymmetries.
[Show abstract][Hide abstract] ABSTRACT: The impact of the divertor geometry on global plasma confinement in type I ELMy H-mode has been investigated in the JET tokamak equipped with ITER-Like Wall. Discharges have been performed in which the position of the strike-points was changed while keeping the bulk plasma equilibrium essentially unchanged. Large variations of the global plasma confinement have been observed, the H98 factor changing from typically 0.7 when the outer strike-point is on the vertical or horizontal targets to 0.9 when it is located in the pump duct entrance. Profiles are mainly impacted in the pedestal but core gradient lengths, especially for the density, are also modified. Although substantial differences are observed in the divertor conditions, none seem to correlate directly with the confinement. Modelling with the EDGE2D-EIRENE and SOLEDGE2D-EIRENE transport codes exhibits differences in the energy losses due to neutrals inside the separatrix, but orders of magnitude are too low to explain simply the impact on the confinement.
No preview · Article · Aug 2015 · Journal of Nuclear Materials
[Show abstract][Hide abstract] ABSTRACT: A unique sequence of 120 almost identical plasmas in the Joint European Torus (JET) recently provided two orders of magnitude more statistically equivalent data than ever previously available. The purpose was to study movement of eroded plasma-facing material from JET's new Beryllium wall, but it has allowed the statistical detection of otherwise unobservable phenomenon. This includes a sequence of resonant-like waiting times between edge-localised plasma instabilities (ELMs), instabilities that must be mitigated or avoided in large magnetically confined plasmas such as those planned for ITER. Here, we investigate the cause of this phenomenon, using the unprecedented quantity of data to produce a detailed picture of the plasma's behaviour. After combining the data, oscillations are clearly observable in the plasma's vertical position, in edge losses of ions, and in Beryllium II (527 nm) light emissions. The oscillations are unexpected, are not obvious in data from a single pulse alone, and are all clearly correlated with each other. They are likely to be caused by a small vertical oscillation that the plasma control system is not reacting to prevent, but a more complex explanation is possible. The clearly observable but unexpected link between small changes in the plasma's position and changes to edge-plasma transport and stability suggest that these characteristics cannot always be studied in isolation. It also suggests new opportunities for ELM mitigation and control that may exist.
No preview · Article · Aug 2015 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: Abstract This paper describes integrated numerical modeling applied to JET ITER-like wall H-mode discharges using the COREDIV code, which self-consistently solves 1D radial transport of plasma and impurities in the core region and 2D multi-fluid transport in the SOL. In this paper we focus on sensitivity studies to give insight on the influence of different code input parameters on the simulations results. Simulations show, that for a given power and density value, there is always a limit on the seeding level, above which the self-consistent solution does not exist. This limit seems to be related to the edge density limit and might impose severe consequences on the accessible JET operational domain. Therefore, numerical investigations have been done to check how this limit can be released. It has been found that changes in the edge density, and in particular in the SOL transport can somehow mitigate this effect.
No preview · Article · Jul 2015 · Journal of Nuclear Materials
[Show abstract][Hide abstract] ABSTRACT: A global signature of the build-up to an intrinsic edge localized mode
(ELM) is found in the temporal analytic phase of signals measured in full flux azimuthal loops in the divertor region of JET. Toroidally integrating, full flux loop signals provide a global measurement proportional to the voltage induced by changes in poloidal magnetic flux; they are electromagnetically induced by the dynamics of spatially integrated current density. We perform direct time-domain analysis of the high time-resolution full flux loop signals VLD2 and VLD3. We analyze plasmas where a steady H-mode is sustained over several seconds during which all the observed ELMs are intrinsic; there is no deliberate intent to pace the ELMing process by external means. ELM occurrence times are determined from the Be II emission at the divertor. We previously [Chapman et al., Phys. Plasmas 21, 062302 (2014); Chapman et al., in 41st EPS Conference on Plasma Physics, Europhysics Conference Abstracts (European Physical Society, 2014), Vol. 38F, ISBN 2-914771-90-8] found that the occurrence times of intrinsic ELMs correlate with specific temporal analytic phases of the VLD2 and VLD3 signals. Here, we investigate how the VLD2 and VLD3 temporal analytic phases vary with time in advance of the ELM occurrence time. We identify a build-up to the ELM in which the VLD2 and VLD3 signals progressively align to the temporal analytic phase at which ELMs preferentially occur, on a
timescale. At the same time, the VLD2 and VLD3 signals become temporally phase synchronized with each other, consistent with the emergence of coherent global dynamics in the integrated current density. In a plasma that remains close to a global magnetic equilibrium, this can reflect bulk displacement or motion of the plasma. This build-up signature to an intrinsic ELM can be extracted from a time interval of data that does not extend beyond the ELM occurrence time, so that these full flux loop signals could assist in ELM prediction or mitigation.
[Show abstract][Hide abstract] ABSTRACT: JOREK 3D non-linear MHD simulations of a D
2 Massive Gas Injection (MGI) triggered disruption in JET are presented and compared in detail to experimental data. The MGI creates an overdensity that rapidly expands in the direction parallel to the magnetic field. It also causes the growth of magnetic islands (
and 3/2 mainly) and seeds the 1/1 internal kink mode. O-points of all island chains (including 1/1) are located in front of the MGI, consistently with experimental observations. A burst of MHD activity and a peak in plasma current take place at the same time as in the experiment. However, the magnitude of these two effects is much smaller than in the experiment. The simulated radiation is also much below the experimental level. As a consequence, the thermal quench is not fully reproduced. Directions for progress are identified. Radiation from impurities is a good candidate.
Full-text · Article · Jun 2015 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: A consistent deterioration of global confinement in H-mode experiments has been observed in JET  following the replacement of all carbon plasma facing components (PFCs) with an all metal ('ITER-like') wall (ILW). This has been correlated to the observed degradation of the pedestal confinement, as lower electron temperature (Te) values are routinely measured at the top of the edge barrier region. A comparative investigation of core heat transport in JET-ILW and JET-CW (carbon wall) discharges has been performed, to assess whether core confinement has also been affected by the wall change.
No preview · Article · Jun 2015 · Plasma Physics and Controlled Fusion
[Show abstract][Hide abstract] ABSTRACT: In a wide variety of natural and laboratory magnetized plasmas, filaments
appear as a result of interchange instability. These convective structures
substantially enhance transport in the direction perpendicular to the magnetic
field. According to filament models, their propagation may follow different
regimes depending on the parallel closure of charge conservation. This is of
paramount importance in magnetic fusion plasmas, as high collisionality in the
scrape-off layer may trigger a regime transition leading to strongly enhanced
perpendicular particle fluxes. This work reports for the first time on an
experimental verification of this process, linking enhanced transport with a
regime transition as predicted by models. Based on these results, a novel
scaling for global perpendicular particle transport in reactor relevant
tokamaks such as ASDEX-Upgrade and JET is found, leading to important
implications for next generation fusion devices. Full manuscript can be found in arXiv (http://arxiv.org/abs/1505.07741).
Full-text · Article · May 2015 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: Single-crystal Diamond Detectors (SDDs) feature high radiation hardness, fast response and
compact size. This makes SDDs ideal candidates for fast neutron detectors in environment
where high neutron flux is an issue such as the next generation burning plasmas experiments.
Neutron detection in SDD is based on the collection of electron-hole pairs produced by charged
particles generated by neutron interaction with 12C nuclei. For neutron energies above about 7
MeV neutron spectroscopy is possible by measuring the deposited energy into the detector via
the reaction 12C(n,α)9Be. This is indeed the cases of SDD measurements of 14 MeV neutrons of
DT plasmas. A single pixel SDD (4.5x4.5x0.5 mm^3) prototype was installed at JET in 2013
and the achieved results allowed to assess also the neutron spectroscopic capability of
deuterium plasmas. A 12-pixels SDD matrix has been recently realized and will be installed in
2015 at JET for DT plasmas as part of the Vertical Neutron Spectrometer project.
In this paper calibration of the SDD matrix with alpha particles in the laboratory and 14 MeV
neutrons performed at the ENEA Frascati Neutron Generator will be presented. These
calibrations have been performed with a fast charge preamplifier combined to a fast digital data
acquisition, which allows for neutron spectroscopy measurements with simultaneously high
energy resolution and high count rate capability. Both requirements are essential for neutron
spectroscopy of high power fusion plasmas. The calibrations results achieved extrapolate
favourably in view of future neutron spectroscopy measurements at JET using diamond
[Show abstract][Hide abstract] ABSTRACT: The physical transport processes at the basis of JET typical inductive H-mode scenarios and advanced hybrid regimes, with improved thermal confinement, are analyzed by means of some of the newest and more sophisticated quasi-linear transport models: trapped gyro Landau fluid (TGLF) and QuaLiKiz. The temporal evolution of JET pulses is modelled by CRONOS where the turbulent transport is modelled by either QuaLiKiz or TGLF. Both are first principle models with a more comprehensive physics than the models previously developed and therefore allow the analysis of the physics at the basis of the investigated scenarios. For H-modes, ion temperature gradient (ITG) modes are found to be dominant and the transport models are able to properly reproduce temperature profiles in self-consistent simulations. However, for hybrid regimes, in addition to ITG trapped electron modes (TEM) are also found to be important and different physical mechanisms for turbulence reduction play a decisive role. Whereas E × B flow shear and plasma geometry have a limited impact on turbulence, the presence of a large population of fast ions, quite important in low density regimes, can stabilize core turbulence mainly when the electromagnetic effects are taken into account. The TGLF transport model properly captures these mechanisms and correctly reproduces temperatures.
No preview · Article · Mar 2015 · Plasma Physics and Controlled Fusion