Exploring fusion-reactor physics with high-power electron cyclotron resonance heating on ASDEX Upgrade

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2MW, 2 s, 140 GHz system to an 8MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q$_{95}$>5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported

Overview of JET results for optimising ITER operation

The JET 2019–2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major neutral beam injection upgrade providing record power in 2019–2020, and tested the technical and procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle (α) physics in the coming D–T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed shattered pellet injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design and operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D–T benefited from the highest D–D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER

Neutral pathways and heat flux widths in vertical- and horizontal-target EDGE2D-EIRENE simulations of JET

This paper further analyses the EDGE2D-EIRENE simulations presented by Chankin et al (2017 Nucl. Mater. Energy 12 273), of L-mode JET plasmas in vertical-vertical (VV) and Vertical-horizontal (VH) divertor configurations. As expected, the simulated outer divertor ionisation source peaks near the separatrix in VV and radially further out in VH. We identify the reflections of recycled neutrals from lower divertor tiles as the primary mechanism by which ionisation is concentrated on the outer divertor separatrix in the VV configuration. These lower tile reflection pathways (of neutrals from the outer divertor, and to an even greater extent from the inner divertor) dominate the outer divertor separatrix ionisation. In contrast, the lower-tile-reflection pathways are much weaker in the VH simulation and its outer divertor ionisation is dominated by neutrals which do not reflect from any surfaces. Interestingly, these differences in neutral pathways give rise to strong differences in the heat flux density width λq at the outer divertor entrance: λq = 3.2 mm in VH compared to λq = 11.8 mm in VV. In VH, a narrow channel exists in the near scrape-off-layer (SOL) where the convected heat flux, driven by strong Er × B flow and thermoelectric current, dominates over the conducted heat flux. The width of this channel sets λq and is determined by the radial distance between the separatrix and the ionisation peak in the outer divertor

Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas

The low temperature boundary layer plasma (Scrape-Off-Layer or SOL) between the hot core and the surrounding vessel determines the level of power-loading, erosion and implantation of material surfaces, and thus the viability of tokamak-based fusion as an energy source. This study explores mechanisms affecting the formation of flattened density profiles, so-called ‘density shoulders’, in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces – and subsequent erosion. There is evidence against local enhancement of ionization inducing shoulder formation. We find that increases in SOL parallel resistivity, Λdiv (=[L||νei Ωi ]/cs Ωe), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in upstream SOL shoulder amplitude, As only under a subset of conditions (D2-fuelled L-mode density scans with outer strike point on the horizontal target). Λdiv fails to correlate with As for cases of N2 seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Λdiv with As was also found for H-mode discharges. Thus, while Λdiv above a threshold of ~1 may be necessary for shoulder formation and/or growth, another shoulder mechanism is required. More significantly we find that in contrast to parallel resistivity, outer divertor recycling as quantified by the total outer divertor Balmer Dα emission, I-Dα, does scale with shoulder amplitude where Λdiv does and even where Λdiv fails. Divertor recycling could lead to SOL density shoulder formation through: a) reducing the parallel to the field flow (loss) of ions out of the SOL to the divertor; and b) changes in radial electric fields which lead to ExB poloidal flows as well as potentially affecting the SOL turbulence birth characteristics. Thus changes in divertor recycling may be the sole process in bringing about SOL density shoulders or in tandem with parallel resistivity

Observations and modelling of ion cyclotron emission observed in JET plasmas using a sub-harmonic arc detection system during ion cyclotron resonance heating

Peer reviewe

Investigations on Spectroscopic Diagnostic of High-Z Elements in Fusion Plasmas

Nuclear fusion of deuterium and Tritium relies on the accomplishment of plasma temperatures in the range of 20keV. In magnetic confinement fusion, the heat transport and radiation losses are compensated by plasma heating. This scheme relies on the control of the loss mechanisms and in particular the plasma radiation. Tungsten (W) is the main candidate for the first wall of a reactor due to its robustness against physical sputtering by the plasma ions, however, when W reaches concentrations of 1E-4 in the plasma, it causes unduly large plasma cooling by radiation. This implies restrictive impurity control for W, which needs reliable diagnostic by plasma spectroscopy. A pre-requisite for interpretation of the W-spectra is the availability of atomic data for W. The most intense spectral lines of highly ionized W are emitted in the VUV and soft X-ray range. To perform calculations on atomic data the code packages incorporated in the ADAS project are used. The electronic structure of nearly all W-ions is calculated by the Cowan-code (Hartree-Fock algorithm). In a second step, the cross sections for electron impact excitation are evaluated via the Cowan-code using the plane wave Born-approximation. A detailed collisional-radiative model is employed to calculate the model-spectra for each ion in equilibrium. Finally, ionization and recombination rates of W are evaluated by semi-empirical formulae, which make use of the electronic structure calculations of the Cowan-code. All atomic data are confronted with experimental measurements from the Garching tokamak ASDEX Upgrade and the Berlin electron-beam ion trap (EBIT). The experimental investigations extend up to 5keV electron temperatures, which is the maximum of the routine operation at ASDEX Upgrade. 'Impurity accumulation', which is characterized by a strong peaking of the impurity density profile, enables unique investigations on the fractional abundance of Ag-like W27+ up to Co-like W47+. According to this findings different sets of ionization and recombination data (originating from independent sources) are evaluated. The recombination rates for few states are corrected empirically satifying boundary conditions which arise from experimental evidence. Focus was put on the two most intense spectral features at 4-6nm (VUV) and at 0.4-0.8nm (soft X-ray). At 5nm the spectral emissions of Ag-like W27+ to Cu-like W45+ are superimposed and the EBIT-data is used to disentangle the emissions of each ionization state. Very rough agreement is found for the emissions below Kr-like W38+ at electron temperatures below 2keV, while the level of agreement improves for the spectral lines emitted by Se-like W40+ to Cu-like W45+ at electron temperatures above 2keV. At these temperatures Kr-like W38+ to about Mn-like W49+ show emissions in the soft X-ray, for which the modelled spectra give good agreement. Both spectral features have been studied also for isoelectronic sequences by injecting the impurities hafnium, tantalum, rhenium, gold, lead and bismuth. Additionally, xenon is targeted by the same code packages, as xenon might be injected in future experiments or a reactor for intentional plasma cooling. The systematical trend for these elements is the same as indicated for W, as the agreement improves for higher charged ionization states and higher electron temperatures. Predictions on radiative plasma cooling (cooling factor CF) have been based up to now on the rough 'Average Ion Model' (AIM) and a further result of the work is the analysis of plasma cooling with the outlined, superior model. All data, which are benchmarked by experimental spectra, are used to calculate the CF of the high-Z elements. The resulting CF does not exhibit large differences to that from the AIM, in particular the new data predicts about factor 2 less radiation in the range 2-5keV while for higher electron temperatures the difference is decreasing to negligible values at about 15keV. This imposes no change on the predictions of the maximum tolerable W concentration in the core plasma of a reactor. Finally, the new atomic data is used to predict spectral lines at higher electron temperatures, which will be important to diagnose the W concentration in the central part of the reactor plasma.Kernfusion von Deuterium und Tritium benötigt Plasmatemperaturen von ca. 20keV. Bei der Fusion in magnetisch eingeschlossenen Plasmen sollen der Wärmetransport und die Strahlungsverluste durch Plasmaheizung ausgeglichen werden. Die Verluste müssen kontrolliert werden. Aus heutiger Sicht ist Wolfram (W) das beste Material für die Oberflächen, die im Kontakt zum Plasma stehen ("Erste Wand"), da hier die Zerstäubung durch Plasmaionen uneffektiv ist. Jedoch darf die Plasmaverunreinigung durch W 1E-4 nicht übersteigen, da sonst die Plasmakühlung durch Strahlung nicht tolerierbar ist. Die W-Konzentration muss also kontrolliert und somit auch diagnostiziert werden. Dies ist durch Plasmaspektroskopie im VUV- und Röntgenbereich möglich, wo die hochionisierten Wolframionen besonders intensive Spektrallinien aufweisen. In der aktuellen Arbeit wurden Atomdaten für diese Emissionen berechnet und anhand entsprechender Messungen qualifiziert. Im Einzelnen, wurden verschiedene Programmpackete aus dem ADAS Projekt benutzt. Dabei wurde zunächst mit dem Cowan-code (Hartree-Fock Algorithmus) die elektronische Struktur der Wolframionen berechnet. Im Weiteren wurden, wiederum mit dem Cowan-code, Elektronenstoßquerschnitte mittels der ebenen Wellen Born-Näherung bestimmt. Durch ein Stoß-Strahlung-Modell wurden Modell-Spektren für alle W-Ionen berechnet. Die Ionisations- und Rekombinationsraten von Wolfram wurden mittels semi-empirischer Formeln und den elektronischen Strukturrechnungen des Cowan-codes gewonnen. Den Atomdaten werden experimentellen Messungen vom Garchinger Tokamak ASDEX Upgrade und der Berliner Elektron-Beam Ion Trap (EBIT) gegenüber gestellt. Die experimentellen Untersuchungen erstrecken sich bis zu 5keV Elektronentemperatur, was der oberen Grenze für Standard-Plasmabetrieb an ASDEX Upgrade entspricht. Durch das Phänomen 'Verunreinigungsakkumulation', was durch ein stark zugespitztes Verunreinigungsdichteprofil charakterisiert ist, ergibt sich die Möglichkeit, die relative Häufigkeit von Ag-ähnlichem W27+ bis Co-ähnlichem W47+ genau zu dokumentieren. Aus verschiedenen Datensätzen für Ionisation und Rekombination konnten die besten Daten identifiziert werden, welche aus einer 'configuration averaged distorted wave'-Rechnung für Ionisation und den hier berechneten Rekombinationsdaten bestehen. Für Ionenstufen unterhalb Se-ähnlichem W40+ sind trotzdem Abweichungen zu beobachten. Durch empirische Anpassung der Rekombinationsdaten werden diese beseitigt, während alle Randbedingungen, die sich aus den Messdaten ergeben, erfüllt sind. Bei der Analyse der Spektren wurde besonderen Wert auf die beiden intensivsten spektralen Strukturen bei 4-6nm (VUV) und bei 0.4-0.8nm (weicher Röntgenbereich, SXR) gelegt. Bei 5nm zeigt sich eine Emissionsstruktur, die sich aus Spektrallinien von Ag-ähnlichem W27+ bis Cu-ähnlichem W45+ zusammensetzt. Durch Analyse von EBIT-Spektren konnten die Emissionen gut den Ionenstufen zugeordnet werden. Im Vergleich zu den Theoriedaten zeigen die Emissionen von Ionenstufen unterhalb Kr-ähnlichem W38+ (2keV) stimmen dagegen gut mit den Modelldaten überein. Bei den Elektronentemperaturen über 2keV werden intensive Spektrallinien im SXR durch Kr-ähnliches W38+ bis ca. Mn-ähnliches W49+ emittiert. Diese werden gut durch die Theorie modelliert. Die spektralen Strukturen wurden systematisch auch für die dem W benachbarten Elemente Hafnium, Tantal, Rhenium, Gold, Blei und Wismut untersucht. Zusätzlich wurde auch das Spektrum von Xenon genauer untersucht, da Xenon in zukünftigen Fusionsplasmen zur Plasmakühlung eingeblasen werden könnte. Für alle untersuchten Elemente zeigt sich der gleiche Trend, dass die Theoriedaten für höhere Ionenladungsstufen und für höhere Elektronentemperaturen besser mit dem Experiment übereinstimmen. Darüberhinaus wurden Spektrallinien vorhergesagt, die im heißen Zentrum eines zukünftigen Reaktors emittiert werden und heute noch nicht charakterisiert sind. Schließlich wurde die Plasmakühlung durch Strahlung aus den ADAS-Daten berechnet, indem alle berechneten Emissionen summiert wurden. Es ergibt sich der sog. Strahlungsleistungsparameter (SP), der mit dem des gröberen 'Average Ion Modell' (AIM) verglichen wird. Dabei zeigt sich, dass zwischen 2-5keV ca. ein Faktor 2 weniger Strahlung durch die neuen Daten vorhergesagt werden. Der Unterschied zwischen den Datensätzen verschwindet für noch höhere Elektronentemperaturen ab ca. 15keV. Die Vorhersage der maximal tolerierbaren W-konzentration im Zentrum eines Fusionsreaktors ändert sich somit nicht