Advanced electron cyclotron heating and current drive experiments on the stellarator Wendelstein 7-X

During the first operational phase (OP 1.1) of Wendelstein 7-X (W7-X) electron cyclotron resonance heating (ECRH) was the exclusive heating method and provided plasma start-up, wall conditioning, heating and current drive. Six gyrotrons were commissioned for OP1.1 and used in parallel for plasma operation with a power of up to 4.3 MW. During standard X2-heating the spatially localized power deposition with high power density allowed controlling the radial profiles of the electron temperature and the rotational transform. Even though W7-X was not fully equipped with first wall tiles and operated with a graphite limiter instead of a divertor, electron densities of n e > 3·1019 m-3 could be achieved at electron temperatures of several keV and ion temperatures above 2 keV. These plasma parameters allowed the first demonstration of a multipath O2-heating scenario, which is envisaged for safe operation near the X-cutoff-density of 1.2·1020 m-3 after full commissioning of the ECRH system in the next operation phase OP1.2

First experiments on plasma production using field-aligned ICRF fast wave antennas in the large helical device

The results of the first experimental series to produce a plasma using the ion cyclotron range of frequency (ICRF) in the large helical device (LHD) within the minority scenario developed at Uragan-2M (U-2M) are presented. The motivation of this study is to provide plasma creation in conditions when an electron cyclotron resonance heating start-up is not possible, and in this way widen the operational frame of helical machines. The major constraint of the experiments is the low RF power to reduce the possibility of arcing. No dangerous voltage increase at the radio-frequency (RF) system elements and no arcing has been detected. As a result, a low plasma density is obtained and the antenna-plasma coupling is not optimal. However, such plasmas are sufficient to be used as targets for further neutral beam injection (NBI) heating. This will open possibilities to explore new regimes of operation at LHD and Wendelstein 7-X (W7-X) stellarator. The successful RF plasma production in LHD in this experimental series stimulates the planning of further studies of ICRF plasma production aimed at increasing plasma density and temperature within the ICRF minority scenario as well as investigating the plasma prolongation by NBI heating

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator’s non-turbulent ‘neoclassical’ energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization

Impact of Magnetic Field Configuration on Heat Transport in Stellarators and Heliotrons

We assess the magnetic field configuration in modern fusion devices by comparing experiments with the same heating power, between a stellarator and a heliotron. The key role of turbulence is evident in the optimized stellarator, while neoclassical processes largely determine the transport in the heliotron device. Gyrokinetic simulations elucidate the underlying mechanisms promoting stronger ion scale turbulence in the stellarator. Similar plasma performances in these experiments suggests that neoclassical and turbulent transport should both be optimized in next step reactor designs

Mikrowellenheizung und Diagnostik von überthermischen Elektronen in einem überdichten Stellarator-Plasma

Eine der Hauptmethoden zur Heizung eines magnetisch eingeschlossenen Plasmas basiert auf der resonanten Einkopplung von Mikrowellen. Von wachsender Bedeutung für Fusionsplasmen hoher Dichte ist die Identifizierung und das Verständnis der Mechanismen zur Heizung von überdichten Plasmen, in denen eine Wellenpropagation im Allgemeinen nicht möglich ist, da die Plasmafrequenz höher als die Heizfrequenz ist. Das Hauptthema in der vorliegenden Arbeit ist die Heizung von überdichten Plasmen am Stellarator WEGA. Die Anregung von Elektron-Bernsteinwellen über den OXB-Konversionsprozess bietet eine Möglichkeit die andernfalls nicht zugängliche resonante Absorptionsschicht im Plasma zu erreichen. Im speziellen Fall von WEGA wird während der OXB-Heizung eine überthermische Elektronenkomponente mit Energien bis zu 80 keV erzeugt. Die schnellen Elektronen sind im Plasmazentrum lokalisiert und weisen innerhalb des weichen Röntgenbereiches eine Maxwellsche Verteilung auf. Die entsprechende mittlere Energie liegt im Bereich von einigen keV. Die OXB-Entladungen sind zusätzlich begleitet von einem breitbandigen Mikrowellenspektrum mit Strahlungstemperaturen von ebenfalls einigen keV. Die Quelle der Strahlung konnte als parametrischer Zerfall der Heizwelle identifiziert werden ohne eine Verbindung zur überthermischen Elektronenkomponente. Zur detailierten Untersuchung der Mikrowellenemission wurde ein quasioptisches Spiegelsystem aufgebaut, das auf die OX-Konversion optimiert ist. Mit Hilfe der breitbandigen Streustrahlung des Zerfallsprozess konnte die OX-Konversionseffizienz mit 0.56 bestimmt werden, das in guter Übereinstimmung mit dem theoretischen Wert ist. Darüber hinaus wurde bei Plasmen ohne eine Elektronen-Zyklotron-Resonanz bezüglich der Heizwelle ein überdichter nicht-resonanter Heizprozess untersucht. Whistler-Wellen oder allgemeiner Wellen mit einer rechtsdrehenden Polarisation sind die einzigen ausbreitungsfähigen Wellen innerhalb der untersuchten stark überdichten Plasmen. Durch Analyse der Heizeffizienz in Abhängigkeit von der magnetischen Flussdichte in der Umgebung der Reflexionsschicht lässt sich auf die Tunnelung als wahrscheinlichster Kopplungsmechanismus schließen. Zur Bestimmung der Heizeffizienz für die verschiedenen Heizszenarien (überdichte nichtresonante Heizung sowie underdichte und überdichte resonante Heizung) wurde eine Vielzahl von Diagnostik-Methoden angewendet. Im Fall von Wärmewellenexperimenten wurden unter anderem die Zeitabhängigkeit der Plasmastrahlung genutzt um zu einer Abschätzung der Heizleistung sowie der Energieeinschlusszeit zu kommen. Die Genauigkeit der Methode kann auf ±10% abgeschätzt werden abhängig von der Messgenauigkeit der Strahlungsmessung.The resonant coupling of microwaves into a magnetically confined plasma is one of the fundamental methods for the heating of such plasmas. Identifying and understanding the processes of the heating of overdense plasmas, in which the wave propagation is generally not possible because the wave frequency is below the plasma frequency, is becoming increasingly important for high density fusion plasmas. This work focuses on the heating of overdense plasmas in the WEGA stellarator. The excitation of electron Bernstein waves, utilizing the OXB-conversion process, provides a mechanism for the wave to reach the otherwise not accessible resonant absorption layer. In WEGA these OXB-heated plasmas exhibit a suprathermal electron component with energies up to 80 keV. The fast electrons are located in the plasma center and have a Maxwellian energy distribution function within the soft X-ray related energy range. The corresponding averaged energy is a few keV. The OXB-discharges are accompanied by a broadband microwave radiation spectrum with radiation temperatures of the order of keV. Its source was identified as a parametric decay of the heating wave and has no connection to the suprathermal electron component. For the detailed investigation of the microwave emission, a quasioptical mirror system, optimized for the OX-conversion, has been installed. Based on the measurement of the broadband microwave stray radiation of the decay process, the OX-conversion efficiency has been determined to 0.56 being in good agreement with full-wave calculations. In plasmas without an electron cyclotron resonance, corresponding to the wave frequency used, non-resonant heating mechanisms have been identified in the overdense plasma regions. Whistler waves or R-like waves are the only propagable wave types within the overdense plasmas. The analysis of the heating efficiency in dependence on the magnetic flux density leads to tunneling as the most probable coupling mechanism. For the determination of the heating efficiencies of the different heating scenarios (overdense non-resonant heating, underdense and overdense resonant heating) a variety of diagnostic methods have been used. Based on heat wave experiments and measuring the response of the total plasma radiation, an estimate of the absorbed heating power and the energy confinement time has been achieved with an accuracy of ±10% dependent on the measurement accuracy of the radiated power

Emphasis of spatial cues in the temporal fine structure during the rising segments of amplitude-modulated sounds II : single-neuron recordings

Recently, with the use of an amplitude-modulated binaural beat (AMBB), in which sound amplitude and interaural-phase difference (IPD) were modulated with a fixed mutual relationship (Dietz et al. 2013b), we demonstrated that the human auditory system uses interaural timing differences in the temporal fine structure of modulated sounds only during the rising portion of each modulation cycle. However, the degree to which peripheral or central mechanisms contribute to the observed strong dominance of the rising slope remains to be determined. Here, by recording responses of single neurons in the medial superior olive (MSO) of anesthetized gerbils and in the inferior colliculus (IC) of anesthetized guinea pigs to AMBBs, we report a correlation between the position within the amplitude-modulation (AM) cycle generating the maximum response rate and the position at which the instantaneous IPD dominates the total neural response. The IPD during the rising segment dominates the total response in 78% of MSO neurons and 69% of IC neurons, with responses of the remaining neurons predominantly coding the IPD around the modulation maximum. The observed diversity of dominance regions within the AM cycle, especially in the IC, and its comparison with the human behavioral data suggest that only the subpopulation of neurons with rising slope dominance codes the sound-source location in complex listening conditions. A comparison of two models to account for the data suggests that emphasis on IPDs during the rising slope of the AM cycle depends on adaptation processes occurring before binaural interaction.13 page(s