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The results of the plasma start-up with ICRH of U-2M RF discharges in H2+He mixture with newly implemented controlled gas H2 concentration are presented. The W7-X like ICRH antenna operated in monopole phasing with applied RF power of ∼ 100 kW. We investigated plasma start-up in the pressure range p = 6×10−4 - 9×10−2 Pa. Plasma production with an average density of up to Ne ∼ 1013 cm−3 was observed at frequencies the fundamental harmonic of the hydrogen cyclotron frequency.

Following the outstanding results from the first experimental period (OP1) of the stellarator Wendelstein 7-X [1-6], and in view of the second experimental period (OP2; planned to begin in 2021), an upgrade of the Electron Cyclotron Resonance Heating (ECRH) installation is underway. A major target of the upgrade is to increase the installed microwave power. To this end, a prototype Continuous-Wave (CW) 1.5 MW, 140 GHz gyrotron is under development, which is planned to be followed by two additional series tubes. In order to keep the risk and cost of the development as low as possible, the design of the 1.5 MW gyrotron is regarded as an upgrade of the successful design of the existing 1 MW, 140 GHz gyrotrons at W7-X [7]. The scientific design of the 1.5 MW tube has been realized by KIT with contributions from IPP and NKUA. The design will be provided to the industrial partner, in order to fully develop and manufacture the prototype. In parallel, a short-pulse mock-up gyrotron, which has an identical scientific design to that of the 1.5 MW CW prototype, is also under development at KIT. This gyrotron is planned to support the development of the prototype and to mitigate the risks by providing the means for the experimental validation of the scientific design of all critical components. The paper reports on the complete scientific design of the 1.5 MW CW prototype and on the progress in the construction of the short-pulse gyrotron at KIT. Possible exchange: A demand for increased gyrotron unit power in ECRH installations is present worldwide, both in existing as well as in future fusion reactors. Although several successful 1 MW CW tubes at 140 or 170 GHz already exist, the number of CW tubes at higher power is still limited. Since EU, Japan, and US are developers of MW-class gyrotrons, the exchange of ideas and experience on advanced tubes, reliably performing well-above the 1 MW power level, can be very fruitful.

This paper reports about a novel approach to the absolute intensity calibration of an electron cyclotron emission (ECE) spectroscopy system. Typically, an ECE radiometer consists of tens of separated frequency channels corresponding to different plasma locations. An absolute calibration of the overall diagnostic including near plasma optics and transmission line is achieved with blackbody sources at LN2 temperature and room temperature via a hot/cold calibration mirror unit. As the thermal emission of the calibration source is typically a few thousand times lower than the receiver noise temperature, coherent averaging over several hours is required to get a sufficient signal to noise ratio. A forward model suitable for any radiometer calibration using the hot/cold method and a periodic switch between them has been developed and used to extract the voltage difference between the hot and cold temperature source via Bayesian analysis. In contrast to the classical analysis which evaluates only the reference temperatures, the forward model takes into account intermediate effective temperatures caused by the finite beam width and thus uses all available data optimally. This allows the evaluation of weak channels where a classical analysis would not be feasible, is statistically rigorous, and provides a measurement of the beam width. By using a variance scaling factor, a model sensitive adaptation of the absolute uncertainties can be implemented, which will be used for the combined diagnostic Bayesian modeling analysis.

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.

The role of toroidal plasma currents for the island divertor scrape-off layer in the stellarator Wendelstein 7-X is investigated using reciprocating electric probes. Experiments show that small amounts (of a few kA) of plasma current are sufficient to significantly affect the scrape-off layer plasma conditions, whereas higher plasma currents above 10kA result in more drastic changes. This behavior is linked to the effect of the plasma current on the rotational transform profile, which can result in significant shifts of the edge magnetic islands. These shifts affect the interaction of the islands with the divertor and can eventually result in a transition from a diverted to a limited plasma configuration. The probe observations are complemented by further edge diagnostics including plasma flow measurements, divertor Langmuir probes, divertor thermography and impurity spectroscopy.