Towards a 1.5 MW, 140 GHz gyrotron for the upgraded ECRH system at W7-X

For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation

A new 3MW ECRH system at 105 GHz for WEST

The aim of the WEST experiments is to master long plasma pulses (1000s) and expose ITER-like tungsten wall to deposited heat fluxes up to 10 MW/m$^2$. To increase the margin to reach the H-Mode and to control W-impurities in the plasma, the installation of an upgraded ECRH heating system, with a gyrotron performance of 1MW/1000s per unit, is planned in 2023. With the modifications of Tore Supra to WEST, simulations at a magnetic field B$_0$∼3.7T and a central density n$_{e0}$∼6 × 10$^{19}$ m$^{−3}$ show that the optimal frequency for central absorption is 105 GHz. For this purpose, a 105 GHz/1MW gyrotron (TH1511) has been designed at KIT in 2021, based on the technological design of the 140 GHz/1.5 MW (TH1507U) gyrotron for W7-X. Currently, three units are under fabrication at THALES. In the first phase of the project, some of the previous Tore Supra Electron Cyclotron (EC) system components will be re-installed and re-used whenever possible. This paper describes the studies performed to adapt the new ECRH system to 105 GHz and the status of the modifications necessary to re-start the system with a challenging schedule

ECCD-induced sawtooth crashes at W7-X

The optimised superconducting stellarator W7-X generates its rotational transform by means of external coils, therefore no toroidal current is necessary for plasma confinement. Electron cyclotron current drive experiments were conducted for strikeline control and safe divertor operation. During current drive experiments periodic and repetitive crashes of the central electron temperature, similar to sawtooth crashes in tokamaks, were detected. Measurements from soft x-ray tomography and electron cyclotron emission show that the crashes are preceded by weak oscillating precursors and a displacement of the plasma core, consistent with a (m, n)=(1, 1) mode. The displacement occurs within 100μs, followed by expulsion and redistribution of the core into the external part of the plasma. Two types of crashes, with different frequencies and amplitudes are detected in the experimental program. For these non-stationary parameters a strong dependence on the toroidal current is found. A 1-D heuristic model for current diffusion is proposed as a first step to explain the characteristic crash time. Initial results show that the modelled current diffusion timescale is consistent with the initial crash frequency and that the toroidal current rise shifts the position where the instability is triggered, resulting in larger crash amplitudes

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

The Spatio-temporal Structure of Electrostatic Turbulence in the WEGA Stellarator

The present work is the first work dealing with turbulence in the WEGA stellarator. The main object of this work is to provide a detailed characterisation of electrostatic turbulence in WEGA and to identify the underlying instability mechanism driving turbulence. The spatio-temporal structure of turbulence is studied using multiple Langmuir probes providing a sufficiently high spatial and temporal resolution. Turbulence in WEGA is dominated by drift wave dynamics. Evidence for this finding is given by several individual indicators which are typical features of drift waves. The phase shift between density and potential fluctuations is close to zero, fluctuations are mainly driven by the density gradient, and the phase velocity of turbulent structures points in the direction of the electron diamagnetic drift. The structure of turbulence is studied mainly in the plasma edge region inside the last closed flux surface. WEGA can be operated in two regimes differing in the magnetic field strength by almost one order of magnitude (57mT and 500mT, respectively). The two regimes turned out to show a strong difference in the turbulence dynamics. At 57mT large structures with a poloidal extent comparable to the machine dimensions are observed, whereas at 500mT turbulent structures are much smaller. The poloidal structure size scales nearly linearly with the inverse magnetic field strength. This scaling may be argued to be related to the drift wave dispersion scale. However, the structure size remains unchanged when the ion mass is changed by using different discharge gases. Inside the last closed flux surface the poloidal ExB drift in WEGA is negligible. The observed phase velocity is in good agreement with the electron diamagnetic drift velocity. The energy in the wavenumber-frequency spectrum is distributed in the vicinity of the drift wave dispersion relation. The three-dimensional structure is studied in detail using probes which are toroidally separated but aligned along connecting magnetic field lines. As expected for drift waves a small but finite parallel wavenumber is found. The ratio between the average parallel and perpendicular wavenumber is in the order of 10^-2. The parallel phase velocity of turbulent structures is in-between the ion sound velocity and the Alfvènvelocity. In the parallel dynamics a fundamental difference between the two operational regimes at different magnetic field strength is found. At 500mT turbulent structures can be described as an interaction of wave contributions with parallel wavefronts. At 57mT the energy in the parallel wavenumber spectrum is distributed among wavenumber components pointing both parallel and antiparallel to the magnetic field vector. In both cases turbulent structures arise preferable on the low field side of the torus. Some results on a novel field in plasma turbulence are given, i.e. the study of turbulence as a function of resonant magnetic field perturbations leading to the formation of magnetic islands. Magnetic islands in WEGA can be manipulated by external perturbation coils. A significant influence of field perturbations on the turbulence dynamics is found. A distinct local increase of the fluctuation amplitude and the associated turbulent particle flux is found in the region of magnetic islands.Ziel der vorliegenden Arbeit ist eine ausführliche Charakterisierung elektrostatischer Turbulenz im Stellarator WEGA, sowie die Identifikation der zugrundeliegenden Instabilität. Die zur Untersuchung der räumlich-zeitlichen Struktur der Turbulenz notwendige Auflösung wird durch eine Vielzahl von Langmuir Sonden erreicht. Die Turbulenz in WEGA wird von Driftwellen dominiert. Dies wird durch die Beobachtung einer Reihe markanter, aus dem physikalischen Mechanismus der Driftwelle resultierender, Eigenschaften gezeigt. Die Phasenverschiebung zwischen Dichte- und Potenzialfluktuationen ist hinreichend klein; Fluktuationen treten vornehmlich im Bereich des Dichtegradienten auf; die poloidale Phasengeschwindigkeit turbulenter Strukturen weist in Richtung der diamagnetischen Drift der Elektronen. Das Augenmerk in den Turbulenzuntersuchungen richtet sich auf den Rand des Plasmas innerhalb der Bereichs geschlossener Flussflächen. WEGA kann in zwei unterschiedlichen Betriebsarten arbeiten, die sich in der Induktion des einschließenden Magnetfeldes unterscheiden (57 mT bzw. 500 mT). Die zwei Modi zeigen starke Unterschiede in der Dynamik der Turbulenz. Bei 57 mT zeigen sich Strukturen mit einer poloidalen Ausdehnung die der räumlichen Ausdehnung des Plasmas nahekommt. Bei 500 mT sind die Strukturen deutlich kleiner, wobei die Ausdehnung der Strukturen nahezu proportional zur inversen Induktion ist. Dies legt eine direkte Verbinding der Strukturgröße zur Dispersions-Skalenlänge der Driftwelle nahe. Gegen diesen Zusammenhang spricht jedoch, dass die Strukturgröße bei einer Variation der Ionenmasse unverändert bleibt. Die poloidale Phasengeschwindigkeit innerhalb des Einschlussbereiches, wo die ExB Drift vernachlässigbar ist, ist in guter Übereinstimmung mit der diamagnetischen Driftgeschwindigkeit der Elektronen. Die Energie im turbulenten Wellenzahl-Frequenzspektrum verteilt sich um die Dispersionsrelation der Driftwelle. Die dreidimensionale Struktur der Turbulenz wird mit Hilfe entlang einer Feldlinie angeordneter Sonden untersucht. Wie für Driftwellen erwarten zeigt sich eine endliche parallele Wellenzahlkomponente. Das Verhältnis von mittlerer paralleler zu poloidaler Wellenzahl liegt bei 10^-2. Die parallele Geschwindigkeit der Strukturen liegt zwischen der Ionenschall- und der Alfvén-Geschwindigkeit. In der parallelen Dynamik der Turbulenz zeigt sich ein grundlegender Unterschied zwischen den beiden Betriebsarten. Bei 500 mT lassen sich die Strukturen als Überlagerung von Wellenpaketen mit parallelen Wellenfronten beschreiben. Bei 57 mT zeigt das parallele Wellenzahlspektrum eine Verteilung der Energie auf Anteile, die sowohl parallel als auch antiparallel zum magnetischen Feld ausgerichtet sind. In beiden Fällen entstehen die Strukturen vornehmlich auf der Niedrigfeld-Seite des Torus. Erste Ergebnisse zu einem neuen Gebiet im Bereich der Plasmaturbulenz werden gezeigt: Der Einfluss resonanter Magnetfeldstörungen und damit verbundener magnetischer Inseln auf die Turbulenz. Magnetische Inseln werden mit Hilfe externer Störspulen manipuliert. Es zeigt sich ein deutlicher Einfluss von Störfeldern auf die Turbulenz. Eine ausgeprägte Erhöhung der Fluktuationsamplitude und des damit verbundenen turbulenten Teilchenflusses tritt im Bereich magnetischer Inseln auf