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An optimized second harmonic cavity for a 170 GHz 1.3MW fusion gyrotron is presented. The suppression of the competing modes at fundamental frequency is achieved solely due to a specific design of the coaxial cavity with corrugated inner conductor and does not require e.g. the injection of an external signal. The design is verified by self-consistent multimode simulations. The results show an excellent suppression of the competing fundamental modes.

Latest research and development on advanced gyrotron developments for Electron Cyclotron Resonance Heating (ECRH) and plasma control performed at Karlsruhe Institute of Technology (KIT) is presented. The status on the development of coaxial cavity gyrotrons for the future demonstration power plant (DEMO), on advanced gyrotron operation, key components, simulation tools and test facilities is reported. Finally, the upstarting research on future megawatt-class gyrotrons operating at the second harmonic of the electron cyclotron resonance frequency is discussed

The paper reports the improvement of the methods for the synthesis of launchers for high power gyrotrons at KIT in the period from 2018 to the present. The spectrum reconstruction method has been developed to greatly increase the curvature radii of numerically optimized launcher wall perturbations in order to reduce stray radiation. The method for the design of Denisov-type launchers has been developed for the conversion of co- and counter rotating modes in the same launcher. The design method for hybrid-type launchers has also been improved for the conversion of co- and counter-rotating modes.

Drifts affect particle, momentum, and energy transport in the scrape-off layer (SOL) of tokamaks and stellarators, altering plasma flows and creating asymmetries between divertors. To understand how drifts affect SOL transport in the W7-X island divertor, an experiment was performed to compare plasmas with matched core parameters but opposite magnetic field directions, and therefore opposite drift transport directions. Parallel flow measurements made with coherence imaging spectroscopy are interpreted with the aid of a diagnostic forward model and a 1D simple SOL model that includes the E × B drift. In low-density plasmas ( n ‾ e < 2 × 10 19 m−3), the poloidal E × B drift induces a large poloidal density asymmetry within the island SOL, as measured by divertor Langmuir probes. This in turn causes the parallel flow stagnation point to shift from the position halfway between targets to the X-point in the drift direction, leading to near-unidirectional flow throughout the SOL. As density increases, the effects of the poloidal E × B drift decrease substantially, resulting in a smaller density asymmetry and the development of a counter-streaming flow pattern. For the entire density range probed in this experiment ( n ‾ e = 1.5 − 6 × 10 19 m−3), the experimental observations are more consistent with the effects of the poloidal E × B drift than the radial E × B drift.

The detachment regime has a high potential to play an important role in fusion devices on the road to a fusion power plant. Complete power detachment has been observed several times during the experimental campaigns of the Wendelstein 7-X (W7-X) stellarator. Automatic observation and signaling of such events could help scientists to better understand these phenomena. With the growing discharge times in fusion devices, machine learning models and algorithms are a powerful tool to process the increasing amount of data. We investigate several classical supervised machine learning models to detect complete power detachment in the images captured by the Event Detection Intelligent Camera System (EDICAM) at the W7-X at each given image frame. In the dedicated detached state the plasma is stable despite its reduced contact with the machine walls and the radiation belt stays close to the separatrix, without exhibiting significant heat load onto the divertor. To decrease computational time and resources needed we propose certain pixel intensity profiles (or intensity values along lines) as the input to these models. After finding the profile that describes the images best in terms of detachment, we choose the best performing machine learning algorithm. It achieves an F1 score of 0.9836 on the training dataset and 0.9335 on the test set. Furthermore, we investigate its predictions in other scenarios, such as plasmas with substantially decreased minor radius and several magnetic configurations.

The 2 MW 170 GHz single-frequency coaxial-cavity short-pulse pre-prototype is upgraded to operate also at 204 GHz. Therefore, the quasi-optical output coupler, which is a gyrotron key component, has been modified. Before the newly manufactured quasi-optical output coupler is installed into the gyrotron, a low-power cold measurement for the verification is performed. Therefore, a mode generator is designed and adjusted to excite the relevant operating gyrotron modes, namely the TE 34,19 mode at 170 GHz and TE 40,23 mode at 204 GHz, with excellent purity and a low counter-rotating amount of < 0.5 % for both modes. The TE 40,23 mode is the mode with the highest eigenvalue ever excited in cold tests. After the successful mode excitation, first the fabricated launcher and then the entire quasi-optical output coupler are verified, showing excellent agreement with the simulation.

The formation of the radial electric field, E r in the scrape-off layer (SOL) has been experimentally studied for attached divertor conditions in stellarator W7-X. The main objective of this study is to test the validity in a complex three-dimensional (3D) island divertor of simple models, typically developed in tokamaks, relating E r in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge E r shear on the reduction of density fluctuation amplitude, a well-established phenomenon according to the existing bibliography. The main diagnostic for measurements in the SOL is a V-band Doppler reflectometer that can provide the measurement of the E r and density fluctuations with good spatial resolution. 3D measurements of divertor parameters have been carried out using infrared cameras, with the exponential decay length of the divertor heat flux ( λ q ) resulting in a suitable proxy for the model-relevant λ T , the exponential decay length of the temperature at the divertor. In the investigated attached regimes, it is shown for the first time that the formation of the E r in the SOL depends on parameters at the divertor, following a E r ∝ T e / λ q qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed E r shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.