Theoretical investigation on possible operation of a 140 GHz 1 MW gyrotron at 175 GHz for CTS plasma diagnostics at W7-X

Collective Thomson scattering is a common diagnostic technique for ion temperature measurements in experimental fusion plasma reactors. Such a system was successfully installed and commissioned at the Wendelstein 7-X stellarator. For this purpose, a 140 GHz gyrotron of the Electron Cyclotron Resonance Heating system was used as a source of the required probing millimeter (mm)-wave beam. However, accurate measurements in the plasma core were not possible at this heating frequency due to the absorption of the mm-waves and the high electron cyclotron emission background. To suppress these effects and to enhance the accuracy of the measurements, it is required to increase the frequency of the probing beam. In this work, the possibility to operate the same gyrotron, which has its nominal operation at 140 GHz, at a higher frequency is comprehensively investigated

Optical detection and spatial modulation of mid-infrared surface plasmon polaritons in a highly doped semiconductor

Highly doped semiconductors (HDSCs) are promising candidates for plasmonic applications in the mid-infrared (MIR) spectral range. This work examines a recent addition to the HDSC family, the dilute nitride alloy In(AsN). Post-growth hydrogenation of In(AsN) creates a highly conducting channel near the surface and a surface plasmon polariton detected by attenuated total reflection techniques. The suppression of plasmonic effects following a photo-annealing of the semiconductor is attributed to the dissociation of the N-H bond. This offers new routes for direct patterning of MIR plasmonic structures by laser writing

Validation of theory-based models for the control of plasma currents in W7-X divertor plasmas

A theory-based model for the control of plasma currents for steady-state operation in W7-X is proposed and intended for model-based plasma control. The conceptual outline implies the strength of physics-based models: it offer approaches applicable to future conditions of fusion devices or next-step machines. The application at extrapolated settings is related to the validity range of the theory model. Therefore, the predictive power of theory-based control models could be larger than for data-driven approaches and limitations can be predicted from the validity range for the prediction of bootstrap currents in W7-X. The model predicts the L/R response when density or heating power is changed. The model is based on neoclassical bootstrap current calculations and validated for different discharge conditions. While the model was found to be broadly applicable for conducted electron-cyclotron-heated discharges in W7-X, limits were found for cases when the polarization of the electron cyclotron heating was changed from X2 to O2-heating. The validity assessment attempts to quantify the potential of the derived model for model-based control in the operational space (density, heating power) of W7-X