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The present COMPASS tokamak at the Institute of Plasma Physics in Prague is equipped with the 2-mm interferometer, which gives a possibility to measure line-average electron densities up to 1.2×1020 m−3. A high magnetic field tokamak COMPASS-U will be designed and built to replace COMPASS. Higher line-average plasma densities in COMPASS-U of about 5×1020 m−3 will require a new design of the interferometer. This will provide measurements in a wide density range and will allow using a real-time gas puff density feedback. A solution based on the solid-state technology is proposed. The system will use two microwave transceivers with close sub-millimeter wavelengths and will fulfill the principle of the unambiguous measurement. An assessment of signal corrections corresponding to non-linearity effects caused by plasma refractive index is performed. The ray-tracing FIESTA-8 code was used to model the propagation of the probing waves through COMPASS-U plasmas. [ABSTRACT FROM AUTHOR]

• Main constraints influencing the conceptual design of diagnostics for COMPASS-U were identified. • Diagnostic solutions for the hot vacuum vessel, high plasma densities and high heat flux densities are proposed. • Spatial constraints at COMPASS-U are reviewed and possible solutions indicated. COMPASS-U, a high magnetic field tokamak with hot walls, will be designed and built at IPP Prague. Unique features of this new device bring noticeable constraints and requirements on plasma diagnostics, which make their development highly demanding. In this paper, the main expected constraints influencing the conceptual design of diagnostic tools for COMPASS-U (high temperature of the vacuum vessel, high plasma density, high heat flux density, strong auxiliary plasma heating, spatial constraints, liquid metals in the divertor) are reviewed and possible solutions are indicated. [ABSTRACT FROM AUTHOR]