Microwave cryogenic calibrator for Strip LSPE: design optimization and verification

The activity presented in this thesis is part of the verification campaign of the Strip instrument, which is one of the subsystems of the LSPE (Large Scale Polarization Explorer) project, dedicated to the detection of the B-modes of the Cosmic Microwave Background (CMB) polarised component, representing one of the most challenging and significant goals of modern cosmology, as it would provide direct evidence of primordial gravitational waves, supporting the inflationary model of the early Universe. In particular, Strip will observe polarised foregrounds and the atmosphere by means of an array of polarimeters based on HEMT low-noise amplifiers (LNAs), operating at central frequencies of 43 GHz and 95 GHz. To properly calibrate Strip’s polarimeters, a cryogenic calibrator is required to provide a stable blackbody reference signal in the microwave band. This thesis focuses on the thermal characterisation of the calibrator cryostat, carried out both in stand-alone configuration and when integrated with the Strip instrument, within the verification campaign at the INAF-OAS Cryowaves Laboratory. My work included the assembly of the system, a series of thermal-vacuum tests, and the development of thermal models and Python-based analysis tools for data interpretation. The experimental results offered insights into the thermal behaviour and coupling of the integrated system under cryogenic conditions. The comparison with thermal simulations highlighted differences that provided useful indications on how to improve the system design and achieve performances closer to the experiment’s requirements. The tests confirmed the mechanical and vacuum compatibility of the two subsystems and demonstrated the capability of Strip’s polarimeters to detect thermal gradients from the calibrator, validating the adopted approach and supporting the ongoing optimisation of the LSPE-Strip instrument.