Biblio

Very sensitive magnetic instruments on the JUICE spacecraft require an extremely low magnetic field emission of the various subsystems. The JUICE solar array includes a photovoltaic assembly and various mechanisms with a magnetic signature. The design of the photovoltaic assembly has been optimised not only with respect to magnetic moment, but also with respect to the emitted magnetic field, by applying the so-called back-wiring technique, alternating string polarity etc. The remanent magnetic field of the mechanisms (hinges, eddy-current damper, hold-down & release mechanism) was tested including a process for demagnetisation. In addition, the temperature coefficient for the magnetic moment was measured, down to the operational temperature of -130°C. The eddy-current damper was also subjected to a field-induced magnetisation test. All the contributors were included in a model to calculate the magnetic field at the instrument location.

The heat load of the original cryomodules for the continuous electron beam accelerator facility is 50% higher than the target value of 100 W at 2.07 K for refurbished cavities operating at an accelerating gradient of 12.5 MV/m. This issue is due to the quality factor of the cavities being 50% lower in the cryomodule than when tested in a vertical cryostat, even at low RF field. Previous studies were not conclusive about the origin of the additional losses. We present the results of a systematic study of the additional losses in a five-cell cavity from a decommissioned cryomodule after attaching components, which are part of the cryomodule, such as the cold tuner, the He tank, and the cold magnetic shield, prior to cryogenic testing in a vertical cryostat. Flux-gate magnetometers and temperature sensors are used as diagnostic elements. Different cool-down procedures and tests in different residual magnetic fields were investigated during the study. Three flux-gate magnetometers attached to one of the cavities installed in the refurbished cryomodule C50-12 confirmed the hypothesis of high residual magnetic field as a major cause for the increased RF losses.