Instabilities in SQUID Series Arrays due to the Internal and External Parasitics

SQUID Series Arrays (SSAs) are widely used amplification devices for cryogenic readout chains such as Transition Edge Sensor (TES) matrices for space observation (Barrett et al., 2023) or quantum science (Hummatov et al., 2023). Either wiring-related external or on-chip internal parasitics often cause SSAs to show instabilities, which may limit the obtainable gain from a SSA design or may constrain the cryogenic setup that can be made stable for measurements. We shall show a rough model of typical parasitics in a setup and estimate the associated stability criteria. Modelling is backed up with experimental data from SSAs fabricated at VTT (M. Kiviranta et al., 2021) and operated at T  =  4.2 K. In experiments the SSA has been augmented with a set of controlled external parasitic elements and the instability threshold observed. Results show correlation between simulations and experimental data, suggesting the SSAs to be prone to instabilities caused by internal and external parasitics.