Development of the Characterization Methods Without Electrothermal Feedback for TES Bolometers for CMB Measurements

Superconducting Transition-Edge Sensor (TES) bolometers are used for cosmic microwave background (CMB) observations. We used a testbed to evaluate the thermal performance of TES bolometers in regard to the saturation power P_sat and intrinsic thermal time constant τ _0 . We developed an evaluation method that is complementary to methods with electrothermal feedback. In our method, the antenna termination resistor of the bolometer is directly biased with DC or AC electric power to simulate optical power, and the TES is biased with small power, which allows P_sat and τ _0 to be determined without contribution from the negative electrothermal feedback. We describe the method and results of the measurement using it. We evaluated P_sat of five samples by applying DC power and confirmed the overall trend between P_sat and the inverse leg length. We evaluated τ _0 of the samples by applying DC plus AC power, and the measured value was reasonable in consideration of the expected values of other TES parameters. This evaluation method enables us to verify whether a TES has been fabricated with the designed values and to provide feedback for fabrication for future CMB observations.