  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 Psat
and intrinsic thermal time constant tau0. 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 Psat and tau0 to be determined without contribution from
the negative electrothermal feedback. We describe the method and results of the
measurement using it. We evaluated Psat of five samples by applying DC power
and confirmed the overall trend between Psat and the inverse leg length. We
evaluated tau0 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.

Instrumentation and Methods for Astrophysics