iased High-Tc Superconducting Infrared Bolo Electrothermal Feedback

* Abstract - In the current generation of high- T, bolometers the thermal conductance is often chosen for a short time-constant rather than for optima1 sensitivity. We describe a novel bolometer bias and readout scheme that promises to relax this constraint. Voltage bias of the superconductor results in strong negative electrothermal feedback that greatly reduces the time-constant of the bolometer. We estimate that a decrease of more than one order of magnitude in time-constant should be possible with existing high-T, thermometers. We give theoretical estimates of the performance gain with voltage bias for several bolometers that have been reported in the literature. We find cases where the sensitivity can be greatly improved (by changing the thermal conductance) while holding the time constant fixed and others where the bolometer can be made much faster while maintaining the sensitivity. Superconducting transition-edge bolometers have traditionally been operated with a constant-current bias, which results in positive electrothermal feedback (ETF).(S) The bias current must therefore be limited to avoid thermal instability. It has recently been pointed out(6) that a constant-voltage bias mode with strong negative ETF(6,7) gives several important operational advantages. The most important of these is a large reduction of the bolometer time-constant due to strong ETF. We estimate a decrease of more than one order of magnitude in time constant should be possible with existing high-Tc thermometers. In the current generation of high-Tc bolometers the thermal conductance is often chosen for a short time-constant rather than for optimal sensitivity. The time-constant decrease due to negative ETF can be used to relax this constraint. In this Daner. we give theoretical estimates of the