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Advanced ACTPol (AdvACT) is a third generation polarization upgrade to theAtacama Cosmology Telescope, designed to observe the cosmic microwavebackground (CMB). AdvACT expands on the 90 and 150 GHz transition edge sensor(TES) bolometer arrays of the ACT Polarimeter (ACTPol), adding both highfrequency (HF, 150/230 GHz) and low frequency (LF, 27/39 GHz) multichroicarrays. The addition of the high and low frequency detectors allows for thecharacterization of synchrotron and spinning dust emission at the lowfrequencies and foreground emission from galactic dust and dusty star forminggalaxies at the high frequencies. The increased spectral coverage of AdvACTwill enable a wide range of CMB science, such as improving constraints on darkenergy, the sum of the neutrino masses, and the existence of primordialgravitational waves. The LF array will be the final AdvACT array, replacing oneof the MF arrays for a single season. Prior to the fabrication of the final LFdetector array, we designed and characterized prototype TES bolometers.Detector geometries in these prototypes are varied in order to inform andoptimize the bolometer designs for the LF array, which requires significantlylower noise levels and saturation powers (as low as ${\sim}1$ pW) than thehigher frequency detectors. Here we present results from tests of the first LFprototype TES detectors for AdvACT, including measurements of the saturationpower, critical temperature, thermal conductance and time constants. We alsodescribe the modifications to the time-division SQUID readout architecturecompared to the MF and HF arrays.

Advanced ACTPol Low Frequency Array: Readout and Characterization of Prototype 27 and 39 GHz Transition Edge Sensors