  Advanced ACTPol (AdvACT) is a third generation polarization upgrade to the
Atacama Cosmology Telescope, designed to observe the cosmic microwave
background (CMB). AdvACT expands on the 90 and 150 GHz transition edge sensor
(TES) bolometer arrays of the ACT Polarimeter (ACTPol), adding both high
frequency (HF, 150/230 GHz) and low frequency (LF, 27/39 GHz) multichroic
arrays. The addition of the high and low frequency detectors allows for the
characterization of synchrotron and spinning dust emission at the low
frequencies and foreground emission from galactic dust and dusty star forming
galaxies at the high frequencies. The increased spectral coverage of AdvACT
will enable a wide range of CMB science, such as improving constraints on dark
energy, the sum of the neutrino masses, and the existence of primordial
gravitational waves. The LF array will be the final AdvACT array, replacing one
of the MF arrays for a single season. Prior to the fabrication of the final LF
detector array, we designed and characterized prototype TES bolometers.
Detector geometries in these prototypes are varied in order to inform and
optimize the bolometer designs for the LF array, which requires significantly
lower noise levels and saturation powers (as low as ${\sim}1$ pW) than the
higher frequency detectors. Here we present results from tests of the first LF
prototype TES detectors for AdvACT, including measurements of the saturation
power, critical temperature, thermal conductance and time constants. We also
describe the modifications to the time-division SQUID readout architecture
compared to the MF and HF arrays.
