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The Atacama Large Millimeter-submillimeter Array (ALMA) is a general purposetelescope that performs a broad program of astrophysical observations.Beginning in late-2016, solar observations with ALMA became available, therebyopening a new window onto solar physics. Since then, the number of solarobserving capabilities has increased substantially but polarimetricobservations, a community priority, have not been available. Weakly circularlypolarized emission is expected from the chromosphere where magnetic fields arestrong. Hence, maps of Stokes V provide critical new constraints on thelongitudinal component of the chromospheric magnetic field. Between 2019-2022,an ALMA solar development effort dedicated to making solar polarimetry atmillimeter wavelengths a reality was carried out. Here, we discuss thedevelopment effort to enable solar polarimetry in the 3 mm band (ALMA Band 3)in detail and present a number of results that emerge from the developmentprogram. These include tests that validate polarization calibration, includingevaluation of instrumental polarization: both antenna based "leakage" terms andoff-axis effects (termed "beam squint" for Stokes V). We also present testpolarimetric observations of a magnetized source on the Sun, the followingsunspot in a solar active region, which shows a significant Stokes V signaturein line with expectations. Finally, we provide some cautions and guidance tousers contemplating the use of polarization observations with ALMA.

Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Polarization Observations at 3 mm