  The polarization properties of radio sources at very low frequencies (<200
MHz) have not been widely measured, but the new generation of low-frequency
radio telescopes, including the Low Frequency Array (LOFAR: a Square Kilometre
Array Low pathfinder), now gives us the opportunity to investigate these
properties. In this paper, we report on the preliminary development of a data
reduction pipeline to carry out polarization processing and Faraday tomography
for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of
this pipeline from the LOTSS preliminary data release region (10h45m - 15h30m
right ascension, 45 - 57 degrees declination, 570 square degrees). We have
produced a catalog of 92 polarized radio sources at 150 MHz at 4.3 arcminute
resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized
sources at such low frequencies. We estimate a lower limit to the polarized
source surface density at 150 MHz, with our resolution and sensitivity, of 1
source per 6.2 square degrees. We find that our Faraday depth measurements are
in agreement with previous measurements and have significantly smaller errors.
Most of our sources show significant depolarization compared to 1.4 GHz, but
there is a small population of sources with low depolarization indicating that
their polarized emission is highly localized in Faraday depth. We predict that
an extension of this work to the full LOTSS data would detect at least 3400
polarized sources using the same methods, and probably considerably more with
improved data processing.
